Gradients as Visual Primitives

Journal of Expmimental Psychology: Human Pefc~cm and Pe~'formance 1997, VoL 23, No. 2, 370-387

Copyright 1997 by the Americaa Psychological Association, Inc. 0096-1523/97/$3.00

Gradients as Visual Primitives Belinda G o o d e n o u g h and Barbara G i l l a m University of New South Wales Following J. J. Gibson (1950), it is implicitly assumed in the literature that texture gradients are directly available as perceptual primitives. Yet, the depth response to compression gradients is poor compared with gradients of linear perspective. This may indicate that mechanisms for directly detecting the differential structure that constitutes a compression gradient do not exist. We tested this hypothesis outside the context of depth perception by measuring the speed with which participants could detect a gradient anomaly as a function of the number of elements in the gradient. Only in the case of linear perspective did anomalies "pop out." This was attributable to the emergent feature of alignment of the ends of the elements forming the gradient rather than the direct detection of its differential structure. It is argued that gradients are not perceptual primitives and that the poor depth response to compression in a variety of contexts (motion parallax, stereo, and perspective) therefore is not surprising.

distributed elements forming a gradient are directly available and do not need to be derived from zero-order properties (individual lengths; Lappin, Ahlstrtm, Craft, & Tschantz, 1994). The current research was designed to test this assumption. Gradients in the two-dimensional projection of the details on a surface receding in depth can be classified as one of two basic types (shown in Figure 1) or their combination. In the first type of gradient, equal extents perpendicular to the depth axis (parallel to the picture plane) form a gradient of extents on the picture plane, such that the projected size of an extent varies in inverse proportion to its distance from the point of observation. Such a gradient, being linear, is specified by first-order information, although second-order information must be processed to estabfish it as first-order. Detecting an anomaly in the gradient also requires secondorder information. This type of gradient, known as linear perspective, has a well-known emergent feature: If the extents projected have collinear end points, the projected end points also will be collinear. The implicit or explicit collinear line joining the end points will converge to a vanishing point shared by the projections of any lines parallel to it. If this collinearity (also a second-order property) is detected directly, the slope of the line may be detected directly and be available for depth perception (see Sedgwick, 1986). The second type of gradient is found in projections of extents on the surface parallel to the depth axis (orthogonal to the picture plane). As the distance of a given extent from the point of observation increases, its projected size on the picture plane decreases approximately as the square of the distance (Gibson, 1950). Such a gradient (usually described as a compression gradient), being approximately quadratic, is specified by second-order information, although thirdorder information must be processed to establish it as second order. Detecting an anomaly in the gradient therefore requires third-order information. Unlike linear perspective gradients, compression gradients have no emergent features. Both linear perspective and compression gradients spec-

The gradients of concern in this article are first derivatives of size with respect to spatial position, often known as texture gradients. Such gradients are of particular interest in vision because of their association with depth perception (e.g., Gibson, 1950, 1966). Following the work of Gibson, Purdy (1960) showed that, mathematically, a texture gradi. ent in the optic array (usually approximated by the picture plane) potentially specifies the slant of a distal surface and the size and distance of objects in contact with that surface, Although there has been considerable discussion about how texture gradients should be defined (e.g., Purdy, 1960; Sedgwick, 1986; Stevens, 1981, 1984), the use of the term usually seems to imply that the gradient is a potential direct source of information or "primitive" for space perception that is not reducible to the successive comparison of the individual elements making up the gradient. For example Treisman, Cavanagh, Fischer, Ramachandran, and yon der Heydt (1990) stated: It seems plausible, however, that the visual system has evolved to extract information even at the earliest stages, in a form which specifies the external world. Thus... the visual system may code such properties directly. Examples may be the gradients of texture and of motion that specify threedimensional shapes and surfaces. (p. 307)

In current terminology, the implicit assumption appears to be that the first- and second-order properties of spatially Belinda Goodenough and Barbara Gillam, School of Psychology, University of New South Wales, Sydney, New South Wales, Australia. This work was supported by Grant B350305 from the Australian Research Council. Experiments 1 and 2 were presented at the 20th Experimental Psychology Conference held in Melbourne, Australia, in April 1993. We thank Sam Lander for technical assistance. Correspondence concerning this article should be addressed to Barbara Gillam, School of Psychology, University of New South Wales, Sydney, New South Wales, 2052 Australia. Electronic mail may be sent via Internet to [email protected]. 370

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Perspective (scaling) Figure 1. Example of a compression gradient and a linear perspective gradient in a texture of circles, depicted with respect to the ground plane. To view these same two gradient types with respect to a wall plane (as represented in the bar stimuli used in the current experiments), turn this diagram 90* cloekwise.

ify various aspects of the spatial layout of a surface. For example, the number of texture dements with which an object placed on a surface makes contact is preserved in the projection of that surface and specifies the size of that object, given an assumption of uniform texture in the environment projected. Such judgments of object size do not require any appreciation of the steepness of the texture gradient. On the other hand, the specification of distance and slant do depend on the steepness of the gradient (see Sedgwick, 1986). However, although Purdy (1960) demonstrated the mathematical equivalence of linear perspective and compression information in specifying the properties of surfaces, it has been increasingly recognized in depth perception research that these two types of gradients differ considerably in perceptual effectiveness. Studies have shown that whereas linear perspective supports accurate perception of slant and distance, compression gradients are poor at supporting such perceptions (Braunstein, 1968; Cutring & Millard, 1984; Gillam, 1968, 1970; Rosinski & Levine, 1976; Vickers, 1971). To our knowledge, the reason for this difference in depth response to perspective and compression gradients has never been investigated. It simply could reflect a difference in the degree to which a depth response is associated with each form of texture gradient. Alternatively, it could reflect a more fundamental difference in the degree to which the two types of gradient are detectable directly. There are several differences between the two types of gradients that could have effects on their direct detectability. Three such differences deserve mention. First, linear perspective gradients involve lengths parallel to the picture plane and to each other, whereas compression gradients involve extents orthogonal to the picture plane and that lie end-on to each other. Second, as described earlier, the mathematical forms of the gradients differ in the two cases, with the former being linear and the latter quadratic. Third, and unlike

371

regular compression gradients, regular linear perspective gradients have an emergent feature (end-point collinearity). In the current research we explored the question of whether the well-known differences in the effectiveness of the two kinds of texture gradients in depth perception arise, at least in part, from a fundamental difference in their immediate detectability as gradients. The testing of this hypothesis required a task not involving a depth response because depth tasks confound gradient detectability with the degree to which particular gradients are associated with depth responses. We assessed gradient detectability using a speeded discrimination task. In this task, participants were asked to indicate as quickly as possible whether a particular gradient represented in a series o f bars (e.g., a sequence of descending heights) was consistent or contained an anomaly (e.g., a bar that was too tall for its particular position in the height sequence). The number of bars was varied. We assumed that if decision time does not increase significantly as more bar elements are added to the gradient sequence, the gradient for that particular dimension is directly available rather than built up from successive scrutiny of element extents. Such an assumption parallels those underlying interpretation of visual search data (e.g., Treisman, 1985; Wolfe, Cave, & Franzel, 1989), and some of the conventions of the visual search paradigm have been adopted in this article. Because our research was not concerned with whether different types of gradients support depth-slant responses (this is already well established), the stimuli we used were deliberately impoverished relative to naturally occurring texture patterns. Despite the simplified nature of the patterns, however, the stimuli retain the same geometric properties that characterize the gradients studied in the context of depth perception. Gradients on three dimensions of a series of horizontally separated vertical bars were tested: the height, width, and separation of bar elements (see Figure 2). The height variation constituted a linear perspective gradient, and variations in either width or separation constituted compression gradients. The aim was to investigate the immediate availability of the gradients in these three dimensions. Furthermore, considering that gradients on these three dimensions of height, width, and separation typically eovary in natural scenes, we also examined whether these gradients would interact in the sense that the state of one would influence the speed and accuracy of response to another. Experiment 1 To investigate the relative ease with which the separate gradients 1 of height, width, and separation (i.e., gaps between elements) can be detected, we showed participants a 1 If the distance between two consecutive points in one dimension is considered the unit (zero-order property) of the gradient, comparison of the successive extents (first,order property) will not be sufficient to establish whether the gradient is linear (as in the case of linear perspective) or (to a first approximation) quadratic, as in the case of compression. Similarly, the comparison of three

372

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pleted six tasks (see Table 1), which made up the three possible two-way combinations of the gradients of height, width, and separation. For example, in one task the participants were instructed to respond to the gradient of height and ignore any differences in the widths of the bars, whereas in another task the participants were required to respond to a width gradient and ignore any differences in the heights of the bars (in both tasks, separation was held constant). Height and separation (with width held constant), and separation and width (with height held constant), were investigated similarly. In each of these six tasks, the varying but irrelevant attribute of the bars was manipulated in one of four ways (see Figure 2): (a) It formed a gradient that decreased in the same direction (left to right) as the attended gradient (coherent); (b) it formed a gradient that decreased in the opposite direction (right to left) from the attended gradient (opposite); (c) it did not form a gradient but, the set of values for the coherent and opposite conditions were randomized across the positions in the series (random); and (d) it did not form a gradient but was held constant (fixed). Therefore, in each of these six tasks, the participants knew which attribute of the bars formed the gradient that was the basis of the response, which attribute of the bars was irrelevant but also manipulated, and which attribute would always be held constant from trial to trial. They did not know from trial to trial, however, whether an anomaly would be present or absent on the attended gradient, how many bar elements constituted the series, or in which of the four ways described earlier that the second irrelevant attribute of the bars would be manipulated.

Me~od series of bar elements in which at least one of these dimensions formed a gradient, and they were instructed to attend to that dimension (either the heights of the bars, the widths of the bars, or the separations between the bars). Participants were asked to indicate the presence or absence of an anomaly on the attended gradient. When present, the anomaly was generated by swapping the values of two adjacent elements in the series with respect to the attended gradient (e.g., the heights of two adjacent bars were swapped; see the Method section). Equally rapid detection of the anomaly regardless of the number of elements in the series (6, 8, or 10) was taken as evidence of direct perception of that particular gradient, whereas decision latency that increased with the number of elements was taken as evidence of a response built up serially from a series of local perceived differences between elements. That is, and to borrow the terminology from the visual search paradigm (e.g., Treisman & Gormican, 1988), we explored whether the anomaly in the attended gradient would "pop out." To investigate whether the ability to respond to a designated gradient (e.g., width) would be affected by the presence of a second gradient in one of the two irrelevant stimulus magnitudes (e.g., height), all participants comsuccessive extents (second-order property) will not be sufficient to establish whether the gradient is quadratic or regular.

Participants. Participants were 5 male and 5 female undergraduates who received credit for their participation. Materials and apparatus. Displays were drawn on a color VGA screen (operating in the high-resolution mode), which was controlled by a 386 IBM-compatible PC. The stimuli were 6, 8, or 10 filled bars drawn in dark blue on an off-white (greenish) background. Although the task did not involve depth judgments in any way, the gradients used were of the type that project to a station point (SP) from a series of equally spaced threedimensional objects extending away from the observer (see Figure 3), with the following specifications (expressed in meters). The total width of the scene projected was held constant at 15 m. The required number of elements (6, 8, or 10) then were fitted into this area to conform to the projection of a series of identical and Table 1 Attended Gradient, Irrelevant but Manipulated Attribute, and the Bar Attribute Held Constant for Each of the Six Tasks (Three Pairs) in Experiment 1 Attended

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GRADIENTS AS VISUAL PRIM1TIVF~

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373

sponses were recorded by pressing a key. The participants used one button with the index finger of the dominant hand to indicate the presence of an anomaly and used a second button with the index t'mger of the nondominant hand to indicate that no anomaly was present. In each task there was one block of 30 practice trials, followed by four blocks of 72 test trials. Before each of the six tasks, the participants were instructed about which gradient they were to attend to and to ignore any other differences in the bar elements. They were asked to respond as quickly as possible without making mistakes. Feedback on errors was given viaa tone, and the gradient anomaly also was highlighted (in red) in the event of an incorrect anomaly-absent response. All six tasks were completed in a single testing session lasting no longer than 2 hr, and the order of task completion was counterbalanced across participants.

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Figure 3. Perspective depth projection used to generate gradients with a fixed visual angle, regardless of the number of elements in the display. This figure shows the relative position of the station points (SPs) for a series (S) of 6 elements (SP6), 8 elements (SP8), and 10 elements (SP10).

equally spaced bars receding in depth. Under this constraint of a fixed visual angle of the display, both the degree of laterality of SP relative to the line on which the elements lay (LSP), and the distance between SP and the picture plane (DPP), varied according to the number of elements that needed to be fitted into the display (see Figure 3): for 6 elements (SP6), LSP = 1.74 m and DPP = 6 m; for 8 elements (SP8), LSP = 1.89 m and DPP = 13.7 m; and for 10 elements (SP10), LSP = 2.07 m and DPP = 25.72 m. Although the gradients were derived from a two-dimensional projection of a three-dimensional scene, for the height gradient the upper and lower edges of the bars were always parallel and horizontal. When present, a height gradient was applied to both the upper and lower edges of the bars. The lengths of the bars went from a maximum of 2.75 ° to a minimum of 0.69°; otherwise, the heights of the bars were held constant at 2.75 °. When a width or a separation gradient was applied, the smallest (rightmost) widthseparation was always 0.08 °, whereas the maximum (leftmost) width-separation was 2.79 ° (6 elements), 1.95 ° (8 elements), and 1.33 ° (10 elements). When a width-separation gradient was held constant, it assumed one of the following values: 0.27 ° (6 elements), 0,21 ° (8 elements), or 0.15 ° (10 elements). 2 The direction of the attended gradient was always from left to right across the screen. Although this invited a left-right reading bias when viewing the display, the influence of this should have been equal for all three gradients across all six tasks. When the irrelevant attribute varied randomly, the same set of values for that attribute when forming a gradient coherent from left to right was assigned randomly across the positions of the bar elements. When present, the position of the anomaly in the attended gradient could be either left, right, or centrally placed. The anomaly was generated by swapping the attributes (only with respect to the attended gradient) of two adjacent bars. A constraint was that the anomaly did not include the left- or rightmost bar elements. There were equal numbers of anomaly-present and anomaly-absent trials. Displays were viewed from a distance of 150 cm. A chin rest was used to restrict head movement. Procedure. Each participant was tested individually. Re-

The data for 1 participant were discarded because of an extremely high error rate (this participant appeared to perform no better than chance, with greater than 60% errors on one task). The analyses proceeded on the remahdng 9 participants using median reaction times ( R T s ) a n d errors averaged across the last three blocks of trials (the first block of trials in each of the six tasks was discarded as practice). Figure 4 plots RT as a function of response type (anomaly present or absent), the number of elements in the gradient (6, 8, and 10), and the nature of the manipulation of the irrelevant attribute (fixed, coherent, opposite, and random). The results are graphed according to the pairs of tasks listed in Table 1. The overall slopes of the functions relating RT to the number of bar elements in the gradient (hereafter referred to as "slope") for each of the three gradients (as averaged across response type and all manipulations of the irrelevant attribute) were 2.1 for height, 128.1 for width, and 92.1 for separation. Slopes, intercepts, r 2 measures of linearity, and percentage of errors for each Condition × Re2 Clearly, in maintaining a constant visual angle, the steepness of the projected gradient varied as a function of the number of elements in the series. The relative steepness of the gradient may be a significant factor if the task requires a depth-related response, but this was not elicited from participants in our experiments. Because pilot work with displays holding the steepness of the gradient constant (with visual angle varying) did not change the character of the results, a constant visual angle was maintained on the basis of assumptions surrounding the visual search paradigm as a diagnostic of attentional processing (e.g., Treisman, 1988; Wolfe, Cave, & Franzel, 1989), that is, the aim to hold constant, as much as possible, the region of the visual field over which attention may be distributed. This rule of constant visual angle was not satisfied, however, for those conditions in which height was held constant and both a width and a separation gradient were applied to the bars. In these cases, both the attended and irrelevant attributes of the bars necessarily contribute to the overall projected width of the display, but not with equal numbers of elements. For example, a gradient of n widths composes n-1 (not n) gaps. Thus, for the combinations of width and separation, the visual angle subtended by the display varied slightly with the number of elements in the gradient: 6.32 ° (6 bars), 7.03 ° (10 bars), and 7.59 ° (10 bars). Although the differences were small, this covariance between display width and the number of elements in the series was addressed in Experiment 3.

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sponse Type in each of the six tasks are listed in Table 2. Separate analyses of variance (ANOVAs) were performed on median RTs and errors. RTs. RTs were significantly faster for anomaly-present trials than for anomaly-absent trials, F(1, 8) = 14.78, p < .05. Planned orthogonal contrasts confirmed the asymmetries evident in Figure 4, listed as follows. Responses to a height gradient (in which the task was to ignore any differences in the separations between the bars) were significantly faster overall than responses to a separation gradient (ignoring height), F(1, 8) = 41.42, p < .05 (see Figure 4A). This asymmetry proved significantly greater in magnitude for anomaly'absent decisions, F(1, 8) = 6.37, p < .05. Responses to a height gradient (ignoring differences in width) were significantly faster than responses to a width gradient (ignoring height), F(1, 8) = 55.01, p < .05 (see Figure 4B), especially on anomaly-absent trials, F(1, 8) = 14.81, p < .05. Responses also were significantly faster for a width gradient (ignoring separation) than to a separation gradient (ignoring width), F(1, 8) = 16.98,p < .05 (see Figure 412). This latter asymmetry between width and separation did not interact with response type. Figure 5 shows the effect of the manipulation of the irrelevant attribute (averaged across the number of elements in the gradient) for anomaly-present trials (see Figure 5A) and anomaly-absent trials (see Figure 5B). Planned orthogonal contrasts showed that, generally, any manipulation of the irrelevant attribute served to increase RT. That is, the average RT for the coherent, opposite, and random conditions was greater than that for the fixed condition, F(1, 8) = 135.57, p < .05, especially for anomaly-absent trials, F(1, 8) = 21.97, p < .05. Random manipulation Of the irrelevant attribute produced the greatest increase in RT (random vs. coherent and opposite), F(1, 8) = 63.31, p < .05, especially on anomaly-absent trials, F(1, 8) = 52.41, p < .05. For the two conditions in which the irrelevant attribute formed a gradient (coherent and opposite), RTs were significantly faster overall if the irrelevant gradient ran in the same (coherent) than the opposite direction to the attended gradient, F(1, 8) = 11.51, p < .05, especially on anomalyabsent trials, F(1, 8) = 7,66, p < .05. There were significant interactions between the effects of condition type (fixed, coherent, opposite, and random) and the asymmetries described earlier. The detrimental impact of the random manipulation (compared with the coherent and opposite conditions) was significantly greater for the task that yielded the slowest RTs in each task pair, as listed in Table 1 and plotted in Figure 4, that is, for separation (ignoring height), F(1, 8) = 20.44, p < .05, for width (ignoring height), F(1, 8) = 6.08, p < .05, and for separation (ignoring width), F(1, 8) = 25.46, p < .05. In each case, this difference was amplified on anomaly-absent trials, as verified by the following significant interactions with response type: F(1, 8) = 7.64, p < .05, for separation (ignoring height), F(1, 8) = 7.31, p < .05, for width (ignoring height), and F(1, 8) = 7.41, p < .05, for separation (ignoring width). When the irrelevant attribute formed a gradient (coherent and opposite), the detrimental impact of an irrelevant opposite versus an irrelevant coherent gradient

375

GRADIENTS AS VISUAL PRIMITIVF~ Table 2

Measures of Linearity and Percentage of Errors for Anomaly-Present and AnomalyAbsent Trials for Each Condition in Experiment I Attended gradient Height

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107.8 -35.9 3.1 -6.9 122.6 -41.4 123.5 91.5 51.1

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199.1 70.0 151.6 32.1 149.7 20.7 208.3 206.9 133.9

-243 1,121 170 1,180 451 1,830 76 901 648

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376

GOODENOUGH AND GILLAM more errors when attending to a gradient of separation (ignoring width) than the reverse, F(I, 8) = 6.96, p < .05. Errors were least frequent when the irrelevant attribute was not manipulated (fixed vs. the coherent, opposite, and random conditions), F(1, 8) = 41.01, p < .05, and most frequent for the random condition, F(1, 8) = 11.52, p < .05, especially on anomaly-present trials, F(1, 8) = 12.76, p < .05. There was no main effect or any significant interaction that showed a tendency for more errors in the opposite than coherent condition (all ps > .05).

Discussion

Figure 5. Results for Experiment 1 showing the effect of the manipulation of the irrelevant attribute, averaged across the number of elements in the gradient, for anomaly-present (Figure 5A) and anomaly-absent (Figure 5B) trials. SEP = separation.

was significantly greater when participants were attending to width (ignoring height) than to height (ignoring width), F(1, 8) = 26.95, p < .05. Neither of these results interacted with response type. The detrimental effect of the opposite over a coherent gradient was equivalent, however, whether attending to width (ignoring separation) or separation (ignoring width; p > .05). Errors. There were more misses than false-positives, F(1, 8) = 50.76, p < .05, with errors also tending to increase as the number of elements in the gradient increased, F(2, 16) = 69.32, p < .05. Generally, the analysis of errors complemented the patterns for RTs. Fewer errors were made when attending to a height gradient ignoring differences in either width, F(1, 8) = 15.02, p < .05, or separation, F(1, 8) = 30.54, p < .05, than on the reverse of these two tasks. There also was a significant tendency for

These results suggest that for the three dimensions tested and using simple bar stimuli, the only gradient type in which anomaly detection was independent of the number of elements was linear perspective (i.e., a gradient of changes in the size of bar heights). We concluded that the perspective gradient was perceived directly. The average slopes for attending to a perspective gradient (see Table 2) frequency satisfied the criterion of less than 15 ms per item, which is used as a diagnostic for parallel processing in visual search 3 (e.g., Enns & Rensink, 1990); however, there was no evidence in our results that the compression gradient present for the width or separation of the bars could be perceived directly. Slopes for this gradient type consistently averaged more than 100 ms per item. It is likely that the magnitude of these slopes primarily reflected the difficulty of the task because it certainly was the case that the participants in this experiment invariably commented that the tasks in which they were asked to attend to changes in width or separation size were comparatively difficult under the demands for accuracy. Certainly, in a related paradigm, visual search, steep slopes (i.e., slopes greater than 200 ms per item; e.g., Braddick & Holliday, 1991; Holliday & Braddick, 1988) have been interpreted simply as underscoring the extreme difficulty of the task for the participants. The slope patterns we observed should be interpreted with respect to the accompanying intercepts. For example, on slopes alone, it might be assumed that detecting an anomaly on a separation gradient (when a coherent height gradient is present) is as easy as detecting an anomaly on a height gradient (when a coherent separation gradient is present; see Figure 4A). It is clear, though, that large intercept differences (favoring height) accompany these search slopes. It could be argued that a shallow slope with a low intercept constitutes more compelling evidence that a fast anomalypresent decision does reflect detection of a feature that can be defined as a visual primitive (i.e., an emergent feature such as linear perspective for bar height). On the other hand, a similarly shallow slope that is accompanied by a high 3 Other, more stringent criteria have been applied, such as 6 ms per item (Treisman & Souther, 1985) and 12 ms per item (Nakayama & Silverman, 1986), but Enns and Rensink (1990) claimed that search slopes less than 15 ms per item can be taken as diagnostic of parallel processing because they are still well below the accepted estimates of the speed of movement of an attentional SCan.

377

GRADIENTS AS VISUAL PRIM1TIVF~ intercept may reflect a strategy that serves to maximize performance for the task at hand but cannot be said to define a visual primitive. The second aim of Experiment 1 was to investigate the nature and type of interactions among the three gradients. A number of asymmetries have emerged (see Figures 4A4C). It is easier to detect an anomaly in a height gradient while ignoring variations in either the widths or separations of the bars than it is to do these tasks in reverse. On the basis of a significant overall difference in RT, it also is easier to detect a width anomaly while ignoring differences in separation than vice versa (see Figure 4C). These asymmetries are reminiscent of those found for simple properties such as line orientation and curvature in visual search (Treisman & Gormican, 1988). Treisman and Gormican found that for a pair of stimuli diffem~g by one feature on a single dimension, search efficiency depended on which of the pair was given the role of the target and the nontarget. In one account of search asymmetry, Treisman and Gormican suggested that the flatter of the two RT functions reflects efficient search based on the presence of an attribute possessed by the target item but lacked by the nontarget items. If this reasoning can be applied to the results of Experiment 1, the relatively flatter functions for detecting anomalies in a height gradient may indicate the presence of an attribute that is lacking in both the width or separation gradients. One candidate is the emergent property of alignment of bar comers that is associated with a height gradient, but not with a width or separation gradient. The possible role for this attribute was the focus of Experiment 2. The response to a width gradient was significantly easier (faster RTs and fewer errors) than to the separation gradient, despite the fact that the metric properties of the gradients were identical. This makes sense in ecological terms. Perception is of objects, surfaces, and boundaries. It is unusual to be asked to attend to the gaps between objects (the featureless ground), although the results of Experiment 1 show that it can be done. The type of gradient formed by the irrelevant attribute was found to be important. An irrelevant gradient was more detrimental to performance if it was running in the opposite than the same direction as the attended gradient, This finding may reflect~the fact that gradients usually ~ v a r y in the real world. Individual gradients generally contribute in a cooperative way to the same percept (e.g., thelperception of depth; cf. Figure 1). Our results suggest that when this usual cooperation between gradients is prevented, the apprehension of any individual gradient in a scene may be impeded significantly. However, the presence of one gradient did not facilitate the response to another gradient. The only suggestion of any such facilitative interactions in this experimCilt were responses on anomaly-absent trials for attending to either a width or a separation gradient and in which an irrelevant coherent height gradient also was present (see Figure 5C). In these two instances, the presence of a coherent but irrelevant height gradient did appear to facilitate RTs relative to when the irrelevant attribute of height was held constant (i.e., fixed). Overall, these findings suggest that the

property of an irrelevant attribute that promotes performance on the attended gradient is the predictability of position in guiding a left-fight serial search' type of processing strategy. Nonetheless, these results suggest overall that gradients do not facilitate one another, although an opposite gradient does iml~tt¢ performance relative to a coherent gradient. This ~ u s i o n , of course, is limited to the task demands and stimulus types of the current experiment. It may be that facilitative gradient interactions would be evident in a task requiring a different response associated with gradients in depth (e.g., the perception of surface slant).

Experiment 2 In Experiment 1, we found a clear advantage for detecting anomalies in a perspective gradient (i.e., changes in bar height), compared with a compression gradient (i.e., changes in bar width or separation): RTs were faster and errors were fewer. This result could have been attributable to an emergent property associated with the heights of the bars, namely, the linear alignment of the bar corners. When a height gradient was applied to the bars, the global shape of the series was a serrated wedge (see Figure 2). Any protrusion from this wedge shape would signal a height anomaly. For the gradients of width or separation, the global shape property of the stimulus could not be used in the same way to signal the presence of an anomaly. In these latter cases, anomaly detection appeared to have depended on scrutiny of the individual elements that comprised the series. If participants could reliably use the emergent property of comer alignment to detect the presence of a height anomaly, this raises the question as to why (irrelevant) manipulations of the width or separation of the bars caused significant interference with detection of height anomalies. This pattern of interference was most marked when the irrelevant gradient was width (see Figure 4B) and when the widths of the bar elements varied randomly. The interference in detecting a height anomaly in the conditions containing an irrelevant width gradient (either coherent or opposite) was manifest as an increase in the value of the y-intew.ept (i.e., an overall increase in RT); search slopes for these conditions still pass the criterion for parallel processing as applied in the visual search paradigm (see Table 2). This interaction between width and height was explored further in Experiment 2. Figure 6 shows the alignment of bar comers for the task in Experiment 1, in which the instruction was to attend to height and to ignore differences in width (separation was held constant). It can be seen that for the fixed-, coherent-, and opposite-width manipulations, the left- and fight-hand comers of the bars are each aligned in a regular fashion. In the fixed-width condition, the left- and right-hand comers are all aligned linearly. In the coherent-width condition, the left-hand comers are aligned linearly, whereas the righthand corners lie on a smoothly curved slightly bowed (concave) line. In the opposite-width condition, the right-hand comers are aligned linearly, whereas the left-hand comers lie on a smoothly curved slightly bowed (convex) line. The

378

~)ODENOUGH AND GILLAM

Width Fixed

Width Opposite

Width Coherent

Width Random

Figure 6. Alignment of the bar comers for each condition in Experiment 1 in which the task was to attend to height and ignore differences in bar width. For each of the four conditions, the linear relationship between the left-most comers of the bars is represented by the lower line of the pair of lines above each gradient, whereas the linear relationship between the right-most corners of the bars is represented by the upper line of the pair of lines, concavity of the implicit line joining the right-hand corners in the coherent=width conditions is equal to the convexity of the implicit line joining the left-hand corners in the opposite-width condition. Thus, for the fixed, coherent, and opposite conditions, an anomalous height could be detected by checking for the presence of a deviation from regularity with respect to one or both of the two sets of corner alignments. This was not the case, however, for detecting a height anomaly in the random-width condition. In this condition, although the left-hand corners were linearly aligned, the right-hand set of corners could not be fitted to any regular pattern, either linear or curved. In Figure 6, there is no height anomaly present in any of the four examples. Yet, it is clear that with the random-width manipulation, decision time might have been potentially slowed because the series contained several marked deviations from a smooth curve or line. For displays of this type, such projections would need to be checked to determine whether they are true height anomalies before a confident decision could be initiated. Therefore, one hypothesis is that a priw~ry determinant of the ease with which an anomaly in a height gradient can be detected is the degree to which the emergent cue of corner alignment can be extracted and reliably used. When attending to a height gradient and ignoring width, the cue of corner alignment is strongest when bar widths are fixed and poorest when bar widths are random (see Figure 6). In the latter case, a deviation from regularity does not reliably signal a height anomaly. Thus, in the random-width condition, greater scrutiny of the individual elements in the series may be required to determine whether a height anomaly is present. In Experiment 2, we tested this hypothesis in the following way: At all times, the participants attended to the gradient of height in a series of bars, ignoring any differences in the widths of the bars (separations between the bars were always held constant). The four manipulations of the irrelevant attribute were the same as in Experiment 1: fixed, coherent, opposite, or random. In contrast to Experiment 1, in which only bars with horizonUd edges were used, in Experiment 2 (see Figure 7), there were four types of bar edges. Edges could be either horizontal or convergent, and each of the bar corners in the series could be aligned or misaligned. In two conditions, the corners of the bars were

aligned. In the horizontal-aligned condition, the left- and the right-hand corners were independently aligned, but in the convergent-aligned condition, the edges were coliinear across all bars. In the other two conditions (horizontal misaligned and convergent misaligned), the corners of the bars were globally misaligned to remove any emergent regular line formed by one or both sets of bar corners. The individual bars were the same as in the two aligned conditions. The rnisalignments were produced by offsetting the relative vertical positions of the bars (see the Method section). If participants were simply relying on the emergent property of regular corner alignment to detect height anomafies in Experiment 1, then three predictions follow for Experiment 2: (a) When the bars are aligned, RTs should be faster when the edges of the bars are convergent rather than horizontal, as the cue of linear corner alignment is most strongly specified for convergent bar edges. That is, RTs in the convergent-aligned condition should be faster than in the horizontal-aligned condition. (b) RTs should be slowest when the bars are deliberately misaligned. In these cases, the emergent feature is destroyed and therefore cannot be used as a basis for detecting a height anomaly. (c) When the emergent cue of corner alignment is made either completely absent (horizontal misaligned and convergent misaligned) or explicit (convergent aligned), the four types of width manipulations should have no effect. That is, if detection of a height anomaly depends entirely on the presence of the emergent cue of corner alignment, it should be no more difficult to detect a deviation from this emergent property when the widths of the bars vary randomly than when the bar widths conform to a gradient. This follows because for the two misaligned conditions, all potential anomalies will have to be checked, and the deliberate misalignment of the bars should mean that false-positives will be equally true of all four types of width manipulations, not just the randomwidth condition, whereas for the convergent-aligned condition, deviation from regularity will always signal the anom-

AlignedHorizontal

Z

AlignedConvergent

MisalignedHorizontal

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Figure 7. The four conditions of bar edge used in Experiment 2. In these examples, the widths of the bars form a gradient that is coherent with the attended height gradient.

GRADIENTS AS VISUAL PRIMITIVES aly and equally so for each of the four types of width manipulations. In short, the only condition that should show an effect of the four irrelevant width manipulations is the one in which the stimuli replicated those in Experiment 1 (horizontal-aligned condition). Unlike the procedure used in Experiment 1, the manipulations of the irrelevant width manipulations were blocked in Experiment 2 rather than presented randomly across trials. For example, all of the width-coherent trials were presented together. This procedural change was implemented only for pragmatic reasons to maintain manageable blocks of trials. The variable that did vary randomly from trial to trial was the type of bar edge (see Figure 7).

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Resul~ Because of the high error rates (approaching 50%), the data for 1 participant were excluded from the analysis, and the data analyses proceeded on the remaining 7 participants. Figure 8A shows RT as a function of the number of elements in the gradient for the two tasks in which the bar edges were aligned (i.e., horizontal aligned and convergent aligned), whereas Figure 8B shows the results for the two conditions in which the bar edges were misaligned (i.e., horizontal misalib,ned and convergent misaligned). Percentage errors and r ~ measures of linearity for each condition are listed in Table 3. The slopes for each condition (averaged over the manipulation of width) for anomaly-present and anomaly-absent trials, respectively, were 18.5 and - 1 . 8 for horizontal aligned, 1.7 and - 4 . 7 for convergent aligned, 177.6 and 132.1 for horizontal misaligned, and 151.5 and 121.5 for convergent misaligned. Median RTs and errors were entered into separate ANOVAs. Although it was difficult to compare the results directly with those of Experiment 1, the slope difference between the aligned and misaligned conditions of this experiment were of the same order as those between the attend-to-height and attend-to-width conditions of Experiment 1. RTs. The two misaligned conditions yielded RTs that, on average, were more than twice as long as those for the two aligned conditions, F(1, 6) = 37.45, p < .05. Regardless of whether the bars were aligned, RTs were faster when bars had convergent than horizontal edges, F(1, 6) = 34.12,

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Figure 8. Results for Experiment 2. Reaction time is plotted as a function of the number of elements in the gradient, for anomalypresent (solid lines) and anomaly-absent (dashed lines) trials. The results are plotted separately for the two aligned (Figure 8A) and two misaligned (Figure 8B) conditions. p < .05. Although this advantage for convergent over horizontal bars seemed to be greater when the bars in the displays were misaligned rather than aligned, this interaction did not reach statistical significance, F(1, 6) = 4.04, p > .05. The effect of the width manipulation (averaged across the number of elements in the gradient) is plotted i n Figure 9 for anomaly-present (see Figure 9A) and anomaly-absent (see Figure 9B) trials. Planned comparisons showed that RTs were faster when the widths of the bars were fixed than manipulated (fixed vs. coherent, opposite, and random), F(1, 6) = 30.07, p < .05. This result did not interact with response type. The detrimental effect of manipulating width was significantly greater when the bars were misaligned than aligned, F(1, 6) = 13.93, p < .05, especially when the edges of the bars were horizontal, F(1, 6) = 5.99, p < .05. The random-width manipulation produced the slowest RTs overall, F(1, 6) = 5.95, p < .05, especially on anomalyabsent trials, F(1, 6) = 8.73, p < .05. This detrimental

380

GOODENOUGH AND GILLAM Table 3

Measures of Linearity and Percentage of Errors for Anomaly-Present and AnomalyAbsent Trials for Each Condition of Experiment 2 Error % Bar edge

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Slope

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Pres Abs Pres Abs Pres Abs Pies Abs Pres Abs Pres Abs Pres Abs Pres Abs

3.0 2.0 -6.0 - 11.8 59.0 -27.5 18.8 29.3 35.3 -7.0 - 8.8 -6.8 -13.8 - 11.0 -5.8 5,0

837 705 928 827 560 1,127 962 877 482 674 814 697 936 728 855 663

.147 .990 .125 1.00 .976 .676 .330 .964 .922 .985 .834 .928 .991 .997 .286 .216

0.6 0.0 0.9 0.0 0.7 0.3 1.3 0.0 0.0 0.1 0.1 0.0 0.1 0.3 0.4 0.1

0.1 0.0 0.7 0.0 1.4 0.1 1.3 0.0 0.1 0.0 0.1 0.1 0.9 0.0 0.1 0.0

0.1 0.0 0.4 0.5 1.4 0.1 1.4 0.4 0.1 0.1 0.0 0.0 0.6 0.0 0.3 0.0

Pies Abs Coherent Ires Abs Opposite Pres Abs Random Pres Abs Convergent Fixed Pres Abs Coherent Pres Abs Opposite Pres Abs Random Pres Abs Note. Pres = present; Abs = absent.

186.2 169.8 154.0 129.3 1523.0 31.5 218.8 199.0 211.0 149.0 136.3 96.0 171.8 66.8 88.3 175.8

- 106 525 322 1,087 523 2,012 93 841 -425 510 421 1,232 159 1,678 767 1,063

1.00 .993 1.00 .950 .997 .180 .931 .967 .998 .995 .986 .989 .959 .990 .921 .893

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3.1 0.9 4.6 1.9 4.3 2.1 4.4 2.4 4.7 1.4 4.3 1.7 4.9 1.9 3.4 1.7

Aligned Horizontal

Width Fixed Coherent Opposite Random

Convergent Fixed Coherent Opposite Random Misaligned Horizontal

Fixed

effect of random width proved to be significantly greater for convergent- than horizontal-edged bars when the bar edges themselves were aligned than misaligned, F(1, 6) = 5.99, p < .05. The general tendency for slower RTs when the width gradient ran in a direction that was opposite rather than coherent with height reached significance, F(1, 6) = 5.90, p = . 0 5 , but showed no significant interaction with response type or type of bar edge (all ps > .05). The main effect for response type (anomaly presence or absence) was significant, F(1, 6) = 16.85, p < .05, and interacted with global edge type (aligned vs. misaligned), F(1, 6) = 17:75, p < .05. In short, RTs for anomaly-absent trials were significantly longer than those for anomalypresent trials, whereas the reverse was true when the bars were misaligned. In this case, RTs for absent trials were generally faster or at least equally fast as those for present trials. There was no interaction between response type and local edge type (convergent vs. horizontal; p > .05). Errors. The trend for more errors on anomaly-absent than anomaly-present trials did not reach significance (p > .05). Errors were more frequent when the bar edges were misaligned, F(1, 6) = 17.72, p < .05, especially on

anomaly-absent trials, F(1, 6) = 17.30, p < .05. The main effect for local edge type (the tendency for more errors with horizontal than convergent edges) was only marginally reliable, F(1, 6) = 5.45, p = .057, and did not interact with response type (p > .05). There was an overwhelming tendency to commit errors when width was manipulated in any way rather than held constant (i.e., fixed vs. coherent, opposite, and random), F(1, 7) = 144.64, p < .05, especially when the local edges of the bars were horizontal, F(1, 6) = 8.05, p < .05, or when the bars were aligned, F(1, 6) = 5.69, p = .05.

Discussion The results of Experiment 2 provide support for the hypothesis that participants relied on the emergent cue of comer alignment to detect a size anomaly on a gradient of bar heights in Experiment 1. When this cue was removed in Experiment 2 (by applying random vertical offset to the bars), detection of a height anomaly was slow and inaccurate. When this emergent cue was made explicit (by making

GRADIENTS AS VISUAL PRIMITIVES

381

basic pattern of results obtained for the same types of displays in Experiment 1. That is, RTs were fastest for fixed width and slowest for random width, with the coherent and opposite widths falling between these two extremes. For the remaining three conditions of bar edge in Experiment 2, however, we predicted that the width manipulation would have no impact. For the convergent-aligned condition, we expected that the emergent property of alignment would be as explicitly usable when the widths of the bars were random as when the widths of the bars were fixed to a constant value. The results for this condition generally support the prediction of no effect of the width manipulation (second from the left in Figure 9). For the two misaligned conditions, we expected that the width manipulation also would have no effect. In these two conditions, a height anomaly should be able to be detected only by comparing adjacent pairs of bars, probably serially from left to right, as the emergent property of bar comer alignment has been deliberately removed. Such a search strategy should leave no room for the influence of the bar width manipulation if, as suggested earlier, the width manipulation served only to dilute the utility of the cue of comer alignment. The results, however, do not support this prediction for the misaligned displays. The impact of the irrelevant width manipulation was, at the least, as large when the bars were misaligned than aligned. This effect of the width manipulation for the misaligned displays replicated the order of difficulty obtained in Experiment 1.

Expedment 3

Results for Experiment 2 showing the effect of the width manipulation on attending to height averaged across the number of elements in the gradient, for anomaly-present (Figure 9A) and anomaly-absent (Figure 9B) trials. HOR = horizontal; CONV = convergent.

Figure 9.

the bar edges themselves collinear), detection of an anomalous height was facilitated. In fact, the facilitation was such that RTs for anomaly-absent trials were faster than those for anomaly-present trials (see Figure 8A). These results strongly suggest that the advantage for the height gradient found in Experiment 1 was not attributable to the mathematical rate of diminution of extent (which was less in the height gradient than in the width or separation gradients), but to the presence of the emergent feature. The hypothesis that the rapid response to a height gradient in Experiment 1 would depend on an emergent property of the display (the alignment of bar comers) cannot, however, fully account for the findings of Experiment 2. There was a significant interaction between height and width. As expecte d - a n d excepting the fast anomaly-absent responses---the results for the horizontal-aligned condition replicate the

The displays used in Experiment 1 were wide. This was especially true when height was held constant and the task was to attend to a separation gradient (ignoring width) or to a width gradient (ignoring separation), in which displays subtended 7.5 ° of visual angle. It might be argued that the lack of evidence of parallel processing for the compression gradients (i.e., detecting anomalies in width or separation of bars) was attributable to the fact that the attended gradient did not lie within the area of greatest visual acuity (the central 2* of the visual field). Experiment 3 was conducted as a check of the hypothesis that the visual angle subtended by the displays in Experiment 1 was detrimentally large. In this experiment, participants completed two tasks. In one (width), the participants attended to the gradient of width (ignoring irrelevant variations in bar separation), and in the other task (separation), they attended to the gradient of separation (ignoring irrelevant variation in width). These two tasks were identical to those run in Experiment 1, except for two modifications: First, the displays were viewed from a greater distance so that the visual angle subtended by the display was no greater than 2 °. Second, the covariance in Experiment 1 between display size and the number of elements in the display was removed (see the Method section), so that all displays, regardless of the number of elements in the gradient, projected the same total visual angle. If the visual angle subtended by the displays was detrimentally large in Experi-

382

GOODENOUGH AND GILLAM

ment 1, then, when visual angle was both reduced and fixed across trials, improved performance on both tasks was expected in Experiment 3 (e.g., shallower slopes).

(greater than 50% in some conditions). Their data were excluded from the analysis and were not replaced. Median RT as a function of the number of elements in the gradient is plotted in Figure 10. Slopes, intercept values, r z measures of linearity, and percentage of errors for each task are listed in Table 4. RTs. Anomaly-absent trials were significantly slower than anomaly-present trials, F(1, 6) = 34.10, p < .05. RTs were significantly slower for the separation than the width task, F(1, 6) = 6.61, p < .05, but this result did not interact with response type. The effect of the manipulation of the irrelevant attribute replicated the basic pattern established in the first two experiments. That is, RTs were significantly lengthened when the irrelevant attribute deviated in any way from a fixed value, F(1, 6) = 12.99, p < .05 (fixed vs. coherent, opposite, and random). Random manipulation of the irrele, rant attribute produced the longest RTs, F(1, 6) = 36.28, p < .05 (random vs. coherent and opposite), especially for the separation task, F(1, 6) = 6,92, p < .05. Finally, an irrelevant gradient running in the opposite than the same direction as the attended gradient produced significantly longer RTs, F(1, 6) = 8.34, p < .05 (opposite vs. coherent), especially on anomaly-absent trials in the separation task, F(1, 6) = 8.96, p < .05. Errors. Errors were no more likely on the separation than on the width task (19 > .05). Participants tended to miss an anomaly rather than commit false-positives, F(1, 6) = 54.28, p < .05. There were more errors when the irrelevant gradient was manipulated in any way that deviated from a fixed value, F(1, 6) = 9.74, p < .05 (fixed vs. coherent, opposite, and random), with the highest rate of errors occurring when the irrelevant attribute was random, F(1, 6) = 20.11, p < .05 (random vs. coherent and opposite). There

Method Participants. The participants were 1 male undergraduate and 7 female undergraduates. , Materials and apparatus. The displays were the same as those used in Experiment 1 for the attend-to-width (ignore separation) and the attend-to-separation (ignore width) tasks. The previous covariance between the number of elements in the gradient and the visual angle subtended by the display was removed. On any given trial, this *was achieved "on-line" by calculating the difference in overall width between the display to be presented and the largest possible display (a gradient containing 10 elements), dividing this obtained residual difference value by the number of elements in the gradient to be presented, and then adding this value as a constant to the irrelevant attribute of the bars (e.g., adding a constant to the widths of the bars if the task was to attend to separation). All displays in both tasks subtended a visual angle of 2", which was achieved by increasing the viewing distance of the displays relative to the viewing distance used in Experiment 1. In Experiment 2, the image drawn on the computer screen could not be seen directly by the participant, but it was projected onto a mirror behind and then in front of the participant. The mirrors were arranged to give the impression of a display viewed from a distance of 4.3 m. Procedure. Both tasks were completed in a single session lasting 1 hr, and the order of testing was counterbalanced across participants. Results The performance of 2 participants (1 man and 1 woman) in the separation task showed unacceptable error rates

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GRADIENTS AS VISUAL PRIMITIVES

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Table 4 Measures of Linearity and Percentage of Errors for Anomaly-Present and AnomalyAbsent Trials for Each Condition in Experiment 3 I

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135 1,332 1,138 1,317 389 2,437 177 489

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4.3 0.4 5.6 0.3 4.4 0.7 6.1 2.4

Note. Pres = present; Abs = absent.

were no significant differences in error rates between the coherent and opposite conditions.

D~c~swn The results of Experiment 3 do not support the hypothesis that the relatively large visual angle subtended by the displays in Experiment 1 precluded parallel processing of a compression gradient in Experiment 1. When the visual angle was maintained at 2", thereby ensuring that the entirety of the attended gradient fell on the region of greatest acuity, there was still no evidence of direct detection of an anomaly in bar width or bar separation. Slopes for detection of an anomaly (cf. Table 4) were still much greater than the previously cited criterion of 15 ms per item. Experiment 4 In Experiments 1-3, it is reasonable to assume that any serial scrutiny of a gradient would have proceeded from left to right (the direction in which the attended gradient decreased), such that participants were probably comparing adjacent pairs of bars (or gaps) in search of a violation of the relevant gradient rule (e.g., "Is this bar (gap) shorter or narrower than the preceding bar (gap)?"). It therefore is important to rule out an alternative hypothesis concerning the obtained differences in processing between perspective and compression gradients. This alternative states that comparisons of two bar heights are easier than comparisons of two bar widths or two bar separations regardless of whether these comparisons are made within the context of a gradient. In Experiment 4 we tested this hypothesis. All participants completed three tasks corresponding to comparative judgments of height, width, or separation. In the task con-

eerned with height, participants were presented with pairs of bars and asked to indicate whether the bar on the right was shorter than the bar on the left (width and separation were held constant). The pairs of bars were actually two-element portions on the height gradients shown in Experiment 1 for the attend-to-beight task (fixed width). When the bar on the right was not shorter than the bar on the left (i.e., requiring the participant to give a "no" decision), it was because the bar on the right was actually taller than the bar on the left. Thus, in this task, a "no" decision corresponded with an anomaly-present trial in Experiment 1 (in Experiment 1, gradient anomalies were generated by swapping the relative attributes of two adjacent elements with respect to the attended gradient). The tasks in Experiment 4 for width and separation were procedurally identical: Adjacent pairs of values from the relevant gradients used in Experiment 1 were presented, with participants being asked to indicate whether the bar (or gap) on the right was narrower than the bar (or gap) on the left. If the speed of comparison of adjacent values on the attended gradient contributed to the results of Experiments 1-3, then we predicted that RTs in Experiment 4 should be fastest for judgments of height than for judgments of width and separation. Furthermore, a comparison of two separations will be significantly slower than a comparison of two widths, accounting for the asymmetry between width and separation in Experiment 1.

Me~od Participants. Participants were 5 male and 3 female undergraduates.

Materials and apparatus. The stimuli were centrally presented samples of pairs of bars taken from gradients in Experiment 1 that

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originally comprised eight elements. Equal numbers of such stimulus samples were extracted from three positions in the gradient. For the width and height tasks, these were the second and third bars, the third and fourth bars, and the fourth and fifth bars. For the separation task (which necessarily required the presentation of three bars to generate two gaps), these were the second, third, and fourth bars; the third, fourth, and fifth bars; and the fourth, fifth, and sixth bars. Procedure. Participants used their dominant index finger to press a key for "yes" decisions (i.e,, the gap [bar] on the fight was narrower [shorter] than the gap [bar] on the left). The nondominant index finger was used on a different key for "no" decisions. A tone sounded immediately after an incorrect response. All participants did all three tasks in counterbalanced order. Each task was made up of two blocks of 120 trials, with the first block discarded as practice.

comparisons of two widths. 4 The asymmetry between width and separation in those experiments differed in character from that between height and separation and from that between height and width. In the latter two cases, and applying the conventions of interpretation from the visual search paradigm, height clearly yielded RT functions that could be classified as parallel, whereas width or separation yielded functions that could be clearly classified as serial. By contrast, the asymmetry between width and separation was tied to intercept rather than slope differences, whereby RTs for attending to separation (ignoring width) were significantly slower than RTs for attending to width (ignoring separation). General Discussion

Results Figure 11 shows RTs for "yes" and "no" trials for each of the three tasks. Median RTs and errors were analyzed separately. There was no main effect or any significant interaction involving response type in the analysis of either RTs or errors (all ps > .05). Overall, judgments of two separations yielded the slowest RTs of the three tasks, F(1, 7) = 8.48, p < .05, as well as the most errors, F(1, 7) = 15.79, p < .05. This seemed to be especially the case for RTs for "yes" decisions, but this interaction did not reach significance, F(1, 7) = 4.54, p > .05. There was no significant difference between height and width in the analysis of either RTs or errors (all ps > .05). Discussion The results of Experiment 4 do not support the hypothesis that the differences in processing between perspective and compressioh gradients in Experiments 1-3 were due to differences in the speed or ease with which comparisons of adjacent values on these gradients could be made. It could be argued, however, that the asymmetry found for width and separation in Experiments 1 and 3 was attributable to some general time cost for comparisons of two separations versus

I ram,a, [~No ]

sso ® 500 E 1~ 450

i.-~ 3 0 0 o

250 200 Height

Width

Separation

ATTRIBUTE OF COMPARISON

Figure 11. Reaction time for "yes" and "no" decisions in each of the three tasks in Experiment 4.

The goal of the current experiments was to investigate the degree to which different types of gradients, such as those that occur in projections of the visual environment, are directly available perceptually, independently of their capacity to elicit a depth response. The results of Experiment 1 suggest that only a linear perspective gradient (in this case, a gradient of height) can be detected easily. That is, the results suggest that a gradient of heights of the kind that occurs in the projections of natural scenes can be perceived directly as a fundamental property, or "primitive," not derived from the sequential comparison of successive heights. (Note that if a ground plane had been used instead of a wall plane, the linear perspective gradient would have been one of width.) The fact that deliberately misaligning the elements destroyed the processing advantage for height gradients implies that this advantage does not derive from the mathematical form of the linear perspective gradient. The results of Experiment 2 indicate that the rapid response to a linear perspective gradient was accounted for by the presence of the emergent property of collinearity of the end points of the bars whose height was varied. The finding that compression gradients (width and separation in this case) were not perceived easily in the speeded discrimination task strongly suggests that the poor depth or slant responses to such gradients (Braunstein, 1968; Cutting & Millard, 1984; GiUam, 1968, 1981; Vickers, 1971) reflected a difficulty with the perceptibility of these gradients per se rather than merely a failure of these gradients to elicit a depth response. Compression does not appear to be a visual primitive. These same studies indicate that linear perspective elicits good depth or slant responses. The difficulty with compression gradients is not restricted to pictorial perception. Judgments of stereoscopic slant * The asymmetry between width and separation is underestimated in both Figure 4 (Experiment 1) and Figure 9 (Experhnent 3). In these figures, reaction time (RT) has been plotted as function of how many bars make up the gradient. Clearly, a series of n bars yields a series of n-1 gaps. Thus, RTs for detecting a separation anomaly may be correctly plotted one step to the left of the divisions on the x-axis in these figures. This would serve to increase intercept differences in the comparative RT functions for separation and width gradients in Experiments 1 and 3.

GRADIENTS AS VISUAL PRIMITIVES around a vertical axis for which differences in horizontal compression between the left and fight eye images must be processed have been shown to have a long latency (Gillam, Chambers, & Russo, 1988), a high threshold (Rogers & Graham, 1983), and under some conditions an attenuated degree of slant (Gillam, 1968; Gillam & Ryan, 1992; Mitchison & McKee, 1990). On the other hand, judgments of stereoscopic slant around a horizontal axis for which differential shearing along the vertical meridian of one image relative to the other must be processed do not have these problems. Note that the term shear is used in the space perception literature (Howard & Kaneko, 1994; Nakayama, Silverman, MacLeod, & Mulligan, 1985; Rogers & Graham, 1983), to mean the lateral displacement of one set of elements relative to another along a specified axis. The convergence of lines to a vanishing point in linear perspective can be regarded as a sheafing of the image in this sense. Positions along the line are increasingly sheared relative to the vanishing point, and the rate of shear depends on the lateral separation of the line from the point of observation. Indeed, perspective is analogous to stereopsis in the same way that structure from motion is analogous to motion parallax. In perspective and structure from motion, the distal stimulus varies in position relative to a fixed point of observation, whereas in stereopsis and motion parallax, the point of observation varies relative to a fixed distal stimulus. Consider a ground plane. In perspective, a single line on the ground plane parallel to the median plane will have a projected slope (rate of shear) that increases as the line is displaced laterally from the point of observation. In stereopsis, a single line parallel to the median plane similarly has different slopes in the two eyes because it has a different lateral position relative to the eye considered as a point of observation. Consider a textured wall plane. It will be projected as a more compressed image horizontally the more it is shifted toward the point of observation. Similarly, the same wall plane seen binocularly will have a more compressed image in the eye to which it is closer laterally. Considering these geometric similarities, it is not surprising that perspective and stereopsis are similarly influenced by stimulus factors. The same difficulty with compression information and the same good response to shear information that occurs in pictorial perspective and stereopsis also is found in the motion parallax domain (Rogers and Graham, 1983). Rogers and Graham found that the degree of depth seen was much lower when given by changing compression as a function of motion than it was when depth was given by changing shear as a function of motion. Nakayama et al. (1985) found that participants also had a higher threshold for detecting a compression as opposed to a shear transformarion in motion, showing, as in our results, that the problem is in the detectability of the gradient rather than in the depth response to it. Lappin et al. (1994) found that participants were much better able to discriminate stimuli in simulated rotation in depth on the basis of differential collineafity information than on the basis of differential compression information. This is highly consistent with our results. It is now obvious

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in several domains that the differential structure associated with compression is not a primitive image property for the visual system in the sense that neural structures exist for its direct detection. Clearly, it can be argued that our findings are specific to the stimuli used and the paradigm in which they were presented. It is possible that each of the three gradient dimensions we studied were directly represented but that our task was not sensitive to this representation. We assumed that if there is a visual mechanism that can respond directly to gradient information (i.e., that a gradient has an accessible preattentive representation that is distinct from that for the components of the gradient), then this mechanism in turn would support the ability to notice a single incongruity in that gradient. 5 Given that there was no difficulty with discrimination of individual pairs of the successive extents that constituted the gradients (Experiment 4), it seems reasonable to conclude that the failure to find anomalies quickly reflected a failure of the visual system to represent the differential structure of the gradient as a whole. A second aim of our experiments was to examine whether the dimensions of height, width, and separation would interact. Evidence was found for such an interaction, but not in the form of facilitation of one by another. The general pattern of the results was that, relative to when an irrelevant attribute was held constant (fixed), the presence of an iffelevant gradient interfered significantly with the ability to attend to the specified gradient, especially when that irrelevant gradient was running counter to the attended gradient (opposite). Performance was poorest overall when the irrelevant attribute varied randomly across the positions in the series (random). The interactions also suggested that performance may become poorer still if two irrelevant gradients are present rather than just one (e.g., attend to width, in which both height and separation are opposite or random). The RTs were so long that this interference is likely to be with a serial process of comparing adjacent elements. Our experiments also complement a recent shift of emphasis in the application of the visual search paradigm, a task that we consider to be conceptually related to the speeded classification task we implemented. Early theoretical accounts of visual search performance (e.g., Treisman & Gelade, 1980; Treisman, Sykes, & Gelade, 1977) prioritized the role of the relationship between the target and the nontarget items (i.e., the attributes of the target item relative to the nontarget set that either permitted or precluded efficient search for that target). Recently, however, the focus of interest is moving away from the relationship between the target and nontarget items (the basis for popout) to the 5 Such a mechanism would not respond to stochastic gradients such as a beach covered with pebbles. These are known to poorly support space perception, however, and there is no reason to believe that there is a mechanism capable of extracting stochastically defined gradients (Clark, Smith, & Rabe, 1956; Gillam, 1968). Our concern has been with the origins of the apparently poor depth response to regular compression gradients relative to regular-sized gradients.

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impact on visual search performance of the relations between the nontarget items themselves. It has now come to be accepted that search performance cannot be neatly partitioned into "parallel" versus "serial" functions (e.g., Fahle, 1991) but that it tends to show a continuum of difficulty. Many researchers are investigating the effects of nontarget heterogeneity per se in producing this range of performance in visual search. Although it has been verified empirically that nontarget heterogeneity can be detrimental to search performance (e.g., Duncan & Humphreys, 1989; Goodenough, 1992; Treisman, 1988; but see Quinlan & Humphreys, 1987; Treisman & Sato, 1990; Wolfe et al., 1989; Wolfe & Friedman-Hill, 1992), recent evidence shows that this expected detrimental impact of nontarget distractor heterogeneity can be reduced or eliminated simply by manipulating the way in which the same set of heterogeneous nontarget stimufi are arranged. For example, Moraglia (1989) reported that a horizontal line target can be found easily among a heterogeneous set of orientations if the nontarget lines are arranged using a specific orientation or position rule (i.e., conforming to patterns of broken concentric circles) rather than being placed in a random configuration. Nothdurft (1985, 1991a, 1991b) reported similar findings for both visual search and texture segregation based on line orientation. Our experiments could be taken as providing another example of how the arrangement of the same set of heterogeneous stimuli can reduce the impact of that heterogeneity in search-related performance. In Experiments 1 and 2, the random and coherent manipulations of the irrelevant attribute presented the same set of values on the irrelevant dimension. Irrelevant heterogeneity in the form of a gradient (rather than random) presented an organized set of heterogeneous items analogous to the orientation or position rules used by Moraglia (1989) with line segments in visual search. Although the coherent organization of an irrelevant gradient did not guarantee immediate apprehension of the attended dimension or gradient, it did seem to make such heterogeneity easier to filter out relative to the random condition. In the case of height, such organization also seemed to allow, borrowing the terminology of visual search, parallel processing of an additional emergent property of comer alignment (of. Experiment 2). References Braddick, O. J., & Holliday, I. E. (1991). Serial search for targets defined by convergence or deformation of optic flow. Perception, 20, 345-354. Braunstein, M. L. (1968). Motion and texture as sources of slant information. Journal of Experimental Psychology, 78, 247-253. Clark, W. C., Smith, A. H., & Rabe, A. (1956). The interaction of surface texture, outline gradient, and ground in the perception of slant. Canadian Journal of Psychology, 10, 1-8. Cutting, J. E., & Millard, R.T. (1984). Three gradients and the perception of flat and curved surfaces. Journal of Experimental Psychology: General, 113, 198-216. Duncan, J., & Humphreys, G. W. (1989). Visual search and stimulus similarity. Psychological Review, 96, 433-458. Enns, J.T., & Rensink, R.A. (1990). Sensitivity to three-

dimensional orientation in visual search. Psychological Science, 1, 323-326. Fahle, M. (1991). Parallel perception of vernier offsets, curvature, and chevrons in humans. Vision Research, 31, 2149-2184. Gibson, J. J. (1950). The perception of the visual worM. Boston: Houghton Mifflin. Gibson, J. J. (1966). The senses considered as perceptual systems. Boston: Houghton Mifflin. Gillam, B. (1970). Judgments of slant on the basis of foreshortening. Scandinavian Journal of Psychology, 11, 31-34. Gillam, B. (1981). False perspectives. Perception, 10, 313-318. Gillam, B., Chambers, D., & Russo., T. (1988). Postfusional latency in stereoscopic slant perception and the primitives of stereopsis. Journal of Experimental Psychology: Human Perception and Performance, 14, 163-175. Gillam, B., & Ryan, C. (1992). Perceptive, orientation disparity, and anisotropy in stereoscopic slant perception. Perception, 21, 427-439. Gillam, B.J. (1968). Perception of slant when perspective and stereopsis conflict. Journal of Experimental Psychology, 78, 297-305. Goodenough, B. J. (1992). Searching for a difference in orientation: Implications for models of visual search. Unpublished doctoral dissertation, University of New South Wales, Sydney, New South Wales, Australia. Holliday, i. E., & Braddick, O. J. (1988). Visual search for targets differing in divergence or shear is serial. Perception, 18, 403. Howard, L P., & Kaneko, H. (1994). Relative shear disparities and the perception of surface inclination. Vision Research, 34, 2505-2517. Lappin, J. S., Ahlstrtm, U.B., Craft, W.D., & Tschantz, S.T. (1994). Spatial primitives for seeing 3-D shape from motion. In T. Papathomas, C. Chubb, A. Gorea, & E. Kowler (Eds.), Early vision and beyond (pp. 145-153). Cambridge, MA: MIT Press. Mitchison, G. J., & MeKee, S. P. (1990). Mechanisms underlying the anisotropy of stereoscopic tilt perception. Vision Research, 30, 1781-1791. Moraglia, G. (1989). Display organization and the detection of horizontal line elements. Perception & Psychophysics, 45, 265-272. Nakayama, K., & Silverman, G.H. (1986). Serial and parallel processing of visual feature conjunctions. Nature, 320, 264-265. Nakayama, K., Silverman, G. H., MacLeod, D. I., & Mulligan, J. (1985). Sensitivity to sheafing and compressive motion in random dots. Perception, 14, 225-238. Nothdurfl, H.-C. (1985). Orientation sensitivity and texture segmentation in patterns with different line orientation. Vision Research, 25, 551-560. Nothdurft, H.-C. (1991a). The role of local contrast in pop-out of orientation, motion and color. Investigative Ophthalmology and Visual Science, 32, 714. Nothdurft, H.-C. (1991b). Texture segmentation and pop-out form orientation contrast. Vision Research, 31, 1073-1078. Purdy, W. C. (1960). The hypothesis ofpsychophysical correspondence in space perception. Ithaca, NY: General Electric Advanced Electronics Center. Quinlan, P.T., & Humphreys, G.W. (1987). Visual search for targets defmed by combinations of color, shape and size: An examination of the constraints on feature and conjunction searches. Perception & Psychophysics, 41, 455-472. Rogers, B. J., & Graham, M. E. (1983). Anisotropies in the perception of three-dimensional surfaces. Science, 221, 1409-1411. Rosinski, R.R., & Levine, N. P. (1976). Texture gradient effec-

GRADIENTS AS VISUAL PRIMITIVES tiveness in the perception of surface slant. Journal of Experimental Child Psychology, 22, 261-271. Sedgwick, H. A. (1986). Space perception. In K. R. Boff, L. Kanfman, & J. P. Thomas (Eds), Handbook of perception and human performance (pp. 21-1-21-57). New York: Wiley. Stevens, K. A. (1981). The information content of texture gradients. Biological Cybernetics, 42, 95-105. Stevens, K. A. (1984). On gradients and "texture" gradients. Journal of Experimental Psychology: General, 113, 217-220. Treisman, A. M. (1985). Preattentive processing in vision. Computer Vision, Graphics, and Image Processing, 31, 156-177. Treisman, A.M. (1988). Features and objects: The Fourteenth Bartlett Memorial Lecture. Quarterly Journal of Experimental Psychology, 40A, 201-237. Treisman, A. M., Cavanagh, P., Fischer, B., Ramachandran, V. S., & yon der Hey&, R. (1990). Form perception and attention: Striate cortex and beyond. In L. Spillman & B. Wootton (Eds.), Visual perception: The neurophysiological foundations (pp. 273-315). San Diego, CA: Academic Press. Treisman, A. M., & Gelade, G. (1980). A feature-integration theory of attention. Cognitive Psychology, 12, 97-136. Treisman, A., & Gormican, S. (1988). Feature analysis in early vision: Evidence from search asymmetries. Psychological Review, 95, 15-48.

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Treisman, A. M., & Sato, S. (1990). Conjunction search revisited. Journal of Experimental Psychology: Human Perception and Performance, 16, 459-478. Treisrnan, A.M., & Souther, J. (1985). Search asymmetry: A diagnostic for preattentive processing of separable features. Journal of Experimental Psychology: General, 114, 285-310. Treisman, A. M., Sykes, M., & Gelade, G. (1977). Selective attention and stimulus integration. In S. Dornic (Ed.), Attention and performance VI (pp. 327-352). Hillsdale, NJ: Erlbaum. Vickers, D. (1971). Perceptual economy and the impression of visual depth. Perception & Psychophysics, 10, 23-27. Wolfe, J. M., Cave, K. R., & Franzel, S. L. (1989). Guided search: An alternative to the feature integration model for visual search. Journal of Experimental Psychology: Human Perception and Performance, 15, 419-433. Wolfe, J. M., & Friedman-Hill, S. R. (1992). Part-whole relationships in visual search. Association for Research in Vision and Ophthalmology Abstracts, 33, 1355. Received June 14, 1993 Revision received September 26, 1995 Accepted November 20, 1995

New Editor Appointed for Contemporary Psychology: 1999-2004

The Publications and Communications Board of the American Psychological Association announces the appointment of Robert L Stemberg (Yale University) as editor of Contemporary Psychology, for a 6-year term beginning in 1999. The current editor, John H. Harvey (University of Iowa), will continue as editor through 1998. All reviews are written by invitation only, and neither the current editor nor the incoming editor receives books directly from publishers for consideration. Publishers should continue to send two copies of books for consideration, along with any notices of publication, to PsycINFO Services Department, APA, Attn: Contemporary Psychology Processing, P.O. Box 91700, Washington, DC 20090-1700 or (for UPS shipments) 750 First Street, NE, Washington, DC 20002-4242.