Visual Search for Conjunctions of Motion and Form

Journal of Experimental Psychology: Human Perception and Performance 1996, Vol. 22, No. 1, 122-132

Copyright 1996 by the American Psychological Association, Inc. 0096-1523/96/$3.00

Visual Search for Conjunctions of Motion and Form: Display Density and Asymmetry Reversal H e r m a n n J. Mtiller and A n d r e w F o u n d Birkbeck College, University of London In visual search for motion-form conjunctions, search rates have been reported to be faster for moving than for stationary targets if the target-nontarget discrimination is easy (45 ° target line tilt from vertical), but this asymmetry is reversed if the discrimination is difficult (9° tilt) (J. Driver & P. McLeod, 1992). Driver and McLeod proposed that gross aspects of form discrimination are performed within a motion filter that represents only the moving items, whereas fine discriminations rely on a stationary form system that is poor at filtering by motion. However, H.J. Mtiller and J. Maxwell (1994) failed to observe the asymmetry reversal, possibly because they used lower density displays. The study reported in this article also did not yield an effect due to varying display density. This lends support to the notion of a unitary form system, with the role of the motion filter being limited to guiding the search to the moving items or, if required by the task, the stationary items.

The original proposal of feature integration theory (FIT; Treisman & Gelade, 1980) was based on the finding of differential search reaction time (RT) functions depending on the featural content of the target items as compared with the nontarget items. Some search tasks showed no increase in RT as the number of nontarget items increased, whereas others showed a linear increase as more items were added to the display. FIT proposed that flat functions, indicating spatially parallel search, occur when the search involves a target that differs from the nontargets in a single feature (e.g., a vertical line among horizontal lines). Linearly increasing functions, indicating serial search, occur when the target is defined by a conjunction of features, each of which is separately present among the nontarget items (e.g., a red vertical line among red horizontal lines and green vertical lines). However, there is no simple dichotomy between parallel and serial search, there is, in fact, a continuum of search RT function slopes (search rates). Several models of visual search have been proposed to account for this continuum, notably revised FIT (Treisman & Sato, 1990), guided search (Cave & Wolfe, 1990; Wolfe, 1994), and similarity theory (Duncan & Humphreys, 1989, 1992). All of these theories can be viewed as general theories of visual search in that they postulate general grouping principles

based on interitem similarity (and proximity), general topdown processes, or both to account for search performance across a range of stimulus attributes without invoking the specific properties of different visual subsystems to explain the results of particular experiments. However, the generality of these accounts has recently been challenged by experiments investigating search for targets defined by a conjunction of motion and form (Berger & McLeod, 1996; Driver & McLeod, 1992; McLeod, Driver, & Crisp, 1988). Search for such conjunctions provides an exception to original FIT (similar to conjunctions involving stereoscopic depth; Nakayama & Silverman, 1986) in that the search RTs are independent of the number of items in the display. For example, in one experiment by Driver and McLeod (1992), half the display items were stationary lines tilted 45 ° from the vertical, and the other half consisted of (upward) moving vertical lines except for one 45 ° line (the target) on "target present" (positive) trials. On such trials, the slope of the search RT function was approximately 3 ms/item, indicating parallel search. The finding of flat search functions for motion-form conjunctions is not controversial. However, what is at issue is Driver and McLeod's finding that the search rates are dependent on both the form discriminability of the target (relative to the nontargets within its set) and whether the target is a member of the moving or stationary items (see also Berger & McLeod, 1996). In particular, Driver and McLeod reported an asymmetry reversal (i.e., crossover interaction) such that search for a "salient" tilted line target (45 ° from the vertical among vertical lines) was easier when it was present in the moving set of items rather than the stationary set. Conversely, when a finer discrimination was required to detect the target (9 ° flit), search was easier when it was present in the stationary set rather than the moving set. The explanation proposed by Driver and McLeod (1992) expands on their original notion of a "motion filter" (McLeod et al., 1988). According to their account, two

Hermann J. Miiller and Andrew Found, Department of Psychology, Birkbeck College, University of London, London, England. This research was supported by Science and Engineering Research Council Grant GR/H/54966 and by a Royal Society research grant. We thank J. Anderton and S. Blackwell for their help with running the experiment and P. Jolicoeur, P. McLeod, and J. Wolfe for their valuable comments on an earlier version of this article. Correspondence concerning this article should be addressed to Hermann J. Mtiller, Department of Psychology, Birkbeck College, University of London, Malet Street, London WC1E 7HX, England. Electronic mail may be sent via Internet to h.muller @psyc.bbk.ac.uk. 122

SEARCH FOR MOTION-FORM CONJUNCTIONS components of the visual system are involved in the search of displays with moving items: The motion filter and the "stationary form system." The motion filter is specialized for segregating moving items from stationary items; it represents only moving stimuli but has a relatively poor representation of their orientation and form. Conversely, the stationary form system has a precise representation of stimulus orientation and supports accurate discrimination between all forms, but it is poor at filtering by movement, representing moving items to some extent as well as stationary items. The asymmetry reversal can then be explained as follows. Detection of a moving target can be accomplished efficiently within the motion filter when the form discrimination required is easy; however, when the discrimination is difficult, the stationary form system (which has a poor representation of moving items) has to come into play. In contrast, detection of a stationary target is comparatively inefficient when the discrimination is easy because the stationary form system has difficulty keeping moving items out of the search. However, when the discrimination is difficult, this difficulty is outweighed by the system's accurate representation of form. On the basis of neuropsychological evidence, McLeod, Heywood, Driver, and Zihl (1989) concluded that the motion filter resides in cortical area MT (or V5), where neurons are known to be especially responsive to motion but only broadly orientation tuned. Driver and McLeod's (1992) account, thus, invokes the specific (physiological and neuropsychological) properties of different visual subsystems. Since general theories do not incorporate such information, they are unable to explain the asymmetry reversal. However, recent work by Miiller and Maxwell (1994) has cast doubt on the existence (or, at least, generality) of Driver and McLeod's (1992) asymmetry reversal. They performed a number of experiments comparing targets of different discriminability (including 45 ° and 9 ° tilted lines) but found no evidence of an asymmetry between moving and stationary search in terms of positive RT function slopes and hence no asymmetry reversal (crossover interaction). Miiller and Maxwell (1994) addressed a number of possible causes for the discrepancy between the two studies. However, although the size of the items and the movement speed were comparable between the two studies, they could not rule out that differences in display parameters were crucial. In particular, Miiller and Maxwell used a larger maximum display area than did Driver and McLeod (1992): 18.1 ° × 11.3 ° compared to 11 ° × 8 °. Because the movement speeds (vertical motion) were equivalent, the important difference was in terms of the horizontal display extension, which determines the horizontal display density. Thus, the display density was less in M011er and Maxwell's experiments. Display density effects in (color-form) conjunction search have been observed by Cohen and Ivry (1991). In search for motion-form conjunctions, display density could influence the pattern of results in a number of ways. First, the effect of density could be to make item localization more difficult when high, or interitem comparison more

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difficult when low. Both of these potential effects of density would presumably influence search efficacy in a fairly linear manner. However, varying display density could also introduce nonlinear effects on search performance because of configural interactions between adjacent stimuli producing qualitative differences in search RT functions at different densities (cf. MUller & Maxwell, 1994). For example, if adjacent items are closely spaced, a moving target (say, a 45 ° tilted line) passing a stationary nontarget (also tilted at 45 °) will result in the transient formation of a tilted line double the length of the other stimuli in the display. However, if the overall display area is maintained and the search is for a 9 ° tilted target, the gap between adjacent stimuli will be larger, insufficient to allow the formation of an apparent double-length line. This is illustrated in Figure 1. Assuming that the movement filter consisted of "extended-line" detectors and that these detectors were more strongly activated when searching for a moving item, this could account for Driver and McLeod's (1992) finding of an asymmetry reversal. In the present experiment, we sought to address the discrepancy between the two studies by systematically examining the effect of display density on the search for moving and stationary targets of differing discriminability. Display density was varied by varying the horizontal width of each (vertical) stimulus track and, thus, the overall horizontal display extension, affecting the separation between adjacent items. Note that the actual separation depends not only on the width of the tracks but also on the horizontal extension of the stimuli themselves (with the horizontal extension of a 45 ° line being over six times larger than that of a 9 ° line). The experiment to be reported examined search for stationary and moving targets of different discriminability (45 ° or 9 ° lines) at various display densities. For both types of target, the maximum horizontal display extensions (with a set size of 24 items) were 20.8 ° , 16.8 ° , and 12.8 ° . For 9 ° targets, two additional levels (8.8 ° and 4.8 ° ) were examined. The greater horizontal width of the 45 ° targets prohibited the use of smaller extensions because adjacent items would have crossed and overlapped one another. Note that the horizontal extensions of 12.8 ° and 20.8 ° were close to those used by Driver and McLeod (1992) and Mtiller and Maxwell (1994), respectively. Method

Participants Eight participants, 4 men and 4 women, (including one of the experimenters) took part in the experiment. Participants' ages ranged from 16 to 35 years. All had normal or corrected-to-normal vision. Participants (except Andrew Found) were paid £4 ($6.25) per hour. To minimize practice effects and to help reduce error rates, all participants were given thorough practice in a preexperimental session.

Apparatus ' Stimuli were presented on a Tektronik 608 monitor CRT with P31 phosphor. The CRT was controlled through a CED 502

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Figure 1. Differential effects of display density (interitem spacing) on the collinearity grouping of adjacent moving and stationary line stimuli in the search for moving 45 ° and 9 ° targets. The left and right panels show the displays at Times 1 and 2, respectively. Moving stimuli are marked by arrows, and targets are encircled (Time 2). With 45 ° targets (upper panels), the temporary formation of a double coUinear line may provide a cue to detecting the presence of a target. With 9 ° targets (lower panels), no such conftgural cue will be formed at the display density shown; the interitem distance at the point of collinearity between the moving target and the stationary nontarget is too large to allow collinearity grouping.

interface using Shepherd's (1984) EMDISP software. The laboratory was dimly illuminated to prevent reflections on the CRT, and the brightness of the stimuli was adjusted to ensure that moving items did not leave a visible persistence "trail." Participants used a hand-sized response keypad to initiate blocks of trials and to make responses during trials. Participants viewed the display from a distance of 33 cm (with head position maintained through the use of a chin rest).

Stimuli The total screen area was 22.3 ° x 16.2 ° of visual angle. Displays contained 8, 16, or 24 line stimuli (set size), half of which were moving and half of which were stationary. The size of the stimuli was 0.62 °. The moving items moved upward at a constant speed of 2.8°/s. The position of the moving items was incremented every 17 ms, giving the impression of smooth, continuous move-

ment. Displays were presented for a maximum time of 2,550 ms unless terminated by a participant's response. The display was subdivided into 24 (equally wide) adjacent vertical tracks. Stationary and moving items were placed on randomly interleaved tracks, and displays were "dense" in the sense that items occupied directly neighboring tracks. When the number of items in the display was 24 (the maximum set size), each track contained a stimulus (with stationary and moving items randomly distributed across tracks). When ,the set size was less than 24 items, the stimuli were placed randomly on adjacent tracks (i.e., each item was flanked by 2 other items except the 2 marginal items to the left and the right). With set sizes less than 24 items, the leftmost stimulus position was varied randomly within a range that permitted all the items to be displayed. The vertical positioning of stimuli was determined randomly within the constraints that the stationary items were spread over the full vertical display extension and the moving items were initially placed toward the lower half of the display (to prevent the uppermost items from scrolling off the display during a trial).

SEARCH FOR MOTION-FORM CONJUNCTIONS

Design and Procedure There was a total of 16 experimental conditions: 10 with 9° targets, Display Density (5 levels) x Moving or Stationary Target, and 6 with 45 ° targets, Display Density (3 levels) x Moving or Stationary Target. Each condition consisted of 240 trials (i.e., 40 trials for each set size [3 levels] by target-absent or target-present combination). Each participant performed all conditions. Experimental sessions were exclusively devoted to either moving search or stationary search. Half the participants started with moving search and half with stationary search. Within each search condition (moving or stationary), half the participants started with 45 ° targets and half with 9 ° targets. Display density was varied in either ascending or descending order. Before each condition, participants were given a block of 48 unrecorded practice trials.

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Furthermore, each experimental block started with 5 unrecorded warm-up trials. Participants were instructed to make a response as quickly and as accurately as possible. When participants made an error on a trial, a computer-generated beep alerted them to the mistake. At the end of each condition, participants were given feedback concerning their performance to help them maintain a balance between response speed and accuracy.

Results Figures 2 and 3 present the (correct) mean RTs and the error percentages for target-absent and target-present responses as a function of set size under all display density

Figure 2. Target-present (pres) and target-absent (abs) reaction times (RTs) and error percentages for 45 ° targets as a function of set size (8, 16, or 24 items) under each display density condition (display extension: 12.8°, 16.8°, or 20.8°), separately for stationary (left) and moving (right) search. Target-present and target-absent RTs are symbolized by Xs and triangles, respectively; false-alarm and miss errors are represented by white and shaded bars, respectively. Also given are the slope (s; in ms/item), y-intercept (y; in ms), and r2 (r; percentage of variance accounted for by the linear component) associated with each search RT function.

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SEARCH FOR MOTION-FORM CONJUNCTIONS and stationary or moving target conditions, along with the slopes and the intercepts of the search RT functions (estimated by linear regression analyses of the mean RTs). Figures 2 and 3 show the data for the 45 ° and 9 ° targets, respectively. The positive (target-present) RT function slopes are also illustrated in Figure 4, which presents search rates as a function of (decreasing) display density for all stationary or moving x 45 ° or 9 ° target conditions. Note that the individual participants' RTs were not screened for outliers. For 45 ° targets, the positive slopes ranged between 5.19 and 8.19 ms/item for stationary targets and between 6.44 and 7.56 ms/item for moving targets. Such slopes are generally taken as being indicative of parallel search. The results for the 9 ° targets show a similar pattern, although both the slopes and the intercepts were increased (as expected, given the increased discrimination difficulty). Slopes ranged between 16.38 and 18.56 ms/item for stationary targets and between 15.25 and 22.36 ms/items for moving targets. Such slopes are generally taken as being indicative of serial search. Multifactorial analyses of variance (ANOVAs) of the RT data performed separately for the 45 ° and 9 ° targets revealed the following effects: RTs increased with decreasing display density (i.e., increasing horizontal display extension) and with increasing set size and were longer for stationary targets than for moving targets and longer for target-absent responses than for target-present responses. The search RT functions were steeper for target-absent than for target-present responses. Decreasing display density affected target-absent responses more than target-present responses and produced an increase in the slopes of the search RT functions (i.e., a decrease in search rates). The most important result of the two ANOVAs was the absence of Significant Display Density X Stationary or Moving T ~ g e t X Set Size interactions: for 45 ° targets, F(4, 28) = 0.97; for 9 ° targets, F(8, 56) = 1.50). Display density had no differential effect on the search rates between stationary and moving targets. A third ANOVA performed to compare the two target types (45 ° and 9 ° ) at their common display densities also exhibited no Significant Stationary or Moving Target × Display Density X Set Size and 45 ° or 9 ° Target X Stationary or Moving Target X Display Density X Set Size interactions (Fs < 1.0), arguing against any differential effects of display density on the search rates with stationary and moving 45 ° and 9 ° targets within the range of densities used by Driver and McLeod (1992) and Miiller and Maxwell (1994). Thus, the overall advantage of search for moving targets over stationary targets (discussed earlier) was not one in terms of search RT function slopes but rather y-intercepts (as in the experiments of Miiller and Maxwell, 1994). The

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intercepts were about 25 ms faster for moving targets than for stationary targets (positive RTs; seven out of three 45 ° targets plus five 9 ° targets display density conditions showed this difference). Figure 5 graphs the slopes of the positive search RT functions for stationary and moving easy (45 ° ) and hard (9 ° ) targets under two conditions of display density (i.e., horizontal display extension): 12.8 ° (high density), which is close to the density used by Driver and McLeod (1992), and 20.8 ° (low density), which is close to the density used by Miiller and Maxwell (1994). For comparison, Figure 5 also graphs the corresponding search rates reported by McLeod and his colleagues (the high-density figure shows the averaged slope data of Driver and McLeod, 1992, and Berger and McLeod, 1996; the low-density figure shows the data of Berger and McLeod). The 12.8 ° condition (of the present study) showed no sign of an interaction between target type and target set; the 20.8 ° condition suggests an interaction but not of the crossover type reported by Driver and McLeod. The arcsine-transformed error rates were also analyzed by using multifactorial ANOVAs performed separately for 45 ° and 9 ° targets. More errors were made on target-present trials (misses) than on target-absent trials (false alarms), and error rates increased with increasing set size (9 ° targets). The set size effects were mainly (9 ° targets) or entirely (45 ° targets) due to increasing miss rates with increasing set size; false-alarm rates remained constant or tended to decrease. Note that there were no effects involving stationary or moving target. This means that the RT effects (or their absence) cannot be attributed to differential speed-accuracy trade-offs between moving and stationary targets.

Discussion It is interesting to consider the positive RT slope data in more detail. For 45 ° targets, the display density effect showed some tendency to be more marked with stationary search than with moving search. Conversely, with 9 ° targets, the display density effect tended to be somewhat more pronounced with moving search than with stationary search. This pattern could be taken (at least) to go in the direction of the search asymmetry reversal reported by Driver and McLeod (1992); however, three points are noteworthy. First, the differential effects of display density between moving and stationary targets were statistically not reliable. The 45 ° and 9 ° target-present RTs exhibited no significant Display Density X Stationary or Moving Target X Set Size interactions: 45 ° target ANOVA, F(4, 28) = 1.86, p > .10; 9 ° target ANOVA, F(8, 56) -- 1.27, p > .10. Second, one of the "differential" effects on the present RTs was contra-

Figure 3. Target-absent (abs) and target-present (pres) reaction times (RTs) and error percentages for 9° targets as a function of set size (8, 16, or 24 items) under each display density condition (display extension: 4.8 °, 8.8 ° , 12.8°, 16.8°, or 20.8°), separately for stationary (left) and moving (right) search. Target-present and target-absent RTs are symbolized by Xs and triangles, respectively; false-alarm and miss errors are represented by white and shaded bars, respectively. Also given are the slope (s; in ms/item), y-intercept (y; in ms), and r2 (r; percentage of variance accounted for by the linear component) associated with each search RT function.

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etc.). The function of the movement filter is simply to "tag" the moving items so processing within the form system may be guided to the moving (or, if required by the task, the stationary) stimuli. When the form discrimination is easy, the quality of the moving items within the form system is sufficient to permit fast search. However, when the discrimination is difficult, their degraded representation makes search more effortful, probably requiring a greater number of eye movements (i.e., a spatially serial component that would produce steeper slopes). Restated, the reduced quality of the moving items within the form system affects search performance only above a certain level of discrimination difficulty; 45 ° targets are below and 9 ° targets are above that level. The tendency for a stronger display density effect with moving 9 ° targets may be consistent with this account. It is possible that decreasing the display density (spreading the items over a wider area) would reduce the quality of moving items' representation within the form system relatively more than that of the stationary items, increasing the need for eye movements.

Comparison With Driver and McLeod (1992) and Berger and McLeod (1996) An important question concerns why McLeod and his colleagues (Berger & McLeod, 1996; Driver & McLeod, 1992) consistently found a crossover interaction (at least with high display densities), whereas we failed to observe such a pattern. Although the display parameters were very similar in Berger and McLeod's and our experiments, they used a between-subjects design, and we used a withinsubject design (one additional difference was that they blocked set size, whereas we randomized it). Our partici-

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pants performed in eight stationary and eight moving conditions, which, one could object, may have washed out subtle differences between conditions (P. McLeod, personal communication, August 13, 1994). However, this is unlikely to be the case. MUller and Maxwell (1994) found little difference between within- and between-subjects designs. However, Mtiller and Maxwell's (1994) participants (and the participants in the present study) were highly practiced. Even in their between-subjects condition, the participants were familiarized with the task in a preexperimental session (of about 480-960 trials). In contrast, the participants in the experiments by McLeod and his colleagues received from 50 (Berger & McLeod, 1996) to 100 (Driver & McLeod, 1992) practice trials under each (blocked) set size condition, which were immediately followed by 50 experimental trials. It is possible, therefore, that our participants performed optimally under conditions that provided difficulty for the relatively unpracticed participants of McLeod and his colleagues, in particular, search for stationary 45 ° targets and moving 9 ° targets at high display densities and all search conditions at low densities (see Figure 5). At low densities, Berger and McLeod's (1996) participants exhibited search RT function slopes that were twice as steep, on average, as those of our participants (see Figure 5); however, Berger and McLeod's participants found the lowdensity conditions rather difficult. In fact, Berger and

McLeod increased the display duration from 2,150 ms (high density) to 4,000 ms (low density), presumably so that their participants could complete searching the 9 ° target displays within the time available. The general difficulty experienced by Berger and McLeod's participants with low-density displays may have rendered their (second) experiment insufficiently sensitive to pick up subtle effects on performance. For example, the miss rate 1 was particularly high in their 9 ° moving condition (20-item displays), suggesting a speed-accuracy trade-off. Compensating for this trade-off would increase the search RT slopes, producing a disadvantage relative to the 9 ° stationary condition. Recall that our highly practiced participants showed some tendency toward such a disadvantage. At high display densities, the participants of McLeod and his colleagues performed more efficiently, achieving search rates within a broadly similar range to our participants (see Figure 5). Yet there were two conditions that they found more difficult than did our participants. Their main difficulty was with stationary 45 ° targets (12 ms/item) relative to moving 45 ° targets (4 ms/item); our participants showed 1 Berger and McLeod (1996) reported only error rates (averaged across miss and false-alarm rates). However, according to our experience, it is safe to assume that the predominant errors at large set sizes were misses.

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fast search rates iri both conditions (5.5 ms/item, on average). Furthermore, the participants of McLeod and his colleagues experienced difficulty with moving 9 ° targets (22 ms/item) relative to stationary 9 ° targets (15 ms/item); again, our participants performed relatively efficiently in both conditions (17.5 ms/item, on average). We suggest that the crossover interaction (especially the advantage for moving over stationary 45 ° targets) found by McLeod and his colleagues is characteristic of task performance during early stages of practice. In particular, early on, participants show poor performance with stationary 45 ° targets because they have difficulty in keeping moving 45 ° nontargets out of the search; furthermore, participants exhibit poor performance with moving 9 ° targets because of the relatively noisy representation of such stimuli within the form system (unfortunately, we did not store our practice data and so were unable to corroborate this suggestion). Note that our account, which is developed below, explains the asymmetry reversal of McLeod and his colleagues within the theoretical frameworks provided by general theodes of visual search.

Alternative Account of the Asymmetry Reversal In more detail, our account is as follows: 1. Search RT function slopes reflect the size of the window within which processing operates inparallel, with steep slopes indicating small windows and increased involvement of some spatially serial component (e.g., see Pashler, 1987; Treisman & Gormican, 1988; see also Humphreys & Miiller, 1993). 2. Target selection operates from an "overall saliency" representation. Selection can be top-down controlled by enhancing the saliency of items sharing target features (e.g., Cave & Wolfe, 1990; Treisman & Sato, 1990). In conjunction search for, say, a moving 45 ° tilted line, moving items will be enhanced through the motion system and items oriented to 45 ° through the form system. 3. The form system represents both the stationary and moving items, but the latter has a degraded quality. The reduced quality for moving items has no effect on performance when the form discrimination required is easy (a moving 45 ° tilted line is clearly different than a moving vertical line); but when the discrimination is difficult (9 ° tilt from vertical), performance may be disproportionately affected (see Driver & McLeod, 1992). 2 4. Search can be efficiently directed to the moving items even when there are stationary items in the display. However, guiding the search to the stationary items in the presence of moving items is difficult. The reason is that, in default mode, the motion system passes the moving items, making them more salient at the stage of selection (positive tagging). If the task requires filtering out of the moving items (thereby making the stationary items more salient), the system must be reconfigured (negative tagging). However, this is demanding and prone to failure (i.e., there is a tendency to revert to the default operation of the system). Consequently, moving nontargets are more likely to intrude

into the search for a stationary target than vice versa (see MOiler & Maxwell, 1994; MOiler & Found, 1994). 5. It follows from paragraphs 3 and 4 that moving 45 ° nontargets are more likely to intrude into the search for a stationary 45 ° target than are moving 9 ° nontargets to intrude into the search for a stationary 9 ° target. Moving 9 ° lines are more likely to be mistaken for vertical lines (because of their noisy representation within the form system) and are, therefore, less likely to receive the top-down enhancement given to items sharing the target orientation. 6. It follows from paragraphs 4 and 5 that when the discrimination required is easy (45 ° target), there is an advantage, in terms of search rates, for moving over stationary search. To prevent the moving 45 ° nontargets from intruding into the search for a stationary 45 ° target, the size of the parallel search window needs to be decreased. 7. It follows from paragraphs 3 and 5 that when the discrimination required is difficult (9 ° target), search is easier for stationary targets than for moving targets. Stationary search benefits from better quality representation of the stationary items in the form system (paragraph 3) and is less prone to intrusions from moving 9 ° lines (paragraph 5). Both of these points permit search using a larger window. In the present study, highly practiced participants achieved faster search rates with stationary 45 ° targets; in fact, they searched for such targets as efficiently as they did for moving 45 ° targets. This finding indicates that they were able to keep moving 45 ° tilted nontargets out of the search because they were more efficient at reversing the (default) operation of the motion system (see paragraph 4). Note, however, that this reversal seems to incur a cost in terms of the y-intercept of the search RT functions, which were raised for stationary relative to moving searches (despite the fact that search condition was blocked). Interestingly, neither the data of Driver and McLeod (1992) nor those of Berger and McLeod (1996) showed any evidence of a y-intercept cost for stationary conditions, supporting the suggestion that their participants performed the task differently. 3 2 The feature search data of Driver and McLeod (1992) under the 45 ° and 9° conditions provide evidence for this assumption. For stationary search, the search rates increased from zero (present response), one (absent response) to two (present response), six (absent response) ms/item; for moving search, the slopes were steeper and increased from one (present response), three (absent response) to four (present response), 12 (absent response) ms/item. The interaction is apparent in both the target-present and targetabsent responses. Note that the feature search data probably underestimate the magnitude of the interaction because there is no need to filter by motion (or its absence; see paragraph 4 of our alternative account). In fact, the saliency of the target may be increased by computing the contrast to all display items rather than merely a subset of (moving or stationary) items (e.g., Cave & Wolfe, 1990; Wolfe, 1994). 3 Recall, in this context, that McLeod and his colleagues blocked the set size, which is unusual in visual search experiments. As a result, some of their participants might have adopted a mixture of strategies to deal with small and large set sizes (e.g., a strategy that might have worked with 8 items in the displays may not be

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Furthermore, our participants achieved faster search rates with moving 9 ° targets. Discriminating such targets from vertical lines is particularly affected by the "noisy" representation of the moving items within the form system. To optimally perform the task, participants have to tune themselves to small differences, which requires practice. Therefore, practice enabled the participants to compensate for the added difficulty with moving 9 ° targets, at least at high densities. At low densities, their search rates showed some tendency to increase more markedly for moving targets than for stationary 9 ° targets, probably because the quality of moving items deteriorates more rapidly with increasing display extension (i.e., item eccentricity) than does that of stationary items.

the finding of an asymmetry reversal with relatively unpracticed participants). Driver and McLeod (1992) may, of course, be right in arguing that viable search theories must increasingly incorporate the specific (physiological and psychophysical) properties of different visual subsystems to account for performance across a wide range of tasks. It remains to be seen to what extent general theories succeed in doing so without losing their very generality (so far, general theories have been able to accommodate the fact that not all subsystems are created alike). However, news of the demise of general theories is premature at present.

Conclusion

Berger R. C., & McLeod, P. (1996). Display density influences visual search for conjunctions of movement and orientation. Journal of Experimental Psychology: Human Perception and Performance, 22, 114-121. Cave, K. R., & Wolfe, J. M. (1990). Modeling the role of parallel processing in visual search. Cognitive Psychology, 22, 225-271. Cohen, A., & Ivry, R. B. (1991). Density effects in conjunction search: Evidence for a coarse location mechanism of feature integration. Journal of Experimental Psychology: Human Perception and Performance, 17, 891-901. Driver, J., & McLeod, P. (1992). Reversing visual search asymmetries with conjunctions of movement and orientation. Journal of Experimental Psychology: Human Perception and Performance, 18, 22-33. Duncan, J., & Humphreys, G. W. (1989). Visual search and stimulus similarity. Psychological Review, 96, 433-458. Duncan, J., & Humphreys, G.W. (1992). Beyond the search surface: Visual search and attentional engagement. Journal of Experimental Psychology: Human Perception and Performance, 18, 578-588. Humphreys, G. W., & MUller, H. J. (1993). SEarch via Recursive Rejection (SERR): A connectionist model of visual search. Cognitive Psychology, 25, 43-110. McLeod, P., Driver, J., & Crisp, J. (1988). Visual search for a conjunction of movement and form is parallel. Nature, 332, 154-155. McLeod, P., Heywood, C., Driver, J., & Zihl, S. (1989). Selective deficit of visual search in moving displays after extrastriate damage. Nature, 339, 466-467. MUller, H.J., & Found, A. (1994). Perceptual integration of motion and form information: Competitive interactions between grouping by "common fate" motion and form similarity. Unpublished manuscript, Birkbeck College, University of London, London, England. MUller, H.J., & Maxwell, J. (1994). Perceptual integration of motion and form information: Is the movement filter involved in form discrimination? Journal of Experimental Psychology: Human Perception and Performance, 20, 397-420. Nakayama, K., & Silverman, G. (1986). Serial and parallel processing of visual feature conjunctions. Nature, 320, 264-265. Pashler, H. (1987). Detecting conjunctions of color and form: Reassessing the serial search hypothesis. Perception & Psychophysics, 41, 191-201. Shepherd, M. (1984). EMDISP: A visual display system with digital and analogue sampling. Behavior Research Methods, Instruments, & Computers, 16, 297-302. Treisman, A., & Gelade, G. (1980). A feature-integration theory of attention. Cognitive Psychology, 12, 97-136.

In summary, we contend that the asymmetries reported by Driver and McLeod (1992) and Berger' and McLeod (1996) do not pose a fundamental challenge to general theories of visual search. Potentially, the most important asymmetry in the data of McLeod and his colleagues was the search rate advantage for moving over stationary 45 ° targets, which they took as indicating that gross aspects of form discrimination are accomplished within the motion system. However, our (highly practiced) participants achieved search rates with both moving and stationary targets that were as fast as those attained by the participants of McLeod and his colleagues with moving 45 ° targets only. In other words, McLeod and his colleagues did not demonstrate a fundamental limit to performance with stationary 45 ° targets. Thus, what requires special consideration is not why search is so fast with moving 45 ° targets but rather why it may be comparatively slow with stationary 45 ° targets. We suggest that it is slow during early stages of practice with the task, when participants find it hard to keep moving nontargets sharing the target line tilt out of the search. One reason why this is true is because filtering out the moving items requires participants to reverse the default setting of the motion system (from positive tagging of the moving items to negative tagging). Practice improves participants' ability to do this, thereby, minimizing interference. Our own experiment, which used highly practiced participants, failed to produce any evidence supporting the existence of an asymmetry reversal between stationary and moving search dependent on target (form) discriminability. No asymmetry reversal was evident at any of the display densities examined, which covered the range of densities used in previous studies. The results are thus in accordance with Mtiller and Maxwell's (1994) previous findings. The failure to find the asymmetry reversal favors the more parsimonious accounts offered by general theories in terms of general top-down processes and similarity grouping of display items (which, as we have shown, could also explain successful with 20 items), obliterating consistent y-intercept effects. In contrast, randomizing the set size in our experiments ensured that participants approached the task with a single strategy that had been proven, during practice, to work with all set sizes.

References

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Treisman, A., & Gormican, S. (1988). Feature analysis in early vision: Evidence from search asymmetries. Psychological Review, 95, 15-48. Treisman, A., & Sato, S. (1990). Conjunction search revisited. Journal of Experimental Psychology: Human Perception and Performance, 16, 459-478.

Wolfe, J. M. (1994). Guided Search 2.0: A revised model of visual search. Psychonomic Bulletin and Review, 1, 202-238. Received March 22, 1994 Revision received October 4, 1994 Accepted December 15, 1994