Aging and Filtering by Movement in Visual Search - CiteSeerX

Journal of Gerontology: PSYCHOLOGICAL SCIENCES 1996, Vol. 51B. No. 4. P20I-P2I6

Copyright 1996 by The Gerontological Society of America

Aging and Filtering by Movement in Visual Search Arthur F. Kramer,1 Robin Martin-Emerson,1 John F. Larish,1 and George J. Andersen2 'University of Illinois at Urbana-Champaign. 2 University of California at Riverside.

We examined the ability ofyounger and older adults to selectively process moving items and ignore stationary items in a task that required the search for a target defined by a conjunction of movement andform (i.e., search for a moving X among moving Os and stationary Xs) in displays of 5,9,17, and 25 stimuli (Experiment 1) and displays of 5,10, and 20 stimuli (Experiment 2). We also investigated subjects' performance in two feature search tasks, the search for a target defined by movement or form. Finally, we examined the influence of practice on feature and conjunction search. Younger and older adults searched the displays at similar rates in the feature and conjunction search tasks. Older and younger adults also benefited equivalently from practice. These data suggest age-equivalence in the processes which underlie feature search in dynamic environments as well as those processes responsible for the segregation of moving and stationary objects in the visual field.

to locate and identify objects in cluttered THEvisualability environments has been of great interest to attention theorists for over two decades. Treisman and Gelade (1980) proposed what was to become the modal view of visual search. In the feature integration theory they suggested that the search for information in the visual environment takes place via two different processes or stages. In the feature extraction stage, features are registered in parallel across the visual field. The available features are coded along a number of separable dimensions such as color, brightness, curvature, direction of motion, etc. In the feature integration stage, the features are conjoined into objects via the sequential focusing of attention on different locations in the visual environment. This relatively simple and elegant theory provides an accurate account of many aspects of visual search performance. For example, the theory suggests that the search for targets that are distinguishable from nontargets by a single feature (e.g., a red flag among blue flags) should be fast and insensitive to the number of nontargets present in the visual field. On the other hand, since the integration of features to form objects takes place via the process of serially focusing attention on different locations, the search for targets defined by a conjunction of features (e.g., detecting a red Honda Accord in a used car lot with red Honda Civics and blue Honda Accords) should be relatively slow and sensitive to the number of nontargets present in the visual scene. In general, both of these predicted patterns of results have been obtained in visual search studies (Treisman & Gelade, 1980; Treisman & Gormican, 1988). Plude and Doussard-Roosevelt (1989) examined agerelated differences in visual search within the context of Treisman and Gelade's (1980) feature integration theory. The aim of their study was to determine if age-related decrements in visual search processes that had previously been observed (Ball, Beard, Roenker, Miller, & Griggs, 1988; Gilmore, Tobias, & Royer, 1985; Madden, 1987; Nebes & Madden, 1983; Plude & Hoyer, 1985, 1986;

Sekuler & Ball, 1986) were the result of decreased efficiency of the feature extraction process, the feature integration process, or both processes. To that end, they had both younger and older adults perform a set of visual search tasks which required subjects to (a) detect a target that differed from nontargets on two features (e.g., red X among blue Os) and (b) detect a target that differed from nontargets on a conjunction of features (e.g., red X among red Os and green Xs). Consistent with feature integration theory, both younger and older adults produced very shallow search functions in the feature search condition. Target present search slopes [i.e., slopes for the function relating reaction time (RT) to the number of nontarget items] were less than 2.5 msec/item for both younger and older adults. On the basis of these data the authors concluded that the parallel feature extraction process was age-insensitive. On the other hand, older adults produced steeper search slopes than younger adults in the conjunction search task (i.e., target present search slopes for younger and older subjects were 13.9 and 25.4 msec/item, respectively), suggesting that the serial feature integration process is susceptible to aging. Since the Plude and Doussard-Roosevelt (1989) examination of age differences in visual search processes, there have been a number of interesting empirical findings and theoretical developments relevant to our understanding of visual search. For example, a number of authors have reported that, under certain conditions, conjunction search can produce search slopes that do not differ from those obtained in feature search tasks (Duncan & Humphreys, 1992; Nakayama & Silverman, 1986; Steinman, 1987; Treisman & Sato, 1990; Wolfe, 1994; Wolfe, Cave, & Franzel, 1989). That is, conjunction searches were found to produce relatively flat search slopes that were interpreted as evidence for parallel search. Triple conjunction searches have also been found to be easier, that is, to produce flatter search slopes, than double conjunction searches (Dehaene, 1989; Quinlan & Humphreys, 1987; Wolfe etal., 1989). Neither of these results can P201

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be easily accommodated by Treisman and Gelade's (1980) original version of the feature integration theory. Several theories have been proposed to account for these search results. For example, Treisman and Sato (1990) proposed a modification of the feature integration theory in which a top-down feature inhibition process can be used to dampen the activation of nontarget items in feature maps when the nontargets are easily distinguishable from the targets. Thus, even in the case of conjunction search, the inhibition process can serve to render the search process parallel by effectively preventing the nontargets from being addressed by the feature integration process. Wolfe et al. (1989; Wolfe, 1994) proposed a similar model in which the activation of the target could be increased via a top-down control process. This would effectively raise the activation of the conjunction target so as to preclude the need for a timeconsuming and serial scan of target and nontarget items. Movement Filters and Visual Search In a relatively recent series of studies, McLeod and colleagues (McLeod, Driver, & Crisp, 1988; McLeod, Driver, Dienes, & Crisp, 1991; McLeod, Heywood, Driver, & Zihl, 1989; see also Muller & Maxwell, 1994) have examined one particular variety of visual search, the search for a target defined by a conjunction of movement and form that is interesting both from a psychological as well as a neuroanatomical and physiological perspective. In an initial series of studies McLeod et al. (1988) had younger adults search for a moving X among moving Os and stationary Xs. Subjects searched through displays of 5, 9, 17, and 25 items with a target occurring on 50% of the trials. Search slopes for conjunction targets were less than 8 msec/item, suggesting parallel search through the display. McLeod et al. (1988) suggested two possible explanations for these relatively flat search slopes for the form by movement conjunction targets. One possibility was that subjects were capable of capitalizing on the grouping engendered by movement of the letters in a specific direction at a fixed velocity. That is, as suggested years ago by the Gestaltists (Wertheimer, 1923), the common fate of the moving items would define a coherent perceptual group which could be selectively attended. Selective attention to the moving items would, in effect, render the task equivalent to feature search, since among the moving items the target (i.e., an X) could be distinguished from the nontargets (i.e., the Os) by its form. The second explanation involved a more specialized mechanism. McLeod et al. (1988) suggested that the cells in the medio-temporal region of the cortex were uniquely suited for the process of filtering via movement, since these cells are selectively sensitive to particular speeds and directions of movement while being relatively insensitive to stationary stimuli (Albright, 1992; Livingstone & Hubel, 1987; Movshon & Newsome, 1992). Unlike the grouping proposal, the movement filter explanation does not require that common fate (i.e., movement in a fixed direction and velocity) be present for subjects to selectively attend to the moving stimuli and ignore the stationary stimuli. McLeod et al. (1991) examined these two hypotheses by requiring subjects to search for a moving X among Os moving in each of the four cardinal directions and stationary Xs. If

filtering was occurring on the basis of common fate or grouping, then subjects should be unable to rapidly search through the display for the conjunction target under these conditions. On the other hand, the movement filter proposal suggests that subjects should be capable of selectively attending to the moving stimuli regardless of whether the movement is in one or many directions. The data were consistent with this latter proposal. The search slope on target-present trials was less than 10 msec/item. Further evidence was provided for the movement filter proposal in a study in which McLeod et al. (1989) had a patient with bilateral medio-temporal lesions perform the form by movement conjunction search task. Search was quite slow with target present slopes greater than 160 msec/item. Thus, these data suggest that search for targets defined by conjunctions of movement and form are mediated by a movement filter supported by the mediotemporal cortex. In effect, this mechanism appears capable of selectively processing moving items, thereby reducing conjunction search to feature search. Experiment 1 An interesting question, and the main topic of the present study, is whether the movement filter is degraded by the process of normal aging? The answer to this question has both theoretical and practical implications. From a theoretical perspective, it is important to determine which visual search mechanisms are relatively resistant to aging as well as those visual search mechanisms that are more susceptible to aging. The results obtained by Plude and DoussardRoosevelt (1989) suggest that feature extraction process is age-invariant while feature integration processes are susceptible to aging (see also Zacks & Zacks, 1993). In the present study, we will revisit the issue of age effects on feature search as well as explore the movement filter, a mechanism that appears to enable fast and efficient conjunction search, in the context of aging. From a practical perspective, both older and younger adults search for conjunctions of motion and form every time they take a walk, drive an automobile, or play tennis. Therefore, it would appear important to examine how the movement filter, which presumably supports such activities, is influenced by aging. Research on age-related differences in motion sensitivity may provide some insight into whether to expect age-related decrements in movement filtering in visual search tasks. Trick and Silverman (1991) used a correlated motion paradigm to examine potential age differences in motion sensitivity. In this task the percentage of dots which move in the same direction are varied and subjects are required to indicate the direction of the coherent motion of these dots. Motion thresholds are defined as the percentage of correlated dots required to perform the task at an accuracy of 75%. Trick and Silverman (1991) found a linear decrease in motion sensitivity of approximately 1% per decade. Gilmore, Wenk, Naylor, and Stuve (1992) also used a correlated motion paradigm and found that while older females had higher motion thresholds than younger adults, the thresholds of older males were not significantly different from those obtained by younger adults (see also Schieber, Hiris, White, Williams, & Brannan, 1990). Another inter-

FILTERING BY MOVEMENT IN VISUAL SEARCH

esting finding in the Gilmore et al. studies was the observation of substantial overlap in the distribution of motion thresholds between younger and older adults, including the elderly females. This finding is consistent with other research, which has found substantial individual differences in age-related effects on motion perception and other visual functions (Spear, 1993). In an effort to examine the relationship between motion sensitivity and movement filtering in visual search, we administered a correlated motion task like that used by Gilmore et al. (1992) and Trick and Silverman (1991) to our younger and older subjects. The motion threshold value was then used as a covariate in the analysis of the relationship between aging and the visual search for moving targets. One additional issue that we explored in the present study was the influence of practice on the visual search performance of younger and older adults. Previous studies have found that both older and younger adults' visual search performance benefits from practice, but age differences in performance (i.e., the magnitude of the visual search slope) are maintained across practice sessions (Plude & Hoyer, 1986; Rogers & Fisk, 1991). However, these studies have used relatively small display set sizes and have not contrasted practice effects on feature and conjunction search tasks. Previous studies have also been confined to the search for static targets in static nontarget displays. Therefore, it is an open question as to the nature of practice effects on younger and older adults' performance in dynamic search tasks. In an effort to examine this issue, subjects received three sessions of practice on our feature and conjunction search tasks.

METHOD

Subjects Fifteen younger (age range 18-25, M = 20.5, SD = 1.3) and 14 older adults (age range 60-75, M = 68.8, SD = 5.1) were recruited from the university community and were paid $5.00 per hour for their participation in the study. Eight of the younger and 8 of the older subjects were women. All of the subjects were screened for use of any medication that would influence performance on the experimental tasks (e.g., psychotropic drugs, beta blockers) and for near and far visual acuity. All of the subjects possessed corrected visual acuities of at least 20/30. The average corrected acuity for the younger and older subjects was 20/21 and 20/24 (Snellen), respectively. The difference was not statistically significant (p > .55). All of the younger and older subjects were asked to indicate the number of years of formal education that they received. The mean number of years of formal education for the younger and older subjects was 15.2 and 16.7, respectively. The older subjects were significantly better educated than the younger subjects [F(l,28) = 4.2, p < .05]. The subjects were also administered the Kaufman Brief Intelligence Test (K-BIT). The average standardized composite scores for the Kaufman Brief Intelligence Test were 115.8 and 118.4 for the younger and older adults, respectively. This difference was not statistically significant (p > .70).

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Apparatus and Stimuli Visual search tasks. — The visual search display was partitioned into 25 imaginary columns subtending 10.7° horizontally and 7.9° vertically at a viewing distance of 90 cm. The stimulus array consisted of 5, 9, 17, or 25 letters (Os and Xs), each subtending .45° horizontally and vertically. One letter was positioned in each of the 25 columns; for set sizes less than 25 letters, the central columns were used to maintain horizontal spacing (i.e., density was maintained across display set sizes). During each trial, the stimulus letters either remained stationary or moved vertically within the display at a fixed velocity (2.047sec) such that, for a 1200-msec display period, the extent of movement was 2.45°. The direction of motion (up or down) was randomly determined on each trial with movement in each direction occurring on 50% of the trials. Moving stimuli were initially arranged over the upper or lower two-thirds of the display area (for stimuli that moved down or up, respectively) to prevent the stimuli from moving off the screen during a trial. Stationary stimuli were distributed over the entire display. The vertical distribution of the letters across columns was random within the extent of the display area occupied by the stimuli. Moving and stationary stimuli were used to construct three visual search tasks where the target was uniquely defined by form, motion, or a conjunction of form and motion. For the motion search task, a single target letter moved among stationary letters (e.g., a moving O among stationary Os). In the form search task, all letters moved within the display (e.g., a moving O among moving Xs). In the conjunction search task, the target and one-half of the remaining letters moved among stationary letters (e.g., a moving O among moving Xs and stationary Os). For set size 5 in the conjunction search task, no more than one moving letter was adjacent to another moving letter; for set sizes 9 and 17, no more than two moving letters were adjacent to one another; and for set size 25, no more than three moving letters were adjacent. The three visual search tasks are illustrated in Figure 1. Motion threshold task. — The stimuli consisted of 170 randomly positioned dots that moved, across frames, either in a coherent direction at a constant speed (signal dots) or in a random direction with random speed (noise dots). The displays subtended a 7.5 degree square region. The size of the dots was 1.25 min arc by 1.86 min arc. Dot density was 3.25 dots/deg2. The displays consisted of 77 frames presented at a rate of 38.5 frames/sec. Display duration was 2 seconds. The signal-to-noise ratio of the moving pattern was varied, from trial to trial, using a two-down one-up adaptive staircase procedure. One hundred percent coherence displays consisted of dots that translated uniformly at a fixed speed (2.4°/sec) either to the right or to the left. The signalto-noise ratio was manipulated by varying the percentage of dots that translated in a uniform direction. For example, displays with a signal-to-noise ratio of 1.0 consisted of 50% of the dots translating in a uniform direction from frame to frame (correlated motion). Those dots, on a given frame, that did not translate in a uniform direction (uncorrelated

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Conjunction Search Figure 1. A graphic illustration of the displays viewed by the subjects in the motion (top), form (middle), and conjunction (bottom) search tasks.

motion) were randomly repositioned within the display. The selection of dots with correlated motion (from frame to frame) was random, with each dot having an equal probability of correlated motion. Thus, for each new frame, new dots were selected to have correlated motion for the next frame. The percentage of correlated motion was varied in units of signal-to-noise ratio defined according to the following equation: i = log(c/100-c), where i is increment change in signal (correlated motion) to noise (uncorrelated motion), and c is the proportion of signal dots in the display. The experiment used a two-down one-up adaptive staircase procedure in which the proportion of signal dots was decreased following two successive correct responses. After an incorrect response the number of signal dots was either increased or decreased according to the following rule. If the previous two responses were correct, then the signal-to-noise ratio was decreased. If the previous response was incorrect, then the signal-to-noise ratio was increased. A reversal refers to two correct successive responses followed by an incorrect response. The decrement value was .2 log units before any

reversal occurred. This value decreased to .1 log units between reversal one and reversal two, and decreased to .05 log units for all subsequent trials in the run. Adjusting the magnitude of the change (i) in signal-to-noise ratio following each reversal allowed us to reach a threshold value with a relatively small number of trials. Design Age group (older or younger adults) was the betweensubjects variable. Target type (present or absent), search task (form, motion, or conjunction), and set size (5,9, 17, or 25) were manipulated within subjects in the visual search task. For one-half of the subjects in each age group, the " X " was designated as the target letter; the " O " was the target letter for the remaining subjects. The presence or absence of the target was uniquely defined by the search task (i.e., by target letter form, target letter motion, or by target letter form and motion). Target-present and target-absent trials occurred with equal frequency, and target type response hand was counterbalanced across subjects. In the form search task, all letters of the stimulus array moved. For target-present trials, the target letter moved among moving nontarget letters. For target-absent trials, the stimulus array consisted of moving nontarget letters. All letters of the stimulus array for the motion feature search task were identical. The target, when present, was a single moving letter among stationary letters. When the target was absent, all letters remained stationary. For the conjunction search task, the target letter was defined by both form and motion. When present, the target letter moved among identical stationary letters and moving nontarget letters. For target-absent trials, nontarget letters moved among stationary target letters. Subjects performed the visual search tasks in three sessions. In each session, four blocks of each feature search task were presented. The subject then performed four blocks of the conjunction search task. Finally, the subjects performed four more blocks of each feature search task. Each block of a feature search task consisted of 24 trials for one set size. Similarly, a block in the conjunction search task consisted of 96 trials for one set size. Thus, subjects performed 192 trials in the motion search task, 192 trials in the form search task, and 384 trials in the conjunction search task in each of the three experimental sessions. In each search task, the order of the four blocks corresponding to set size was randomly determined. The order of the feature search tasks in a session was counterbalanced across sessions and subjects. Procedure The experimenter began the first experimental session with a brief description of the tasks that the subjects would complete in the course of the four-session study. Subjects were then administered the acuity tests and the education and demographics questionnaire. Next, subjects were administered the Kaufman Brief Intelligence test, which took approximately 30 min to complete. Finally, subjects performed the motion threshold test. In the motion threshold test, the subjects were instructed to respond by pressing one of two keys to indicate the direction the dots were moving in, either to the left or to the right. Each


subject was presented with two runs of the adaptive staircase procedure. The first run began with a 1.77 signal-to-noise ratio display and continued until subjects responded with five reversals of the two-down one-up procedure. A preliminary threshold was derived based on the average signal-to-noise ratios for the first five reversals. The second run started at the midpoint signal-to-noise ratio value halfway between the preliminary threshold determined from the first run and the original 1.77 signal-to-noise ratio value. The second run continued until five reversals occurred and a second preliminary threshold was derived based on these reversals. A final threshold was determined based on the average of the two preliminary thresholds. The motion threshold testing procedure took about 10 min to complete. The second experimental session began with a review of the general format of each of the next three sessions and provided both written and verbal instructions for each visual search task. In addition, each block of trials was preceded by an instruction screen specifying the task, target, and set size. The third and fourth experimental sessions were similarly conducted. Each visual search trial within a block began with the subject's response to the instruction "SPACE BAR" shown in the center of the display. After pressing the space bar, the instruction was replaced by a blank screen for 500 msec. A small fixation cross was then displayed for 500 msec and followed by a 100-msec blank screen interval prior to the presentation of the stimulus array. The stimuli were presented for 1200 msec. A blank screen was displayed after a subject's response. After a response, the next trial was signaled to the subject with the space bar instruction. If the subject failed to respond within 2400 msec of the stimulus array onset, an error was recorded and the space bar instruction was displayed, indicating the subject should begin the next trial. An invalid key press was signaled by a beep, but no other trial feedback was provided. Subjects were instructed to respond with a specified key press of one hand to the presence of the target and to the absence of the target with the other hand. Subjects were asked to respond as quickly as possible without sacrificing accuracy. At the end of each block, a performance graph displayed mean response time and accuracy. Subjects were required to take at least a oneminute rest after each block of trials. Subjects were also instructed that they could take a break, if needed, at any other time during the experimental sessions.

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Motion Threshold Task The motion thresholds obtained for each of the younger and older subjects are presented in Figure 2. The thresholds obtained by the younger subjects ranged from .5% to 12.8%, whereas those obtained by the older subjects ranged from .7% to 8.4%. The mean motion thresholds for the younger and older subjects were 1.8% and 2.4%, respectively. The motion thresholds obtained by the younger and older subjects were not statistically different (p > .5). Thus, it would appear that any performance differences obtained in the search tasks cannot be attributed to differential motion thresholds for the younger and older subject groups. Nonetheless, given the increased variability in motion thresholds for the older adults, we decided to take a conservative approach to the analysis of the search performance data by employing subjects' motion threshold scores as a covariate in these analyses. Motion Search Task The mean RT data for the motion search conditions are presented in the top panels of Figures 3 and 4. Slopes, intercepts, and R2s for motion searches are presented in Tables 1 and 2. Finally, the accuracy data for each of the motion search conditions are presented in Tables 3 and 4. As can be seen in the figures, RTs appear to be relatively insensitive to display set size for both younger and older subjects. RTs also appear slower for older than younger subjects and for target-absent than target-present responses. Mean RTs and accuracies were submitted to four-way ANCOVAs with age as a between-subjects factor and display set size (5, 9, 17, and 25), target type (present and absent), and practice (session 1, 2, and 3) as within-subjects factors. Motion threshold served as a covariate in these and

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£1 RESULTS

The results will be organized in the following manner. First, we will describe the results of the motion threshold task that was administered to the younger and older adults. This analysis will provide an index of age-related differences in the sensitivity to motion. Second, we will present the results obtained from the form and motion feature search tasks. These results will indicate the extent to which parallel or bottom-up search processes are influenced by age. Finally, we will present the RT and accuracy data for the conjunction search conditions for the younger and older adults. These results will indicate the degree to which younger and older adults are able to capitalize on movement filtering to modify the task from that of a conjunction search to a feature search.

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Age Figure 2. Correlated motion thresholds for each of the younger and older subjects. Note that the separation of points along the x-axis for the younger and older groups was done to prevent overplotting of similarly valued data points and does not imply differences in age with the older and younger adult groups.

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Motion Feature Search 1400 1300 1200 W

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D i s p l a y Size Figure 3. Mean RTs for the target-present trials for each of the experimental conditions in the motion (top panel), form (middle panel), and conjunction search (bottom panel) tasks in Experiment 1. Older subjects' RTs are represented by the solid lines and filled symbols. Younger subjects' RTs are represented by the dotted lines and unfilled symbols. Session 1,2, and 3 RTs are represented by the circles, squares, and triangles, respectively.

Display Size Figure 4. Mean RTs for the target-absent trials for each of the experimental conditions in the motion (top panel), form (middle panel), and conjunction search (bottom panel) tasks in Experiment 1. Older subjects' RTs are represented by the solid lines and filled symbols. Younger subjects' RTs are represented by the dotted lines and unfilled symbols. Session 1, 2, and 3 RTs are represented by the circles, squares, and triangles, respectively.


Table 1. Slopes, Intercepts, and R2s for the Search Functions in Each of the Target-Present Conditions in Experiment 1

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Table 3. Accuracy for Search Task Target-Present Trials by Age Group for Each Session in Experiment 1 Motion

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Motion Search Intercept Slope R2

512 0.7 .53

452 1.1 .95

470 0.4 .82

670 0.6 .15

599 0.3 .34

565 0.4 .65

Older Session 1 .96 .97 .97 .96 .96 .96 .96 .91 .95 .90 .82 .63 Session 2 .98 .98 .98 .98 .98 .99 .97 .93 .96 .95 .84 .68 Session 3 .98 .98 .99 .98 .99 .98 .97 .95 .97 .97 .87 .72

Form Search Intercept Slope R2

492 5.9 .96

451 5.6 .98

439 6.3 .97

648 10.6 .99

588 9.3 .96

553 9.1 .92

Younger Session 1 .95 .93 .97 .97 .97 .97 .99 .95 .96 .93 .90 .80 Session 2 .97 .95 .91 .94 .96 .97 .97 .98 .95 .95 .93 .88 Session 3 .94 .95 .94 .94 .97 .96 .97 .96 .95 .96 .92 .87

Conjunction Search Intercept 560 Slope 12.4 R2 .99

526 10.3 .99

520 10.3 .99

794 13.3 .98

69 i 14.6 .99

669 14.4 .99

Table 4. Accuracy for Search Task Target-Absent Trials by Age Group for Each Session in Experiment 1 Motion Age Group

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Table 2. Slopes, Intercepts, and R s for the Search Functions in Each of the Target-Absent Conditions in Experiment 1 Older

Younger 1

2

3

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567 2.4 .72

496 1.5 .68

490 1.4 .97

749 3.9 .76

648 3.6 .99

661 3.2 .92


457 13.8 .99

426 10.6 .99

414 11.3 .96

623 18.5 .99

605 15.1 .99

553 16.0 .97


502 18.5 .99

463 21.2 .99

767 24.2 .99

690 26.5 .99

664 27.0 .99

Session

all other analyses of the visual search performance data. Consistent with Figures 3 and 4, RTs were faster for younger than older subjects [F( 1,27) = 26.3, p < .01], target-absent than target-present responses [F(l,27) = 79.1, p < .01], small than large display sizes [F(l,27) = 13.1, p < .01], and with more practice [F(l,27) = 62.9, p < .01]. Two significant two-way interactions were also obtained. Older adults benefited more from practice than younger adults [F(2,54) = 4.4, p < .05]. RTs increased more with increasing display size on target-absent than on targetpresent trials [^(3,81) = 7.6, p < .01]. Most interestinglyj however, we failed to find a significant age-by-display-size effect. This is perhaps not surprising since, as indicated in Tables 1 and 2, display size slopes were less than 4 msec/ item in all of the conditions in the motion search task (and less than 1.5 msec/item in the target-present conditions). Search slopes of less than 10 msec/item have often been interpreted as evidence for parallel search (Treisman & Gelade, 1980; Treisman & Gormican, 1988). Thus, the failure to find an age-by-display-size interaction, along with the relatively flat search slopes, strongly suggests that both

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Older Session 1 .96 .99 .94 .98 .99 .98 .98 .97 .96 .97 .94 .90 Session 2 .97 .99 .99 .99 .99 .99 .99 1.00 .97 .98 .95 .92 Session 3 .98 .98 .99 .98 .99 .98 .99 .99 .98 .98 .96 .92 Younger Session 1 .94 .96 .94 .94 .94 .98 .98 Session 2 .96 .96 .98 .97 .98 .98 .98 Session 3 .96 .98 .98 .98 .98 .97 .98

.97 .96 .95 .95 .93 .98 .94 .97 .98 .96 .99 .96 .97 .98 .97

younger and older adults were searching in parallel for the moving targets. None of the three- or four-way interactions was significant (ps > .40). As suggested by Tables 3 and 4, older adults were significantly more accurate than younger adults [F(l,27) = 5.1, p < .05]. Accuracies improved with practice [F(2,54) = 4.4, p < .01]. None of the other main effects or interactions was statistically significant (ps > .30). Form Search Task The mean RT data for the form search conditions are presented in the middle panels of Figures 3 and 4. Slopes, intercepts, and Rh for form searches are presented in Tables 1 and 2. Finally, the accuracy data for each of the form search conditions are presented in Tables 3 and 4. As can be seen in the figures, older adults are slower to search for the form target than younger adults. As in the motion search conditions, RTs appear to decrease across sessions. Mean RTs and accuracies were submitted to four-way ANCOVAs with age as a between-subjects factor and display set size (5,9, 17, and 25), target type (present and absent), and practice (session 1, 2, and 3) as within-subjects factors. Consistent with Figures 3 and 4, older adults responded more slowly than younger adults [F(l,27) = 38.3,/? < .01], RTs decreased with practice [F(2,54) = 38.0,/? < .01], RTs were slower for target-absent than for target-present responses [F(l,27) = 70.9, p < .01], and responses slowed with increasing display set size [F(3,81) = 172.6, /? < .01].

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As indicated in Tables 1 and 2, RT slopes were steeper on target-absent than on target-present trials [F(3,81) = 67.8, p < .01]. Unlike the data obtained for the motion targets, older adults were more negatively influenced by increasing numbers of distractors than were younger subjects [F(3,81) = 5.6, p < .01]. However, with the exception of session one, RT slopes were less than 10 msec/item for both younger and older adults in all of the target-present conditions. Accuracy improved with practice [F(2,54) = 4.4, p < .05]. Furthermore, the improvements in accuracy with practice were largest for the target-present trials on the large display size trials [F(6,162) = 3.2, p < .01]. None of the other main effects or interactions was significant (ps > .25). Conjunction Search Task The mean RT data for the conjunction search conditions are presented in the bottom panels of Figures 3 and 4. Slopes, intercepts, and R2s for conjunction searches are presented in Tables 1 and 2. Finally, the accuracy data for each of the conjunction search conditions are presented in Tables 3 and 4. Mean RTs and accuracies were submitted to four-way ANCOVAs with age as a between-subjects factor and display set size (5, 9, 17, and 25), target type (present and absent), and practice (session 1,2, and 3) as within-subjects factors. Consistent with Figures 3 and 4, older adults performed the conjunction search task more slowly than younger adults [F(l,27) = 55.9, p < .01], RTs decreased with practice [F(2,54) = 27.6, p < .01], target-absent responses were slower than target-present responses [F(l,27) = 151.5, p < .01], and RTs increased with display set size [F(3,81) = 358.9,p< .01]. A number of significant interactions were also obtained. Display set size had a larger impact on RTs on the targetabsent than on the target-present trials [F(3,81) = 73.6, p< .01]. Performance benefited more from practice on the target-present than on the target-absent trials [F(2,54) = 4.8, p < .01]. Similar to the form feature search task, display size had a larger impact on older than younger adults' RTs [F(3,81) = 11.8, p < .01]. Interestingly, however, this effect was qualified by a three-way interaction among age, set size, and target-type factors [F(3,81) = 3.8, p < .01 ]. The age-related difference in the display size effect was larger on the target-absent than on the target-present trials. Search slopes averaged across sessions for the targetpresent trials for younger and older adults were 11.0 and 14.1 msec/item, respectively. The comparable search slopes for the target-absent trials were 19.8 and 25.9 msec/item. Post hoc comparisons indicated that the differences in slope as a function of age were significant for the target-absent but not for the target-present trials (all post hoc comparisons were computed with the Bonferroni /-test). The steeper search slopes on the target-absent trials for the older rather than for the younger adults have previously been attributed to a more cautious response strategy (i.e., re-search the display) for the older adults when a target is not detected (Ford, Roth, Molls, Hopkins, & Kopell, 1979; Pfefferbaum, Ford, Roth, & Kopell, 1980). Several significant effects were obtained for conjunction search accuracy. Accuracies decreased with increasing dis-

play set sizes [F(3,81) = 63.5, p < .01]. Accuracies also improved with practice [F(2,54) = 12.4, p < .01] and were higher for target-absent than for target-present trials [F(l,27) = 51.3, p < .01]. Accuracy reductions with increasing display set sizes were larger for target-present than target-absent trials [F(3,81) = 33.3,/? < .01]. Finally, older adults showed a larger decrease in accuracy with increasing display set size than did younger adults [F(3,81) = 18.5, p< .01]. Comparison of Form Feature and Conjunction RTs Older adults displayed steeper display size search functions than younger adults in both the form and conjunction search tasks (see Tables 1 and 2). Interestingly, however, the difference between older and younger search slopes during target-present trials did not increase from the form to the conjunction search tasks as would be expected if older subjects were more negatively influenced by the requirement to detect and identify conjunction targets than were younger adults. In fact, the ratio of the older-to-younger search slopes averaged across the three sessions for the target-present trials actually decreased from 1.6 in the form search task to 1.3 in the conjunction search task. Such a result suggests that the older adults benefited to at least the same extent, if not more, than younger adults from the use of movement in the conjunction search task. DISCUSSION

Consistent with the findings of Plude and DoussardRoosevelt (1989), we obtained age-equivalent search slopes for the feature search for a single moving item among stationary nontargets. Search slopes were less than 4 msec/ item for both younger and older adults in all conditions. Interestingly, however, older subjects were slower to search for a target defined by form than were younger subjects. This result is in contrast with Plude and Doussard-Roosevelt's report of age-equivalent search slopes for feature search. Search slopes on the target-present trials for the younger and older adults in the Plude and Doussard-Roosevelt study were -.8 and 2.4/msec item, respectively. The comparable search slopes for the form target were 6.3 and 9.1 msec/item in the last session of our study. There are several reasons our younger and older subjects may have produced steeper slopes than those reported by Plude and Doussard-Roosevelt (1989). First, the stimuli in our form search task were moving while the stimuli in the Plude and Doussard-Roosevelt study were stationary. Previous studies have reported that cells in the medio-temporal cortex that respond to movement are less sensitive to aspects of form (i.e., these cells have lower spatial resolution and orientation sensitivity) than are cells in other visual areas (Albright, 1992; Livingstone & Hubel, 1987). Therefore, it is not surprising that search for a target distinguishable from nontargets by form should be less efficient in moving than in stationary stimuli. Second, the target in Plude and Doussard-Roosevelt's (1989) feature search task was distinguishable from nontargets by both color and form (e.g., a red X target among green O nontargets), while the target in our form search task was distinguishable from nontargets on only a single feature


(e.g., a moving X target among moving O nontargets). Within the context of Wolfe's (1994) guided search model, search for a double-feature target would be more efficient than search for a single-feature target. A larger difference signal would be produced in the feature maps (i.e., bottomup activation) for a double-feature target since the target would be distinguishable from nontargets on two feature maps — color and form in the Plude and DoussardRoosevelt study. Additionally, more top-down activation could be directed to the double-feature target since, on the basis of knowledge of the target characteristics, activation could be directed to both the color and form maps. Thus, it would appear likely that the more efficient feature search in the Plude and Doussard-Roosevelt (1989) study than in our form search task can be attributed to both the characteristics of the motion processing system in mediotemporal cortex as well as the physical characteristics used to define targets and nontargets in the two studies. Nonetheless, the important point is that search was still quite efficient, that is, the search slope was less than 10 msec/item, in our form search task. The conjunction search results are particularly interesting given McLeod and colleagues' (McLeod et al., 1988, 1989, 1991) proposal of a movement filter that can be used to segregate moving from stationary objects, thereby effectively turning a conjunction search into a feature search (i.e., the search for a moving X among moving Os). Their results were consistent with this proposal since search slopes were less than 10 msec/item across a variety of conditions and studies when subjects searched for a target defined by a conjunction of movement and form (McLeod et al., 1988, 1989, 1991; see also Muller & Maxwell, 1994). The younger adults in our study displayed comparable performance data, producing 10.3 msec/item search slopes in the targetpresent conditions in the last session of the study. The older subjects displayed slightly (i.e., 11.0 vs 14.1 msec/item for younger vs older adults averaged across the three sessions for the target-present trials) but nonsignificantly larger search slopes than the younger subjects in our conjunction search task. Therefore, it would appear that both younger and older adults are capable of employing the movement filter to effectively inhibit or ignore the stationary nontarget objects, thereby selectively processing the moving objects (see Appendix, Note 1). There is, however, an important qualification to the conclusion of age equivalence in conjunction search that is revealed by inspection of the accuracy data. As indicated in Table 3, older adults showed a significantly larger decrease in accuracy with increasing numbers of nontargets on the target-present trials than did the younger adults. The important question, of course, is whether this differential agerelated accuracy effect belies our conclusion that both younger and older adults are able to effectively employ the movement filter to facilitate search for targets defined by a conjunction of movement and form. One way to address this question is to compare the RT search slopes in conditions in which younger and older adults achieved comparable levels of accuracy. If, under such conditions, older adults display substantially steeper search slopes than younger adults, then we can conclude

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that the search slopes obtained by the older adults across the four display sizes were artifactually suppressed by a speed/accuracy tradeoff (i.e., older adults reduced search time, in the larger display size trials, by sacrificing accuracy). Of course, such a pattern of results would suggest that younger adults more effectively employed the movement filter to selectively process the moving stimuli than did the older adults. As can be seen in Table 3, display sizes 5 and 9 meet the criterion of equivalent younger/older accuracy. In fact, older subjects were slightly, but nonsignificantly, more accurate in these conditions than were younger adults. The search slopes obtained in this subset of display size conditions for the last session of the study were 10.1 for the younger and 14.6 msec/item for the older adults. These slopes did not differ significantly from the slopes obtained from the full set of display set sizes. Therefore, it appears reasonable to conclude that, at least for displays of 9 or fewer items, both younger and older adults are able to effectively use the movement filter to enhance search. However, a question remains as to why the older adults showed a more dramatic decrease in accuracy with increasing display set size than did the younger adults. One possibility that has been suggested by a number of researchers is that older adults take smaller perceptual samples than younger adults when scanning the visual field for relevant information (Rabbitt, 1965). This perceptual window or useful field of view hypothesis is consistent with the results of a number of studies which have found that older adults have more difficulty searching for and selectively attending to target stimuli in cluttered displays, especially when the targets appear at eccentric locations (Ball et al., 1988; Scialfa, Kline, & Lyman, 1987; Sekuler & Ball, 1986). A reduction in the size of the perceptual window with age would, in turn, necessitate that older adults make additional saccades to scan the display. Since saccade latencies increase with age, it is likely that older adults will take longer and make additional errors given limited viewing time than younger adults when scanning large cluttered displays (Carter, Obler, Woodward, & Albert, 1983; Scialfa, Thomas, & Joffe, 1994). The perceptual window hypothesis is consistent with the increased error rates for the older compared to the younger adults for the larger display sizes in the conjunction search task. In fact, the accuracy data for the target-present conjunction search task might provide a clue as to the size of the perceptual window for younger and older adults. Older adults show a rapid decline in accuracy between display sizes of 9 and 17, while younger adults show a rather gradual decline in accuracy between 9 and 25 display items. These data would appear to suggest that, as a group, younger adults' perceptual window is from four to eight items larger than that of older adults. Of course, further research with a more systematic manipulation of the number and density of objects per unit of space will be required to more precisely characterize age-related differences in the size of the perceptual window or useful field of view. However, it is important to note that, while a simple notion of a perceptual window provides a reasonable account of the conjunction search data, it does not account for the

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data set in its entirety. For example, there was no evidence for age-related differences in the size of the perceptual window in the motion feature search task. Older and younger adults possessed equivalent search slopes, and older subjects were more accurate than younger adults in their performance. Similarly, in the form feature search task, older and younger adults displayed equivalent accuracies and only a small difference in slopes (i.e., 3.8 msec/item for targetpresent trials averaged across the three sessions). To account for the feature and conjunction search data, the notion of an age-sensitive perceptual window needs to be modified so that the size of the perceptual window and its sensitivity to aging is a function of the nature of the task (see also Plude & Doussard-Roosevelt, 1989). Within the framework of Wolfe's (1994) guided search model, we suggest that the perceptual window will be large and insensitive to age whenever the search task can be performed solely on the basis of local (i.e., bottom-up) differences in activation between targets and nontargets. On the other hand, if topdown guidance (i.e., knowledge of target arid distractor characteristics) is required to perform the search task, we suggest that the perceptual window will be both reduced in size and age-sensitive. The present data are consistent with this proposal. Experiment 2 A second study was conducted to further examine the question as to why older adults showed a more dramatic decrease in accuracy than younger adults with increasing display set size in the conjunction search task. If our taskspecific perceptual window hypothesis is correct, we would expect increased accuracies, especially with the larger display set sizes, for older adults in situations in which they perceived that there would be sufficient time to carefully scan the display for a potential target stimulus. In Experiment 1, in which only 1200 msec were available to view the display, older adults may have either had insufficient time to scan the larger displays or rushed their responses because they perceived that they would have insufficient time to complete their search for the target. In either case, this would tend to lead to increased guessing and, therefore, increased error rates with larger display sets during the search for conjunction targets. In the present study, we attempted to remedy this potential problem by extending the display viewing time to 2100 msec. According to the RT distributions obtained in Experiment 1, this should provide sufficient viewing time for older adults to find a target in the conjunction search condition. We also employed three (5, 10, and 20 objects) rather than four display set sizes in the present study in an effort to provide subjects with additional practice with each of the display sets. Finally, we strongly encouraged subjects to respond accurately in the present study by stressing their accuracy during presentation of performance feedback. METHOD

Subjects Seventeen younger (age range 18-25, M = 21.5, SD = 1.8) and 17 older adults (age range 60-75, M = 67.6, SD

= 4.6) were recruited from the university community and were paid $5.00 per hour for their participation in the study. Eleven of the younger and 9 of the older subjects were women. All of the subjects were screened for use of any medication that would influence performance on the experimental tasks (e.g., psychotropic drugs, beta blockers) and for near and far visual acuity. All of the subjects possessed corrected visual acuities of at least 20/30. The average corrected acuity for the younger and older subjects was 20/ 21 and 20/28 (Snellen), respectively. The younger adults' acuity was significantly better than that of the older adults [F(l,33) = ll.0,p< .01]. All of the younger and older subjects were asked to indicate the number of years of formal education that they received. The mean number of years of formal education for the younger and older subjects was 15.4 and 14.5, respectively. This difference was not statistically significant. The subjects were also administered the Kaufman Brief Intelligence Test. The average standardized composite scores for the test were 111.0 and 110.5 for the younger and older adults, respectively. This difference was not statistically significant. Apparatus and Stimuli The apparatus and stimuli were the same as those used in Experiment 1, with the following exceptions. First, we used three different display sizes: 5, 10, or 20 letters. Second, the movement velocity was slightly faster than that used in Experiment 1. The letters moved at a rate of 2.45°/sec. Third, moving stimuli were initially arranged over the upper (for downward movements) or lower (for upward movements) one-half of the screen to prevent stimuli from moving off the screen during a trial. Fourth, the motion threshold test was not employed in the present study, since motion thresholds were uncorrelated with performance on the feature and conjunction search tasks. Design Age group (older or younger adults) was the betweensubjects variable. Target type (present or absent), search task (form, motion, or conjunction), and set size (5, 10, or 20) were manipulated within subjects in the visual search task. For one-half of the subjects in each age group the " X " was designated as the target letter; the " O " was the target letter for the remaining subjects. The presence or absence of the target was uniquely defined by the search task (i.e., by target letter form, target letter motion, or by target letter form and motion). Target-present and target-absent trials occurred with equal frequency, and target-type response hand was counterbalanced across subjects. Subjects performed the visual search task in three sessions. In each session, six blocks of each feature search task were presented. The subject then performed six blocks of the conjunction search task. Finally, subjects performed six additional blocks of each of the feature search tasks. Each block of a feature search task consisted of 16 trials for each set size. Similarly, a block in the conjunction search task consisted of 64 trials for one set size. Thus, subjects performed 192 trials in the motion search task, 192 trials in the form search task, and 384 trials in the conjunction search task in each of the


three experimental sessions. In each search task, the order of the six blocks (two blocks for each set size) was randomly determined. The order of the feature search tasks was counterbalanced across sessions and subjects. Procedure Each visual search trial within a block began with the subject's response to the instruction "SPACE BAR" shown in the center of the display. After pressing the space bar, the instruction was replaced by a blank screen for 500 msec. A small fixation cross was then displayed for 500 msec and followed by a 100-msec blank screen interval prior to the presentation of the stimulus array. The stimuli were presented for 2100 msec. The longer presentation duration used in the present study than that used in Experiment 1 was meant to ensure that subjects had sufficient time to search for the target stimulus in the large display set size condition. After a response, the subject was signaled to begin the next trial with the space bar instruction. If the subject failed to respond within 4100 msec of the onset of the stimulus array, an error was recorded and the words "TOO SLOW" were displayed for 500 msec prior to the space bar instruction. With an incorrect response, the words "INCORRECT RESPONSE" were presented on the display accompanied by a 100-msec tone. An invalid key press was signaled by display of the words "INVALID RESPONSE." RESULTS

The results will be organized in the following manner. First, we will present the results obtained from the form and motion feature search tasks. These results will indicate the extent to which parallel or bottom-up search processes are influenced by age. Second, we will present the RT and accuracy data for the conjunction search conditions for the younger and older adults. These results will indicate the degree to which younger and older adults are able to capitalize on movement filtering to modify the task from that of a conjunction search to a feature search. More specifically, we are interested in determining whether the same pattern of RT slopes will be obtained in the present study as in Experiment 1 but with more accurate performance, particularly for the older adults, at the larger display set sizes. Motion Search Task The mean RT data for the motion search conditions are presented in the top panels of Figures 5 and 6. Slopes, intercepts, and R2s for motion searches are presented in Tables 5 and 6. Finally, the accuracy data for each of the motion search conditions are presented in Tables 7 and 8. The pattern of data obtained in the present study is quite similar to that obtained for the motion search conditions in Experiment 1. As can be seen in the figures, RTs appear to be relatively insensitive to display set size for both younger and older subjects. RTs also appear slower for older than younger subjects and for target-absent than target-present responses. Mean RTs and accuracies were submitted to four-way ANOVAs, with age as a between-subjects factor and display set size (5, 10, and 20), target type (present and absent), and practice (session 1,2, and 3) as within-subjects

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factors. RTs were faster for younger than older adults [F(l,32) = 35.3, p < .01], target-present than targetabsent responses [F(l,32) = 57.2, p < .01], small than large display sizes [F(2,64) = 34.3, p < .01], and with more practice [F(2,64) = 12.8, p< .01]. Two interactions were also significant. Older adults improved more with practice than did younger adults [F(2,64) = 4.6, p < .01]. Age interacted with display size and target type [F(2,64) = 5.4, p < .01]. Post hoc comparisons indicated that this interaction can be attributed to the steeper search slope with increasing display set size for the older than for the younger adults on the target-absent trials. Display size effects were equivalent for the younger and older adults on the target-present trials. Two points seem worth noting with respect to this interaction. First, although search slopes were steeper for older than younger adults on targetabsent trials, in no case did a search slope exceed 4 msec/ item. Search slopes of less than 10 msec/item have often been interpreted as evidence for parallel search (Treisman & Gormican, 1988). Second, younger and older adults were equally efficient at searching for and identifying a moving target, an effect that was also obtained in Experiment 1. Two main effects were obtained for accuracy. Accuracy improved with practice [F(2,64) = 3.4,/? < .05] and targetabsent trials were responded to more accurately than targetpresent trials [F(l,32) = 10.3,p< .01]. Form Search Task The mean RT data for the form search conditions are presented in the middle panels of Figures 5 and 6. Slopes, intercepts, and R2s for form searches are presented in Tables 5 and 6. Finally, the accuracy data for each of the form search conditions are presented in Tables 7 and 8. The pattern of RT and accuracy data appear quite similar to that obtained in Experiment 1. As can be seen in the figures, older adults are slower to search for the form target than younger adults. As in the motion search conditions, RTs appear to decrease across sessions. Mean RTs and accuracies were submitted to four-way ANOVAs with age as a between-subjects factor and display set size (5, 10, and 20), target type (present and absent), and practice (session 1, 2, and 3) as within-subjects factors. Older adults responded more slowly than younger adults [F(l,32) = 78.8, p < .01], RTs decreased with practice [F(2,64) = 33.1, p < .01], RTs were slower on targetabsent than on target-present trials [F(l,32) = 51.1, p < .01], and responses slowed with increasing display set sizes [F(l,32) = 167.7,p< .01]. As suggested in Tables 5 and 6, search slopes were steeper on target-absent than on target-present trials [F(l,32) = 48.0, p < .01]. Older adults also produced steeper search slopes, on both target-present and target-absent trials, than did younger adults [F(2,64) = 48.0,/? < .01]. However, as was the case in Experiment 1, target-present search slopes were quite shallow for both younger and older adults during form search trials. In fact, younger and older adults differed by only 2.4 msec/item in their search slopes across three sessions of practice. The fact that such a small effect was significant attests to the power of our experimental design. However, given that both younger and older adults' target-

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Display Size Figure 5. Mean RTs for the target-present trials for each of the experimental conditions in the motion (top panel), form (middle panel), and conjunction search (bottom panel) tasks in Experiment 2. Older subjects' RTs are represented by the solid lines and filled symbols. Younger subjects' RTs are represented by the dotted lines and unfilled symbols. Session 1, 2, and 3 RTs are represented by the circles, squares, and triangles, respectively.

20

Display Size Figure 6. Mean RTs for the target-absent trials for each of the experimental conditions in the motion (top panel), form (middle panel), and conjunction search (bottom panel) tasks in Experiment 2. Older subjects' RTs are represented by the solid lines and filled symbols. Younger subjects' RTs are represented by the dotted lines and unfilled symbols. Session 1, 2, and 3 RTs are represented by the circles, squares, and triangles, respectively.


Table 5. Slopes, Intercepts, and R2s for the Search Functions in Each of the Target-Present Conditions in Experiment 2

Session

1

2

Table 7. Accuracy for Search Task Target-Present Trials by Age Group for Each Session in Experiment 2 Motion

Older

Younger 1

3

2

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Form

3

Age Group

5

10

20

5

10

20

:

10

20

.97 .97 .98

.97 .98 .98

.97 .98 .97

.97 .99 .99

.97 .98 .98

.96 .97 .98

.96 .98 .99

.96 .97 .97

.88 .90 .93

.97 .96 .98

.96 .96 .97

.97 .98 .96

.97 .97 .98

.98 .99 .98

.99 .98 .98

.97 .97 .98

.96 .97 .98

.92 .94 .95


418 1.6 .97

408 1.6 .81

404 0.9 .98

582 1.4 .77

536 1.0 .85

516 1.5 .90

Older Session 1 Session 2 Session 3


435 3.7 .96

428 3.7 .85

399 3.8 .98

601 6.2 .97

544 6.0 .98

528 6.3 .94

Younger Session 1 Session 2 Session 3


458 10.5 .98

447 8.8 .99

610 16.6 .98

579 13.4 .99

661 13.3 .99

Table 8. Accuracy for Search Task Target-Absent Trials by Age Group for Each Session in Experiment 2 Motion Age Group

2

Table 6. Slopes, Intercepts, and R s for the Search Functions in Each of the Target-Absent Conditions in Experiment 2

1

2

3

1

2

3

Older Session 1 Session 2 Session 3

461 2.9 .94

462 0.8 .39

448 -O.I .03

712 1.1 .64

622 3.5 .92

619 2.7 .96

Younger Session 1 Session 2 Session 3

426

408

396

605

526

8.6

6.4

5.6

.98

.98

.96

11.8 .99

12.6 .98

519 12.3 .98


429 19.7 .98

425 17.4 .99

606 29.9 .98

540 31.5

Older

Younger Session Motion Search Intercept Slope R2 Form Search Intercept Slope R2

.99

Conjunction

520 30.7 .99

present search slopes were consistently below 6.5 msec/ item, it would appear to be reasonable to conclude that form search with moving stimuli is both parallel and relatively unaffected by normal aging. Only target type yielded a significant accuracy effect [F(l,32) = 21.7, p < .01]. Responses were more accurate on the target-absent than on the target-present trials. Conjunction Search Task The mean RT data for the conjunction search conditions are presented in the bottom panels of Figures 5 and 6. Slopes, intercepts, and R2s for conjunction searches are presented in Tables 5 and 6. Finally, the accuracy data for each of the conjunction search conditions are presented in Tables 7 and 8. Mean RTs and accuracies were submitted to four-way ANOVAs with age as a between-subjects factor and display set size (5, 10, and 20), target type (present and absent), and practice (session 1, 2, and 3) as within-subjects factors. Consistent with the data presented in Figures 5 and 6, older adults performed more slowly than younger adults [F(\ ,32)

Form

10

20

Conjunction

10

20

5

10

20

.97 .98 .99

.97 .98 .99

.98 .98 .99

.96 .98 .99

.98 .99 .99

.98 1.00 .99

.98 .98 .98

.98 .99 .99

.97 .98 .99

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.97 .97 .97

.98 .98 .98

.98 .99 .98

= 52.9, p < .01], RTs decreased with practice [F(2,64) = 40.6, p < .01, target-absent responses were slower than target-present responses [F(l,64) = 144.6,/? < .01], and RTs increased with display set size [F(2,64) = 457.1, P < 01]. A number of significant interactions was also obtained. Display size had a larger influence on RTs on target-absent responses than it did on target-present responses [F(2,64) = 133.2, p < .01]. Increases in display size had a larger influence on older adults' than younger adults' RTs [F(2,64) = 24.7, p < .01]. However, consistent with the data obtained in Experiment 1, this effect was qualified by a significant three-way interaction among age, display set size, and target type [F(2,64) = 10.9, p < .01]. The age-related difference in the display size effect was larger on the targetabsent than on the target-present trials. Search slopes averaged across sessions for the target-present trials for younger and older adults were 10.3 and 14.4 msec/item, respectively. The comparable search slopes for the target-absent trials were 19.5 and 30.7 msec/item. Post hoc comparisons indicated that the differences in slope as a function of age were significant for the target-absent but not for the targetpresent trials. Compared to Experiment 1, both older and younger adults were substantially more accurate in performing the conjunction search task, especially with the larger display set sizes. Accuracies decreased with increasing display set size [F(2,64) = 35.3, p < .01], were higher for target-absent

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than for target-present trials [F( 1,32) = 47.7, p< .01], and increased with practice [F(2,64) = 14.5, p < .01]. Furthermore, the decrease in accuracy with increasing set size was larger for the target-present than for the target-absent trials [F(2,64) = 54.7, p< .01]. Comparison of Form Feature and Conjunction RTs Older adults displayed steeper display size search functions than younger adults in both the form and conjunction search tasks (see Tables 5 and 6). Consistent with the data obtained in Experiment 1, however, the difference between older and younger search slopes during target-present trials did not increase from the form to the conjunction search tasks as would be expected if older subjects were more negatively influenced by the requirement to detect and identify conjunction targets than were younger adults. In fact, the ratio of the older-to-younger search slopes averaged across the three sessions for the target-present trials actually decreased from 1.7 in the form search task to 1.4 in the conjunction search task. Such a result suggests that the older adults benefited to at least the same extent, if not more, than younger adults from the use of movement in the conjunction search task. DISCUSSION

The results obtained for the motion and form search tasks in the present study mimicked those obtained in Experiment 1. Both older and younger adults searched the displays rapidly, with search rates on target-present trials never exceeding 6.5 msec/item. Shallow search rates such as these are usually interpreted in terms of parallel search of the visual display (Treisman & Sato, 1990; Wolfe, 1994). Therefore, it appears reasonable to conclude, consistent with Plude and Doussard-Roosevelt (1989), that the feature search process is relatively age-insensitive. Our findings viewed in conjunction with those of Plude and DoussardRoosevelt further suggest that age insensitivity is realized in both static and dynamic situations. The main motivation for the present study was to determine if older and younger adults' search rate for conjunction targets would still be statistically equivalent in a situation in which older adults' accuracies, particularly with large display sets, were substantially higher than in Experiment 1. In the last session of Experiment 1, older adults were 15% less accurate than were younger adults at the largest display set size on the target-present trials (72% correct for older and 87% correct for younger adults). Such a differential pattern of accuracies calls into question the interpretation of the statistically equivalent search slopes for older and younger adults that were obtained in the target-present conjunction search conditions. In the present study, we introduced a number of modifications to our paradigm in an attempt to enhance the accuracy of performance of the older adults. Perhaps most importantly, we increased the amount of time that the stimulus array could be viewed from 1200 msec in Experiment 1 to 2100 msec in the present experiment. Such a modification could have had at least two different effects on the performance of the older adults. First, if subjects did not actually have sufficient time to search the stimulus array for the target

at the larger display sizes in Experiment 1, we would expect both increased accuracy as well as an increased search slope with the longer presentation duration in the present study. In this case, it could no longer be argued that use of a movement filter (McLeod et al., 1988, 1989, 1991) to effectively transform conjunction searches into feature searches is relatively unaffected by the processes of normal aging. However, it also appeared reasonable to suppose that older adults might have rushed their responses in Experiment 1, leading to reduced accuracy at the larger display sizes, because they perceived that they might not have sufficient time to complete their search before the offset of the stimulus array. If this were the case, then lengthening the viewing time of the stimulus array might serve to reduce the perceived time stress and result in enhanced accuracy without a concomitant increase in response time. The data obtained in the present study were consistent with this proposal. Accuracies were substantially higher on the target-present conjunction search trials for the older adults in the present study than they were in Experiment 1. Furthermore, the difference in accuracy between older and younger adults was reduced from 15% in Experiment 1 to 2% in the present study (see Tables 3 and 7) in the last session of the experiment (see Appendix, Note 2). When viewed in conjunction with the statistically equivalent RT search rate data in the targetpresent conditions for the older and younger adults, the present data provide additional support for our conclusion that the movement filter, which presumably supports the search for conjunctions of movement and other stimulus properties, is relatively insensitive to aging. General Discussion One additional issue that we have not yet discussed concerns the effects of practice on visual search performance in the feature and conjunction search tasks. However, before discussing our results, we will briefly describe previous investigations of practice effects on visual search. Across two studies, Plude and Hoyer (1986) found a main effect of practice on search performance. Given that practice did not interact with display set size, the Plude and Hoyer results suggest an age-equivalent improvement in the speed of processes other than those involved in visual search (i.e., a practice effect on the intercept rather than the slope). On the other hand, Rogers and Fisk (1991), in their Experiment 1, observed larger practice effects for older than younger adults on both the intercept and slope in a consistently mapped letter search task. However, the visual search slope was shallower for the younger than the older adults at the end of practice. In their Experiment 2, Rogers and Fisk observed a larger practice effect on both the intercept and slope for younger adults when younger and older adults performed a consistently mapped category search task. Although these results are difficult to compare given differences in the nature of the task (i.e., letter vs category search), the amount of practice, and size of the displays, in three out of the four studies the older adults benefited at least to the same degree as younger adults from practice. Our results are consistent with this overall assessment. The older adults benefited from practice to at least the same degree as


the younger adults, for both RT and accuracy measures, in each of our three search tasks and two studies. Furthermore, with the exception of the accuracy measure in the form search task in Experiment 1, practice did not interact with display set size, which suggests that, in general, performance improvements can be attributed to processes other than those underlying visual search. The fact that the search slopes were insensitive to practice suggests that feature search, as well as the search for targets defined by a conjunction of movement and form, does not benefit from stimulus-specific practice. This is in marked contrast to the findings of Rogers and Fisk (1991), in which visual search slopes for letters or category labels improve with practice. It is conceivable that we might have found further reductions in search slopes with additional practice, at least in the conjunction search task. However, this would appear unlikely, since practice effects are generally well fit by a power function, with the largest performance benefits occurring early in practice. Instead, we suggest that performance is supported by a different subset of processes in feature and movement conjunction tasks and the letter search task employed by Rogers and Fisk (1991). Consistent with visual search models, feature search can be carried out solely on the basis of an operation that performs a parallel comparison of targets and nontargets on different features such as color, orientation, movement, etc. This parallel comparison operation is well established early in life (Colombo, Ryther, Frick, & Gifford, 1995) and is quite general (i.e., its operation is independent of specific stimuli; Treisman, Viera, & Hayes, 1992). Similarly, the movement filter, which presumably subserves the search for targets in our conjunction task, performs a relatively general operation, the segregation of moving and stationary objects. Given the limits of sensitivity of cells in the medio-temporal cortex, the functionality of movement filter is independent of the specific nature of the moving and stationary objects (McLeod et al., 1988, 1992; Muller& Maxwell, 1994). On the other hand, the decreased sensitivity of search performance to the number of display objects in the Rogers and Fisk (1991) study appears to be the result of the stimulus-specific strengthening of targets and inhibition of nontargets that develop with consistently mapped practice (Czerwinski, Lightfoot, & Shiffrin, 1992; Rogers & Fisk, 1991). This process has been referred to as priority learning (Schneider, 1985). Given our present state of knowledge, it appears that the mechanisms that subserve the operations of feature comparison and movement filtering are relatively age-insensitive, while the mechanisms that underlie priority learning are more susceptible to aging. ACKNOWLEDGMENTS

This research was supported by a grant from the National Institute on Aging (AG-12203). We gratefully acknowledge the assistance of Lynn Bardell for running subjects in these studies. John F. Larish is now associated with American Telephone and Telegraph. Address correspondence to Arthur F. Kramer, Beckman Institute, University of Illinois, 405 North Mathews Avenue, Urbana, 1L 61801. E-mail: akramer(a>s. psych, uiuc.edu

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798-813. Received June 14, 1995 Accepted January 2, 1996

Appendix Notes 1. It is interesting to note that the slopes obtained for the search for a target defined by a conjunction of movement and form were also substantially shallower, particularly for the older adults, than slopes obtained in previous studies of age-related differences in visual search. For example, Plude and Hoyer (1986) found search slopes of 37 and 58 msec/item when younger and older subjects searched for a consistently mapped letter target among two to five letter distractors. Rogers and Fisk (1991, Experiment 1) obtained similar data, search slopes of 23 and 57 msec/item, when subjects searched for letter targets among two or three letter distractors (see also Zacks & Zacks, 1993). Finally, Plude and DoussardRoosevelt (1989) found search slopes of 14 and 25 msec/item when younger and older adults searched for a target defined by a conjunction of color and form. 2. It might be argued that it is unfair to compare the accuracies obtained in the largest set sizes in the two studies since the largest display set size in Experiment 1 was 25 but only 20 items in Experiment 2. One way to equate the comparisons across studies would be to estimate, by linear extrapolation, the accuracy that would have been obtained had there been a 20-item condition in Experiment 1. Such an estimate can, in fact, be considered to be a relatively conservative estimate of accuracy for 20 items in Experiment 1, given that it would appear (see Table 3) that accuracy is negatively accelerating at the larger set display set sizes. In any event, the percentage of correct difference value (i.e., the difference in accuracy between younger and older adults for a display size of 20 items) derived from a linear extrapolation of the accuracies obtained in Experiment 1 is 9%. This may be contrasted with a 2% difference in accuracy between older and younger adults at display size 20 on the target present trials in Experiment 2.

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