102 TREISMAN AND GELADE “purple"distractors in one condition,.“O'in“N'and“T'distractors in another)and for targets specified by a conjunction of features,a"pink O"'(Opink,in distractors Oeen and Npink).The function relating search when a single feature was target,but increased ir inearly w hen a conjunction of features was required.Experiment I replicates this study with some changes in the design,to confirm and generalize the conclusions.The most important change was in the feature search condition'subiects were now asked to search concurrently for two targets,each defined by different single fe color(blue) and a shape(S).Thus the were forced to attend to both dim sions in the feature condition as well as in the conjunction condition,although they had to check how the features were combined only when the target was a conjunction (T).The dis- tractors were identical in the two conditions(X and T. )to ensure that differences between feature and conjunction fromgrn a possibility in the previous experiment. fr ty o the distractors in the conjunction condition. Another question which has become important in evaluating information-processing hypotheses is how stably they apply across differ- ent stages of practice.Neisser,Novick,and Lazar (1963).Rabbitt (1967). and Shiffrin and Schneider(1977)have all showr alitative chang in performance as subjects repeatedly perform a particu task. Search ap- pears to change from conscious. limited capacity,serial decision making to automatic,fast,and parallel detection.LaBerge (1973)studied the effects of practice on priming in a visual successive matching task.He found that familiarity with the stimuli eventually made indepe dent of ex suggested that this was due to unitization of th features of highly fami iar stimuli.We propose that feature unitization may account also for the change with practice from serial to parallel processing in a display,in conditions in which such a change occurs.Thus the development of new unitary detectors for what were previously con nctions of features would fre eus from the constraints of focal attentior the re th 00 memory and in a phys cally present display ent I explored the possi ility that extended practice on a par ticular shape -color conjunction (Tgreen)could lead to a change from serial to parallel detection,which would suggest the possible emergence of a unitary "green T"detector. Method Stimuli.The stimulus displays were made by hand.using letter stencils and colored ink on white cards.The distractors were scattered over the card in positions which appeared random,although no systematic randomization procedure was used Four different display 15,and 30 items in cach An area s th f
102 TREISMAN AND GELADE “purple” distracters in one condition, “0” in “N” and “T” distracters in another) and for targets specified by a conjunction of features, a “pink 0” COpink in distracters O,,,, and Npink). The function relating search times to display size was flat or nonmonotonic when a single feature was sufficient to define the target, but increased linearly when a conjunction of features was required. Experiment I replicates this study with some changes in the design, to confirm and generalize the conclusions. The most important change was in the feature search condition: subjects were now asked to search concurrently for two targets, each defined by a different single feature: a color (blue) and a shape (S). Thus they were forced to attend to both dimensions in the feature condition as well as in the conjunction condition, although they had to check how the features were combined only when the target was a conjunction (Tg,.,,,). The distractors were identical in the two conditions (X,,,, and Thrown), to ensure that differences between feature and conjunction search could not result from greater heterogeneity of the distracters in the conjunction condition. (This had been a possibility in the previous experiment.) Another question which has become important in evaluating information-processing hypotheses is how stably they apply across different stages of practice. Neisser, Novick, and Lazar (1963), Rabbitt (1967), and Shiffrin and Schneider (1977) have all shown qualitative changes in performance as subjects repeatedly perform a particular task. Search appears to change from conscious, limited capacity, serial decision making to automatic, fast, and parallel detection. LaBerge (1973) studied the effects of practice on priming in a visual successive matching task. He found that familiarity with the stimuli eventually made matching independent of expectancy, and suggested that this was due to unitization of the features of highly familiar stimuli. We propose that feature unitization may account also for the change with practice from serial to parallel processing in a display, in conditions in which such a change occurs. Thus the development of new unitary detectors for what were previously conjunctions of features would free us from the constraints of focal attention to these features both in memory and in a physically present display. Experiment I explored the possibility that extended practice on a particular shape-color conjunction (T preen) could lead to a change from serial to parallel detection, which would suggest the possible emergence of a unitary “green T” detector. Method Stir?&. The stimulus displays were made by hand, using letter stencils and colored inks on white cards. The distracters were scattered over the card in positions which appeared random, although no systematic randomization procedure was used. Four different display sizes, consisting of 1,5, 15, and 30 items were used in each condition. An area subtending 14 x 8” was used for all display sizes, so that the displays with fewer items were less densely
ATTENTION AND FEATURE INTEGRATION 103 packed,but the average distance from the fovea was kept approximately constant.Each letter subten ded 0.8 ×0.6°.To ensure that the target locati across cond the arca cachcard w nd an in al hau tended 5 For each condition and each display size,eight cards were made,one with a target randomly placed in each of the resulting eight areas(top outer,top inner,left outer,left inner,right outer.etc.).Another eight cards in each condition and display size The distra and X each card as possible.The uarget in the coniunction condition was T it was either a blue letter or an S.The blue letter(Tor)matched half the distractors in shape,and the S(S orS)matched half the distractors in color.The fact that there were ou ougr conditio the conjun edure.The stimulus cards and RT were pide Development thre At the beginning of each trial,subjects viewed a plain white card in the tachistoscope,and each of their index fingers rested on a response key.The experimenter gave a verbal cady and pressec button to display a sccond whitc care and was then imm with the dominant handif arget and with the nondominnt and to respond as quickly as possible without making any errors.RT was recorded to the earest millis con on a digital timer [Advance Electronics.TC11],which was triggered by he onset of the earch array and stoppeo a response cey was Trials on whic dummytrial was given.the results of which were not recorded.Subje and whether or not they were correct after each trial:they were not however informed of the dummy trials procedure.the purpose of which was to exclude slow posterror responses from te on conjunc s and on ures in separate der conjunction targets.Six subiects did 3 blocks of 128 trials each in each condition.then two of these subjects volunteered to continue for another 4 blocks in the conjunction condition and wo for another 10 blocks,making 13 altogether(a total of 1664 trials).The mean RTs for on the cks approxin ed th e group means display sizes wasnd of differen d thus in alternative targets were,but did not know what the array size would be on any given trial. Each block contained 16 positive and 16 negative trials for each display size. Subjects.The six subjects,four men and two women,were members of the Oxford Subject Panel,ages between 24 and 29.Three of them had previously taken part in the Results Figure I shows the mean search times for the six subiects over the second and third blocks in each condition;the first block was treated as e details of linear regress t that search time inereseed linearly with dieplay ion analyses s on these
ATTENTION AND FEATURE INTEGRATION 103 packed, but the average distance from the fovea was kept approximately constant. Each letter subtended 0.8 x 0.6”. To ensure that the target locations did not vary systematically across conditions, the area of each card was divided into eight sections. This was done by superimposing a tracing of the two diagonals and an inner elliptical boundary, which subtended 8.5” x 5.5”. For each condition and each display size, eight cards were made, one with a target randomly placed in each of the resulting eight areas (top outer, top inner, left outer, left inner, right outer, etc.). Another eight cards in each condition and display size contained no target. The distracters in both conditions were Tbruwn and X,,,,, in as near equal numbers on each card as possible. The target in the conjunction condition was T,,,,,; in the feature condition, it was either a blue letter or an S. The blue letter (T hlue or X,,,,,) matched half the distracters in shape, and the S (Shruun or S g& matched half the distracters in color. The fact that there were four possible disjunctive targets in the feature condition (although the definition specified only “blue or S”), should, if anything, impair performance relative to the conjunction condition. Procedure. The stimulus cards were presented in an Electronics Development threefield tachistoscope and RT was recorded as described below. At the beginning of each trial, subjects viewed a plain white card in the tachistoscope, and each of their index fingers rested on a response key. The experimenter gave a verbal “Ready” signal and pressed a button to display a second white card bearing a central fixation spot, which remained in view for 1 set and was then immediately replaced in the field of view by a card bearing a search array. Subjects were instructed to make a key press with the dominant hand if they detected a target and with the nondominant hand otherwise, and to respond as quickly as possible without making any errors. RT was recorded to the nearest millisecond on a digital timer [Advance Electronics, TCll], which was triggered by the onset of the search array and stopped when a response key was pressed. Trials on which an error was made were repeated later in the testing session, and following each error a dummy trial was given, the results of which were not recorded. Subjects were told their RT and whether or not they were correct after each trial; they were not however informed of the dummy trials procedure, the purpose of which was to exclude slow posterror responses from the data. Each subject was tested both on conjunctions and on features in separate sessions following an ABBAAB order. Half the subjects began with the feature targets and half with the conjunction targets. Six subjects did 3 blocks of 128 trials each in each condition, then two of these subjects volunteered to continue for another 4 blocks in the conjunction condition and two for another 10 blocks, making 13 altogether (a total of 1664 trials). The mean RTs for these two subjects on the first 3 blocks closely approximated the group means. Within each block the presentation order of positive and negative trials and of different display sizes was randomized; thus in each block the subject knew what the target or the two alternative targets were, but did not know what the array size would be on any given trial. Each block contained 16 positive and 16 negative trials for each display size. Subjects. The six subjects, four men and two women, were members of the Oxford Subject Panel, ages between 24 and 29. Three of them had previously taken part in the search experiment described in Treisman et al. (1977). Results Figure 1 shows the mean search times for the six subjects over the second and third blocks in each condition; the first block was treated as practice. Table 1 gives the details of linear regression analyses on these data. The results show that search time increased linearly with display
104 TREISMAN AND GELADE SEARCH FOR COLORED SHAPES 2400T 200 COLOR SAPE 30 DISPLAY SIZE FiG.1.Search times in Experiment I. size in the conjunction condition,the linear component accounting for more than 99%of the variance due to display size.The ratio of the posi- tive to the negative slopes in the conju ondition was 0.43.which is quite close to half.These results suggest that search is serial and self- terminating with a scanning rate of about 60 msec per item.The variances increased more steeply for positive than for negative trials,and for posi- tives the root mean square of the RTs increased linearly with display size as predicted for serial self-terminating search. With the feature targ ets.the results we ery different.For the Dosi tive displays, rch times were rdly affect d by the number dis tractors,the slopes averaging only 3.1 msec.Deviations from linearity were signi cant,and the linear component accounted for only 68%of the variance due to display size.For the negatives,the linear component accounted for 96%of the variance due to display size,and departures from linearity did not reach significance.The slope was,however,less than
104 TREISMAN AND GELADE SEARCH FOR COLORED SHAPES - CONJUNCTION --- DISJUNCTION I 1 I I I 5 15 30 DISPLAY SIZE FIG. 1. Search times in Experiment I. size in the conjunction condition, the linear component accounting for more than 99% of the variance due to display size. The ratio of the positive to the negative slopes in the conjunction condition was 0.43, which is quite close to half. These results suggest that search is serial and selfterminating with a scanning rate of about 60 msec per item. The variances increased more steeply for positive than for negative trials, and for positives the root mean square of the RTs increased linearly with display size as predicted for serial self-terminating search. With the feature targets, the results were very different. For the positive displays, search times were hardly affected by the number of distractors, the slopes averaging only 3.1 msec. Deviations from linearity were significant, and the linear component accounted for only 68% of the variance due to display size. For the negatives, the linear component accounted for 96% of the variance due to display size, and departures from linearity did not reach significance. The slope was, however, less than
ATTENTION AND FEATURE INTEGRATION 105 TABLE 1 Linear Regressions of Reaction Times on Display Size in Experiment I Slope Positives 28.7 398 997 Conjunction Negatives 67.1 397 99.6 Feature Positives 3.1 448 67.9m mear Negatives 25.1 514 96.6 Feature color Positive 3.8 455 61.09 Feature shape Positive 441 78.5 half the slope for conjunction negatives.The ratio of positive to negative slopes with feature targets was only 0.12.In both conditions,all subjects showed the same pattern of results,with individuals varying mainly in the absolute values of slopes and intercepts. Errors in the feature condition averaged 2.2%false positives and 2.1% false negatives:for the conjunction condition there were 0.8%false posi- tives and 4.9%false negatives.There were no systematic effects of dis- play size on errors,except that false negatives in the conjunction condi- tion were higher for display size 30 than for 15,5,or 1(8.2%com 3.8%).The highest ear erro e conjunction condition and 3.5%in the feature condition. It is important to the theory that the difference between conjunction and feature conditions is present only when more than one stimulus is presented.The mean positive RT for display size 1 was 422 msec for the ed to 426 for and 446 tion.The negatives with display size 1were as faster in the conjunction than in the feature conditions,473 msec com- pared to 500 msec.Thus the difficulty of search for conjunctions arises only when more than one stimulus is presented. The effects of practice on conjunction search are shown in Fig.2.The positive slopes and intercepts decrease over the first 7 blocks and change little for the remaining 6 hlocks The ative slopes flu ctuate acros first 9 block <s and stab ize at ock 10.Both pos tive and negative remained linear throughout:the proportion of the variance with display
ATTENTION AND FEATURE INTEGRATION 105 TABLE 1 Linear Regressions of Reaction Times on Display Size in Experiment I Slope Intercept Percentage variance with display size which is due to linearity Positives 28.7 398 99.7 Conjunction Negatives 67.1 397 99.6 Feature Positives 3.1 448 67.9” mean Negatives 25.1 514 96.6 Feature color Positive 3.8 455 61.0” Feature shape Positive 2.5 441 78.5 ” Cases where deviations from linearity are significant at p < .Ol. The positive shape feature also deviates considerably from linearity, but the significance level here is only .08. half the slope for conjunction negatives. The ratio of positive to negative slopes with feature targets was only 0.12. In both conditions, all subjects showed the same pattern of results, with individuals varying mainly in the absolute values of slopes and intercepts. Errors in the feature condition averaged 2.2% false positives and 2.1% false negatives; for the conjunction condition there were 0.8% false positives and 4.9% false negatives. There were no systematic effects of display size on errors, except that false negatives in the conjunction condition were higher for display size 30 than for 15,5, or 1 (8.2% compared to 3.8%). The highest mean error rate for an individual subject was 5.5% in the conjunction condition and 3.5% in the feature condition. It is important to the theory that the difference between conjunction and feature conditions is present only when more than one stimulus is presented. The mean positive RT for display size 1 was 422 msec for the conjunction targets, compared to 426 msec for shape and 446 msec for color in the feature condition. The negatives with display size 1 were also faster in the conjunction than in the feature conditions, 473 msec compared to 500 msec. Thus the difficulty of search for conjunctions arises only when more than one stimulus is presented. The effects of practice on conjunction search are shown in Fig. 2. The positive slopes and intercepts decrease over the first 7 blocks and change little for the remaining 6 blocks. The negative slopes fluctuate across the first 9 blocks and stabilize at block 10. Both positive and negative slopes remained linear throughout: the proportion of the variance with display
106 TREISMAN AND GELADE SLOPES NEGATI POSITIVE FiG.2.The effects of practice on the slope and intercept of the function relating search size that was due to linearity was above 0.99 in every block except posi- tive blocks3and 1,when it wasand7,respectively.Thus there is little indication of any change in the pattern of s and no sign switch from serial to paralle I search over the 13 blocks of practice.The mean results for the two subjects who volunteered for this extensive practice were typical of the group as a whole on blocks 2 and 3(negative and positive slopes of 67 and 31,respectively,compared to the group means of 67 and 29;intercepts 423 and 389 compared to 397 and 398). Discussion rially,is the means by which the rect in egration of features into esetec especially on positive trials,fit these predictions well.Despite the major changes in the feature search condition between this experiment and the
106 TREISMAN AND GELADE P I , I I I I I I I 1 1 I I I INTERCEPTS 440 - NEGATIVE - POSITIVE - 360 - 320 - 280 - 240 : > I 1 I I I I 1 3 5 7 9 11 13 BLOCKS FIG. 2. The effects of practice on the slope and intercept of the function relating search time to display size. (The dotted lines are the data for the four subjects who did 7 sessions and the solid lines for the two subjects who continued for 13 sessions.) size that was due to linearity was above 0.99 in every block except positive blocks 3 and 12, when it was 0.98 and 0.97, respectively. Thus there is little indication of any change in the pattern of results and no sign of a switch from serial to parallel search over the 13 blocks of practice. The mean results for the two subjects who volunteered for this extensive practice were typical of the group as a whole on blocks 2 and 3 (negative and positive slopes of 67 and 31, respectively, compared to the group means of 67 and 29; intercepts 423 and 389 compared to 397 and 398). Discussion We suggested that focal attention, scanning successive locations serially, is the means by which the correct integration of features into multidimensional percepts is ensured. When this integration is not required by the task, parallel detection of features should be possible. The results, especially on positive trials, fit these predictions well. Despite the major changes in the feature search condition between this experiment and the