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 sub￾tended 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 conjunc￾tion condition. Procedure. The stimulus cards were presented in an Electronics Development three￾field 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 fol￾lowing 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