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 the106 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 posi￾tive 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 se￾rially, is the means by which the correct integration of features into mul￾tidimensional 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