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 than104 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 posi￾tive 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 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 targets, the results were very different. For the posi￾tive displays, search times were hardly affected by the number of dis￾tractors, 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