Visual search of heading direction.

Abstract

When we move along we frequently look around. How quickly and accurately can we gaze in the direction of heading? We studied the temporal aspects of heading perception in expanding and contracting patterns simulating self-motion. Center of flow (CF) eccentricity was 15 degrees. Subjects had to indicate the CF by making a saccade to it. A temporal constraint on the response time was introduced, because stimuli were presented briefly (1 s). On average, subjects needed two saccades to find the CF. Initial saccades covered about 50-60% of the distance between the fixation point and the CF. Subjects underestimated the eccentricity of the CF. The systematic radial error ranged from -2.4 degrees to -4.9 degrees. The systematic tangential error was small (about 0.5 degree). The variable radial error ranged from 2.7 degrees to 4.6 degrees. We found a relation between saccade onset time and saccade endpoint error. Saccade endpoint error decreased with increasing saccade onset time, suggesting that saccades were often fired before the heading processing had been completed. From the saccade onset times, saccade endpoint errors and an estimate for the saccadic dead time (interval prior to the saccade during which modification is impossible 70 ms), we estimated the heading processing time (HPT 0.43 s). In three out of four subjects, HPT was longer for trials simulating backward movement than for trials simulating forward movement. For each saccade we determined whether it reduced the distance error. The second saccade reduced the error more effectively per time unit than the initial saccade. On the basis of this finding, we suggest that visual processing that occurs during the saccadic dead time of the first saccade is used in the preparation of the second saccade.