Visual search of heading direction

When we move along we frequently look around. How quickly and accurately ca n we gaze in the direction of heading? We studied the temporal aspects of h eading perception in expanding and contracting patterns simulating self-mot ion. Center of flow (CF) eccentricity was 15 degrees. Subjects had to indic ate 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 aver age, subjects needed two saccades to find the CF. Initial saccades covered about 50-60% of the distance between the fixation point and the CE Subjects underestimated the eccentricity of the CE The systematic radial error rang ed from -2.4 degrees to -4.9 degrees. The systematic tangential error was s mall (about 0.5 degrees). 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 o nset time, suggesting that saccades were often fired before the heading pro cessing had been completed. From the saccade onset times, saccade endpoint errors and an estimate for the saccadic dead time (interval prior to the sa ccade during which modification is impossible 70 ms), we estimated the head ing processing time (HPT 0.43 s). In three out of four subjects, HPT was lo nger for trials simulating backward movement than for trials simulating for ward movement. For each saccade we determined whether it reduced the distan ce error. The second saccade reduced the error more effectively per time un it 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 sa ccade is used in the preparation of the second saccade.