PSYCHOPHYSICAL AND SACCADIC INFORMATION ABOUT DIRECTION FOR BRIEFLY PRESENTED VISUAL TARGETS

For saccades, the difference between desired and actual eye movement (constant error), as well as the variability in amplitude on repeated trials (variable error), presumably represent a combination of errors in processing target position (sensory error) and errors in execution (motor error). To examine whether the saccadic system uses the same information about target position as visual perception, subjects made saccades to and psychophysical judgments about the location of targets presented for 17-200 msec. Mean saccadic amplitude markedly decreased and inter-trial variability increased when saccadic targets were presented briefly and followed by a spatial mask. Judged target position (from psychophysical vernier and bisection tasks) also shifted to lesser eccentricities and was more variable. Although qualitatively alike, changes with duration in saccadic constant errors were larger than could be accounted for by the psychophysical results, suggesting similar but separate processing of position information for the saccadic and perceptual systems. Differences in the sizes of collicular receptive fields that preferentially respond to targets at short and long durations can account qualitatively for the observed changes in saccadic amplitude as well as the common occurrence of saccadic undershoots.