Saccadic and psychophysical discrimination of double targets

Saccades are rapid, conjugate eye movements that shift the foveas onto peripheral targets of interest. However, the initial saccades to a peripheral pair of targets have been reported to direct the foveas between the targets, despite instructions to look at one or the other. This phenomenon, referred to as saccadic averaging, can occur for target separations of 10 degrees or more and is usually interpreted as evidence that the spatial information available to the saccadic system is of inherently poor resolution. We compared the amplitudes of initial saccades to single peripheral targets and to pairs of targets, with and without prior auditory cues that provided unambiguous information about which target represented the saccadic goal. Target duration ranged from 33 to 100 ms, followed by a mask that was intended to limit neural processing time. The results show that supplementary auditory cues reduce the extent of saccadic averaging to target separations as small as 2 degrees. Saccadic averaging is more pronounced for targets of brief duration, consistent with the degradation of psychophysical resolution that occurs for targets of short duration. We conclude that saccadic averaging results in large part because of ambiguity about which of two targets represents the saccadic goal and, to a lesser extent, because of the limited time that is available before the saccade to process target position.