Asymmetry in saccadic latency during smooth pursuit: A signature of visual spatial attention?

Abstract

Previous studies reported faster saccadic reaction times (SRTs) for targets presented in the same compared to the opposite direction of pursuit, suggesting that the locus of visual spatial attention might be ahead of the pursuit target. We revisited these findings to investigate if task instruction or saccadic amplitude influenced the difference in SRTs between forward and backward saccades during ongoing pursuit. The pursuit target was a single letter from the set, CDHKNORSVZE, which traversed the screen horizontally in sinusoidal motion with a peak-to-peak amplitude of 10 degrees and a frequency of 0.3 Hz. Every 500 ms the target changed to a different letter. Each time the letter crossed the straight-ahead position, it jumped either forward or backward with an amplitude of 2.5 or 4 degrees. A dual Purkinje image eye tracker was used to track subjects' (N = 4) eye movements at a sampling rate of 357 Hz. Two different tracking conditions were run in blocks, each lasting for 24 seconds and consisting of 8 pursuit cycles with 16 target jumps. In the 'track only' condition subjects were instructed to hold their gaze on the center of the changing letter throughout the course of stimulus motion. In the 'read' condition the subjects were encouraged only to read the changing string of letters. In both viewing conditions, saccadic reaction times were on average 30-40 ms less for saccades to targets in the direction of pursuit compared to the saccades to targets that jumped in the opposite direction. This difference could not be explained by a difference in the amplitudes of forward vs. backward saccades. The results concur with those of previous studies and suggest that attention remains ahead of the pursuit target. Meeting abstract presented at VSS 2016