Shared response preparation for pursuit and saccadic eye movements.

Tracking a single target in the visual world requires coordination between pursuit and saccadic eye movements. The constraints imposed on pursuit and saccade decisions by visual processing and response preparation are difficult to compare because latency differences between the two movements provide different amounts of visual sampling time. The present study compares pursuit and saccade decisions when visual processing was directly manipulated. Human observers were asked to select between two stationary stimuli presented simultaneously at two different locations based on which had the higher contrast. The stimuli were presented for a brief, variable interval and then occluded by masks. Because the masks moved horizontally and were offset vertically, subjects were obliged to make both pursuit and saccadic eye movements to track the mask covering the target stimulus. For each of the exposure durations, we constructed oculometric curves for pursuit and saccades. We found that both systems had similar oculometric thresholds and response biases. The initial pursuit decisions differed from the subsequent saccade decisions on 1-13% of the trials but were the same more often than predicted by independent mechanisms. Moreover, pursuit reversed direction on discordant trials, so that the pursuit decision always matched that of the saccade by the time the saccade was started. These results support the view that, in addition to overlap in early visual areas and the final motor pathways, the pursuit and saccadic systems share processing at the level of response preparation. This shared processing may help ensure the coordination of pursuit and saccadic eye movements in selecting a single target.