Spatial mapping of the remote distractor effect on smooth pursuit initiation.

In order to extract information from the visual world, it is necessary to bring the images of objects of interest to rest on the high acuity part of the retina, the fovea. Primates, including humans, use two types of eye movement, saccades and smooth pursuit, to accomplish this. While classically conceived of as being separate and distinct, various lines of evidence indicate a close linkage between these two eye movement systems. They are often investigated at a behavioural level by presenting subjects with single targets to saccade or to track. We investigated the effect of presenting a single stationary distractor at various positions in the visual field at the same time as a moving target which subjects were instructed to track. We found that while a stationary distractor presented in the contralateral visual field and part of the ipsilateral visual field increased pursuit latency in an eccentricity dependent manner, a distractor presented in the ipsilateral visual field, within 45 degrees of the axis along which the pursuit target moved, had no effect on latency. We found no evidence that within this region distractors modified eye velocity during the early part of the pursuit response. This spatial pattern of the effect of a stationary distractor on pursuit latency is very similar to the effect of distractors on saccade latency. Our results provide behavioural evidence supporting the hypothesis that the processes that determine when an eye movement is made are linked, but that those determining the form of that eye movement are substantially independent.