An Eye-Movement-Defined Hierarchy of Visual Stimuli

Abstract

Reverse correlation of stimulus velocity with eye velocity can be used to infer pursuit latency in a task in which a subject attempts to follow a target undergoing random motion. Using a binocular dual Purkinje image eye tracker equipped with stimulus deflectors, we measured tracking responses to four-dimensional motion, produced by delivering an independent random walk to each eye, analyzing the results in terms of horizontal and vertical vergence and version. High-contrast luminance-defined stimuli produce the shortest latencies (around 100 msec); varying texture contrast increases the delay by approximately 50 msec/log unit. Nulling the first-order luminance contrast in a second-order constrast-modulated target produces a dramatic increase in latency (over 100 msec additional delay), and abolishes the vertical vergence response - the only one of the four types of movement that cannot be executed voluntarily. We propose a model of the system in which a fast reflexive system responds to a limited class of stimuli, while a slower voluntary system is capable of tracking anything that can be seen.