Title: "The Eye-Movement Correlogram: A new physiological tool for probing the time course of visual processing".

Jeffrey B. Mulligan

NASA Ames Research Center

Abstract:

It is well known that visual sensory processing is slower for stimuli with low contrast, or low luminance, or purely chromatic contrast. The eye-movement correlogram provides a precise way to quantify these delays. The subject attempts to maintain fixation on a target that moves randomly in one or two dimensions while eye position is monitored. The eye velocity is computed, and the smooth component is interpolated in the neighborhood of saccades. The resulting smooth

eye velocity is cross-correlated with the stimulus velocity. Averaging across presentations with different motion signals reveals a signal that resembles a multiphasic impulse response. The time of the peak provides a simple measure of pursuit latency. Under the assumption that delays due to motor processing are constant as stimulus features are varied, we can use this techniques to measure increases in sensory latency. For example, we observe an increase in latency of approximately 20 milliseconds each time we halve the contrast of a small white spot. Even larger delays have been observed for equiluminant chromatic targets, and flicker-defined second-order-motion targets. With independent binocular stimulation, we can measure independent responses for vergence and version. The horizontal version response is generally the fastest (100 msec),

followed within 10-20 msec by V version. Vergence responses are delayed by an additional 50-70 msec, with V vergence 10-20 msec faster than H vergence. Responses to the spot are delayed 20 msec with respect to the response for an extended texture, while the pure disparity stimulation provided by binocularly correlated dynamic noise produces an additional delay of 50-80 msec (vergence only). When multiple targets are presented and the subject is instructed to pursue only one, low-latency responses are still measured to both targets, with an additional long-latency response seen to the attended target.