The effect of a temporary absence of target velocity information on visual tracking

P M Cisarik,S B Stevenson,F E Visco,S Kasthurirangan,H E Bedell

doi:10.1167/4.8.650

Abstract

Purpose: To investigate the timing of velocity sampling for the programming of saccades and pursuit in a step-ramp paradigm by briefly extinguishing the target at various times. Methods: Responses to horizontal “step-ramp” motions of a laser spot on a blank field were recorded with a dual-Purkinje eye tracker at 120 Hz. Nine step-ramp conditions were produced by combining −2, 0 and +2 deg steps with −8, 0 and +8 deg/s ramps. For each step-ramp, five target blanking conditions were used: either no gap or a 50-ms gap beginning at 25, 50, 75 or 100 ms after the step. All 45 conditions were presented in shuffled order in a single block of trials, and blocks were repeated 25 times for each of three subjects. Results: A 50-ms gap within the first 100 ms tended to increase the latency of the initial saccade. A gap starting at 25 ms reliably delayed pursuit onset when ramp motion was toward the fovea, whereas later gaps had little or no effect. With no gap, saccades were reliably larger in the step-ramp than the step-only condition, indicating that saccade programming incorporated ramp velocity information. Addition of a gap had no effect on saccade accuracy for step-only trials but often caused a relative undershoot in step-ramp trials. Steady state pursuit gain was unaffected by a gap. Conclusion: Gaps in velocity information generally affect saccades occurring 100 ms after gap onset. In contrast, only the earliest gap had an influence on pursuit latency, consistent with the shorter latency of these responses. The effects of gaps on saccade latency and position error are largely influenced by individual differences in oculomotor strategy adopted to track the stimulus.

Highlights

A moving visual stimulus creates both retinal position and velocity errors, often resulting in a combination of saccadic and pursuit eye movements (Dodge, 1903)
More recent evidence indicates that retinal position and velocity errors both contribute to each type of eye movement
Eye acceleration decreased when the target disappeared behind a visible occluder, but in this condition the later reduction of eye velocity was much less. These results show that a 200-ms gap in target visibility is sufficient to eliminate the drive signal for pursuit, still unknown is how the timing of a brief temporal gap in the visibility of target motion influences the latency and metrical properties of pursuit and saccades

Summary

Introduction

A moving visual stimulus creates both retinal position and velocity errors, often resulting in a combination of saccadic and pursuit eye movements (Dodge, 1903). Several investigations showed that both position and velocity information affect the dynamics of a saccade to a moving stimulus (Gellman and Carl, 1991; Guan, et al, 2005; Keller and Johnsen, 1990; Newsome, et al, 1985). Analysis of saccades initiated by targets moving in simple ramp motion showed that saccades with longer latencies are more accurate than saccades with shorter latencies, suggesting that additional time for target motion processing refines the saccadic error signal (Gellman and Carl, 1991). For targets that step in one direction and undergo ramp motion in the opposite direction, both the direction and latency of the saccade are determined by the time the target crosses the fixation location, which

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