Abstract
Experiments with the Rashbass ‘step-ramp’ paradigm have revealed that the initial catchup saccade that occurs near pursuit onset uses target velocity as well as position information in its programming. Information about both position and motion also influences smooth pursuit. To investigate the timing of velocity sampling near the initiation of saccades and smooth pursuit, we analyzed the eye movements made in nine ‘step-ramp’ conditions, produced by combining –2, 0 and +2 deg steps with –8, 0 and +8 deg/s ramps. Each trial had either no temporal gap or a 50-ms gap during which the laser target was extinguished, beginning 25, 50, 75 or 100 ms after the step. Six subjects repeated each of the resulting 45 conditions 25 times. With no temporal gap, saccades were larger in the step-ramp-away’ than the ‘step-only’ condition, confirming that saccade programming incorporates ramp velocity information. A temporal gap had no effect on the accuracy of saccades on ‘step-only’ trials, but often caused undershoots in ‘step-ramp’ trials. A 50-ms gap within the first 100 ms also increased the latency of the initial saccade. Although initial pursuit velocity was unaffected by a temporal gap, a gap that started at 25 ms reliably delayed pursuit onset for ramp motion of the target toward the fovea. Later gaps had a minimal effect on initial pursuit latency. The similar timing of the temporal gaps in target motion information that affect the initiation of saccades and pursuit provides further behavioral evidence that the two types of eye movements share pre-motor neural mechanisms.
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
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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