A variety of experiments have shown that subjects tend to perceive a target flash as mislocalized when the flash is presented just before, during or shortly after the occurrence of a saccade. The characteristics of this mislocalization suggest that it arises from an anticipatory, slow extraretinal signal, i.e., the signal starts to change before a saccade and continues to change during and after the saccade. However, a target flash creates a visual signal that can persist for as long as 300 ms. Interaction of this visual persistence with the extraretinal signal could have a significant influence on the perceived location of the target flash, and thus on features of the extraretinal signal as inferred from the perceived location. In this study, several different types of models were used to explore how retinal signal persistence together with an extraretinal signal might affect perception. According to these models, the anticipatory, slow extraretinal signal may be an artifact of using a target flash, and the actual extraretinal signal may begin to change only after saccade onset and relatively quickly.
Subjects made smooth pursuit eye movements with a target moving horizontally at 15 deg/sec. At a specified location the target either: (1) suddenly vanished; or (2) jumped to the fovea with target retinal velocity and feedback becoming 0 (target stabilized at the fovea). In each type of trial, the subjects either: "looked" at the target, "pushed" the target, or "passively" gazed. When the target vanished, eye velocity decreased exponentially with a short time-constant (tau approximately 0.10 sec), regardless of whether the subjects were "looking," "pushing" or "passively" gazing. However, some subjects while "pushing" (using an imaginary target) did generate low velocity smooth movement (1-2.5 deg/sec) late in the offset. When the target was stabilized at the fovea, eye velocity also decreased, but with a relatively long time-constant (tau = 0.4-0.8 sec). The time-constant was the same with both "looking," and "pushing", but was shorter for some subjects with "passive" gazing (tau = 0.1-0.5 sec). These findings show that smooth pursuit offset is influenced by the presence of a target, but is relatively independent of attentional mode. All of the pursuit offset responses can be simulated using a model of the pursuit system with target velocity and position inputs, and an internal positive feedback loop enabled by target presence.
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