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* To whom correspondence should be addressed. E-mail: g.r.barnes{at}umist.ac.uk.
We compared the predictive behaviour of smooth pursuit (SP) and suppression of the vestibulo-ocular reflex (VOR) in humans by examining anticipatory smooth eye movements, a phenomenon that arises after repeated presentations of sudden target movement preceded by an auditory warning cue. We investigated whether anticipatory smooth eye movements also occur prior to cued head motion, particularly when subjects expect interaction between the VOR and either real or imagined head-fixed targets. Subjects were presented with horizontal motion stimuli consisting of a visual target alone (SP), head motion in darkness (VOR), or head motion in the presence of a real or imagined head-fixed target (HFT and IHFT, respectively). Stimulus sequences were delivered as single cycles of a velocity sinusoid (frequency 0.5Hz or 1.0Hz) that were either cued (a sound cue 400ms earlier) or non-cued. For SP, anticipatory smooth eye movements developed over repeated trials in the cued, but not the non-cued, condition. In the VOR condition, no such anticipatory eye movements were observed, even when cued. In contrast, anticipatory responses were observed under cued, but not non-cued, HFT and IHFT conditions, as for SP. Anticipatory HFT responses increased in proportion to the velocity of preceding stimuli. In general, anticipatory gaze responses were similar in cued SP, HFT and IHFT conditions and were appropriate for expected target motion in space. Anticipatory responses may represent the output of a central mechanism for smooth eye movement generation that operates during predictive SP as well as VOR modulations that are linked with SP even in the absence of real visual targets.
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