When human subjects move both their eyes and head to follow a target the vestibular-ocular response (VOR) is suppressed. When a target is visible, pursuit maintenance is achieved through a combination of visual feedback and internal (extra-retinal) mechanisms. If a target is extinguished, hence visual feedback is removed, the continuation of eye and head movement is under the sole control of extra-retinal mechanisms. Present experiments use very brief initial target exposures to investigate the rapid sampling of target velocity and its subsequent use by extra-retinal mechanisms. Subjects were presented with a step-ramp stimulus where a target stepped either left or right then moved in the opposite direction at 10-40deg/sec for an initial randomised period of 50-200ms. The target then continued on its trajectory but was extinguished for either 400 or 600ms before reappearing and continuing to move for 200ms. There was a randomised period of 1-3seconds between stimuli. Subjects were required to move their eyes and head to follow the target using the initial visual information to maintain pursuit when the target was extinguished. Results were compared with control conditions where the target was continuously illuminated and also to conditions where the target disappeared after its initial brief exposure and did not reappear, where subjects did not expect it to reappear but were instructed to continue their eye and head movements. Conditions were fully randomised, making it very difficult for subjects to predict events. Subjects were able to appropriately grade their eye and head responses to different target velocities, even at the briefest initial presentation. Smooth eye movements were initiated around 130ms after target onset, whereas the head movements were initiated later, around 200ms. The initial pursuit, driven by visual feedback, was sufficiently rapidly sampled to produce eye and head velocities that were sustained and even increased when the target was not visible, when there was an expectation of the target reappearing. An interesting point is that VOR was suppressed even in the absence of visual feedback, implying the extra-retinal drive in the maintenance of gaze velocity. Therefore, expectation seemed to modulate the internally generated drive for eye and head movements, which has implications that do not support an efference copy model of pursuit. Rather, the sampled velocity information output is used to facilitate continued eye and head responses.
University of Cambridge (2008) Proc Physiol Soc 11, PC97
Poster Communications: The role of visual motion information in the control of eye and head movement during head-free pursuit
R. Ackerley1, S. Collins1, G. R. Barnes1
1. Faculty of Life Sciences, University of Manchester, Manchester, United Kingdom.
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Where applicable, experiments conform with Society ethical requirements.