The main goal of the proposed research is to identify the role of recently identified, short-latency vestibular signals in the frontal eye fields (FEF) of the frontal cortex. The FEF have been implicated in maintaining a stable percept of the outside world (i.e., spatial constancy) via a process called visual remapping. Here, the receptive field location of an FEF neuron is changed based on the amplitude and direction of intervening saccadic eye movements. These shifting receptive fields allow one to keep track of the locations of objects in space across saccades. But do these remapping cells only maintain spatial constancy for intervening saccadic eye movements, or do they shift their receptive field locations for any movement that change the direction of gaze (i.e., the line of site)? For example, could the vestibular signals identified in the FEF subserve visual remapping for vestibular-related movements? Nearly all remapping studies to date have examined this phenomenon with the head-immobilized and intervening saccadic eye movements. The proposed experiments are unique because they will examine visual remapping using more natural, whole-body movements as well as pursuit eye movements. Animals will be trained to perform visual remapping tasks in which spatial constancy must be maintained despite intervening saccades, whole-body rotations, whole-body translations, or pursuit eye movements. While they perform these tasks, single-unit responses from visual and visual-motor neurons in the FEF will be recorded. Three outcomes are possible: (1) The same FEF cells that remap for saccadic eye movements will also show remapping for other movement types. This would indicate that saccade, vestibular and pursuit signals converge on the same FEF cells, (2) A subset of FEF cells remap for saccades while a completely separate subset remap for vestibular-related and pursuit eye movements. This would indicate that saccadic and vestibular/pursuit signals input onto different cell populations. Both (1) and (2) would indicate that a similar remapping mechanism is used across different gaze-changing movements. They also show that gaze shifts (i.e., changes in the orientation of the eye-in-space) remap receptive field locations, and not simply a change in the orientation of the eye-in-head (previous saccade studies could not differentiate between these two possibilities). (3) Remapping cells in FEF operate exclusively for saccades (i.e., no updating is observed for vestibular-related or pursuit movements), which would argue for completely separate updating mechanisms. Whatever the results, this study will be highly informative as it tackles a new and growing branch of vestibular research - that of examining the functions of vestibular signals in the cortex.
Without the brain's ability to maintain a stable percept of the outside world, objects in the environment appear to move uncontrollably and simple tasks such as picking up a cup are nearly impossible. Patients who suffer from cortical strokes and other disease-induced lesions experience both perceptual and motor deficits and are a testament to the struggles of producing inaccurate, mislocalized movements. These experiments investigate how the brain uses vestibular signals to keep the world stable despite our own movements.
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