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The Journal of Neurophysiology Vol. 84 No. 3 September 2000, pp. 1599-1613
Copyright ©2000 by the American Physiological Society
Department of Neurobiology, Pharmacology and Physiology, University of Chicago, Chicago, Illinois 60637
Belton, Timothy and
Robert A. McCrea.
Role of the Cerebellar Flocculus Region in Cancellation of the
VOR During Passive Whole Body Rotation. J. Neurophysiol. 84: 1599-1613, 2000. A series of studies were
carried out to investigate the role of the cerebellar flocculus and
ventral paraflocculus in the ability to voluntarily cancel the
vestibuloocular reflex (VOR). Squirrel monkeys were trained to pursue
moving visual targets and to fixate a head stationary or earth
stationary target during passive whole body rotation (WBR). The firing
behavior of 187 horizontal eye movement-related Purkinje (Pk) cells in
the flocculus region was recorded during smooth pursuit eye movements
and during WBR. Half of the Pk cells encountered were eye velocity Pk
cells whose firing rates were related to eye movements during smooth pursuit and WBR. Their sensitivity to eye velocity during WBR was
reduced when a visual target was not present, and their response to
unpredictable steps in WBR was delayed by 80-100 ms, which suggests
that eye movement sensitivity depended on visual feedback. They were
insensitive to WBR when the VOR was canceled. The other half of the
Purkinje cells encountered were sensitive to eye velocity during
pursuit and to head velocity during VOR cancellation. They resembled
the gaze velocity Pk cells previously described in rhesus monkeys. The
head velocity signal tended to be less than half as large as the eye
velocity-related signal and was observable at a short (
40 ms)
latency when the head was unpredictably accelerated during ongoing VOR
cancellation. Gaze and eye velocity type Pk cells were found to be
intermixed throughout the ventral paraflocculus and flocculus. Most
gaze velocity Pk cells (76%) were sensitive to ipsilateral eye and
head velocity, but nearly half (48%) of the eye velocity Pk cells were
sensitive to contralateral eye velocity. Thus the output of flocculus
region is modified in two ways during cancellation of the VOR. Signals
related to both ipsilateral and contralateral eye velocity are removed,
and in approximately half of the cells a relatively weak head velocity
signal is added. Unilateral injections of muscimol into the flocculus
region had little effect on the gain of the VOR evoked either in the
presence or absence of visual targets. However, ocular pursuit velocity and the ability to suppress the VOR by fixating a head stationary target were reduced by approximately 50%. These observations suggest that the flocculus region is an essential part of the neural substrate for both visual feedback-dependent and nonvisual mechanisms for canceling the VOR during passive head movements.
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