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1 Neurobiology, Washington University Medical School, St Louis, MO, USA
* To whom correspondence should be addressed. E-mail: angelaki{at}pcg.wustl.edu.
Under natural conditions, the vestibular and pursuit systems work synergistically to stabilize the visual scene during movement. How translational vestibular signals (translational vestibulo-ocular reflex, TVOR) are processed in the premotor pathways for slow eye movements continues to remain a challenging question. To further our understanding of how premotor neurons contribute to this processing, we recorded neural activities from the prepositus and rostral medial vestibular nuclei in macaque monkeys. Vestibular neurons were tested during 0.5 Hz rotation and lateral translation (both with gaze stable and during VOR cancellation tasks), as well as during smooth pursuit eye movements. Data were collected at two different viewing distances, 80 and 20 cm. Based on their responses to rotation and pursuit, eye movement-sensitive neurons were classified into three groups; position-vestibular-pause (PVP) neurons, eye-head (EH) neurons and burst-tonic (BT) cells. We found that only approximately half of the type II PVP and EH neurons with ipsilateral eye movement preference were modulated during TVOR cancellation. In contrast, few of the EH and none of the type I PVP cells with contralateral eye movement preferences modulated during translation in the absence of eye movements. Nor did any of the BT neurons change their firing rates during TVOR cancellation. Of the type II PVP and EH neurons that modulated during TVOR cancellation, cell firing rates increased for either ipsilateral or contralateral displacement, a property that could not be predicted on the basis of their rotational or pursuit responses. In contrast, under stable gaze conditions, all neuron types, including EH cells, were modulated during translation according to their ipsilateral/contralateral preference for pursuit eye movements. Differences in translational response sensitivities for far versus near targets were only seen in type II PVP and EH cells. There was no effect of viewing distance on response phase for any cell type. When expressed relative to motor output, neural sensitivities during translation (although not during rotation) and pursuit were equivalent, particularly for the 20 cm viewing distance. These results suggest that neural activities during the TVOR were more motor-like compared to cell responses during the RVOR. We also found that neural responses under stable gaze conditions could not always be predicted by a linear vectorial addition of the cell activities during pursuit and VOR cancellation. The departure from linearity was more pronounced for the TVOR under near viewing conditions. These results extend previous observations for the neural processing of otolith signals within the premotor circuitry that generates the RVOR and smooth pursuit eye movements.
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