Inputs from the ipsilateral and contralateral vestibular apparatus to
behaviorally-characterized abducens neurons in rhesus monkeys.
Broussard, Dianne M., R. Christopher deCharms, and Stephen G. Lisberger.
Department of Physiology, W.M. Keck Foundation Center for Integrative
Neuroscience, and Neuroscience Graduate Program, University of California, San
Francisco, CA 94143, Playfair Neuroscience Unit, The Toronto Hospital, Western
Division, Toronto, Canada M5T 2S8.
APStracts 2:0215N, 1995.
SUMMARY AND CONCLUSIONS
1. We made extracellular recordings from neurons in the abducens nuclei of
alert rhesus monkeys during electrical stimulation of the vestibular
labyrinths with brief current pulses and during smooth pursuit, steady
fixation, and the vestibulo-ocular reflex (VOR) evoked by passive head turns.
The responses to electrical stimuli were compared with quantitative measures
of the sensitivity of each neuron to eye position and eye velocity. We also
compared the strength of the vestibular inputs from the labyrinths ipsilateral
and contralateral to the side of recording. 2. Abducens neurons showed
transient excitation after a current pulse was applied to the contralateral
labyrinth and transient inhibition after stimulation of the ipsilateral
labyrinth. The latency of excitation had a mean value of 1.7 and a median
value of 1.5. Latency was unimodally distributed with little variation among
neurons. Neurons with large responses showed a second phase of excitation that
started 2.5 ms after the stimulus. 3. In two of three monkeys, the excitatory
responses of abducens neurons to electrical stimulation of the contralateral
labyrinth were approximately three times as large as their inhibitory
responses to stimulation of the ipsilateral labyrinth. The difference in
response size was not observed in the third monkey. The asymmetry in the size
of the electrically evoked inputs from the two labyrinths was associated with
a smaller asymmetry in responses of abducens neurons during the VOR evoked by
passive head turns. The increase in firing rate during head rotation away from
the side of the recording was almost always larger than the decrease in firing
rate during head rotation toward the side of the recording. 4. The size of the
neuronal response to electrical stimulation was correlated with the magnitude
of the change in discharge rate during eye movements. Single or multiple
regression of measures of response amplitude against eye position threshold,
sensitivity to eye position, sensitivity to eye velocity, and baseline
discharge rate yielded correlation coefficients that ranged from 0.26 to 0.92
in different monkeys. The existence of positive correlations is consistent
with a role of the intrinsic properties of abducens neurons in determining
recruitment order. However, the existence of large amounts of variability
within most of the samples suggests that the recruitment order of abducens
neurons also depends on the discharge properties of the afferents to each
abducens neuron.
Received 15 December 1995; accepted in final form 20 July 1995.
APS Manuscript Number J783-4.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 10 August 1995.