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The Journal of Neurophysiology Vol. 88 No. 4 October 2002, pp. 1880-1892
Copyright ©2002 by the American Physiological Society
The Smith-Kettlewell Eye Research Institute, San Francisco, California 94115
Missal, M. and
E. L. Keller.
Common Inhibitory Mechanism for Saccades and Smooth-Pursuit Eye
Movements. J. Neurophysiol. 88: 1880-1892, 2002. The premotor pathways subserving saccades and
smooth-pursuit eye movements are usually thought to be different.
Indeed, saccade and smooth-pursuit eye movements have different
dynamics and functions. In particular, a group of midline cells in the
pons called omnipause neurons (OPNs) are considered to be part of the
saccadic system only. It has been established that OPNs keep premotor
neurons for saccades under constant inhibition during fixation periods. Saccades occur only when the activity of OPNs has completely stopped or
paused. Accordingly, electrical stimulation in the region of OPNs
inhibits premotor neurons and interrupts saccades. The premotor relay
for smooth pursuit is thought to be organized differently and omnipause
neurons are not supposed to be involved in smooth-pursuit eye
movements. To investigate this supposition, OPNs were recorded during
saccades and during smooth pursuit in the monkey (Macaca mulatta). Unexpectedly, we found that neuronal activity of OPNs decreased during smooth pursuit. The resulting activity reduction reached statistical significance in ~50% of OPNs recorded during pursuit of a target moving at 40°/s. On average, activity was reduced
by 34% but never completely stopped or paused. The onset of activity
reduction coincided with the onset of smooth pursuit. The duration of
activity reduction was correlated with pursuit duration and its
intensity was correlated with eye velocity. Activity reduction was
observed even in the absence of catch-up saccades that frequently occur
during pursuit. Electrical microstimulation in the OPNs' area induced
a strong deceleration of the eye during smooth pursuit. These results
suggest that OPNs form an inhibitory mechanism that could control the
time course of smooth pursuit. This inhibitory mechanism is part of the
fixation system and is probably needed to avoid reflexive eye movements
toward targets that are not purposefully selected. This study shows
that saccades and smooth pursuit, although they are different kinds of
eye movements, are controlled by the same inhibitory system.
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