Department of Neurology, University of Connecticut Health Center, Farmington, Connecticut 06030-3974, USA
Address for correspondence: David M. Waitzman, MD, PhD, Department of Neurology, University of Connecticut Health Center, 263 Farmington Avenue, Farmington, CT 06030-3974. Voice: 860-679-4313 (Lab), 860-679-8011; fax: 860-679-4446.
waitzman{at}nso2.uchc.edu
Ann. N.Y. Acad. Sci. 956: 111-129 (2002).
Converging lines of evidence support a role for the intermediate
and deep layers of the superior colliculus (SC) and the mesencephalic
reticular formation (MRF) in the control of combined head and
eye movements (i.e., gaze). Recent microstimulation, single-cell
recording, and lesion experiments are reviewed in which monkeys
are free to move their heads. Cells in the SC discharge in advance
of combined head and eye movements and most likely provide a
gaze error signal to downstream structures. In contrast, the
neurons in the MRF are of at least two types. Eye cells have
features that are similar to neurons in the rostral portion
of the SC, but fire before the onset of horizontal eye movments.
A second group of MRF neurons begin to fire after the onset
of the gaze shift and are most closely associated with movements
of the head. The peak discharge of these late-onset MRF neurons
occurs near the peak head velocity. Stimulation in the rostral
SC generates eye movements with fixed amplitude and direction.
A similar response is noted after stimulation of the more dorsal
portion of the caudal MRF. Stimulation in the caudal portion
of the SC produces combined head and eye movements of fixed
amplitude. Electrical activation of the more ventral portions
of the caudal MRF generates goal-directed and centering eye
movements. Temporary inactivation of the SC with the GABA agonist
muscimol generated hypometria and curved trajectories of contralateral
eye movements. Inactivation of the caudal MRF produced contralateral
hypermetria and ipsilateral hypometria of saccades. Release
of the monkey's head demonstrated a profound contralateral head
tilt. Taken together, these data suggest that the gaze signal
generated in the SC is filtered by neurons in the MRF to generate
a feedback signal of eye motor error. The head signal found
in the MRF could cancel a portion of the gaze signal coming
from the SC in the form of head velocity feedback.
