Journal of Neuroscience, Vol 13, 351-370, Copyright © 1993 by Society for Neuroscience
Orienting head movements resulting from electrical microstimulation of the brainstem tegmentum in the barn owl
T Masino and EI Knudsen
Department of Neurobiology, Stanford University, California 94305-5401.
The size and direction of orienting movements are represented
systematically as a motor map in the optic tectum of the barn owl (du Lac
and Knudsen, 1990). The optic tectum projects to several distinct regions
in the medial brainstem tegmentum, which in turn project to the spinal cord
(Masino and Knudsen, 1992). This study explores the hypothesis that a
fundamental transformation in the neural representation of orienting
movements takes place in the brainstem tegmentum. Head movements evoked by
electrical microstimulation in the brainstem tegmentum of the alert barn
owl were cataloged and the sites of stimulation were reconstructed
histologically. Movements elicited from the brainstem tegmentum were
categorized into one of six different classes: saccadic head rotations,
head translations, facial movements, vocalizations, limb movements, and
twitches. Saccadic head rotations could be further subdivided into two
general categories: fixed- direction saccades and goal-directed saccades.
Fixed-direction saccades, those whose direction was independent of initial
head position, were elicited from the midbrain tegmentum. Goal-directed
saccades, those whose direction changed with initial head position, were
elicited from the central rhombencephalic reticular formation and from the
efferent pathway of the cerebellum. Particular attention was paid to sites
from which fixed-direction saccadic movements were elicited, as these
movements appeared to represent components of orienting movements.
Microstimulation in the medial midbrain tegmentum elicited fixed-direction
saccades in one of six directions: rightward, leftward, upward, downward,
clockwise roll, and counterclockwise roll. Stimulation in and around the
interstitial nucleus of Cajal (InC; a complete list of anatomical
abbreviations is given in the Appendix) produced ipsiversive horizontal
saccades. Stimulation in the ventral InC and near the dorsal and medial
edges of the red nucleus produced upward saccades. Stimulation in the
reticular formation near the lateral edge of the red nucleus produced
downward saccades. Stimulation in the ventromedial central gray produced
ipsiversive roll saccades. The metrics and kinetics of fixed-direction
saccades, but not their directions, could be influenced by stimulation
parameters. As such, direction was an invariant property of the circuits
being activated, whereas movement latency, duration, velocity, and size
each demonstrated dependencies on stimulus amplitude, frequency, and
duration. The data demonstrate directly that at the level of the midbrain
tegmentum there exists a three-dimensional Cartesian representation of
head-orienting movements such that horizontal, vertical, and roll
components of movement are encoded by anatomically distinct neural
circuits.(ABSTRACT TRUNCATED AT 400 WORDS)
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