The Journal of Neuroscience, 2002, 22:RC226:1-7
RAPID COMMUNICATION
Semicircular Canal Afferents Similarly Encode Active and Passive
Head-On-Body Rotations: Implications for the Role of Vestibular
Efference
Kathleen E.
Cullen1 and
Lloyd B.
Minor2
1 Aerospace Medical Research Unit, Department of
Physiology, McGill University, Montreal, Quebec, Canada H3G 1Y6, and
2 Department of Otolaryngology
Head and Neck Surgery,
Department of Biomedical Engineering, and Department of Neuroscience,
The Johns Hopkins University, Baltimore, Maryland 21093
The vestibular receptors in the labyrinth receive innervation from
centrifugally projecting efferent fibers. The influence of these
efferents on information processing by vestibular afferents in primates
has not been determined. One commonly held notion is that efferent
activation during large-amplitude, active head movements would result
in an increase in the resting discharge rate and in a reduction of the
rotational sensitivity of afferents. Such an effect would increase the
dynamic range of afferents involved in the encoding of head movements.
To test this hypothesis, we recorded from afferents innervating the
semicircular canals in alert macaques during passive head-on-body
rotations and during active head movements that included gaze shifts
and gaze pursuit. Extracellular, single-unit recordings were obtained
from 24 afferent fibers innervating the horizontal, superior, and
posterior canals. Based on the normalized coefficient of variation of
the interspike interval for these units, our sample contained six
regularly discharging, six intermediate, and 12 irregularly discharging
afferents. Responses were analyzed using a least squares regression to
determine the bias discharge rate of each unit and sensitivity
to head velocity and acceleration. We found no difference in bias
discharge rate or rotational sensitivity of the afferent responses for
the different stimulus conditions tested. Our results indicate that
semicircular canal afferents encode information about head rotation
similarly for self generated and passively applied head-on-body movements.
Key words:
vestibular; labyrinth; semicircular canals; vestibular
afferents; vestibular efferents; active head movement; gaze shifts; pursuit
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