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The Journal of Neurophysiology Vol. 84 No. 4 October 2000, pp. 2001-2015
Copyright ©2000 by the American Physiological Society
1Jenks Vestibular Physiology Laboratory, Massachusetts Eye and Ear Infirmary, Department of Otology and Laryngology, Harvard Medical School, Boston, Massachusetts 02114; and 2Neurological Sciences Institute, Oregon Health Sciences University, Portland, Oregon 97209
Zupan, L. H.,
R. J. Peterka, and
D. M. Merfeld.
Neural Processing of Gravito-Inertial Cues in Humans. I. Influence of the Semicircular Canals Following Post-Rotatory Tilt. J. Neurophysiol. 84: 2001-2015, 2000. Sensory
systems often provide ambiguous information. Integration of various
sensory cues is required for the CNS to resolve sensory ambiguity and
elicit appropriate responses. The vestibular system includes two types
of sensors: the semicircular canals, which measure head rotation, and
the otolith organs, which measure gravito-inertial force (GIF), the sum
of gravitational force and inertial force due to linear acceleration.
According to Einstein's equivalence principle, gravitational force is
indistinguishable from inertial force due to linear acceleration. As a
consequence, otolith measurements must be supplemented with other
sensory information for the CNS to distinguish tilt from translation.
The GIF resolution hypothesis states that the CNS estimates gravity and
linear acceleration, so that the difference between estimates of
gravity and linear acceleration matches the measured GIF. Both otolith
and semicircular canal cues influence this estimation of gravity and
linear acceleration. The GIF resolution hypothesis predicts that
inaccurate estimates of both gravity and linear acceleration can occur
due to central interactions of sensory cues. The existence of specific
patterns of vestibuloocular reflexes (VOR) related to these inaccurate estimates can be used to test the GIF resolution hypothesis. To investigate this hypothesis, we measured eye movements during two
different protocols. In one experiment, eight subjects were rotated at
a constant velocity about an earth-vertical axis and then tilted 90°
in darkness to one of eight different evenly spaced final orientations,
a so-called "dumping" protocol. Three speeds (200, 100, and
50°/s) and two directions, clockwise (CW) and counterclockwise (CCW),
of rotation were tested. In another experiment, four subjects were
rotated at a constant velocity (200°/s, CW and CCW) about an
earth-horizontal axis and stopped in two different final orientations (nose-up and nose-down), a so-called "barbecue" protocol. The GIF
resolution hypothesis predicts that post-rotatory horizontal VOR eye
movements for both protocols should include an "induced" VOR
component, compensatory to an interaural estimate of linear acceleration, even though no true interaural linear acceleration is
present. The GIF resolution hypothesis accurately predicted VOR and
induced VOR dependence on rotation direction, rotation speed, and head
orientation. Alternative hypotheses stating that frequency segregation
may discriminate tilt from translation or that the post-rotatory VOR
time constant is dependent on head orientation with respect to the GIF
direction did not predict the observed VOR for either experimental protocol.
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