Neurology Department, Royal Prince Alfred Hospital, Camperdown NSW-2050, Sydney, Australia
In order to test the human angular vestibulo-ocular reflex in
the dynamic range of normal head movements, we measured 3-dimensional
compensatory eye-movement responses to low-amplitude (10-12°),
high-acceleration (3000-4000°/s/s), passive, manually delivered
head rotations (head "impulses") in the three planes of the
semicircular canals in normal subjects, in subjects who had
recovered from surgical unilateral vestibular deafferentation,
and in patients after acute unilateral peripheral vestibulopathy,
that is, from vestibular "neuritis." We found that canal-plane
head impulses away from an intact semicircular canal, that is,
toward a lesioned semicircular canal, invariably produce a vestibulo-ocular
reflex with permanently low gain, typically less that 0.4 if
the lesion is complete. These results are a necessary consequence
of primary semicircular canal afferents being driven into inhibitory
saturation by rapid angular accelerations. With practice, clinicians
can learn to recognize the telltale compensatory saccades that
patients with unilateral loss of semicircular canal function
will make if asked to look at an earth-fixed target during head
impulses in any one of the three semicircular canal planes.
