Principalcomponent analyses were determined on a series of points measured from the dissected bony labyrinth of ten human skulls, resulting in planar equations for each of the six semicircular canals. Following this, angles were calculated between the ipsilateral canal planes, between opposite synergisticany acting canal planes and between each canal and the Reid stereotaxic planes. Results indicated that pairs of ipsilateral canals were nearly perpendicular, with the exception of the angle formed between the anterior and horizontal canal (mean=l 1 I"). Pairs of contralateral synergistic canal planes formed angles of 19" between right and left horizontal canal planes and 23-24' between vertical canal pairs. horizontal plane. Mathematical equations of the semicircular canals were used to predict the optimal head position for rotational and caloric stimulation.

No abstract

The physiologic response of first-order vestibular canal neurons, recorded within the internal auditory canal with glass microelectrodes, was studied in anesthetized cats. Neurons from all three canals were subjected to velocity trapezoidal rotations about the canal axis, and about different axes extending up to 90 degrees on either side of the canal axis in "roll" and 30 degrees on either side of "pitch." Each cell examined exhibited a spontaneous discharge and responded to constant angular acceleration in a fashion predictable from the direction of the in-plane acceleration vector and the known receptor hair cell polarization. Under conditions of prolonged constant acceleration, (5 degrees/s2 for 40 s) about 30% of the units which could be classified showed adaptation, 55% did not, and 14%, termed reverse adapting cells, demonstrated a fast rise followed by a slower, continual increase during stimulation. Secondary responses (undershoot or overshoot) were noted in most adapting neurons, but were absent in the reverse adapting group. Adapting neurons were distinguished from the nonadapting group by significantly lower resting rates, more irregular interspike-interval distributions, and greater sensitivity to acceleration. When compared with nonadapting neurons, reverse adapting cells had higher spontaneous rates, less irregular spike intervals, and higher sensitivities. The mean canal sensitivity to angular acceleration for all cells was 2 spikes . s-1/deg . s-2 (range 0.3-7.4 spikes . s-1/deg . s-2). Significant differences in mean sensitivity values between canal neurons were demonstrated, with those from the anterior being the most sensitive, followed by the posterior and horizontal canals, respectively. Time constants for all canals governing the transitory rise (or fall) in rate with constant acceleration averaged 3.8 s. Small differences in mean values were noted between canals but these were not significant. Incremental time constants were found to be slightly but significantly longer (mean = 3.9 s) than decremental time constants (mean = 3.6 s). Some cells showed different tine constants to many trials of one stimulus as well as to different levels of stimulus. Most canal unitary responses were approximately linearly related to stimulus magnitudes over the range of 2-18 degrees/s2. This being the case, the angle between the canal plane and plane of stimulus become the main determinant in the first-order neural response. Here, a linear cosine relationship descriged the three-dimentionsal unitary response curve: maximum canal response was elicited with rotation about the canal axis, while no response was evoked with rotation about an axis approximately 90 degrees to canal axis. Between these two extremes, the response of a cell was determined by the cosine of the angle between the canala axis and the axis of rotation.

the 2Max-Planck-fnstitut fur Hirnforschung, FrankfurtlM., West Germany and the 3Department Abstract. Formulation of semicircular canal transfer functions have to date been restricted to idealized cases. Recent information in cat, guinea pig and man on the precise orientation of the semicircular canals allows a more realistic specification of the forces acting on the cupulae during any head rotation.

The gain of the vestibulo-ocular reflex in the cat continues to increase for some time after birth. The reason for this increase is not presently known and one possibility if that it occurs because the cat semicircular canals increase in size. The present study examined this possibility by measuring the radii of curvature (R) of individual semicircular canals and the angular relationships of the semicircular canal planes within a labyrinth in cats and guinea pigs during postnatal growth. It was found that the labyrinths do move apart substantially during postnatal development in both species, but neither the planar relations nor the radii of curvature change significantly during postnatal development. The stability of semicircular canal structure during postnatal skull growth indicates that postnatal developmental changes in canal-related function, such as increased gain in the vestibulo-ocular reflex, in these species are probably due to receptor cell or neural maturational factors.