Response characteristics of semicircular canal and otolith systems in cat. I. Dynamic responses of primary vestibular fibers.

1. The activity of cat semicircular canal and otolith afferents was studied during yaw and roll rotations, respectively, to examine their dynamic behavior. 2. A sinusoidal analysis of the canal afferent activities showed that their dynamic characteristics are similar to those of second order vestibular neurons, except for a two to three-fold lower absolute gain. This agrees with earlier studies using angular acceleration steps. 3. Both divisions of the eighth nerve were sampled so as to examine afferents from both the utriculus and sacculus. Within the range of inputs used (+/- 25 degrees lateral tilt), the presumed sacular afferents (inferior division) showed either a gamma- or beta-response. However, the gain of their response was generally much less than for the afferents of the superior division (mostly utricular). This behavior is to be expected on the basis of receptor orientations and the components of gravity acting upon the macular receptors. 4. In response to ramp changes in angular position, some otolith units showed a phasic-tonic response pattern, i.e., an overshoot followed by an adaptation to a new steady state level of activity. The majority of units showed predominantly tonic responses proportional to displacement. 5. During sinusoidal rotations the predominantly tonic units showed small phase leads of 0 to 15 degrees at 0.025 Hz which remained constant or decreased to 0 to -15 degrees at 1.0 Hz. The gains were flat or increased by up to 2 fold. The phasic-tonic units showed greater phase leads, 10 to 50 degrees, and gains which increased from 2 to 8 fold. 6. This behavior of otolith afferents suggests that they can provide information about both the magnitude and the rate of change of linear acceleration stimuli.