Spontaneous activity and frequency selectivity of acoustically responsive vestibular afferents in the cat.

1. Recordings were made from single afferent fibers in the inferior vestibular nerve. Firing rates of a substantial portion of the afferents with irregular background activity increased in response to moderately intense tone bursts. 2. Spontaneous activity from acoustically responsive vestibular afferents was statistically analyzed and compared with data from a more widespread sampling of primary afferents in the cat's vestibulocochlear nerve. Acoustically responsive vestibular afferents had interspike interval histograms with modes > 10 ms, coefficients of variation > 0.15, and skews > 0.88. On the basis of spontaneous activity, these afferents were easily distinguishable from cochlear afferents and regular vestibular afferents, but no obvious features differentiated them from other irregular vestibular afferents. 3. The distributions of spike intervals in the spontaneous activity of acoustically responsive vestibular afferents were fitted by Erlang probability density functions describing the second-order interarrival times of a Poisson process initiated after a finite delay (refractory period). 4. Acoustically responsive vestibular afferents had broad, V-shaped tuning curves with best frequencies between 500 and 1,000 Hz, thresholds of > or = 90 dB SPL, and shapes comparable with the tuning-curve "tails" of cochlear afferents. In contrast to cochlear-nerve afferents, acoustically responsive vestibular afferents did not show a strong relationship between spontaneous rate and threshold. 5. We compare the acoustic frequency selectivity of vestibular and cochlear afferents in terms of their functional and evolutionary relationships. Our data and those of others indicate that acoustically responsive vestibular afferents are likely to provide an input to the acoustic activation of the sternocleidomastoid muscle in humans, and they may provide an input to other acoustic reflexes such as the middle-ear-muscle reflexes.