The vestibular nerve of the chinchilla. IV. Discharge properties of utricular afferents.

1. Extracellular recording techniques were used in the chinchilla to study the discharge properties of utricular afferents, including their discharge regularity, background discharge, and responses to both externally applied galvanic currents and centrifugal forces. 2. A normalized coefficient of variation (CV*), independent of discharge rate, was used to classify units as regularly (CV* less than 0.10), intermediate (0.10 less than or equal to CV* less than or equal to 0.20), or irregularly discharging (CV* greater than 0.20). In some circumstances, it was useful to recognize a group of very regularly discharging afferents (CV* less than 0.05). The CV* ranged from less than 0.020 to greater than 0.60. Regular units outnumbered irregular units by an approximate 3:1 ratio. The distribution of CV*s was bimodal: there was a major peak at CV* = 0.03 and a minor peak at CV* = 0.3. 3. Background rates were measured with the head in a horizontal position. Those of regular units usually fell between 40 and 80 spikes/s (mean: 54 spikes/s); those of irregular units were more broadly distributed (mean: 47 spikes/s). 4. Units were categorized in terms of the tilt directions resulting in increased discharge. There is a broad distribution of excitatory tilt directions with some units excited by ipsilateral rolls, others by contralateral rolls, some by nose-up pitches, and still others by nose-down pitches. In the chinchilla, there are almost equal numbers of units excited by ipsilateral or contralateral tilts. This is in contrast to previous findings in the cat and squirrel monkey, where the former units predominant by a 3:1 ratio. The difference can be related to the fact that the medial zone of the macula, where units excited by ipsilateral tilts reside, makes up a smaller proportion of the sensory epithelium in the chinchilla than in the monkey. 5. Galvanic sensitivity (beta *) and discharge regularity (CV*) were related by a power law, beta* = (CV*), with an exponent, b = 0.70. 6. Responses to sinusoidal centrifugal forces in the frequency range, f, between DC and 2 Hz were characterized by their gains (gf) and phases (phi f), taken with respect to peak linear force. Response linearity was studied by varying the amplitude of a 0.1-Hz sinusoid from 0.05 to 0.4 g. Nonlinear distortion was small (approximately 10%), as was the variation of gain (+/- 10%) and phase (+/- 5 degrees) with amplitude. 7. Response dynamics vary with discharge regularity. Very regular units are tonic. Their gains are typically 50 spikes.s-1/g and almost constant (+/- 10%) over the entire frequency range. Phases hover near zero with small (5 degrees) phase leads at low frequencies and slightly larger (10 degrees) phase lags at high frequencies. Irregular units are more phasic.(ABSTRACT TRUNCATED AT 400 WORDS)