Responses of gerbil utricular afferents to translational motion.

In the present study, we report the sensitivity of utricular afferents to sinusoidal translational motion in the horizontal plane. The head orientation was altered relative to the direction of translational travel in 30 degrees increments to allow determination of the head orientation that elicited the maximal and minimal responses of each afferent neuron. We determined gain and phase relationships at a constant peak linear acceleration of 0.1 g applied at frequencies between 0.20 and 2.0 Hz for multiple head orientations. The response dynamics and vector of maximal sensitivity for the utricular afferents are consistent with those reported for other mammalian species. Irregularly (CV>0.3) and intermediate (0.1</=CV</=0.3) discharging units demonstrated gain enhancement at higher frequencies. Regular units (CV<0.1) showed lower gains and flat response dynamics. The mean gains of the irregular, intermediate, and regular units at 0.5 Hz were 256, 118, and 69 spikes s(-1) g(-1), respectively. The phase of the response was independent of the vector of orientation except near the null response orientation where phase and gain were difficult to accurately measure. Phase leads (relative to acceleration) in irregular units at lower frequencies were reduced at higher frequencies. All afferents demonstrated simple one-dimensional tuning with their vectors of maximal sensitivity distributed throughout the 360 degrees of the horizontal plane, though the majority were directed out of the contralateral ear.