Quantification of Different Classes of Canal‐Related Vestibular Nuclei Neuron Responses to Linear Acceleration

A change in otolith activity modifies the dynamic responses of both the and the vertical4 vestibuloocular reflexes (VOR). In response to rotations in vertical planes, dynamic otolith activity is necessary for compensatory eye movements in the rabbit5 and the cat.6 Therefore, significant convergence of otolith and canal information in the VOR pathway must occur. The activity of single vestibular nuclei neurons in the decerebrate rat were recorded extracellularly during sinusoidal linear translation in the horizontal head plane. Details of the experimental procedure are presented elsewhere.’ Neurons from the four groups-(1) type I and (2) type I1 horizontal canal related, (3) vertical canal related, and (4) purely otolith-were systematically tested for their responses to translation at various horizontal head orientations. These responses were then used to describe a response ellipse*JO in which the semimajor axis (S1) defined the cell’s direction of maximum sensitivity and its associated gain and phase and the semiminor axis (S2) defined the minimum sensitivity of the cell in the horizontal head plane. When the magnitude of the S2 vector was zero, the response was referred to as narrowly tuned and was characterized by gain values that were proportional to the cosine of the angle between S1 and the stimulus direction and phase values that were constant with respect to stimulus direction. Whereas, a response with a nonzero magnitude of the S2 vector was referred to as broadly tuned and was characterized by a response phase that varied as a function of stimulus angle. The accuracy with which the response ellipse quantitatively described the data was assessed by comparing the direction, gain, and phase values of the maximum response determined empirically with those calculated from the fitted curves (compare the data points with the curve in FIGURE 1). The calculated and experimentally measured responses had high linear regression coefficients (r = 0.93014.9976) and slopes close to unity. Broadly tuned neurons were observed in each of the four groups of neurons studied. The ratio of S2 and S, response magnitudes (tuning ratio) was calculated for all neurons. The distribution of ratios was similar for all neuron groups. In addition,