Analysis of vestibulocollic reflexes by sinusoidal polarization of vestibular afferent fibers

1. Canal vestibular-neck (vestibulocollic) reflexes have been studied in decerebrate cats by applying modulated polarizing current to individual ampullary nerves, usually the horizontal nerve. 2. Computer-generated stimuli sometimes consisted of sine or square waves at frequencies of 0.01--5 Hz. More often we used a compound wave form of nine superimposed sinusoids within an available frequency range of 0.009--6.11 Hz. The frequencies used were odd, relatively prime multiples of a base frequency selected to minimize distortion or interaction of responses. The response to each of the nine stimulating frequencies could be obtained in subsequent data analysis. 3. Responses to these stimuli were studied by recording the EMG of contralateral neck muscles and extracellular activity of second-order neurons. These neurons were identified by their monosynaptic responses to single-shock stimulation of ampullary nerves. 4. EMG was modulated sinusoidally. Below 0.1 Hz the response was variable, most likely due to differences in the preparations. In the frequency range of 0.1--0.4 Hz there was usually a phase lag, which decreased with incresing frequency and often reversed to a lead at 3 and 6 Hz. Gain decreased from the lowest frequencies, occasionally with an upturn at 3 or 6 Hz. 5. Second-order neuron firing was approximately in phase with the stimulus at the lowest frequencies. Phase advanced with increasing frequency to a lead of 30--50 degrees at 6 Hz. Gain generally increased with frequency. 6. By recording simultaneously from muscle and from second-order neurons, or by comparing the mean behavior of the two, it was possible to determine the central phase lag and gain of the vestibulocollic reflex. The lag was variable at low frequencies, had an average of 50 degrees at 0.18 Hz, and decreased to 20 degrees at 6 Hz. These results are comparable to those obtained by others using natural stimulation at frequencies of 1.0 Hz and below, and provide new information about the behavior of the central processer at higher frequencies. 7. The medial vestibulospinal tract (MVST), which contains the axons of crossed second-order vestibular neurons, was transected in six experiments. In agreement with previous workers there was no effect on phase at frequencies up to 0.4 Hz. There was also no selective effect of phase or gain at the higher frequencies. This shows that the disynaptic pathways in the MVST do not play any role that cannot be taken over by parallel pathways.