Distribution of frequency following responses in cat cochlear nucleus to sinusoidal acoustic signals.

We have investigated the frequency following response (FFR) to pure tones in the cochlear nucleus of anesthetized cats with regard to the spatial distribution of amplitude and phase. Amplitude distributions were variable from animal to animal and showed a range of responses including single and multiple peaks and nulls, broad plateaus, and gradually tapering slopes. In general, amplitudes were larger in the ventral cochlear nucleus and VIIIth nerve than in dorsal cochlear nucleus. Phase distributions displayed gradual phase shifts across the nucleus as well as abrupt phase changes in the order of 0.5 cycle. Stimulus frequency had a greater effect upon the dorso-ventral profile of both phase and amplitude of the FFR than did the horizontal position of the electrode track within the cochlear nucleus. Changing stimulus intensity altered the amplitude profile of the FFR at a given frequency but had relatively little effect upon the phase distribution. Our data are consistent with the following hypotheses: (1) the FFR is generated as an envelope of the VIIIth nerve action potentials as the nerve arborizes and terminates in the cochlear nucleus. Phase shifts may be caused by traveling action potentials along the nerve and/or by volume conduction from different fiber bundles to regions lying between these bundles; (2) the FFR is generated postsynaptically in cochlear nucleus cells innervated by the VIIIth nerve. Once again, phase shifts may reflect the sequential innervation of adjacent dendrites, a 'traveling' postsynaptic wave, or the response of a few generators at different loci driven at different phases, in which case spatial phase shifts are caused by the summing of responses as currents are volume conducted to the recording electrode. © 1967