Analysis of the frequency following response in the cat.

The generators of the frequency following response (FFR) were characterized for three frequency ranges by studying changes in FFR response after lesioning the nuclei within the central brainstem auditory pathway. Responses to low frequency (200-500 Hz) stimulation demonstrated changes in the complexity of the FFR waveform in both time and frequency domains following lesions in the brainstem auditory pathway. The results indicate that the complexity of the low frequency FFR is due to activity from multiple sites within the auditory pathway. The intermediate frequency (700-1500 Hz) responses showed unpredictable amplitude changes following similar lesions and no conclusion could be drawn about the generators of the FFR in this frequency range. The responses to high frequency (3-8 Hz) stimulation showed no reduction in amplitude following serial lesioning. These results, combined with other experimental evidence presented, indicate that the high frequency FFR response originates from the cochlear microphonic. Different electrode configurations were used to evaluate the low frequency FFR. In contrast to multiple generator sources recorded with the standard vertex-mastoid electrode configuration, we were able to record a response contributed primarily by the inferior colliculi with a less peripherally sensitive electrode configuration (vertex-linked-pinnae) at low intensity stimulation. The fact that auditory brainstem nuclei contribute to the FFR in varying amounts depending on the electrode configuration may explain some of the conflicting characterizations of this response in the literature. Despite this difficulty, the FFR neural generators were identified and characterized in the low frequency range using our most sensitive electrode configuration (vertex-mastoid) and in the high frequency range where the single generator is the cochlear microphonic.