An experimental study on the generator of amplitude-modulation following response.

To clarify the contribution of the auditory cortex and the inferior colliculus to amplitude-modulation following response (AMFR), lesion experiments were conducted on 8 cats. Bilateral auditory cortices of 2 cats were aspirated, and the right inferior colliculus of 3 cats, and the left inferior colliculus of another 3 cats were electrocoagulated to make lesions. Auditory brainstem response (ABR) evoked by clicks and AMFR elicited with 1000 Hz sinusoidally amplitude-modulated tones, at modulation frequencies (MF) of 20-200 Hz in 20-Hz steps, were examined before and after making lesions. Stimulus intensity was fixed at 90 dB SPL in both responses. AMFR was detected by phase spectral analysis using fast Fourier transformation. No change was observed in the phase spectral analysis of AMFR after bilateral cortical lesions. Remarkable changes were observed in AMFRs at MFs lower than 80 Hz and 200 Hz after lesions in the ipsilateral inferior colliculus and at all MFs after contralateral lesions of the inferior colliculus, while no such change was observed in ABR. The results suggest that the auditory cortex is not the source of AMFR, but that the inferior colliculus has great impact on the generation of AMFR at modulation frequencies of 20 to 200 Hz.