AN EXPERIMENTAL-STUDY ON THE GENERATOR OF AMPLITUDE-MODULATION FOLLOWING RESPONSE

To clarify the contribution of the auditory cortex and the inferior co lliculus to amplitude-modulation following response (AMFR), lesion exp eriments were conducted on 8 cats. Bilateral auditory cortices of 2 ca ts were aspirated, and the right inferior colliculus of 3 cats, and th e 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 intensify was fixed at 90 d B 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. Remarkabl e changes were observed in AMFRs at MFs lower than 80 Hz and 200 Hz af ter lesions in the ipsilateral inferior colliculus and at all MFs afte r contralateral lesions of the inferior colliculus, while no such chan ge was observed in ABR. The results suggest that the auditory cortex i s not the source of AMFR, but that the inferior colliculus has great i mpact on the generation of AMFR at modulation frequencies of 20 to 200 Hz.