Steady-state responses from the cat auditory cortex

Responses to 350 ms trains of clicks with 10-100 Hz repetition rate were recorded from the auditory cortices of six cats. Click trains of 30-50 and 90-100 Hz elicited a clear steady-state response (SSR) in awake state. SSRs were small or absent below 30 Hz and in 60-70 Hz stimulus range. In slow wave sleep the optimal rate to elicit SSR shifted towards lower frequencies. 90 Hz SSR was largest in paradoxical sleep. SSRs were strongly suppressed by barbiturate anesthesia. The amplitude of the SSR from the medial geniculate body (MGB) in two cats gradually decreased from 20 to 100 Hz and was more resilient to barbiturate anesthesia than the cortical SSRs. Only low amplitude or no SSRs could be recorded from vertex, visual and association cortices and from the hippocampus in control recordings. The results suggest different generation mechanisms for SSRs recorded from cat auditory cortex and MGB. Human auditory SSRs resemble cat auditory cortical SSRs more than those recorded from cat MGB. The results imply that auditory SSRs in humans are generated in the cortex.