Cortical Responses to Repeated Clicks and Bursts of Noise

Abstract

Synchronous neural activity has been recorded by gross electrodes from the auditory nerve in response to repeated impulsive stimuli at rates up to 2000/sec. However, cortical activity, recorded by gross electrodes exhibits no directly identifiable stimulus-locked responses at rates as low as 100/sec. For rates higher than 5/sec the amplitude of cortical responses to repeated stimuli decreases. Overlapping of and interference between successive responses as well as a lack of synchrony contribute to the reduction in size of the cortical responses. Gross electrodes were placed on and in the auditory cortex of cats and steady-state responses recorded to repeated clicks, bursts of noise, and bursts of tone. Stimuli were delivered at intensity levels at which the auditory system is clearly mechanically linear. Responses from both anesthetized and unanesthetized preparations were recorded on magnetic tape and processed by a special analog computer in order to obtain the characteristics of the average response. This method permits the detection of small responses in the presence of large ongoing activity. In unanesthetized preparations synchronous cortical responses were found at rates up to 200/sec. [This work was supported in part by the U. S. Army (Signal Corps), U. S. Air Force (Office of Scientific Research, Air Research, and Development Command), and the U. S. Navy (Office of Naval Research).]