FM-EVOKED RESPONSES AND FM-TONOTOPIC CHARACTERISTICS OF THE CAT AUDITORY CORTEX

Averaged responses evoked by frequency modulation (FM) of tones were recorded from different points on the surface of AI zone of the cat auditory cortex and response thresholds in terms of modulation depth (△ f_m) for different carrier frequencies (f) were determined. FM-evoked potentials were similar in wave form to those evoked by tone bursts. They appeared either as on-, or as off- (in correspondency to the beginning or termination of the rectangular modulating wave), or as on-off-type of response. △ f_m varied greatly with f and with the site of recording. The majority of the △ f_m-versus-f curves were marked by the presence of deep valley or valleys, where the location of the nadirs, which represented the characteristic f's of PM-responses f's of best △ f_m), differed with recording points in the auditory cortex. High, middle and low characteristic f points of PM-responses intermingled in the AI zone, a distribution entirely different from the tonotopic mapping of the auditory cortex in tuning responses. Comparison of △ f_m with the tuning threshold for the same f and for the same recording point revealed that best △ f_m were obtained mostly for those f's whose tuning sensitivity was in the middle range (threshold-40-60 dB).
The smallest △ f_m obtained in cats with the FM-response method were less than 0.01 f for f> 1000 Hz, and below 10 Hz for f< 1000 Hz, quite comparable to the smallest frequency differential thresholds of cats determined by some authors in conditioning avoidance experiments.
The divergence in topographic distribution,of characteristic f's of both FM- and tuning responses in the cat auditory cortex is discussed in the view that there is fine neuro-struc-tural differentiation at the cortex level for these two kinds of functions.