Functional topography of cat primary auditory cortex: representation of tone intensity

The neuronal response to tones as a function of intensity was topographically studied with multiple-unit recordings in the primary auditory cortex (AI) of barbiturate-anesthetized cats. The spatial distribution of the characteristics of rate/level functions was determined in each of three intensely studied cases and their relationship to the distribution of spectral parameters (sharpness of tuning and responses to broadband transients) in the same animals was determined. The growth of the high-intensity portion of rate/level functions was estimated by linear regression. Locations with monotonically growing high-intensity portions were spatially segregated from locations with nonmonotonic rate/level functions. Two noncontiguous areas with a high degree of nonmonotonicity were observed. One was located at the dorsoventral center of AI, and a second in the dorsal third of AI. The more ventral aggregate of high nonmonotonicity coincided with the region of sharp frequency tuning. The stimulus levels that produced the highest firing rate (strongest response level, SRL) at any sampled location ranged from 10 to 80 dB sound pressure level (SPL). Several spatial aggregates with either high or low SRLs were observed in AI. The region of sharpest tuning was always associated with a region of low SRLs. The response threshold to contralateral tones at the characteristic frequency (CF) ranged from -10 dB SPL to 85 dB SPL with the majority between 0 and 40 dB SPL. The spatial distribution of response thresholds indicated several segregated areas containing clusters with either higher or lower response thresholds. The correlation of response threshold with integrated bandwidth and transient responses was only weak. Low- and high-intensity tones of the same frequency are represented at different locations in AI as judged by the amount of evoked neuronal activity and are largely independent of the frequency organization. The spatial distribution of locations with high monotonicity and low strongest response levels were aligned with the organization of the integrated excitatory bandwidth and covaried with the response strength to broadband stimuli.