FUNCTIONAL TOPOGRAPHY OF CAT PRIMARY AUDITORY-CORTEX - DISTRIBUTION OF INTEGRATED EXCITATION

Neuronal responses to tones and transient stimuli were mapped with microelectrodes in the primary auditory cortex (AI) of barbiturate anesthetized cats. Most of the dorsoventral extent of AI was mapped with multiple-unit recordings in the high-frequency domain (between 5.8 and 26.3 kHz) of all six studied cases. The spatial distributions of 1) sharpness of tuning measured with pure tones and 2) response magnitudes to a broadband transient were determined in each of three intensively studied cases. The sharpness of tuning of integrated cluster responses was defined 10 dB above threshold (Q10 (dB), integrated excitatory bandwidth). The spatial reconstructions revealed a frequency-independent maximum located near the center of the dorsoventral extent of AI. The sharpness of tuning gradually decreased toward the dorsal and ventral border of AI in all three cases. The sharpness of tuning 40 dB above response threshold was also analyzed (Q40 (dB)). The Q40 (dB) values were less than one-half of the corresponding Q10 (dB) value. The spatial distribution showed a maximum in the center of AI, similar to the Q10 (dB) distribution. In two out of three cases, restricted additional maxima were recorded dorsal to the main maximum. Overall, Q10 (dB) and Q40 (dB) were only moderately correlated, indicating that the integrated excitatory bandwidth at higher stimulus levels can be influenced by additional mechanisms that are not active at lower levels. The magnitude of excitatory responses to a broadband transient (frequency-step response) was determined. The normalized response magnitude varied between <1% and up to 100% relative to a characteristic frequency (CF) tone response. The step-response magnitude showed a systematic spatial distribution. An area dorsal to the Q10 (dB) maximum consistently showed the largest response magnitude surrounded by areas of lower responsivity. A second spatially more restricted maximum was recorded in the ventral-third of each map. Areas with high-transient responsiveness coincided with areas of broad integrated excitatory bandwidth at comparable stimulus levels. The distribution of excitation produced by narrowband and broadband signals suggest that there exists a clear functional organization in the isofrequency domain of AI that is orthogonal to the main cochleotopic organization of the AI. Systematic spatial variations of the integrated excitatory bandwidth reflect underlying cortical processing capacities that may contribute to a parallel analysis of spectral complexity, e.g., spectral shape and contrast, at any given frequency. Similarities and consistencies in the spatial distribution of responses to narrowband and broadband signals indicate that the integrated excitatory bandwidth, as determined with the multiple-unit technique, is a useful tool to explore global functional properties of AI. It is suggestive that systematic changes in 1) the convergence of the projections to the cortical locations and 2) the extent of inhibitory influences are mainly responsible for the functional organization described along the isofrequency domain.