Basic functional organization of second auditory cortical field (AII) of the cat

C. E. Schreiner and M. S. Cynader The spatial representations of response characteristics to acoustic stimuli within the ventral primary auditory cortex (AI) and the second auditory cortical field (AII) were mapped in the left hemispheres of 14 anesthetized adult cats using closely spaced (200-500 micron) microelectrode penetrations to record cortical responses from small cell clusters. Responses were activated by 200-ms tone bursts (1/s). The obtained response characteristics were l) frequency selectivity, i.e., characteristic frequency (CF) for narrowly tuned responses and center frequency (arithmetic mean of bandwidth 10-15 dB above threshold) for broadly tuned responses; 2) threshold at CF; 3) bandwidth 10 dB above threshold (Q10); 4) binaural interaction class. CF maps. In AI a strictly tonotopically organized CF map was observed with a frequency gradient from high to low along a rostral/caudal axis. At the dorsal end of AII a 0.5- to 1-mm-wide stripe with clear but not strict tonotopic organization was seen, paralleling the organization of AI. In the ventral part of AII a tonotopic organization concordant with AI was found to be still present. However, it was significantly blurred by the strong variability of the CFs, local low-frequency islands, and increasing bandwidth of receptive fields. Q10 maps. In general, narrowly tuned responses were obtained in AI with increasing bandwidth toward the AI/AII border. In AII, broadly tuned responses were usually obtained with a further decrease of the Q10 value along a dorsal/ventral axis. The Q10 at the transition from AI to AII can be described by Q10 = 1.6 X CF0.36 X Q10 in AI is usually higher and Q10 in AII is usually lower than this transition value. Threshold. The sensitivity of AI responses to tonal stimuli was slightly greater than in AII. Across the AI/AII border, a drop in the sensitivity of 10-15 dB was usually observed. The variability in the thresholds of adjacent locations was much higher in AII than in AI. Binaural interaction. Aggregates of neurons with similar binaural properties in ventral AI were "band" shaped and ran roughly orthogonal to isofrequency contours. The course of the binaural bands showed major variations with possible interruptions. The most ventral AI band was EI (contralateral excitation, ipsilateral inhibition) in about 70% of the investigated cortices. In AII binaural aggregates had polymorphic shapes. The sizes and principal orientations of these binaural patches varied w