Comparison of responses in the anterior and primary auditory fields of the ferret cortex.

1. Characteristics of an anterior auditory field (AAF) in the ferret auditory cortex are described in terms of its electrophysiological responses to tonal stimuli and compared with those of primary auditory cortex (AI). Ferrets were barbiturate-anesthetized and tungsten microelectrodes were used to record single-unit responses from both AI and AAF fields. Units in both areas were presented with the same stimulus paradigms and their responses analyzed in the same manner so that a direct comparison of responses was possible. 2. The AAF is located dorsal and rostral to AI on the ectosylvian gyrus and extends into the suprasylvian sulcus rostral to AI. The tonotopicity is organized with high frequencies at the top of the sulcus bordering the high-frequency area of AI, then reversing with lower BFs extending down into the sulcus. AAF contained single units that responded to a frequency range of 0.3-30 kHz. 3. Stimuli consisted of single-tone bursts, two-tone bursts and frequency-modulated (FM) stimuli swept in both directions at various rates. Best frequency (BF) range, rate-level functions at BF, FM directional sensitivity, and variation in asymmetries of response areas were all comparable characteristics between AAF and AI. Responses in both areas were primarily phasic. 4. The characteristics that were different between the two cortical areas were: latency to tone onset, excitatory bandwidth 20 dB above threshold (BW20), and preferred FM rate as parameterized with the centroid (a weighted average of spike counts). The mean latency of AAF units was shorter than in AI (AAF: 16.8 ms, AI: 19.4 ms). BW20 measurements in AAF were typically twice as large as those found in AI (AAF: 2.5 octaves, AI 1.3 octaves). The AI centroid population had a significantly larger standard deviation than the AAF centroid population. 5. We examined the relationship between centroid and BW20 to see whether wider bandwidths were a factor in a unit's ability to detect fast sweeps. There was significant (P < 0.05) linear correlation in AAF but not in AI. In both fields the variance of the centroid population decreased with increasing BW20. BW20 decreased as BF increased for units in both auditory fields.