Representation of amplitude modulation in the auditory cortex of the cat. II. Comparison between cortical fields

The responses of neuronal clusters to amplitude-modulated tones were studied in five auditory cortical fields of the anesthetized cat: the primary auditory field (AI), second auditory field (AII), anterior auditory field (AAF), posterior auditory field (PAF) and the ventro-posterior auditory field (VPAF). Modulation transfer functions (MTFs) for amplitude-modulated tones were obtained at 172 cortical locations. MTFs were constructed by measuring firing rate (rate-MTFs) and response synchronization (synchronization-MTFs) to sinusoidal and rectangular waveform modulation of CF-tones. The MTFs were characterized by their 'best-modulation frequency' (BMF) and a measure of their quality of 'sharpness' (Q2dB). These characteristics were compared for the five fields. Rate and synchronization MTFs for sinusoidal and rectangular modulation produced similar estimates of BMF and Q2dB. Comparison of averaged BMFs between the cortical fields revealed relatively high BMFs in AAF (mean: 31.1 Hz for synchronization to sinusoidal AM) and moderately high BMFs in AI (14.2 Hz) whereas BMFs encountered in AII, VPAF and PAF were generally low (7.0, 5.2, and 6.8 Hz). The MTFs were relatively broadly tuned (low Q2dB) in AAF and sharper in a low modulation group containing AII, PAF and VPAF. The ventro-posterior field was the most sensitive to changes in the modulation waveform. We conclude that there are significant differences between auditory cortical fields with respect to their temporal response characteristics and that the assessment of these response characteristics reveals important aspects of the functional significance of auditory cortical fields for the coding and representation of complex sounds.