Spectral-edge sensitivity of primary auditory cortex neurons in alert cats

Although psychophysical studies have revealed involvement of spectral edges in auditory perception, little is known about neural processing. This study investigates how spectral edges are processed in neurons of alert cat primary-auditory-cortex (A1) with sustained response property. Stimuli are low-pass, high-pass and band-pass tones with sharp spectral edges whose edge-frequencies were systematically shifted, constructing edge-frequency response functions. Pure- and two-tone stimuli served to delineate excitatory and inhibitory subfields of the frequency response field (FRF). Based on the response function characteristics, cells were divided into edge-sensitive and edge-insensitive cells: the edge sensitive cells had narrow tuning to the high-edge (type-II cells) or low-edge (type-III cells) frequencies, while the edge insensitive cells were driven by any static stimuli with energy on FRF (type-I) or only very narrowband stimuli with energy confined to FRF (type-IV cells). Edge-sensitive cells showed a close correlation between the best frequencies of the single-frequency (BFSF) and edge-frequency (BFEF) response functions and between their half-height bandwidths, suggesting that the edge-frequency identification is processed along the tonotopic axis in A1. BFSF shifted (mean 0.11 octaves) into the stimulus band from the BFEF (closely corresponding to pitch shift into stimulus band from the edge frequency in human psychophysical data of edge-pitch), suggesting central mechanism of edge-pitch sensation. Type-I cells had non-significant inhibitory subfields of FRF; type-II cells had the significant inhibitory subfield on the higher frequency side; type-III cells, on the lower frequency side; and type-IV cells, on both sides, suggesting that the inhibitory mechanism characterizes the cell-type specific spectral-edge sensitivity.