OPTICAL RECORDING OF SOUND-INDUCED DYNAMIC ACTIVITIES IN THE AUDITORY CORTEX

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Using an optical recording method with a 12 x 12-channel photodiode array and a voltage-sensitive dye (RH795), dynamic activities of neurons in anterior (A : primary) and dorsocaudal (DC) fields of the guinea pig auditory cortex were recorded under anesthetized condition (Nembutal 30 mg/kg, doroperidol 0.3 mg/kg/h and pentazocine 1.1 mg/kg/h). For sound stimulation, pure tones, frequency-modulated (FM) sounds, sounds with step-wise frequency changes (SF) and sequential two tones were presented to the ear contralateral to the recording side.The response to a pure-tone appeared as a band (isofrequency band) in both fields A and DC.The response was dynamic : the active spot moved ventrally or dorsally along the isofrequency band. The responses to SF sounds were the sequential activation of the isofrequency bands corresponding to the frequencies of the components in the SF sounds. Each response was similar to those to pure tones. The active spot in response to FM sounds moved obliquely crossing the isofrequency bands. When the direction of the frequency sweep was changed, the active spot moved in the opposite direction. No cortical area specifically responding to the FM sounds was found.The responses to sequenial two tones showed that inhibition between and within isofrequency bands occurs after the excitatory responses. When bicuculline, a GABA_A antagonist, was applied to the auditory cortex, the excitatory response to a pure tone was spread widely beyond the isofrequency band. This spread was prevented by further application of an NMDA antaonist.These results indicate that sounds were represented by dynamic neural acivities in the auditory cortex and these activities were created by excitatory and inhibitory interactions between isofrequency bands. A network model to simulate these results was proposed.