Spatiotemporal representation of binaural difference in time and intensity of sound in the guinea pig auditory cortex.

Abstract Neural activity of the auditory cortex (AC) in response to a change of interaural intensity difference (IID) and interaural time difference (ITD) of sound stimuli was observed by optical recording with a 12×12 photodiode array and the voltage-sensitive dye, RH795. Guinea pigs (280–450 g) were anesthetized with sodium pentobarbital (30 mg/kg) and supplemental doses of neuroleptic solutions. When both ears were stimulated dichotically by tone bursts (14 kHz, 75 dB SPL), excitatory optical signals appeared in both anterior (A) and dorsocaudal (DC) fields of AC. An increase of intensity of ipsilateral stimulation from 65 to 95 dB SPL caused a decrease of neural activity of isofrequency bands in both fields. An increase of ipsilateral leads from −2.5 to 10 ms resulted in a gradual decrease of the amplitude of the excitatory responses. A strong inhibition was observed in field DC and the ventral portion of field A. These results show the different spatiotemporal representation of IID and ITD sensitivities in AC. However, the ipsilateral lead inducing a large inhibition was much longer than the time difference (80 μs) calculated from the interaural distance of the guinea pig. This indicates that the longer binaural inhibition observed in AC would have a different functional significance from that of the neural system of ITD detection in the guinea pig.