Spatio-temporal representation of sound intensity in the guinea pig auditory cortex observed by optical recording.

The spatio-temporal representation of sound intensity in the guinea pig auditory cortex was studied by optical recording with the aid of a voltage-sensitive dye. Changes in light intensity due to neural activity induced by sounds at various pressure levels were recorded with a 12 x 12 array of photodiodes. The amplitudes of the responses were displayed as sequential two-dimensional images. The iso-intensity bands revealed by neural activity were orthogonal to the isofrequency bands of tonotopical organization in the anterior field and dorsocaudal field. The frequency-intensity representation through the orthogonal organization varied with time. This mechanism may be adapted to the auditory cortex to sequentially process the three parameters of sound: frequency, intensity, and time. Furthermore, a new field was found in the ventrocaudal region of the auditory cortex. This field showed tonotopicity, but high thresholds.