Neural activity of multiple fields in the auditory cortex of anesthetized guinea pigs in response to pure tones was visualized by optical recording using a voltage-sensitive dye (RH795). Ten auditory fields were identified based on the tonotopic organization and response latency: the core fields consisting of the primary (AI) and secondary (AII) fields and surrounding belt areas consisting of dorso-anterior (DA), dorsal (D), dorso-posterior (DP), posterior (P), ventro-posterior (VP), ventro-medial (VM), ventro-anterior (VA) and ventral (V) fields. Tonotopic organization was observed in all the fields apart from DA, D, DP and V. Spatio-temporal displays suggest that the auditory information spreads from the core fields of AI and AII to belt fields via three distinct (dorsocaudal, caudal and ventrorostral) pathways.

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Spatiotemporal patterns of neuronal responses to asynchronous two-tone stimuli in the anterior field of the auditory cortex of anesthetized guinea pigs were studied using an optical recording method (12 x 12 photodiode array, voltage sensitive dye RH795). Interactions between the onset response to the first tone (masker; 5, 8, 10, 12 and 15 kHz, 200 ms) and to the second tone (probe; 10 kHz, 30 ms) with onset delays relative to the masker onset (0, 5, 10, 15 and 20 ms) were investigated. In general, two-tone interaction was suppressive rather than facilitative. At 0-10 ms probe delays, two-tone responses induced in the probe isofrequency area on the cortex tended to fuse with the masker response. At 15-20 ms probe delays, the probe response was apparently reduced, but was spatially focused and separated from the masker response. This spatial focusing of the probe response may have been due to neuronal inhibition originating after the masker onset response. These results are in agreement with psychoacoustical observations in human subjects, such as auditory segregation, and indicate that the spatial focusing of the cortical response provides a neuronal basis for detecting slightly asynchronous auditory inputs.

Spatiotemporal response patterns in the anterior and dorsocaudal fields of the guinea pig auditory cortex after two-tone sequences were studied in anesthetized animals (Nembutal 30 mg kg-1) using an optical recording method (voltage-sensitive dye RH795, 12 x 12 photodiode array). Each first (masker) and second (probe) tone was 30 ms long with a 10-ms rise-fall time. Masker-probe pair combinations of the same or different frequencies with probe delays of 30-150 ms were presented to the ear contralateral to the recording side. With same-frequency pairs, responses to the probe were inhibited completely after probe delays of less than 50 ms and the inhibition lasted for more than 150 ms, and the inhibition magnitudes in different isofrequency bands of the anterior field were essentially the same. With different-frequency (octave-separated) pairs, responses to the probe were not inhibited completely even after probe delays as short as 30 ms, and the inhibition lasted only for 110-130 ms. Inhibition magnitudes were different from location to location.