Organization of auditory areas in the superior temporal gyrus of marmoset monkeys revealed by real-time optical imaging.

The prevailing model of the primate auditory cortex proposes a core-belt-parabelt structure. The model proposes three auditory areas in the lateral belt region; however, it may contain more, as this region has been mapped only at a limited spatial resolution. To explore this possibility, we examined the auditory areas in the lateral belt region of the marmoset using a high-resolution optical imaging technique. Based on responses to pure tones, we identified multiple areas in the superior temporal gyrus. The three areas in the core region, the primary area (A1), the rostral area (R), and the rostrotemporal area, were readily identified from their frequency gradients and positions immediately ventral to the lateral sulcus. Three belt areas were identified with frequency gradients and relative positions to A1 and R that were in agreement with previous studies: the caudolateral area, the middle lateral area, and the anterolateral area (AL). Situated between R and AL, however, we identified two additional areas. The first was located caudoventral to R with a frequency gradient in the ventrocaudal direction, which we named the medial anterolateral (MAL) area. The second was a small area with no obvious tonotopy (NT), positioned between the MAL and AL areas. Both the MAL and NT areas responded to a wide range of frequencies (at least 2-24 kHz). Our results suggest that the belt region caudoventral to R is more complex than previously proposed, and we thus call for a refinement of the current primate auditory cortex model.