Identification and localisation of auditory areas in guinea pig cortex

The organisation of guinea pig auditory cortex was studied by combining histological methods with microelectrode mapping. This allowed the location of seven auditory areas to be determined in relation to the visual and primary somatosensory areas. The auditory areas were identified by single-unit recordings and their borders defined by evoked potential mapping. The visual areas were identified by their relatively high densities of myelinated fibres, while the primary somatosensory cortex was identified by its characteristic barrels of high cytochrome oxidase (CYO) activity in layer IV. The auditory region had moderate levels of CYO and myelin staining. When staining was optimal, there was a clear edge to the moderate CYO activity, which apparently corresponds to the dorsal border of the primary auditory area (AI) and the other core field that lies dorsocaudal to it (DC). Thus the primary somatosensory area and the visual and auditory regions were separated from each other by a region with lower levels of CYO and myelin staining. The ventral borders of AI and DC could not be determined histologically as there were no sharp transitions in the levels of CYO or myelin staining. The two core areas were partially surrounded by belt areas. The dorsorostral belt and most of the belt around DC responded more strongly to broad-band stimuli than pure tones, while the ventrorostral belt, small field and a belt zone ventral to the rostral part of DC responded better to pure tones. Units in the small field (S) typically had higher thresholds and broader tuning to pure tones than AI, while units in the ventrorostral belt typically had longer onset latencies and gave more sustained responses than units in AI.