Ventral temporal cortex in the rat: Connections of secondary auditory areas Te2 and Te3

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Abstract
The present study in the rat deals with the hodological organization of two cytoarchitectonically distinct areas lying caudoventrally (Te2) or ventrally (Te3) to the primary auditory area (Te1). The afferent and efferent systems of connections were identified by using the properties of retrograde and anterograde transport of wheat germ agglutinin conjugated with horseradish peroxidase (WGA‐HRP). Large tracer deposits in the ventral temporal cortex involving Te2, Te3, and the dorsal bank of the perirhinal cortex induced a dense retrograde and anterograde pattern of labeling in the following nuclei of the medial geniculate (MG) complex: caudodorsal (MGCD), dorsal (MGD), medial (MGM), suprageniculate (SG), and peripeduncular area (PPA). The ventral nucleus (MGV) was only slightly labeled in its caudal division. Several extrageniculate structures were also labeled. Retrograde cell labeling occurred in centers giving rise to ascending systems of diffuse projections: locus coeruleus (LC), dorsal raphe nucleus (DR), and basal magnocellular nucleus (B). Slight anterograde labeling was present in the dorsal and external cortices of the inferior colliculus (IC), central gray, deep layers of the superior colliculus (SC), reticular thalamic nucleus (RT), and caudate putamen (CPU). Callosal connections were also noted with the contralateral homotopic cortex. In the cases in which there was a notable extension of the zone of diffusion of the tracer into the dorsal bank of the perirhinal cortex, a characteristic pattern of labeling in the subparafascicular, reuniens and paraventricular thalamic nuclei, mammillary complex, lateral and dorsal hypothalamic nuclei, amygdaloid complex, laterodorsal tegmental nucleus, subiculum, and retrosplenial cortex was displayed. Tracer deposits restricted to Te2 induced a dense labeling of the caudal, ventrolateral MGD, lateral PPA and, to a lesser extent, MGCD. The MGM and SG were only slightly labeled. Extrageniculate afferents essentially consist of sparse projections from LC, DR, and B, whereas efferent fibers are directed to the dorsal cortex of the IC, central gray, deep SC layers, and CPU. Callosal connections were also identified. Following tracer deposits restricted to Te3, dense labeling occurred in the MGD, mostly in its medial division, in the caudal MGM, and in the PPA. The MGCD, SG, and MGV were only sparsely labeled. Extrageniculate afferents arise from LC, DR, and B, and efferents are directed to the RT and dorsal cortex of the IC. Contralateral connections with the homotopic cortical area were also noted. Te2 and Te3 share some degree of similitude in their pattern of connections with the MG complex. However, several individual characteristics can be noted. Te2 is preferentially connected to the ventrolateral MGD and lateral PPA, whereas Te3 is preferentially related to the medial MGD and medial PPA. Finally, only Te3 is connected densely with the rostral part of the MGD and sparsely with the caudal MGV. In conclusion, the present results and recent electrophysiological observations (Kelly and Sally: J. Neurophysiol. 59:1756–1769, 1988; Sally and Kelly: J. Neurophysiol. 59:1627–1638, 1988) suggest the existence of secondary auditory areas Te2 and Te3 located ventrally to Te1. In contrast to Te1, which receives a dense, topographically organized projection from the MGV, Te2 and Te3 receive dense projections from the remaining nuclei of the MG complex, with no evidence of precise topographic arrangement.

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