Efferent connections of the ventral auditory area in the rat cortex: implications for auditory processing related to...

In the rat auditory cortex, ventral (VA) and posterodorsal (PD) areas are the two major auditory fields that receive thalamic afferents from the dorsal division of the medial geniculate body (MGD). VA and PD are presumed to serve distinct functions in tandem as the pair of major cortical recipients of extralemniscal thalamic inputs. To deduce the functional significance of VA, efferent connections of VA were examined with the anterograde tracer biocytin. VA lies primarily in the ventral margin of area Te1 and represents frequencies primarily < 15 kHz [Donishi, T., Kimura, A., Okamoto, K. & Tamai, Y. (2006) Neuroscience, 141, 1553-1567.] Biocytin was iontophoretically injected into cortical regions which were defined as VA based on histological location, auditory response and thalamocortical connectivity. Anterograde labelling revealed two important aspects of cortical projections. First, VA sent a projection to a well-confined region in the caudal end of the insular cortex (Ins) pivotal for fear memory formation during aversive conditioning. Second, VA sent parallel projections to cortical regions that probably comprise the other nonprimary auditory fields, including PD. The results suggest that VA relays auditory input from the MGD to the Ins for affective memory formation and at the same time dispatches the auditory signal, which may represent emotional content, to the remaining nonprimary auditory fields. PD is assumed to play a pivotal role in auditory spatial processing for directed attention (Kimura et al., 2004). As the counterpart of PD, VA is assumed to give rise to another major stream of cortical information processing, most probably related to emotion.