Feedforward and feedback projections of caudal belt and parabelt areas of auditory cortex: refining the hierarchical model
Thalamic and amygdaloid connections of three association auditory areas (AA1, AA2, AA3) of the superior temporal gyrus (STG) were investigated. In order to define the projections of the particular areas, injections of fluorescent tracers were made in three monkeys. Distribution of labeling indicates that area AA1 differs from areas AA2 and AA3 in patterns of both thalamo-cortical and amygdalo-cortical connections. Area AA1 receives its predominant inputs from the ventral and dorsal nuclei of the medical geniculate body (MGB). The amygdaloid projection to the area AA1 originates from the basal nuclei, whereas input from the lateral nucleus was not found. The characteristic thalamic projections to areas AA2 and AA3 originate from the dorsal MGB nucleus and the polymodal nuclei of the posterior thalamus. The density of projections from the dorsal nucleus gradually decreases from area AA1 to area AA3 while projections from the Plm, Sg and Lim nuclei increase in the same direction. Areas AA2 and AA3 are the source of strong connections with the lateral nucleus of amygdala, which density increases progressively when injections shift from area AA2 to AA3. The basal and accessory basal nuclei are the source of a less significant amygdalofugal projections to both cortical areas. Thus, our experimental data indicate that influence of the polymodal thalamic nuclei increases substantially in the direction of the higher order association areas. The strong relation of the same cortical areas with the lateral amygdaloid nucleus might suggest that areas AA2 and AA3, in addition to auditory input are the site of transfer of complex sensory information to the amygdala.