Parahippocampal projections to posterior auditory association cortex (area Tpt) in Old-World monkeys.

Retrograde and anterograde tracing methods were used to study the connections between the parahippocampal gyrus and the posterior auditory association cortex (area Tpt) in the Old-World monkey. Nine monkeys received injections of tritiated amino acids in different areas of the parahippocampal gyrus, and three monkeys received injections of the retrograde tracer fast blue in area Tpt of the auditory association cortex. It was observed that the medial part of the posterior portion of the parahippocampal gyrus (area TH) gives rise to projections that terminate in layers I-III of area Tpt. Using complementary retrograde tracing experiments, the cells of origin for this projection were identified in layers V and VI of area TH. The findings reveal a direct structural coupling between auditory association cortices and a temporal region thought to play a role in learning and memory. These connections may form part of a neural system in the monkey that is related to acoustic learning and memory. Homologous systems in humans may be involved in auditory learning and language acquisition.