Hierarchical and serial processing in the spatial auditory cortical pathway is degraded by natural aging.

The compromised abilities to localize sounds and to understand speech are two hallmark deficits in aged individuals. The auditory cortex is necessary for these processes, yet we know little about how normal aging affects these early cortical fields. In this study, we recorded the spatial tuning of single neurons in primary (auditory cortex, A1) and secondary (caudolateral field, CL) auditory cortical areas in young and aged alert rhesus macaques. We found that the neurons of aged animals had greater spontaneous and driven activity, and broader spatial tuning compared with those of younger animals. Importantly, spatial tuning was not sharpened between A1 and CL in aged monkeys as it is in younger monkeys. This implies that a major effect of normal aging is a degradation of the hierarchical processing between serially connected cortical areas, which could be a key contributing mechanism of the general cognitive decline that is commonly observed in normal aging.