Cholinergic innervation of the frontal cortex: differences among humans, chimpanzees, and macaque monkeys.

Cholinergic innervation of the frontal cortex is important in higher cognitive functions and may have been altered in humans relative to other species to support human-specific intellectual capacities. To evaluate this hypothesis we conducted quantitative comparative analyses of choline acetyltransferase-immunoreactive axons in cortical areas 9, 32, and 4 among humans, chimpanzees, and macaque monkeys. Area 9 of the dorsolateral prefrontal cortex is involved in inductive reasoning and specific components of working memory processes, while area 32 of the medial prefrontal cortex has been implicated in theory of mind. Area 4 (primary motor cortex) was also evaluated because it is not directly associated with higher cognitive functions. The findings revealed no quantitative species differences in the three cortical areas examined, indicating that human cognitive specializations are not related to a quantitative increase in cortical cholinergic input. However, species-specific morphological specializations were observed. Clusters of cholinergic fibers that may be indicative of cortical plasticity events were present in chimpanzees and humans, but not in macaques. The other significant morphology noted was the common and distinctive oval or ovoid perisomatic staining in macaque cortices. This feature was also sporadically observed in chimpanzee cortex. Our findings suggest a potential alteration of cortical cholinergic afferents within the prefrontal cortex of humans and chimpanzees, to the exclusion of macaque monkeys.