Evidence for Cortical Adaptations Supporting Language
Marmoset monkeys vocalize more often and exhibit more complex vocal motor behavior than macaque monkeys. To gain insight into the central control of vocalization, we used retrograde transneuronal transport of rabies virus from the cricothyroid muscle to identify cortical areas that influence this muscle in the two species. This comparative approach revealed that five cortical motor areas in the frontal lobe of both species are involved in the descending control of this laryngeal muscle. These five areas are: the primary motor cortex (M1), ventral area 6 (6V), the supplementary motor area (SMA), the ventral cingulate motor area (CMAv), and the rostral cingulate motor area (CMAr). Although the same five cortical areas influenced the CT muscle in both monkeys, 6V and the SMA contained many more output neurons in the marmoset than in the macaque. These results suggest that the increased relative contribution of these two premotor areas supports the complex vocal motor abilities of highly vocal primates.