Resumo

Objectives: Primate species often rely on vocal communication to mediate social interactions. Although much is known about the auditory pathway responsible for processing species-specific vocalizations in the primate brain, the same cannot be said about the neural control of the vocal pathway. Our aim was to map the brain areas involved with vocal control in a New World primate species, the common marmoset, by means of immediate-early gene expression (egr-1, a.k.a. zif-268, krox-24, NGFI-A and ZENK) in association with a controlled yet naturalistic behavioral protocol. Methods: Six adult male common marmosets were housed in sound-attenuating cages and stimulated with playbacks of freely recorded natural long distance vocalizations (phee calls; 45 min total duration); a non-stimulated animal was used as a negative control (C). Animals that spontaneously vocalized in response to the playback comprised the "hearing and vocalizing" group (H/V, n=3), while animals that remained silent during the playback composed the "hearing only" group (H/n, n=3). After the stimulation (or no stimulation in the case of the C animal), marmosets were perfused with phosphate-saline buffer and 4% paraformaldehyde. The tissue was frontally sectioned at 20 μm in a cryostat and submitted to immunohistochemistry for the Egr-1 protein. Results: We found that areas previously reported to be related to vocal control in Old World primates, namely the anterior cingulate cortex (ACC), the dorsomedial prefrontal cortex (DMPFC) and the ventrolateral prefrontal cortex (VLPFC), presented a much more intense pattern of Egr-1 immunoreactivity in the group H/V, when compared to the animals H/n and C. Conclusions: The role of the ACC and the DMPFC seems to be related to the voluntary initiation of vocal output. Activation of the VLPFC seems to be the most interesting with regard to the evolution of human language. The VLPFC corresponds in the marmoset brain to the 12/45 area, which encompasses vocal and visual areas (J Comp Neurol. 495(2):149, 2006). The cytoarquitecture of homologous areas in other primates, such as macaques, chimpanzees and humans (Eur J Neurosci. 16(2):291, 2002), suggests a progressive evolution towards refined vocal control. Our results push the debate on the evolution of language one step back in the primate branch, namely to the New World monkeys.