Activation of frontal neocortical areas by vocal production in marmosets

Abstract

Primates often rely on vocal communication to mediate social interactions. Although much is known about the acoustic structure of primate vocalizations and the social context in which they are usually uttered, our knowledge about the neocortical control of audio-vocal interactions in primates is still incipient, being mostly derived from lesion studies in squirrel monkeys and macaques. To map the neocortical areas related to vocal control in a New World primate species, the common marmoset, we employed a method previously used with success in other vertebrate species: Analysis of the expression of the immediate early gene Egr-1 in freely behaving animals. The neocortical distribution of Egr-1 immunoreactive cells in three marmosets that were exposed to the playback of conspecific vocalizations and vocalized spontaneously (H/V group) was compared to data from three other marmosets that also heard the playback but did not vocalize (H/n group). The anterior cingulate cortex, the dorsomedial prefrontal cortex and the ventrolateral prefrontal cortex presented a higher number of Egr-1 immunoreactive cells in the H/V group than in H/n animals. Our results provide direct evidence that the ventrolateral prefrontal cortex, the region that comprises Broca's area in humans and has been associated with auditory processing of species-specific vocalizations and orofacial control in macaques, is engaged during vocal output in marmosets. Altogether, our results support the notion that the network of neocortical areas related to vocal communication in marmosets is quite similar to that of Old world primates. The vocal production role played by these areas and their importance for the evolution of speech in primates are discussed.

Keywords: Egr-1; auditory; broca; immediate early gene; neocortex; primate; speech; vocalization.

Figure 1

Methods. (A) Experimental design. (B) Sonograms of a phee call (left) and a twitter call (right). (C) Cortical section from an H/V animal, immunoreacted for Egr-1. Notice the labeled cells with typical nuclear staining; scale bar = 100 μm.

Figure 2

Neuroanatomical location of the cortical areas investigated and positioning of the regions of interest (ROIs) according to cortical depth. (A–C) auditory cortex. (D–F) anterior cingulate cortex. (G–I) dorsomedial prefrontal cortex. (J–L) ventrolateral prefrontal cortex. (A, D, G, and J) Lateral view of the brain (left) showing the location of each area (red) and the level at which the brain was sectioned coronally for histological analysis (vertical line; A + 5.5 mm for AC; A + 13.8 mm for ACC, DMPFC, and VLPFC). The schematic sections (right) depict the coronal view of each area. (B, E, H, and K) Outline of a coronal section at the same coordinate as in A, D, G, and J respectively, showing the placement of the ROIs (red boxes). (C, F, I, and L) Nissl-stained samples of brain sections corresponding to the dashed box in B, E, H, and K. The ROIs were positioned perpendicularly with respect to the pial surface, so as to assess Egr-1 levels at different cortical depths.

Figure 3

Representative samples of each cortical area investigated. Brain sections of one H/V and one H/n animal were immunoreacted in the same batch for the Egr-1 protein. Scale bar = 500 μm.

Figure 4

Box plots of the counts of immunoreactive cells per ROI, comprising all ROIs of all animals in each group, normalized by the total number of labeled cells per immunoreaction batch, and separated by group for each cortical area investigated.

Figure 5

Egr-1 expression in the AC. (A) Box plots of the raw counts of immunoreactive cells per ROI as a function of cortical depth in the H/V animals, * indicates significant differences (p < 0.05, Mann–Whitney tests with Bonferroni correction) (B) Maps of average (H/V)/(H/n) ratios of normalized values of immunoreactive cells. (C) Box plots of the raw counts of immunoreactive cells per ROI in each hemisphere of H/V animals (Mann–Whitney test).

Figure 6

Egr-1 expression in the ACC. (A) Box plots of the raw counts of immunoreactive cells per ROI as a function of cortical depth in the H/V animals (Mann–Whitney tests with Bonferroni correction). (B) Maps of average (H/V)/(H/n) ratios of normalized values of immunoreactive cells. (C) Box plots of the raw counts of immunoreactive cells per ROI in each hemisphere of H/V animals, ** indicates p < 0.01, *** indicates p < 0.001 (Mann–Whitney test).

Figure 7

Egr-1 expression in the DMPFC. (A) Box plots of the raw counts of immunoreactive cells per ROI as a function of cortical depth in the H/V animals, * indicates p < 0.05, ** indicates p < 0.01 (Mann–Whitney tests with Bonferroni correction) (B) Maps of average (H/V)/(H/n) ratios of normalized values of immunoreactive cells. (C) Box plots of the raw counts of immunoreactive cells per ROI in each hemisphere of H/V animals, ** indicates p < 0.01 (Mann–Whitney test).

Figure 8

Egr-1 expression in the VLPFC. (A) Box plots of the raw counts of immunoreactive cells per ROI as a function of cortical depth in the H/V animals, * indicates p < 0.05, ** indicates p < 0.01, *** indicates p < 0.001 (Mann–Whitney tests with Bonferroni correction). (B) Maps of average (H/V) (H/n) ratios of normalized values of immunoreactive cells. (C) Box plots of the raw counts of immunoreactive cells per ROI in each hemisphere of H/V animals, ** indicates p < 0.01, *** indicates p < 0.001 (Mann–Whitney test).