Evidence for hemispheric specialization in the marmoset (Callithrix penicillata) based on lateralization of behavioral/neurochemical correlations.

A correlative study between behavioral, neurochemical and hormonal measures was conducted on male black tufted-ear marmoset monkeys (Callithrix penicillata). Behavioral analysis was performed in order to examine the effects of confrontation with a natural predator (taxidermized oncilla cat, Felis tigrina). The subjects were subjected to four trials without predator, six confrontation trials with predator present, and four trials with the predator removed. Handedness was analyzed by the frequency with which they performed scratching, grooming and hanging behaviors with the left or right hands. The animals' brains were subjected to ex vivo neurochemical analysis of several structures from both hemispheres. The content of monoamines, acetycholine and metabolites were analyzed by HPLC-ED. Plasma levels of cortisol and adrenocorticotrophic hormone (ACTH) were analyzed by chemoluminescence immunoassay. Testosterone plasma concentration was determined by radioimmunoassay. Higher levels of dopamine and acetylcholine were detected in the right caudate/putamen, in comparison to the left. For the remaining areas, similar levels were observed in both hemispheres. A hand preference between and within the behaviors scored was not detected. However, correlative analyses revealed complex interactions between the behavioral and neurochemical measures, particularly in the left hemisphere. Lateralized correlations were found in relation to brain site, type of behavior, neurochemical parameter and treatment condition, thus providing evidence for functional brain asymmetries in this species. Interhemispheric comparisons of neurochemical/behavioral correlations appear to be a promising approach towards delineating hemispheric specialization of functions in this, and perhaps, other species.