Left Hemisphere Specialization for Oro-Facial Movements of Learned Vocal Signals by Captive Chimpanzees

The left hemisphere of the human brain is dominant in the production of speech and signed language. Whether similar lateralization of function for communicative signal production is present in other primates remains a topic of considerable debate. In the current study, we examined whether oro-facial movements associated with the production of learned attention-getting sounds are differentially lateralized compared to facial expressions associated with the production of species-typical emotional vocalizations in chimpanzees.Still images captured from digital video were used to quantify oro-facial asymmetries in the production of two attention-getting sounds and two species-typical vocalizations in a sample of captive chimpanzees. Comparisons of mouth asymmetries during production of these sounds revealed significant rightward biased asymmetries for the attention-getting sounds and significant leftward biased asymmetries for the species-typical sounds.These results suggest that the motor control of oro-facial movements associated with the production of learned sounds is lateralized to the left hemisphere in chimpanzees. Furthermore, the findings suggest that the antecedents for lateralization of human speech may have been present in the common ancestor of chimpanzees and humans approximately 5 mya and are not unique to the human lineage

The measurement of handedness by preference and performance tests

Hand preference and hand proficiency were compared on 50 male subjects (30 self-classified left-handers and 20 self-classified right-handers) according to their responses to a 20 item questionnaire and their performances on a handwriting and a grip-strength task. Performance differences between the two sides for each group were not related to overall preference assessments but were positively associated with hand preferences on corresponding questionnaire items. Distributions of the performance differences between the two sides for the right- and left-handers were well separated for handwriting but completely overlapped for grip strength. Significant correlations between the performances on each side were recorded on both tasks. The results stress the importance of using the same criteria in making preference vs. performance comparisons and highlight the need to recognize and control the influence of previous training or experience