Brain Lateralization: A Comparative Perspective

Comparative studies on brain asymmetry date back to the 19th century but then largely disappeared due to the assumption that lateralization is uniquely human. Since the reemergence of this field in the 1970s, we learned that left-right differences of brain and behavior exist throughout the animal kingdom and pay off in terms of sensory, cognitive, and motor efficiency. Ontogenetically, lateralization starts in many species with asymmetrical expression patterns of genes within the Nodal cascade that set up the scene for later complex interactions of genetic, environmental, and epigenetic factors. These take effect during different time points of ontogeny and create asymmetries of neural networks in diverse species. As a result, depending on task demands, left- or right-hemispheric loops of feedforward or feedback projections are then activated and can temporarily dominate a neural process. In addition, asymmetries of commissural transfer can shape lateralized processes in each hemisphere. It is still unclear if interhemispheric interactions depend on an inhibition/excitation dichotomy or instead adjust the contralateral temporal neural structure to delay the other hemisphere or synchronize with it during joint action. As outlined in our review, novel animal models and approaches could be established in the last decades, and they already produced a substantial increase of knowledge. Since there is practically no realm of human perception, cognition, emotion, or action that is not affected by our lateralized neural organization, insights from these comparative studies are crucial to understand the functions and pathologies of our asymmetric brain.

Left Nipple Preferences in Infant Pan paniscus and P. troglodytes

We investigated laterality in nipple preference in 16 mother-infant dyads, 8 bonobos, and 8 chimpanzees via an event sampling procedure in infants 5-55 mo of age. Both bonobo and chimpanzee infants preferred the left nipple, and the results were stable over time. Maternal age, experience, and rearing history did not influence laterality in offspring nipple preferences. We discuss the results in the context of the role of early mother-infant interactions on the development of laterality in primates.

Comparative and familial analysis of handedness in great apes

Historically, population-level handedness has been considered a hallmark of human evolution. Whether nonhuman primates exhibit population-level handedness remains a topic of considerable debate. This paper summarizes published data on handedness in great apes. Comparative analysis indicated that chimpanzees and bonobos show population-level right handedness, whereas gorillas and orangutans do not. All ape species showed evidence of population-level handedness when considering specific tasks. Familial analyses in chimpanzees indicated that offspring and maternal (but not paternal) handedness was significantly positively correlated, but this finding was contingent upon the classification criteria used to evaluate hand preference. Overall, the proportion of right handedness is lower in great apes compared with humans, and various methodological and theoretical explanations for this discrepancy are discussed.

Assessment of hand preference in two language-trained chimpanzees (Pan troglodytes): a multimethod analysis

The nonhuman primate literature concludes that monkeys and apes do not exhibit handedness preferences at the population level. This discrepancy between human and nonhuman primate findings may be due to differences in the methods of assessment of handedness, lack of reliability between various measures of handedness, and a paucity of studies involving great apes. This paper presents the results of extensive hand preference studies with two language-trained chimpanzees (Pan troglodytes). Both naturalistic and experimental paradigms are described. In particular, various methodologic factors involved in handedness assessment, including the use of a hand preference measure previously used with human subjects were examined. Both chimpanzees exhibited a right-hand preference for fine motor tasks requiring manual dexterity.

Hand preferences in the skilled gathering tasks of mountain gorillas (Gorilla g. berengei)

Manual dexterity of 44 wild gorillas of all ages and both sexes was investigated on six naturally acquired feeding tasks of varied logical structure, which included multi-stage and bimanually coordinated procedures. At least 400 min observation of feeding per animal, and analysis at the level of bouts rather than individual actions, can be expected to produce statistically robust results; 22 years background data allowed effects of genealogy and injury to be investigated. Five tasks elicited very strong hand preferences in most animals (weakest on the simplest task), while one was usually performed with a strategy in which left and right hands were used symmetrically; the preferences were fully established at 3 years old. Preferences were highly correlated within each of two sets of tasks, but between the two sets there was no correlation across animals in direction or strength of preference. No population trends towards left or right handers were found in either set of tasks, or both sets pooled; distributions were U-shaped and approximately symmetric, with a slight bias towards right-handed fine manipulation in one set of tasks. Processing efficiency was only slightly greater with the preferred hand, and similar in left- and right-handed animals. Neither direction nor strength of hand preference showed a tendency to run in families, but females showed greater strength of preference. Major injury masked strong hand preference, but injuries could not account for the overall distribution of preference; instead this may reflect inbreeding. Feedback acting on initially random hand choices, and imitation at the "program" level rather than slavish copying of acts, are proposed to account for the results.

Spontaneous hand usage and handedness in a troop of baboons

Hand usage was studied in a troop of 18 Guinea baboons (10 adults and 8 young) for spontaneous activities. Handedness was determined by an analysis of unimanual activities and bimanual asymmetric activities. The distribution of preferential biases gave 5 right-handers and 2 left-handers, other subjects being ambidextrous. Main effects were age related: the strength of the preference was greater for adults than for young; moreover, bimanual activities performed by the adult group were more asymmetric than those realized by the subgroup of young. Laterality thus appears to develop during ontogeny in baboons.