Claudio Cantalupo

Asymmetric Broca's area in great apes

Brodmanns area 44 delineates part of Brocas area within the inferior frontal gyrus of the human brain and is a critical region for speech production, being larger in the left hemisphere than in the right — an asymmetry that has been correlated with language dominance. Here we show that there is a similar asymmetry in this area, also with left-hemisphere dominance, in three great ape species (Pan troglodytes, Pan paniscus and Gorilla gorilla). Our findings suggest that the neuroanatomical substrates for left-hemisphere dominance in speech production were evident at least five million years ago and are not unique to hominid evolution.

Handedness in chimpanzees (Pan troglodytes) is associated with asymmetries of the primary motor cortex but not with homologous language areas.

The neurobiology of hand preferences in nonhuman primates is poorly understood. In this study, the authors report the first evidence of an association between hand preference and precentral gyrus-morphology in chimpanzees (Pan troglodytes). Hand preferences did not significantly correlate with other asymmetric brain regions associated with language functions in humans including the planum temporale and frontal operculum. The overall results suggest that homologous regions of the motor cortex control hand preferences in humans and apes and that these functions evolved independently of left-hemisphere specialization for language and speech.

Individual and setting differences in the hand preferences of chimpanzees (Pan troglodytes): A critical analysis and some alternative explanations

Several recent papers have been critical at a theoretical and empirical level of the evidence of population-level right-handedness in chimpanzees and other great apes. For example, Palmer (2002) has recently argued that the evidence of population-level handedness in chimpanzees is weak because there are sampling biases in the data. McGrew and Marchant (1997) argue that all the evidence of right-handedness in apes is from captive animals and therefore the observed phenomenon has little ecological validity. In this paper, we address recent issues regarding the presentation and interpretation of other hand preference data and argue that chimpanzees are right-handed for some measures. We further argue that purported differences in hand use between wild and captive chimpanzees due to rearing environments are unfounded and we emphasise that more cooperative work between researchers working in captive and feral populations is needed to facilitate collection of data on common measures of hand preference.

Grip Morphology and Hand Use in Chimpanzees (Pan troglodytes): Evidence of a Left Hemisphere Specialization in Motor Skill

Three experiments on grip morphology and hand use were conducted in a sample of chimpanzees. In Experiment 1, grip morphology when grasping food items was recorded, and it was found that subjects who adopted a precision grip were more right-handed than chimpanzees using other grips. In Experiment 2, the effect of food type on grasping was assessed. Smaller food items elicited significantly more precision grips for the right hand. In Experiment 3, error rates in grasping foods were compared between the left and right hands. Significantly more errors were made for the left compared with the right hand. The cumulative results indicate that chimpanzees show a left-hemisphere asymmetry in motor skill that is associated with the use of precision grips.

Neuroanatomical Correlates of Handedness for Tool Use in Chimpanzees (Pan Troglodytes) Implication for Theories on the Evolution of Language

It has been hypothesized that cognitive mechanisms underlying lateralized complex motor actions associated with tool use in chimpanzees may have set the stage for the evolution of left-hemisphere specialization for language and speech in humans. Here we report evidence that asymmetries in the homologues to Brocas and Wernickes areas are associated with handedness for tool use in chimpanzees. These results suggest that the neural substrates of tool use may have served as a preadaptation for the evolution of language and speech in modern humans.

Chimpanzees are right‐handed when recording bouts of hand use

Whether nonhuman primates exhibit population‐level handedness remains a topic of considerable debate. Previous research has shown that chimpanzees are right‐handed when frequencies of hand use are recorded but some have questioned the validity of this approach. In this study, we evaluated handedness in 180 captive chimpanzees for a task measuring bimanual actions. Bouts rather than frequency of hand use were recorded in each subject. Population‐level right‐handedness was found using both continuous and nominal scales of measurement. Neither sex nor rearing history had a significant effect on hand use. These results indicate that chimpanzees are right‐handed, even when using a more conservative measure of handedness. Limitations in the use of bouts in handedness assessment are also discussed.

Further evidence for mirror-reversed laterality in lines of fish selected for leftward or rightward turning when facing a predator model.

In the teleost fish Girardinus falcatus eye preferences for inspecting a potential predator is highly heritable and this consented to select lines with opposed laterality. In previous studies individuals from a RD line (rightward turning when facing a dummy predator) and those from a LD line (leftward turning) were subjected to several other laterality tests (most of which, possibly all, were visually based). Since they obtained opposite scores in all tests, it was suggested that LD and RD fish have complete mirror-reversed organizations of the brain. Here, we studied fish from selected lines in a new set of laterality tests including some non-visual tests. They included measures of (1) rotational preference in the home tank (2) direction of spontaneous swimming in the dark (3) escape trajectories after delivery of an auditory stimulus (4) escape turning direction to a fast approaching visual stimulus. The results of the first three tests are congruent with the finding of previous studies in that fishes of the two lines showed opposite direction of lateralisation. When tested for laterality in the escape response to a fast approaching stimulus, fish of the LD and RD lines showed no differences and both were biased toward leftward escape. Overall these results suggest the existence in G. falcatus of a single mechanism controlling a co-ordinate placement of the great majority of lateralised functions. Yet the results of the fourth test suggest that a few lateralized functions are controlled by different mechanisms that were unaffected by artificial selection.

Origins and functions of laterality: interactions of motoric systems.

To evaluate lateral motoric bias in response systems at different levels of the neuraxis and assess the extent of interaction between these levels in the small-eared bushbaby (Otolemur garnettii), 27 animals were tested for lateral bias in hand use and whole-body turn bias in two postural conditions. Subjects retrieved mealworms quadrupedally by reaching downwards into glass jars and bipedally by reaching upwards to baited straws. Eye bias was assessed separately. Behaviours were scored from videotape. Two subgroups were identified: SHIFTERS changed hand preference with posture and had correlations of hand/eye bias in quadrupedal posture and of hand/turn bias, with more bimanual reaching, in bipedal posture; NONSHIFTERS were consistent in hand preference and more strongly lateralised in reach and turn than SHIFTERS. Subgroups did not differ in reach efficiency. Results are interpreted to support the value of the analysis of motoric levels and their interactions in the study of the evolution of laterality. Assuming natural selection for coordinated and targeted behaviours to be the source of lateralisation, several proposals in support of a motoric theory of laterality origins and functions are advanced.

Asymmetries in the hippocampus and amygdala of chimpanzees (Pan troglodytes).

Magnetic resonance imaging was used to measure the hippocampal and amygdalar volumes of 60 chimpanzees (Pan troglodytes). An asymmetry quotient (AQ) was then used to calculate the asymmetry for each of the structures. A one-sample t test indicated that there was a population-level right hemisphere asymmetry for the hippocampus. There was no significant population-level asymmetry for the amygdala. An analysis of variance using sex and rearing history as between-group variables showed no significant main effects or interaction effects on the AQ scores; however, males were more strongly lateralized than females. Several of these findings are consistent with results found in the human literature.

Handedness for Tool Use Correlates With Cerebellar Asymmetries in Chimpanzees (Pan troglodytes)

Recent studies have shown that great ape species possess patterns of macrostructural neocortical asymmetries that are similar to those found in humans. However, little is known about the asymmetry of subcortical structures in great apes. To address this lack of data, the authors assessed left-right asymmetry of the anterior and posterior aspects of cerebellum from MRI brain scans of 53 chimpanzees (Pan troglodytes). No population-level bias was found for either the anterior or the posterior region of the cerebellum. However, a significant inverse association was found in the asymmetry quotients of the anterior and posterior regions, indicating that the cerebellum was torqued at the individual level. Additionally, handedness for tool use but not other measures was associated with variation in cerebellar asymmetries. Last, older chimpanzees had a smaller cerebellum after brain volume was adjusted for. The results are discussed in the context of brain changes in primate evolution related to tool use.