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It has been suggested in humans that sex and hand preference may affect the size of the corpus callosum (CC). The extant literature in humans suggests that females and non-right handed individuals exhibit a larger CC due to a lesser degree of lateralization, or hemispheric specialization for certain tasks. To test whether similar sex and handedness effects are present in chimpanzees, we measured CC morphology from magnetic resonance images (MRI) in a sample of 70 captive chimpanzees. Hand preference was assessed by performance on two tasks. The chimpanzees were classified as left-handed, right-handed, or ambidextrous based on their consistency of hand use across the two tasks. The results revealed no sex difference in CC area but significant effects for hand preference were found with left-handed chimpanzees having a significantly smaller CC area than ambidextrous and right-handed chimps. Regarding strength of hand preference, the chimpanzees were classified as consistently handed (CH) or mixed preferent (MP) based on their preferences for the two-handedness tasks. Subsequent analyses revealed an interaction between sex and strength of handedness. CH males had smaller CC areas than CH females, while MP males had larger CC areas than MP females. Overall, we found results both consistent and inconsistent with the human literature to date.
The corpus callosum (CC), the main structure connecting the brain hemispheres, has been linked to functional and structural lateralization. In humans, research suggests that females2 have a larger CC than males. In addition, within males it has been reported that non-consistently right-handed individuals have a larger CC area than consistently handed individuals.3, 6 In the great apes,1 a larger CC in females has been reported, but one limitation of that study was that the sample size was small (n = 15) and was comprised of three different genuses. In addition, to date no comparative studies have assessed the effect of handedness on CC morphology in non-human primates. Using two measures of chimpanzee handedness4, 5 and MRI analysis, we tested whether or not sex and hand preference are associated with the size of the CC in chimpanzees. Given the human literature on this topic and the assumption that CC morphology is derived from biological factors, we hypothesized that female chimpanzees would have a larger CC than males and inconsistently handed males would show larger CC areas than consistently handed males.
Seventy chimpanzees (32 males and 38 females) from Yerkes National Primate Research Center were included. MRI images were taken with a Phillips, Model NT (1.5-Tesla) scanner on anesthetized chimpanzees (n = 49) or cadaver brains (n = 21).
MRI Analysis
Using ANALYZE 6.0, the midsagittal CC area of each subject was measured. The region of interest function was used to divide the CC into the seven sub-regions described by Witelson.6 Each region was traced using a mouse driven pointer and an area measure was obtained. Brain volume was derived using an automated segmenting program within ANALYZE.
Measures of Handedness
Two handedness measures referred to as bimanual feeding and the TUBE task were used to determine hand preference. The procedures for testing have been described elsewhere.4,5 Chimpanzees showing a right or left hand preference on both tasks were classified as right-handed (n = 26) and left-handed (n = 19). Those preferring the left hand on one task and the right hand on the other were classified as ambidextrous (n = 16). In terms of strength of hand preference, the right- and left-handed individuals were combined to form one group referred to as consistent. The ambidextrous subjects were considered mixed preferent.
Dependent Measures
Because CC size co-varies with the size of the brain, we regressed brain size on the overall and regional CC area measures and saved the residual error values. These values reflected the degree to which CC’s were larger or smaller than predicted for an individual of that brain size. Residual values could be positive or negative with positive values indicating CC’s larger than predicted while negative values reflect CC’s smaller than predicted for a given brain size. The absolute values of the residuals reflect the magnitude of variation.
Left-handed subjects have a significantly smaller overall CC area than ambidextrous and right-handed individuals (F(2,54) = 5.88, p < .005).
Sex = strength of handedness interaction on overall CC size (F(1,57) = 4.20, p < .05). Males and females show opposite patterns of CC morphology in relation to strength of handedness.
There were significant interactions between sex and strength of hand preference for regions 3 (F(1,54) = 8.73, p < .005) and 4 (F(1,54) = 12.60, p < .001) of the CC. In both cases, males and females show opposite patterns much like the overall CC.
In contrast to the human literature,3, 6 we found that left-handed chimpanzees had a smaller total CC than both ambidextrous and right-handed subjects. One interpretation of these findings is that there are species differences in the relationship between CC morphology and handedness between chimpanzees and humans. Alternatively, the differences might reflect factors associated with the measures of hand use.
In accordance with the human literature,3, 6 more consistently handed males have a smaller overall CC than females. Moreover, these results were specific to regions 3 and 4 of the CC, areas that connect the premotor, supplementary motor, and primary motor areas of the two hemispheres.6
The strength of handedness results suggest that in chimpanzees, males have more lateralized brains than females and this may have functional consequences for plasticity and organization of the central nervous system in primates. Additional research involving other behavioral and cognitive measures of lateralization should be considered to further test this hypothesis.
Funding Attributions This research was supported by Howard Hughes Medical Institute Grant No. 52003727 and NIH grant NS-36605.
1. de Lacoste, M.C. & Woodward, D. (1988). The corpus callosum in nonhuman primates: Determinants of size. Brain, Behavior, and Evolution, 31, 318-323.
2. de Lacoste-Utamsing, C. & Holloway, R.L. (1982). Sexual dimorphism in the human corpus callosum. Science, 216, 1431-1432.
3. Habib, M., Gayraud, D., Olivia, A., Regis, J., Salamon, G., & Khalil, R. (1991). Effects of handedness and sex on the morphology of the corpus callosum: A study with brain magnetic resonance imaging. Brain and Cognition, 16, 41-61.
4. Hopkins, W. D. (1994). Hand preference for bimanual feeding in 140 captive chimpanzees (Pan troglodytes): Rearing and ontogenetic determinants. Developmental Psychology, 27, 395-407.
5. Hopkins, W. D. (1995). Hand preferences for a coordinated bimanual task in 110 chimpanzees (Pan troglodytes): Cross-sectional analysis. Journal of Comparative Psychology, 109, 291-297.
6. Witelson, S.F. (1989). Hand and sex differences in the isthmus and genu of the human corpus callosum: A postmortem morphological study. Brain, 112, 799-835.
In the brain there is a structure that connects the right and left halves. It is called the corpus callosum and it helps the two halves communicate processes to one another. Some research indicates that the corpus callosum is affected by the process of lateralization which is when one half of the brain takes over all processes associated with a certain task, such as language. The researchers have found that when an organism is more lateralized, that is, processes have localized in one hemisphere, they have a smaller corpus callosum because the hemispheres do not need to communicate together as much since one is doing all of the work for that function. The hypothesis are that women and people who don't always use their right hand are less lateralized, so they will have a bigger corpus callosum. Our study used chimpanzees to test the effects of sex and hand preference on corpus callosum size. The chimpanzees' hand preferences were measured by two tasks where we watched the hands they feed with. MRI scans were also taken of the chimpanzees so that we could measure their corpus callosum which is in the middle of the brain. By dividing, tracing, and measuring the corpus callosum, we were able to get an overall area for the corpus callosum. What we found is that male and female chimpanzees do not differ in overall size of the corpus callosum. We did find, however, that left-handed chimpanzees have the smallest corpus callosum areas. This is interesting because, according to the human literature, they should have larger areas than the right handed chimpanzees. We also found that male chimpanzees who consistently prefer one hand over another (consistently use their right or consistently use their left) have smaller corpus callosum areas than females who consistently use one hand. The males who show no preference for one hand over the other have a larger corpus callosum than the females who show no preference. The most amazing part is that these results are most pronounced in two particular parts of the corpus callosum which research indicates connects to the motor areas of the brain.
I used the MRI analysis program ANALYZE 6.0. Within the program, I used a region of interest function to locate the midsagittal brain slice and then used placer, divider, and tracer functions to create seven sub-regions and to obtain area measurements.
corpus callosum, handedness, chimpanzee, sexual dimorphism, magnetic resonance imaging (MRI)
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