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natural science summary(hemispheric spec.)

natural science summary(hemispheric spec.) - CU R RE N T D...

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Theoretical Speculations on the Evolutionary Origins of Hemispheric Specialization William D. Hopkins 1,2 and Claudio Cantalupo 3 1 Division of Psychobiology, Yerkes National Primate Research Center, Emory University, 2 Department of Psychology, Agnes Scott College, and 3 Department of Psychology, Clemson University ABSTRACT— Hemispheric specialization has long been considered a uniquely human trait associated with the evolution of language and handedness. Recent studies in a host of vertebrate species have reported evidence of population-level behavioral and brain asymmetries, challenging the claims for the uniqueness of hemispheric specialization to the human species. Here we summarize the findings in nonhuman species and discuss the adap- tive significance and potential costs of lateralization of function. Hemispheric specialization refers to sensory, motor, and cog- nitive abilities that are specialized to either the left or right cerebral hemisphere. Two of the most pronounced manifes- tations of hemispheric specialization in humans are right- handedness and left-hemisphere lateralization for language (Corballis, 2002). Although there is cultural variation, a strong tendency to right-handedness is a universal trait in humans (Annett, 2002). Moreover, most right-handed individuals are left-hemisphere dominant for language. The intrinsic link be- tween right-handedness and left-hemisphere dominance for language has historically led to the view that hemispheric spe- cialization is unique to humans. To put it simply, the typical evolutionary scenario went something like the following: Hu- mans are right-handed and only humans have language; animals do not have language and there is no evidence that animals show population-level behavioral asymmetries; therefore, right- handedness and left hemisphere specialization for language emerged together and are unique to human evolution, with no antecedents in the animal kingdom. In the past 30 years, the very anthropocentric view that hemispheric specialization is unique to humans has been challenged on both the behavioral and neurological level. For example, left-hemisphere asymmetries in the processing of species-specific signals have now been documented in birds, frogs, mice, rats, and nonhuman primates (Rogers & Andrew, 2002). Differential involvement of the cerebral hemispheres in the discrimination of visual stimuli on the basis of global con- figuration (which shows right-hemisphere advantage) or local features (which show left-hemisphere advantage) has been re- ported in birds and nonhuman primate species (Vallortigara & Rogers, 2005). Evidence of right-hemisphere bias in emotional processing has also been reported in many species, including toads, birds, rats, and nonhuman primates (Rogers & Andrew, 2002).
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