About 95% of right-handers and 75% of left-handers show left-hemispheric language dominance ( Bethmann et al., 2007), a feature that was widely thought to be uniquely human ( Corballis, 2009). In the present review, we argue that an interdisciplinary comparative approach, combining findings from psychology, biology, neuroscience, and genetics, provides a uniquely powerful tool in order to advance understanding of the ontogenetic and phylogenetic processes responsible for lateralization.įor example, one field of research in which the integration of findings from diverse animal species has influenced current views about evolution and development of human lateralization is the study of language lateralization. Unfortunately, there has never been a strong integration of research in humans and non-human animals in the field of hemispheric asymmetries, a circumstance that may be rooted in the assumption of human exceptionalism that dominated the field from early on ( Taylor et al., 2010). These discoveries yield tremendous possibilities regarding the employment of model species in order to investigate the ontogenesis and phylogenesis of human brain asymmetry. For example, chicks recognize familiar birds better with the left than with the right eye ( Vallortigara, 1992) and react faster to a predator approaching from the left than from the right side ( Vallortigara, 2006), while most species fish show a consistent tendency to turn preferentially to one side when facing an obstacle while fleeing from a predator ( Bisazza et al., 2000). Lateralization is highly relevant for animal behavior and possibly survival. Recent evidence for asymmetrical organization in only distantly related invertebrate species, ranging from Octopus vulgaris ( Byrne et al., 2002) to the honey bee Apis mellifera ( Rogers and Vallortigara, 2008 Frasnelli et al., 2010) and the nematode Caenorhabditis elegans ( Taylor et al., 2010) – just to name a few examples – revealed that lateralization is indeed not restricted to humans, but constitutes a fundamental principle of nervous system organization. In contrast to this view, left–right asymmetries of brain and behavior have now been observed in all vertebrate classes including mammals ( Corballis, 2009), birds ( Rogers, 2008 George, 2010 Güntürkün and Manns, 2010), reptiles ( Bisazza et al., 1998 Bonati et al., 2008, 2010 Csermely et al., 2010, 2011), amphibians ( Bisazza et al., 1998 Vallortigara, 2006), bony fishes ( Vallortigara and Rogers, 2005 Lippolis et al., 2009 Dadda et al., 2010a), as well as cartilaginous, and jawless fishes ( Concha and Wilson, 2001). After this initial discovery in the language system, hemispheric asymmetries were thought to be uniquely human. This result indicated for the first time that the left hemisphere is highly relevant for language production. Historically, the scientific exploration of hemispheric asymmetries started with a seminal paper by a French surgeon called Broca (1861), who described a patient called Monsieur Tan because the only syllable he was able to generate was “tan.” Post-mortem analysis of this massively speech-impaired patient’s brain revealed a large lesion in the left posterior inferior frontal gyrus, an area now known as Broca’s area. Several explanations for the emergence of hemispheric asymmetries have been given, including an enhancement of an individual’s ability to perform two different tasks at the same time ( Rogers et al., 2004), an increase in neural capacity due to an avoidance of unnecessary duplication of neural networks ( Vallortigara, 2006) and the greater speed of uni-hemispheric processing since no interhemispheric transfer via the corpus callosum is needed ( Ringo et al., 1994). In addition to these functional hemispheric asymmetries, anatomical differences between the two sides of the brain (e.g., in volume or size of a certain area), so-called structural hemispheric asymmetries, have can be found in a wide range of brain regions (e.g., Amunts, 2010). For example, most individuals show a right-hemispheric dominance for visuo-spatial processing (e.g., Vogel et al., 2003) and a left-hemispheric dominance for production and processing of language (e.g., Bethmann et al., 2007 Ocklenburg et al., 2011a). Relative functional differences between the left and the right side of the brain, so-called functional hemispheric asymmetries, have been observed for several cognitive functions ( Corballis, 2009). The two hemispheres of the human brain are not equivalent.
0 kommentar(er)
