Cerebral asymmetries in processing language and time

Abstract

Initially it was thought that only 'higher functions' are lateralized, but investigators have recently been noting very 'low-level' perceptual asymmetries. Some have speculated that these low-level asymmetrics underlie hemispheric specialization for higher functions such as language processing. Experiments 1 and 2 testing this hypothesis by administering tests of low-level temporal asymmetries in the visual (1) and auditory (2) modalities, concurrent with a dichotic-listening test of linguistic laterality. As predicted, individuals demonstrated significant left hemisphere advantages (LHAs) on both the visual and auditory temporal tasks, and in both cases, these LHAs correlated significantly with linguistic asymmetries. A recent theory by Ringo et al. (1994) claims that the evolutionary pressure favouring hemispheric specialization came from a lateralized system's relative superiority at processing stimuli requiring fine temporal precision. This theory would then predict that individuals with greater interhemispheric transmission times (IHTTs) would exhibit greater lateralization for time critical tasks such as language processing. Experiment 3 provided support for the prediction that longer IHTTs from the right to left hemisphere in the auditory modality are associated with greater left hemispheric specialization for linguistic perception. Experiment 4 tested two predictions. The first prediction, that preferred hand for throwing (but not preferred hand for writing) would be associated with linguistic lateralization, was only supported by individuals who normally write with their right hand. The second prediction, that complementarity of functional asymmetries should not be causal in nature, was also supported. There was a weak positive association between what are normally right and left hemispherically dominated tasks. Taken together, these results support the position that the brain is lateralized to facilitate temporal processing,