The beginning of study of cerebral organization of mental functions of left-handed people dates back to mid-19th century, when Paul Broca formulated rule that hemisphere that controls speech is at opposite side of a leading hand. In 1899 Bramwell described aphasia as a cross in a 36-year-old left-handed person with right-sided hemiparesis (Bramwell, 1899). Later on, it was discovered that there is no direct connection between handedness and hemispheric dominance in speech.The Wada test and dichotic listening are considered to be two main methods for differentiation of a dominant-in-speech hemisphere. Wexler and Halwes (1983) proved high test-retest reliability of dichotic listening method. Zatorre (1989) carried out dichotic listening on a sample of right- and left-handed patients who also took Wada test. The finding about speech laterality according to results of dichotic listening coincided with Wada test values in 36 of 38 individuals (95%). These empirical data give grounds for judging dichotic listening test to be highly accurate as an instrument for differentiating a dominantin-speech hemisphere.Until now, many contradictory facts have been gathered from results of dichotic listening with left-handers. For instance, according to findings of Kimura (1983) 50 percent of left-handers have left-hemispheric dominance in speech, and remaining 50 percent have right-hemispheric dominance. According to findings of Warrington and Pratt (1973) this proportion is 75% and 25%, correspondingly. On analyzing findings of Hecaen and Sauguet (1971), Kinsbourne (1988) reported that 70 percent of left-handers have bilateral representation of speech, and most of remaining 30 percent have left-hemispheric dominance. Moffat, Hampson, and Lee (1998) reported that, according to their research, 54 percent of left-handers have left-hemispheric dominance in speech, and 46 percent have right-hemispheric dominance. Dos Santos Sequeira and colleagues (2006) showed exactly opposite results (left-hemispheric dominance in speech in 46.4% of left-handers, and right-hemispheric dominance in remaining 53.6%of left-handers). For their part, 64% of right-handers have left-sided speech control, and 36% have right-sided speech control according to this research.Kimura explains her findings in following way. When two diverse stimuli are introduced in different auditory canals, difference in capacity of ducts increases so much that transmission in ipsilateral canal is suppressed (Kimura, 1961). This supposition explains right ear's advantage. In his study Kotik (1975) comes to following conclusion: both left and right hemispheres of brain take part in audioverbal processes, particularly in process of perception and reproduction of dichotic verbal stimuli. Thus, both hemispheres are capable of coding audioverbal information. However, tracking in a subdominant hemisphere has its own particular features; in case of dichotic perception, verbal stimuli form the track--that is, acoustic iconic figure in right hemisphere. When transmitting to left hemisphere this track is subject to linguistic encoding, and it can be reproduced. At same time, stimuli coming into right ear and transmitted immediately to left hemisphere have a shorter transmission path and, probably, this path is shorter on one link of encoding. Thus, one can suppose that stimuli perception is equally efficient in both auditory canals, but hemispheres are not equal in process of reproduction. This phenomenon creates an advantage for right ear (Kotik, 1974).When results of dichotic listening in left-handers who do not have any left-handedness in their family were analyzed, advantage of right ear appeared, but there was no distinction in stimuli perception by right and left ears within hereditary left-handers. …
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Dec 30, 2015
John M Hudson + 1
Open Access
