BRAIN LATERALISATION

The view that one of the hemispheres of the brain has or develops a special responsibility for language. In most human beings, language appears to be particularly associated with the left hemisphere. Evidence comes from:

Brain damage. When the left hemisphere is damaged by an accident, a stroke or invasive surgery, it often has a serious impact upon the victim’s language.

Commisurotomy. In this operation, the corpus callosum joining the two hemispheres was severed to reduce the effects of epilepsy. Patients who underwent the operation could name objects in their right field of vision (connecting with the left hemisphere) but not those in their left field of vision.

Wada injections of sodium amytol were once used to de-activate one side of the brain prior to surgery, in order to ascertain which hemisphere was the dominant one for language. It was usually the left one.

Dichotic listening. When different messages are presented to the two ears, the right ear (the ear that links to the left hemisphere) usually overrides the left. The right ear appears to be dominant for speech generally but the left for music, rhythm and intonation.

There appears to be a partial relationship between lateralisation and handedness. Rather fewer left-handers than right-handers appear to have a dominant left hemisphere for language.

When damage occurs to the left hemisphere of the brain before the age of five, the victim’s powers of speech sometimes recover completely, and language dominance appears to re-establish itself in the right hemisphere. Some researchers have concluded that the language faculty may be generally distributed at birth, and that lateralisation occurs gradually as part of maturation. The hypothesis has been linked to the notion of a critical period. First language acquisition, it is argued, has to take place during the period of lateralisation if complete command of L1 is to be achieved. If acquisition is delayed, the result may be the inadequate mastery observed in cases of language deprivation.

However, the ‘plasticity’ hypothesis has been challenged by evidence that a degree of left-hemisphere lateralisation exists from birth. Furthermore, recent studies have identified cases of infants who have not recovered some aspects of their linguistic competence after left-brain damage, and of adults who seem to have recovered their language by relocating it to their right hemisphere.

 In addition, we now have more evidence about right-hemisphere damage. While damage to the left hemisphere affects primary language functions such as syntax and lexis, damage to the right appears to affect the processing of discourse. It also seems that, while phoneme-level processing is chiefly the prerogative of the left hemisphere, the processing of suprasegmental features, especially prosody, takes place on the right. One current view of the lateralisation issue is thus that the left hemisphere specialises in more rapid language operations, and the right in those connected to higher-level meaning processes. This division of functions makes sense when the brain has to deal with two closely associated processes such as processing phonemes (left side) and prosody (right side). It also accords with evidence from modern imaging techniques showing that language is much more widely distributed in the brain than the original lateralisation hypothesis supposed.

 It may be that the human brain is more flexible than once believed, even in adulthood when the critical period is long over. In the exceptional circumstances of simultaneous translation, some profes sionals appear to distribute their two operating languages between the two hemispheres– usually reserving the left for the native language and the right for the second. In this way, they keep their two languages apart and manage to cope with the lack of synchronicity between the incoming auditory signal and the outgoing translation. Again, the key seems to be the need to keep two closely associated operations distinct.

 It was once believed that lateralisation might be peculiar to human beings and afford an explanation of why language is unique to our species. However, an enlarged left hemisphere has been found to occur in other species, including birds. Frogs have a dominant hemisphere which appears to be associated with croaking. This suggests that, across species, the larger side of the brain has special functions related to vocalisation and the processing of rapid auditory stimuli.

See also: Brain: localisation, Critical period, Deprivation

 Further reading: Aitchison (1998); Deacon (1997); Dingwall (1998); Obler and Gjerlow (1999); Springer and Deutsch (1997)

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The encyclopaedic view of meaning

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