Saturday, April 25, 2009

Language Areas of the Brain

There seems to be a general agreement that human language is a recent evolutionary adaptation. As such, one would expect to find that the human brain has undergone some sort of task reorganization to accommodate this new function (Worden 1998:150). While some researchers state that brain reorganization would have been a slow process (i.e. Aitchison 1998:22; Worden 1998:148), others have argued that language emerged when humans suddenly discovered an extra use for their increasingly complex brains (i.e. Bickerton 1998; Gould 1987; Chomsky 1968). According to Müller (1996:629), the human brain is larger than that of other primates and so has more interconnections, however some qualitative adaptations have taken place. For example, Broca’s area, traditionally associated with speech production, is now regarded as a cover term for several overlapping areas which involve different abilities (ibid.).

Most studies and theories of language origin focus upon specific language ‘areas’ of the brain which appear to control grammar and syntax (Kim et. al. 1997; Locke 1997; Stiles et. al. 1981). The consensus in this research indicates that the left hemisphere (LH) is specialized for most aspects of language (ibid.). Specifically, the left frontal area shows specialization for expressive language (Broca’s hypothesis), and the left temporal area appears to be specialized for receptive language (Wernicke hypothesis) (Loritz 1999; Locke 1997; Müller 1996). However, the right hemisphere (RH) appears to regulate the comprehension and production of humor, metaphor and idioms (ibid.). Additionally, the RH seems to control the cohesion and coherence in narratives (Stiles et. al. 1981).

It is often assumed that the (adult) pattern of brain organization, for higher cognitive functions, is a result of ‘innate’ properties (Müller 1996; Stiles et. al. 1981). While the basic auditory/vocal production mechanisms employed by speech can be ‘mapped’ through evolutionary history, the evolution of the language ‘area’ is still questionable (Loritz 1999). There is some evidence that this language area may have emerged from both the perceptual and motor systems (Barton 2000; Worden 1998). For example, the various ‘subdivisions’ of Broca’s area and Wernicke’s area, in addition to carrying out linguistic functions, appear to participate in the planning and execution of one or more non-speech specific task (Loritz 1999; Tirassa 1999).

However, there are several instances of functional plasticity (Loritz 1999; Pinker 1999, 1994; Stiles et. al. 1981): 1) while it is estimated that the LH plays a dominant role in the mediation of language in 95-98% of normal individuals, 19% of left-handers have RH language control; 2) normal adults have homologous areas of activation on both sides of the brain in many language tasks, although activation is typically greater in the LH; 3) this pattern can appear in either hemisphere after an early brain injury; and 4) children with various hemispheric brain lesions outperform adults with homologous injuries.

It appears that in the acquisition and development of the linguistic system, children draw on a broader array of brain structures. However, the brain mechanisms responsible for language learning are not the same mechanisms which govern the maintenance and fluent use of language in normal adults (Stiles et. al. 1981:142). Learning what a word means for the first time requires that the child to put together information from many different sources (i.e. auditory signal, the visual and tactile properties of the object, social emotional cues). This learning process may recruit brain areas which are no longer needed once the learning itself is complete. Regardless, the majority of research indicates that the left temporal lobe is of major importance to the emergence of the LH specialization for language, under normal conditions.

In contrast, one would assume that the ability to perform spatial tasks would be a much older evolutionary adaptation. Studies of children with RH and LH lesions indicate that they make the same types of spatial errors as adults with similar lesions (Stiles et. al. 1981:146). Thus, it appears that these spatial mechanisms are ‘hardwired’ in specific brain regions. In general, researchers tend to separate visual, spatial and affect recognition (and production) from language development. It would seem that living in large social groups, coupled with a long period of development to the adult stage, enabled the development of linguistic manipulation, above and beyond that of simple verbal communication. As the individual gains social experience over time, specific areas of the brain, predominantly in the LH, become specialized for processing linguistic functions. However, it must be noted that these are not simply ‘fixed’ locations, since there is evidence of plasticity to compensate for early, developmental, injuries to either brain hemisphere.