Review

Cerebral Asymmetry and Language Development: Cause, Correlate, or Consequence?

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Science  14 Jun 2013:
Vol. 340, Issue 6138, 1230531
DOI: 10.1126/science.1230531

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Structured Abstract

Background

Most children learn language effortlessly, but a minority struggle to master their native tongue for no obvious reason. This is known as specific language impairment. Affected children often have trouble learning to read and may also be diagnosed with developmental dyslexia. These language and literacy impairments are highly heritable, but their neurobiological basis is poorly understood. A popular notion is that the usual pattern of left-sided brain lateralization for language is disrupted in some children, which leads to problems with language and literacy. Until recently, it was difficult to test this idea, because we lacked easy methods to assess cerebral asymmetry directly in children.

Embedded Image

Functional transcranial Doppler ultrasound allows imaging of blood flow to the left and right sides of the brain. Still image from a video. [CREDIT: Journal of Visualized Experiments; see Additional Resources]

Advances

Functional transcranial Doppler ultrasound has been developed as a method for assessing relative blood flow to the left and right cerebral hemispheres during a language-activation task. This technique has made it much easier to assess cerebral asymmetry directly and has confirmed the reduction of the usual left-sided language lateralization in specific language impairment and dyslexia. Studies performed with functional magnetic resonance imaging have provided converging evidence. The challenge now is to understand the nature of this association. Individual differences in cerebral lateralization are often assumed to be highly heritable, which leads to the hypothesis that genetic risk factors for language and literacy impairments may work by disrupting typical lateralization. In effect, reduced cerebral asymmetry would be an endophenotype for language and literacy impairment. In this Review, I argue that the evidence for the endophenotype account is unconvincing, not least because there is little support for strong genetic influences on individual differences in cerebral asymmetry. Research on this topic is in its infancy, and most of the relevant studies are underpowered, so it would be premature to conclude that genes play no role. Nevertheless, we need to question common scientific views that ignore evidence for a substantial effect of nongenetic influences on individual differences in brain lateralization. Furthermore, cerebral lateralization is not fixed from birth; it can change with age. Thus, it seems likely that atypical cerebral asymmetry in language and literacy impairments is at least partly a consequence of poor language development.

Outlook

We now have the tools available to examine lateralization of language processing in real time. Yet, we still do not know whether cerebral asymmetry is a unitary trait or a multidimensional construct, nor how it changes as language develops. Before we can grasp the opportunities presented by technological developments in neuroscience and genetics, we need basic research to clarify how best to conceptualize and reliably measure cerebral asymmetry. Only then will we be able to reconcile the paradoxical findings that those with language and literacy impairments have atypical cerebral lateralization, yet in the general population, most individuals with atypical cerebral lateralization have typical cognitive development.

Abstract

In most people, language is processed predominantly by the left hemisphere of the brain, but we do not know how or why. A popular view is that developmental language disorders result from a poorly lateralized brain, but until recently, evidence has been weak and indirect. Modern neuroimaging methods have made it possible to study normal and abnormal development of lateralized function in the developing brain and have confirmed links with language and literacy impairments. However, there is little evidence that weak cerebral lateralization has common genetic origins with language and literacy impairments. Our understanding of the association between atypical language lateralization and developmental disorders may benefit if we reconceptualize the nature of cerebral asymmetry to recognize its multidimensionality and consider variation in lateralization over developmental time. Contrary to popular belief, cerebral lateralization may not be a highly heritable, stable characteristic of individuals; rather, weak lateralization may be a consequence of impaired language learning.

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