Language Development in Children with Unilateral Brain Injury

Aphasia (defined as the loss or impairment of language abilities following acquired brain injury) is strongly associated with damage to the left hemisphere in adults. This well-known finding has led to the hypothesis that the left hemisphere is innately specialized for language, and may be the site of a specific "language organ". However, for over a century we have known that young children with left-hemisphere damage (LHD) do not suffer from aphasia, and in most studies do not differ significantly from children with right-hemisphere damage (RHD). This result provides strong evidence for plasticity, i.e., brain reorganization in response to experience, and constitutes a serious challenge to the language organ hypothesis. This chapter reviews the history of research on language outcomes in children vs. adults with unilateral brain injury, addressing some discrepancies in the literature to date, including methodological confounds that may be responsible for those discrepancies. It also reviews recent prospective studies of children with unilateral injury as they pass through the first stages of language development. Prospective studies have demonstrated specific correlations between lesion site and profiles of language delay, but they look quite different from lesionsymptom correlations in adults, and gradually disappear across the course of language development. The classic pattern of brain organization for language observed in normal adults may be the product rather than the cause of language learning, emerging out of regional biases in information processing that are relevant for language, but only indirectly related to language itself. If those regions are damaged early in life, other parts of the brain can emerge to solve the language learning problem. Aphasia, or the loss of language abilities following brain injury, has been studied systematically in adults for over a century, and its existence has been docu-mented since the first Egyptian surgical papyrus more than 4000 years ago (Goodglass, 1993; O'Neill, 1980). There is now a large body of research on adult aphasia, and although there is still substantial controversy regarding its nature and causes, consensus has emerged on at least two points: injuries to the left hemisphere are overwhelmingly more likely to cause aphasia than injuries to the right, which in turn suggests that the left hemisphere plays a privileged role in language processing by normal adults. The second conclusion has been independently confirmed in the 20th century by methods ranging from sodium amytal (WADA) tests and/or point-topoint electrical stimulation in adult candidates for neurosurgery (Ojemann, 1991), to neural imaging studies of normals, including positron emis-sion tomography (PET), functional magnetic resonance imaging (fMRI), magnetoencephalo-graphy (MEG) and event-related brain potentials (ERP) (for reviews, see Brown and Hagoort, 1999; Xiong et al., 1998). The privileged status of the left hemisphere for language processing is now beyond dispute (with esti-mates averaging from 95%-98% of normal adults, independent of handedness), but the origins and develop-ment of this specialization are still poorly understood. There must be something about the left hemisphere that makes it especially suited for language -but what is that “something”? Is it present at birth, or does it develop gradually? Is it possible to develop normal language in the absence of a normal left hemisphere? And if an intact left hemisphere is not required for language development, then when, how, and why does it become necessary for language use in adults? Finally, if alternative forms of brain organization for language can emerge in the presence of early left-hemisphere damage, is there some critical period in which this must occur? The sparse but growing body of evidence on language development in children with leftvs. right-hemisphere damage is relevant to all these points, and it has yielded two very puzzling results: (1) most children with early lefthemisphere damage go on to acquire language abilities within the normal range (although performance is often at the low end of the normal range), and (2) most studies fail to find any significant differences in language outcomes when direct comparisons are made between children with leftvs. righthemisphere damage. These unexpected findings in children are hard to reconcile with one of the most popular ideas in neuropsychology: that the left hemi-sphere of the human brain contains an innate and highly specialized organ for language (e.g., Fodor, 1983; Gopnik, 1990; Gopnik and Crago, 1991; Newmeyer, 1997; Pinker, 1994; Rice, 1996). The language-organ hypothesis is appealing on many grounds. Aside from its value in explaining left-hemisphere specialization, the existence of a specialized language organ might help to explain why all normal adults are virtuosi in this domain. For example, adult speakers of English produce an average of 150 words per minute, each rapidly select-ed from a pool of 20-40,000 lexical options. As quickly as these words are spoken (often blurred together, without well-marked boundaries), the average listener can parse these unbroken streams of sound into words and phrases, accessing the meaning of each word (from that same large pool), while simultaneously processing all the complex grammatical cues necessary for com-prehension. This is an ability no other species on the planet appears to have, and one that today’s largest and fastest computers have yet to master.