Genetic Factors and Normal Variation in the Organization of Language

In this essay we present two themes. The first is a factual review of the behavioral and neurological differences in language and cognition between people with and without familial left handedness: These differences begin to justify the claim that there is a continuum of how language and cognition are represented in the brain, reflecting a quantitative difference in the role of the right hemisphere, and consequent potential qualitative differences. The second theme involves the implications of this finding. Various cases of rare neurological organization for language have called into question the idea that there is a single form of representation: These include cases of left-hemis-pherectomy in which the patients with a lone right hemisphere can grow up to be normal linguistically (Curtiss et al. 2001, Devlin et al. 2003) with normal developmental stages (Curtiss & Shaeffer 1997) as well as unique instances such as the infamous formerly hydrocephalic mathematician whose neocortex was a thin layer of tissue lining the skull (Lewin 1980) — clearly the topology and connections of different cortical areas are very different in these cases from the norm. Even classic and recent studies call into question the unique location and function of a linguo-central structure such as Broca's and Wernicke's areas (Penfield & Roberts 1959, Bogen & Bogen 1976, Anderson 2010, Rogalsky & Hickok 2011). But people with familial left-handedness comprise 40% of the population, so we cannot consign their unique behavioral and neurological structures to an odd distaff 'minority'. A profound implication for language of these considerations is the possibility that the existence of language is not causally dependent on any particular unique neurological organization. Rather, especially the sentence construction mechanism of syntax is a computational type that recruits different neurological structures. On this view the possibility for syntax emerges as a function of the availability of propositional relations, combined with an explosive growth in the number of lexical items that can externalize the internally represented categories. The syntactic computational architecture is represented neurologically via co-option and integration of multiple brain regions that are collectively suited to the type of computation that language requires. On this view, there can be significant lability of how language will be represented in an individual's brain, if there is significant variability in how the computationally relevant areas function or are interconnected.