What is missing from the otherwise excellent discussion presented by Christiansen & Chater (C&C) are insights from current studies on how brains might actually work. The traditional Broca-Wernicke theory, though straightforward, is wrong.
Neural circuits
Complex brains contain distinct neuroanatomical structures that perform local operations, such as processing tactile, visual, or auditory stimuli. However, an isolated neural structure or cortical area generally is not the “seat” of a complex behavior. Instead, a particular neural structure may support many anatomically segregated groups – populations – of neurons that carry out a similar “local” operation. Each neuronal population is linked to – projects to – an anatomically distinct neuronal population in another region of the brain, forming a neural circuit. The circuit regulates an observable aspect of behavior, such as walking, talking, dancing, comprehending syntax, and so on. Moreover, within a given neural structure, distinct anatomically segregated neuronal populations may occur, that project to different brain structures forming multiple circuits that regulate other behaviors. Therefore, a given neural structure may be involved in different aspects of behavior, each regulated by a different circuit.
A class of cortico-striatal-cortico circuits involves links between cortical areas and the subcortical basal ganglia. Cummings (Reference Cummings1993), in his review article, identifies five parallel basal ganglia circuits that are involved in motor control, cognition, attention, and other aspects of behavior. Disruptions in behavior seemingly unrelated, such as obsessive-compulsive disorder (Greenberg et al. Reference Greenberg, Murphy and Rasmusen2000), schizophrenia (Graybiel Reference Graybiel1997), and Parkinson's disease (Jellinger Reference Jellinger, Streifler, Korezyn, Melamed and Youdim1990), derive from the impairment of these neural circuits. Behavioral changes usually attributed to frontal lobe cortical dysfunction can be observed in patients having damage to basal ganglia (e.g., Flowers & Robertson Reference Flowers and Robertson1985; Lange et al. Reference Lange, Robbins, Marsden, James, Owen and Paul1992).
Reiteration
The basal ganglia enable humans to generate a potentially unbounded number of motor acts, such as words or dances, by the process of reiteration, selecting and sequencing a finite set of “pattern generators” that each specify a sub-movement. Individual motor pattern generators may entail selectional constraints, yielding hierarchical structures. Walking, for example, involves executing a heel strike at an appropriate point in a sequence of leg and foot sub-movements. The basal ganglia also reiterate cognitive pattern generators that constitute sub-elements of thought processes. Parkinson's disease (PD), which damages the basal ganglia, mostly sparing cortex, thus yields deficits such as tremors, rigidity, and disruptions in speech production. PD patients can have difficulty producing and comprehending sentences that have complex syntax (Grossman et al. Reference Grossman, Carvell, Gollomb, Stern, Vernon and Hurtig1991; Reference Grossman, Carvell, Gollumb, Stern, Reivich, Morrison, Aalavi and Hurti1993; Illes et al. Reference Illes, Metter, Hanson and Iritani1988, Lieberman et al. Reference Lieberman, Friedman and Feldman1990; Reference Lieberman, Kako, Friedman, Tajhman, Feldman and Jiminez1992; Natsopoulos et al. Reference Natsopolous, Grouuios, Bostantzopoulou, Mentenopouloue, Katsarou and Logothetis1993). As PD progresses, patients are unable to readily form or change cognitive sets (Cools et al. Reference Cools, Barker, Sahakian and Robbins2001; Flowers & Robertson Reference Flowers and Robertson1985).
Neuroimaging studies confirm the essential cognitive role of the basal ganglia. Monchi et al. (Reference Monchi, Petrides, Perre, Worsley and Dagher2001) used functional magnetic resonance imaging (fMRI) to monitor brain activity of neurologically intact subjects in a version of the Wisconsin Card Sorting Test (WCST), which evaluates a person's ability to form and shift cognitive criteria. Bilateral activation was observed in the prefrontal cortex, basal ganglia, and thalamus. Dorsolateral prefrontal cortical areas were active when the subjects had to relate the current match with earlier events stored in working memory. Mid-ventrolateral prefrontal cortex, caudate nucleus, putamen, and thalamus were active when subjects had to shift to a different matching criterion. Stowe et al. (Reference Stowe, Paans, Wijers and Zwarts2004) used positron emission tomography (PET) imaging of neurologically intact subjects in a sentence comprehension study. The basal ganglia to dorsolateral prefrontal cortex circuit was active when subjects had to change their interpretation of an ambiguous sentence, confirming that basal ganglia set shifting manifests itself in language. Other neuroimaging studies show basal ganglia linked to cortex during sentence comprehension and word retrieval tasks (Klein et al. Reference Klein, Zatorre, Milner, Meyer and Evans1994; Kotz et al. Reference Kotz, Meyer, Alter, Besson, Von Cramon and Frederici2003; Rissman et al. Reference Rissman, Eliasesen and Blumstein2003; Stowe et al. Reference Stowe, Paans, Wijers and Zwarts2004). In short, the basal ganglia act in concert with cortical areas of the brain to reiterate pattern generators in motor, cognitive, and linguistic tasks.
The “narrow faculty of language”
The evidence noted here (cf. Lieberman Reference Lieberman2000; Reference Lieberman2002; Reference Lieberman2006) suggests that the productive aspects of speech and syntax derive from cortico-striatal-cortico circuits that confer reiteration. This negates the most recent version of Universal Grammar (UG), the “narrow faculty of language” (NFL) (Hauser et al. Reference Hauser, Chomsky and Fitch2002). The human and language-specific NFL supposedly is the key to recursion, which involves inserting identical elements, usually sentences (Ss), into a underlying phrase marker. Generative grammars since Chomsky (Reference Chomsky1957) claim the relative clause in a sentence such as, The boy who was talking fell down, derives from a sentence node “S” inserted into a hypothetical underlying sentence. In the 1957 system, transformational rules acted on the underlying sentence to yield the sentence that might be uttered or written. In Chomsky's (Reference Chomsky1995) current “minimalist” grammar, the syntactic rule merge inserts S nodes; the rule move and subsequent operations then rewrite the string of words to yield the sentence that one actually hears or reads. But is there any neurophysiological evidence for inserted S nodes in a hypothetical abstract structure formed by serial algorithms that have no demonstrable neural bases?
In contrast, “hard” data from imaging and behavioral studies of the linguistic deficits of PD, hypoxia (which has a profound effect on the basal ganglia; Lieberman et al. Reference Lieberman, Kako, Friedman, Tajhman, Feldman and Jiminez1992; Reference Lieberman, Morey, Hochstadt, Larson and Mather2005), and the FOXP2 transcriptional gene studies (e.g., Lai et al. Reference Lai, Gerrelli, Monaco, Fisher and Copp2003; Vargha-Khadem et al. Reference Vargha-Khadem, Watkins, Price, Ashbruner, Alcock, Connelly, Frackowiak, Friston, Pembrey, Mishkin, Gadian and Passingham1998) show that neural circuits involving the basal ganglia provide the productive capacity of syntax, as well as speech, and seemingly unrelated distinctly human traits such as music and dance.
The mark of evolution
Should we be surprised to discover that the basal ganglia, subcortical structures that can be traced back to anurans similar to present-day frogs, play a critical role in language? As Darwin (Reference Darwin1859/1964) first observed, organs initially adapted to control one function take on “new” tasks. Seen in this light, the local motor sequencing operations in the subcortical basal ganglia appear to be precursors for similar operations in cognitive domains. In the course of evolution, they have been adapted to serve as a reiterative engine in cognitive domains including language.
Has anyone ever seen a chimpanzee dancing?
