CONTRIBUTIONS OF APHASIOLOGY TO LANGUAGE EVOLUTION RESEARCH

Brain imaging techniques have greatly improved our understanding of cerebral language processing. Nevertheless, finding the neural correlates of linguistic operations seems to be no less a quest than in the field of consciousness, for which the mapping between psychological experiences and neural activation has been labelled the hard problem (Chalmers, 1995).

Poeppel and Embick (2005) argued that psycholinguistics and the neurosciences face a granularity mismatch and an ontological incommensurability problem. This lacking of a common basis concerns both the granularity levels at which the two disciplines investigate processes and the fundamental elements used and, consequently, prevents the formulation of theoretically motivated, biologically grounded and computationally explicit descriptions of language processes in the brain. To better align the two areas Poeppel and Embick suggest to use computational models, whose operations must be plausibly executable by neural assemblies and represent subroutines of linguistic computation.

We appreciate Poeppel and Embick's analysis and support their proposal to identity computationally explicit processes. Resorting to computer linguistics is however no guarantee that biologically plausible implementations are identified. Artificial neural networks can be built in a way that mimic human behaviour without basing on biology-inspired architecture, as exemplified by the Rumelhart & McClelland (1986) model of English past tense acquisition.

In order to aid the identification of the relevant atomic processes involved in language perception and production we evaluate a model of language evolution with findings from agrammatism research. Johansson (2005) suggested to work backwards from current grammars through a sequence of possible protogrammars by removing pivotal structural features. Johansson's model consists of a four-step hierarchy, which supposedly reflects the appearance of fundamental properties in human language, for the existence of which there is wide agreement across different grammar theories: the emergence of structural constraints, most notably with respect to word order; the emergence of hierarchies, i.e. the occurrence of structured units within larger-scale structures; the emergence of flexibility in the transformational sense, allowing structures to be moved around; and lastly, the occurrence of recursion, proposed to be the only domain-specific computational capacity involved in language processing (Hauser, Chomsky & Fitch, 2002, but see also e.g. Kinsella, 2009).

We hypothesise that the biologically oldest capacities recruited for language processing are least likely to suffer selective impairments from brain injuries as they presumably evolved pre-linguistically. More recent abilities, notably the proposed capacity for recursion, are more likely to have been selected specifically for linguistic purposes. In contrast to their biologically older counterparts, such capacities may be more vulnerable to break-down and affect specific aspects of language production or perception in isolation.

We use findings from studies investigating the patterns of grammar deficits in aphasia patients to assess the proposed hypothesis and to demonstrate how far aphasiology can contribute to the quest of defining linguistic capacities that are evolutionarily layered and real in both linguistic-computational and neurological terms.

Note from Publisher: This article contains the abstract and references.