The representation of grammatical categories in the brain

Abstract

Language relies on the rule-based combination of words with different grammatical properties, such as nouns and verbs. Yet most research on the problem of word retrieval has focused on the production of concrete nouns, leaving open a crucial question: how is knowledge about different grammatical categories represented in the brain, and what components of the language production system make use of it? Drawing on evidence from neuropsychology, electrophysiology and neuroimaging, we argue that information about a word's grammatical category might be represented independently of its meaning at the levels of word form and morphological computation.Understanding how words are represented and retrieved in the brain will be crucial to any eventual neurobiological theory of language. The problem goes beyond determining how neural circuits encoding a particular conceptual representation are mapped to a unique and arbitrary set of representations for sounds, letters, or gestures: although this might suffice for the production of isolated words, it fails to specify how words are combined into phrases and sentences. How do we know, for example, that 'the singing are birds' is not a well-formed alternative to 'the birds are singing'? (And why are such exchanges rare even among speech errors? [1]) The most obvious answer is that bird and sing belong to different grammatical categories -noun and verb, respectively -with different syntactic roles, and that this information is encoded in the brain and accessed during the course of language production.Even though grammatical categories are not created equal [2], most research in the area of lexical access has used the production of concrete nouns as a proxy for word production in general. Therefore, it is not too surprising that our current picture of the cortical organization of lexical knowledge leaves little room for mechanisms that might be sensitive to differences between words of different categories. A recent review of neuroimaging studies of lexical access parcels the left perisylvian cortex into areas involved in retrieving word forms from concepts and in various stages of phonological processing [3], but sheds little light on the question of how and where the brain processes a word's grammatical role.This picture might change as more researchers use neuroscientific methods to address finer-grained questions about the representation of words in the brain. To date, however, experiments on the retrieval of grammatical category information have yielded conflicting results. Although electrophysiological measures have suggested that nouns and verbs are processed by distinct neural generators, neuroimaging has turned up little evidence for an anatomical distinction. The neuroimaging results have led some to argue that words are stored in distributed networks, in which categorical distinctions are not explicitly represented [4] -a conclusion that is at odds with what is known from the study of patients with language deficits following brain damage. Are we at an empirical impasse...

Differential effects of age-of-acquisition for concrete nouns and action verbs: Evidence for partly distinct representations?

“…This result thus lends credibility to the view that the two types of words may be represented in partly distinct neural substrates (Pulvermüller, 1999a;Shapiro & Caramazza, 2003). The result of our second experiment further specifies that these differential effects occur only for verbs that describe actions belonging to the human motor repertoire.…”

“…While nouns are pointer to objects, persons, places, things, or ideas, verbs generally refer to actions or states of being. In apparent conformity with this word class partition, neuropsychological studies have provided evidence that processing of nouns and verbs can be independently disrupted, suggesting that the two types of words may be represented in partly distinct neural substrates (Pulvermüller, 1999a;Shapiro & Caramazza, 2003). Hence, while damage to frontal, motor-processing areas in the left hemisphere correlates more often with difficulties in processing action verbs, damage to left temporal regions correlates with difficulties in processing nouns (Caramazza & Hillis, 1991;Damasio & Tranel, 1993;Daniele, Giustolisi, Silveri, Colosimo, & Gainotti, 1994;Miceli, Silveri, Villa & Caramazza, 1984;Shapiro, Pascual-Leone, Mottaghy, Gangitano, & Caramazza, 2001).…”

Cortical signatures of noun and verb production

“…We have contended that true noun-verb differences should emerge in word production tasks in which on-line access to grammatical category information plays a crucial role (17,18). For instance, we have shown that some patients with impairments in noun or verb naming are also unable to use pseudowords as members of the impaired category; thus, a patient with a nounnaming deficit was able to say he wugs, but not the wugs (17,19).…”

“…Alternately, such an area might represent the unique configuration of lexical properties that define a word's grammatical use (18). The former assumption would be favored if areas of selective activation for nouns and verbs were observed in a part of the brain known to be important for syntactic processing, such as the left posterior frontal cortex (18,19).In the present study we used event-related functional MRI (fMRI) to identify brain regions that were activated when subjects produced nouns and verbs in the context of short phrases or sentences, like many doors and he weeps. The manipulation of morphological inflections in this task (like the noun plural and verb agreement marker -s) ensured that subjects accessed grammatical category information.…”

“…Alternately, such an area might represent the unique configuration of lexical properties that define a word's grammatical use (18). The former assumption would be favored if areas of selective activation for nouns and verbs were observed in a part of the brain known to be important for syntactic processing, such as the left posterior frontal cortex (18,19).…”