A Neural Dissociation within Language: Evidence that the Mental Dictionary Is Part of Declarative Memory, and that Grammatical Rules Are Processed by the Procedural System

Abstract

Language comprises a lexicon for storing words and a grammar for generating rule-governed forms. Evidence is presented that the lexicon is part of a temporal-parietalhnedial-temporal "declarative memory" system and that granlmatical rules are processed by a frontamasal-ganglia "procedural" system. Patients produced past tenses of regular and novel verbs (looked and plagged), which require an -ed-suffixation rule, and irregular verbs (dug), which are retrieved from memory. Word-finding difficulties in posterior aphasia, and the general declarative memory impairment in Alzheimer's disease, led to more errors with irregular than regular and novel verbs. Grammatical difficulties in anterior aphasia, and the general impairment of procedures in Parkinson's disease, led to the opposite pattern. In contrast to the Parkinson's patients, who showed sup pressed motor activity and rule use, Huntington's disease patients showed excess motor activity and rule use, underscoring a role for the basal ganglia in grammatical processing.

Grammatical processing of nouns and verbs in left frontal cortex?

“…This impairment extends to both morphologically regular verbs (like walked) and irregular verbs (like ran), contrary to the prediction that anterior brain damage should lead to greater difficulty with regular forms [55], and also to pseudo-words (like wug) that have no stored representation. R.C.…”

“…has more difficulty producing irregular forms of verbs than regular forms. According to one influential hypothesis, regular morphological transformations are computed by neural circuits in left frontal brain regions that form part of a putative anterior procedural memory system, while the retrieval of irregular forms is mediated by posterior circuits that subserve semantic memory [55]. It is obviously hard to square R.C.…”

Specific Language Impairment is not Specific to Language: the Procedural Deficit Hypothesis

“…Specifically, it has been posited that important aspects of the difference between these two types of mappings can be captured by the distinction between procedural memory and declarative memory, another well-studied memory system (Ullman, 2001a(Ullman, , 2001cUllman et al, 1997). According to this view -referred to as the Declarative/Procedural (DP) model -idiosyncratic mappings are stored in a memorized "mental lexicon" that depends on declarative memory, whereas the learning and use of rule-governed computations involves a "mental grammar" that depends on procedural memory.…”

“…Thus the DP model predicts that these structures should play analogous functional roles for lexical memory -namely, in the selection, retrieval or search for lexical knowledge (Ullman, in press-a). Dissociations between lexicon and grammar should therefore be particularly striking when these functions are not required, such as in lexical recognition tasks.We have argued elsewhere that a broad range of converging evidence, from psycholinguistic, neurological, and neuroimaging studies, supports the DP perspective (Ullman, 2001c, in press-a;Ullman et al, 1997). Therefore this evidence will not be presented or discussed here.…”

“…Similarly, abnormally low over-regularization rates in SLI have been shown in German pluralization (Clahsen et al, 1993), and in the production of complex forms in Japanese (Fukuda and Fukuda, 2001). These deficits suggest that the use of regular morphologically complex forms, which tends to rely on the procedural system (Ullman, 2001c;Ullman et al, 1997), is impaired relative to the use of morphological forms hypothesized to depend on lexical memory. Importantly, although the production of regular past-tense forms is particularly impaired (thus implicating regular morpho-phonology), both regulars and irregulars are produced less successfully by language-impaired children and adults than by control subjects (Ullman and Gopnik, 1994;Ullman and Gopnik, 1999;van der Lely and Ullman, 2001;Vargha-Khadem et al, 1995).…”