Naming Tools and Using Rules: Evidence That a Frontal/Basal-Ganglia System Underlies both Motor Skill Knowledge and Grammatical Rule Use

We investigated the hypothesis that two well-studied brain systems (see Feinberg & Farah, 1997) underlie the lexicon/grammar dichotomy (Ullman et al., 1997). In this view, frontal/basal-ganglia circuitry implicated in the learning and expression of motor and cognitive skills also underlie the acquisition and use of grammatical rules, and temporal-lobe circuits implicated in the learning and use of factual (conceptual) knowledge also underlie the learning and use of memorized words. The hypothesis predicts co-occurring word/rule and fact/skill double dissociations in patients with damage to one or the other system and similar degrees of impairment to words and facts and to skills and rules. Lexicon and grammar, and conceptual and motor skill knowledge, were probed in patients with Parkinson's disease (PD) or Alzheimer's disease (AD). PD is associated with frontal/basal-ganglia damage, which may explain PD impairments at learning new and expressing established motor skills and at syntactic processing. In contrast, in (low-demented) PD patients there is typically little damage to temporal-lobe structures, and word and fact use remain relatively spared. AD is associated with temporal-lobe damage , which may explain AD impairments at learning new words and facts and at using established ones. In contrast, there is a relative sparing of frontal/basal-ganglia structures, learning new and using established motor and cognitive skills, and syntactic processing (see Feinberg & Farah, 1997). Testing for grammar/lexicon dissociations has been problematic, because tasks probing lexicon and grammar usually differ in ways other than their use of the two capacities. Therefore grammar and lexicon were probed with English regular and irregular past tense. Regular forms (e.g., play–played) require an-ed-suffixation rule, whereas irregulars (dig–dug) undergo largely arbitrary transformations and are memorized in the lexicon. Crucially, regulars and irregulars are matched in complexity (one word), meaning (past), and syntax (tensed) (Pinker, 1991).