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

Language comprises a lexicon for storing words and a grammar for gener ating rule-governed forms. Evidence is presented that the lexicon is p art of a temporal-parietal/medial-temporal ''declarative memory'' syst em and that grammatical rules are processed by a frontal/basal-ganglia ''procedural'' system. Patients produced past tenses of regular and n ovel verbs (looked and plagged), which require an -ed-suffixation rule , and irregular verbs (dug), which are retrieved from memory. Word-fin ding difficulties in posterior aphasia, and the general declarative me mory impairment in Alzheimer's disease, led to more errors with irregu lar than regular and novel verbs. Grammatical difficulties in anterior aphasia, and the general impairment of procedures in Parkinson's dise ase, led to the opposite pattern. In contrast to the Parkinson's patie nts, who showed suppressed motor activity and rule use, Huntington's d isease patients showed excess motor activity and rule use, underscorin g a role for the basal ganglia in grammatical processing.