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Introduction For the past couple of decades, the study of verb past tense has become a proving ground for two competing accounts of verb representation. Differences between these two theories are revealed in the manner in which they explain the processing of regular versus irregular verbs. In English, there are two broad classes of verbs distinguished by their inflectional morphology. Regular verbs, constituting the majority of verbs, form their past tense by adding the regular affixes /d/, /ed/, or /t/, depending on the stem. For example, “fix/fixed”, “rain/rained”, and “help/helped”. Therefore, a regular past tense verb is composed of its stem and the affix, such as “open-“ and “-ed” in “opened”. Such verbs are phonologically predictable since they all form their past tense in a standard way. On the other hand, the past tense of irregular verbs is inconsistent, and cannot be predicted by the stem. For example “hit/hit”, “run/ran”, “drive/drove”. These irregulars only number approximately 160 but are among the most commonly encountered. Whereas the formation of regular past tense verbs seems to be governed by rules, it is not the case for irregular verbs. We will review the current dual-mechanism account of past tense inflection, followed by recent neuropsychological evidence, as well as some implications for the connectionist model. The rule-based account The dual-route, or rule-based approach, postulates two separate mechanisms for the past tense inflection of regular and irregular verbs (Marslen-Wilson & Tyler, 1998; Tyler, Randall, & Marslen-Wilson, 2002; Tyler et al., 2002). Starting in the 1960’s, research on language development established the early premises for a computational rule-based account the past tense. Studies of language acquisition in children show the past tense as learned in three stages (Berko, 1958; Ervin & Miller, 1963; Kuczaj, 1977). In Stage 1, it appears that the past tense forms of verbs are learned as items separate from their present tense forms. In essence, the child memorizes a list of distinct items. Stage 2 is characterized by implicit knowledge of a rule for the past tense. Evidence comes from the observation that children over generalize the rule that applies to regulars, to irregular verbs as well, adding the affix /ed/ to irregular verbs, such as “goed” and “bringed”. It is in Stage 3 that the child correctly uses regular and irregular past tense. Emphasis has been put on Stage 2, where overgeneralization of the rule to irregular verbs is taken as evidence for a cognitive process in verb representation. The argument is that the misapplication of the rule on irregulars could not have been learned though imitation, since the child is not exposed to such errors; therefore what was learned must have been the rule.
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