The role of Broca's area in regular past-tense morphology: An event-related potential study

It has been suggested that damage to anterior regions of the left hemisphere results in a dissociation in the perception and lexical activation of past-tense forms. Specifically, in a lexical-decision task in which past-tense primes immediately precede present-tense targets, such patients demonstrate significant priming for irregular verbs (spoke–speak), but, unlike control participants, fail to do so for regular verbs (looked–look). Here, this behavioral dissociation was first confirmed in a group of eleven patients with damage to the pars opercularis (BA 44) and pars triangularis (BA 45) of the left inferior frontal gyrus (i.e., Broca's area). Two conditions containing word-onset orthographic–phonological overlap (bead–bee, barge–bar) demonstrated that the disrupted regular-verb priming was accompanied by, and covaried with, disrupted ortho-phonological priming, regardless of whether prime stimuli contained the regular inflectional rhyme pattern. Further, the dissociation between impaired regular-verb and preserved irregular-verb priming was shown to be continuous rather than categorical; priming for weak-irregular verbs (spent–spend) was intermediate in size between that of regular verbs and strong verbs. Such continuous dissociations grounded in ortho-phonological relationships between present- and past-tense forms are predicted by single-system, connectionist approaches to inflectional morphology and not predicted by current dual-system, rule-based models. Event-related potential data demonstrated that N400 priming effects were intact for both regular and irregular verbs, suggesting that the absence of significant regular-verb priming in the response time data did not result from a disruption of lexical access, and may have stemmed instead from post-lexical events such as covert articulation, segmentation strategies, and/or cognitive control.

Research highlights▶ A group of 11 patients with damage to Broca's area demonstrated a dissociation between disrupted regular-verb and preserved irregular-verb priming. ▶ Disrupted regular-verb priming co-varied with disrupted phonological priming, suggesting that word-onset ortho-phonological overlap between primes and targets was a critical factor. ▶ Examination of two types of irregular verbs (weak irregular and strong) suggested that the regular–irregular dissociation was continuous, as in single-system models, and not a categorical distinction, as in dual-system models. ▶ The regular–irregular dissociation did not extend to the N400 event-related potential component, which was preserved, suggesting that the deficit related to post-lexical events such as covert articulation, segmentation strategies, and/or cognitive control.