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

Abstract

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.

Figure 1

Brain lesions of each of the eleven patients with damage to Broca’s area, defined as the pars opercularis (BA 44) and pars triangularis (BA 45) of the LIFG, are shown in red on a standard brain image. An overlay of the eleven brain lesions illustrates the regions of maximum overlap in the LIFG and insula. These images were produced using MRIcro (Rorden & Brett, 2000).

Figure 2

Response time data, first as a function of Word Type (upper four plots) and second for the irregular verbs alone, separated into weak irregular verbs and strong verbs (lower four plots). Controls are presented to the left and patients to the right. Bar graphs illustrate the grand mean response times for each group, with error bars representing standard error. Scatter plots illustrate individual response time data, using a difference score in order to represent each person by a single point. Horizontal bars represent the mean difference for the group.

Figure 3

ERPs to target words within the nine-electrode region of interest, demonstrating the N400 effect as a function of Word Type (regular verbs, irregular verbs, pseudopast, and orthophono) and Priming. Controls are presented to the left and patients to the right.

Figure 4

Topographic maps of the scalp distributions displaying the N400 priming effect (unprimed-primed) between 350–600 ms and 600–850 ms as a function of Word Type (regular verbs, irregular verbs, pseudopast, and orthophono) and Priming. Controls are presented to the left and patients to the right.