Revisiting the role of Broca's area in sentence processing: syntactic integration versus syntactic working memory

Abstract

Most previous neuroimaging studies of sentence processing have associated Broca's area with syntactic processing; however, the exact nature of the processes subserved by this brain region is yet not well understood. Although some authors suggest that Brodmann area (BA) 44 of the left inferior frontal gyrus (i.e., Broca's area) is relevant for syntactic integration processes, others claim that it is associated with working memory mechanisms relevant for language processing. To dissociate these two possible functions, the present study investigated hemodynamic responses elicited while participants processed German indirect wh-questions. Activation increases were observed in left BA 44 together with superior temporal areas and right hemispheric homologues for sentences with noncanonical word order, in which a verb argument was dislocated from its canonical position over a relatively long distance. In these sentences, syntactic working memory load was assumed to be greatest. In contrast, no activation increase was elicited by object-initial as opposed to subject-initial sentences that did not differ with respect to working memory costs but with respect to syntactic integration costs. These data strongly suggest that Broca's area plays a critical role in syntactic working memory during online sentence comprehension.

Figure 1

Event‐related brain potentials elicited by object wh‐questions (dashed line) as compared to subject wh‐questions (solid line) for questions with a short (left) or a long (right) distance between the dislocated element and the second noun phrase. Figure adapted from Fiebach [ 2001]. ERPs represent averaged data from 14 participants. Negative voltages are plotted upward. Experimental procedures, data acquisition, and data analysis were equivalent to procedures described in the ERP study of Fiebach et al. [ 2002]. The statistical analysis in the time window of 1,000–3,400 ms (in which the sustained negativity was observed for the Long‐object questions and that was also used in Fiebach et al. [ 2002]) indicates an interaction of word order and length (F[1,13] = 5.79; P < 0.05) that was resolved to show a significant negativity only for the Long‐object sentences (F[1,13] = 13.82; P < 0.005). ERPs to Short‐subject and Short‐object sentences did not differ in this time window (F < 0.5). Analysis of behavioral results indicates that responses to object wh‐questions took 46 ms longer than did responses to subject wh‐questions (F[1,13] = 6.39; P < 0.05), and that more errors were made for object than for subject questions (F[1,13] = 7.22; P < 0.05). Although there was a considerable difference in this effect between short questions (2.2% more errors in object than in subject questions) and long questions (5.2% more errors in object questions), the interaction of sentence type and length did not reach significance (F[1,13] = 2.27; P = 0.16).

Figure 2

SPM{Z} for group statistics representing activity that was greater for long object wh‐questions than for short object wh‐questions. The statistical map was thresholded at Z > 2.81 (P < 0.0025 uncorrected) for visual display and rendered onto a high‐resolution structural MR scan of a representative individual brain. 3‐D renderings are presented in sagittal (A), axial (B), and coronal sections (C).

Figure 3

A: Multidimensional scaling map displaying clusters of more inferior (1) and more superior (2) voxels activated in the left inferior frontal gyrus. Relative distances as determined by the MDS method are represented by the x‐ and y‐axes. Absolute distances indicated by the values on the axes are arbitrary and therefore cannot be interpreted in any way. B: Coronal, sagittal, and axial slices representing the results of the multidimensional scaling analysis. The border between the superior and inferior portion of BA 44 is clearly visible.