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Neural correlates of sentence comprehension in agrammatic aphasia

Eduardo Europa1* and Cynthia K. Thompson1, 2
  • 1 Northwestern University, Communication Sciences and Disorders, United States
  • 2 Northwestern University, Department of Neurology and Cognitive Neurology and Alzheimer Disease Center, United States

Background Agrammatic aphasia is characterized by difficulty comprehending sentences that deviate from a canonical word order. This deficit has been linked to damage (e.g., stroke) in the left perisylvian cortex (Schwartz et al., 1980; Dronkers et al., 2004). While some postulate that recruitment of homotopic right hemisphere (RH) regions is maladaptive (Heiss & Thiel, 2006), others argue that RH activation is recruited into the language network to support language processing (Karbe, et al., 1998; Raboyeau et al., 2008; Thompson et al., 2010). The present study used a region-of-interest (ROI) fMRI analysis to test the hypothesis that, in agrammatic aphasia, increased RH activity in perisylvian regions would be associated with better sentence comprehension. Methods FMRI data were acquired from 8 participants with agrammatic aphasia secondary to a single LH stroke (Figure 1) during a sentence-picture verification task. Auditory sentences were in a canonical (actives or subject-clefts) or noncanonical (passives and object-clefts) word order. A baseline condition included scrambled pictures and time-reversed speech. Selection of ROIs was based on a syntactic processing network, found by contrasting canonical>baseline in a cohort of 21 cognitively healthy participants (Figure 1), and included the right superior parietal lobule (SPL), superior lateral occipital cortex (LOCs), temporo-occipital part of the middle temporal gyrus (MTGto), pars triangularis of the inferior frontal gyrus (IFGtri), and the precentral gyrus (PreGyr). The right subcallosal cortex (SCC) was included as a non-language control ROI (Figure 1). Mean contrast estimates (canonical>baseline) were extracted from each of these seven ROIs and served as dependent variables in a mixed effect regression model with group (healthy, aphasic), ROI, and their interactions modeled as fixed effects, and participant as a random effect. Results Accuracy for aphasic participants was greater than chance for canonical sentences (p<0.001) and below chance for noncanonical sentences (p<0.001). The healthy group was significantly more accurate for canonical compared to noncanonical sentences (p<0.005), but near ceiling for both. The model explained a significant proportion of variance (p<0.001) with significant main effects of group (p<0.05) and ROI (p<0.001), and a significant interaction effect driven by neural activity in IFGtri in the aphasic group (p=0.001). A post-hoc two-sample t-test of canonical>baseline contrast estimates for the IFGtri between the healthy group (M=-0.03, SD=0.12) and aphasic group (M=0.31, SD=0.28) confirmed the interaction (p=0.01). Discussion In summary, the right pars triangularis of the inferior frontal gyrus was more active in the aphasic group relative to the healthy group when processing sentences in a canonical order. Although the study only included 8 aphasic participants, this finding provides some evidence for compensatory neural activity for sentence processing in one homotopic RH region in stroke aphasia. However, the relation between damage to the LH sentence processing network, RH activation, and other variables related to recovery of function and their contribution to syntactic processing deficits in agrammatic aphasia remains unclear.

Figure 1
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Acknowledgements

This research was supported by the NIH-NIDCD Clinical Research Center Grant, P50DC012283 (PI: C.K. Thompson), and the Graduate Research Grant and the School of Communication Graduate Ignition Grant from Northwestern University (awarded to E.H. Europa). The authors would also like to thank Drs. Steven Zecker, Swathi Kiran, Darren Gitelman, and members of the Aphasia and Neurolinguistics Research Laboratory for their contributions to this study.

References

Dronkers, N. F., Wilkins, D. P., Van Valin Jr., R. D., Redfern, B. B., & Jaeger, J. J. (2004). Lesion analysis of the brain areas involved in language comprehension. Cognition, 92, 145-177.
Heiss, W. D., & Thiel, A. (2006). A proposed regional hierarchy in recovery of post-stroke aphasia. Brain and Language, 98, 118-123.
Karbe, H., Thiel, A., Weber-Luxenburger, G., Herholz, K., Kessler, J., & Heiss, W.-D. (1998). Brain plasticity in poststroke aphasia: what is the contribution of the right hemisphere? Brain and Language, 64(2), 215-230.
Raboyeau, G., De Boissezon, X., Marie, N., Balduyck, S., Puel, M., Bezy, C., . . . Cardebat, D. (2008). Right hemisphere activation in recovery from aphasia Lesion effect or function recruitment? Neurology, 70(4), 290-298.
Schwartz, M. F., Saffran, E. M., & Marin, O. S. (1980). The word order problem in agrammatism: I. Comprehension. Brain and language, 10(2), 249-262.
Thompson, C. K., den Ouden, D. B., Bonakdarpour, B., Garibaldi, K., & Parrish, T. B. (2010). Neural plasticity and treatment-induced recovery of sentence processing in agrammatism. Neuropsychologia, 48(11), 3211-3227.

Keywords: agrammatic aphasia, noncanonical sentences, sentence comprehension, Syntactic complexity, functional magnetic resonance imaging

Conference: Academy of Aphasia 55th Annual Meeting , Baltimore, United States, 5 Nov - 7 Nov, 2017.

Presentation Type: poster presentation

Topic: Consider for student award

Citation: Europa E and Thompson CK (2019). Neural correlates of sentence comprehension in agrammatic aphasia. Conference Abstract: Academy of Aphasia 55th Annual Meeting . doi: 10.3389/conf.fnhum.2017.223.00122

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Received: 02 May 2017; Published Online: 25 Jan 2019.

* Correspondence: Mr. Eduardo Europa, Northwestern University, Communication Sciences and Disorders, Evanston, IL, United States, eduardo.europa@sjsu.edu