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Inhibitory or excitatory connections between hemispheres? Evidence from sentence comprehension in patients with aphasia

Ron Chu1, 2, Jed A. Meltzer1, 2, 3, 4 and Tali Bitan3, 5*
  • 1 Rotman Research Institute, Baycrest Health Sciences, Canada
  • 2 University of Toronto, Psychology Department, Canada
  • 3 University of Toronto, Speech Pathology Department, Canada
  • 4 Canadian Partnership for Stroke Recovery, Canada
  • 5 University of Haifa, Psychology Department, IIPDM, Israel

The role of the right hemisphere (RH) in compensating for Left hemisphere (LH) damage during language processing in patients with aphasia (Abo et al., 2004; Blasi et al., 2002; Cappa et al., 1997; Winhuisen et al., 2005), has been called into question in the last decade. Some have hypothesized that increased RH activation is due to release from LH inhibition and the RH may play a maladaptive role in language recovery by exerting transcallosal suppression on homologous LH regions (Selnes, 2000; Thiel et al., 2006). However, maladaptive involvement of the RH in post-stroke aphasia is not necessarily mediated by transcallosal inhibition on LH homotopic regions. In principle, a negative effect of RH activity on language performance can also result from inefficient processing occurring in the RH and interfering with LH processing through excitatory coupling of both homotopic and heterotopic connections (Chiarello & Maxfield, 1996; Clarke, Lufkin, & Zaidel, 1993). In the current study we used effective connectivity analysis of fMRI data to assess inter-hemispheric connections in seven post-stroke patients with aphasia, during a sentence comprehension task. Sentence comprehension typically relies on both hemispheres (Friederici, 2011; Price, 2010; Vigneau et al., 2011), and is therefore expected to show strong inter-hemispheric connectivity in both patients and controls. The patients were compared to 19 young and five age-matched control participants. Dynamic Causal Modeling was used to assess the direction of effects among six bilateral regions: inferior frontal gyri (IFG), primary auditory cortices (A1) and temporo-parietal-junction (TPJ). The results showed only excitatory connections between homotopic regions in both controls (RA1>LA1 and RIFG<>LIFG; see Figure 1a) and patients (RTPJ<>LTPJ; see Figure 1b). These results, which are not consistent with the transcallosal suppression hypothesis, may reflect the bilateral nature of sentence comprehension processes. We also tested the correlation between inter-hemispheric connections in patients, and their performance on the experimental task and on standardized language tests. In all of these correlations connectivity patterns in high performing patients were consistent with connectivity in controls (see Figure 1 c). Specifically, patients showed: a) a positive correlation with the typically excitatory right-to-left connection (RA1-LA1); b) a negative correlation with the typically inhibitory top-down connections (LIFG-RA1; LIFG-LA1); and c) a negative correlation with a typically null connection (RA1-LTPJ). Overall, these correlations, in which high performing patients show similar connectivity to controls, are not consistent with the emergence of compensatory influences of RH regions onto lesioned LH language areas. In contrast, they suggest that changes in both excitatory and inhibitory inter-hemispheric connections may be maladaptive for language performance. However, such maladaptivity is not the result of right to left transcallosal suppression, but may be the result of ineffective excitatory right-to-left connectivity, or reduction of inhibitory left-to-right connectivity, thoroughly changing the balance between the two hemispheres. Figure 1: Parameter estimates of connections for A) Young controls, B) and B) Patients. C) Correlations between parameter estimates of connections and performance on the experimental task or score of Western Aphasia Battery: Auditory Verbal Comprehension score. ** Significant at p<0.05 (corrected), * p<0.05 (uncorrected)

Figure 1
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References

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Keywords: Aphasia, inter-hemispheric connectivity, Transcallosal inhibition, left hemisphere stroke, Dynamic Causal Model (DCM)

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

Presentation Type: oral presentation

Topic: Consider for student award

Citation: Chu R, Meltzer JA and Bitan T (2019). Inhibitory or excitatory connections between hemispheres? Evidence from sentence comprehension in patients with aphasia. Conference Abstract: Academy of Aphasia 55th Annual Meeting . doi: 10.3389/conf.fnhum.2017.223.00060

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Received: 21 Apr 2017; Published Online: 25 Jan 2019.

* Correspondence: Dr. Tali Bitan, University of Haifa, Psychology Department, IIPDM, Haifa, Israel, bitantali@gmail.com