The role of the superior parietal lobule in lexical processing of sign language: Insights from fMRI and TMS

Abstract

There is strong evidence that neuronal bases for language processing are remarkably similar for sign and spoken languages. However, as meanings and linguistic structures of sign languages are coded in movement and space and decoded through vision, differences are also present, predominantly in occipitotemporal and parietal areas, such as superior parietal lobule (SPL). Whether the involvement of SPL reflects domain-general visuospatial attention or processes specific to sign language comprehension remains an open question. Here we conducted two experiments to investigate the role of SPL and the laterality of its engagement in sign language lexical processing. First, using unique longitudinal and between-group designs we mapped brain responses to sign language in hearing late learners and deaf signers. Second, using transcranial magnetic stimulation (TMS) in both groups we tested the behavioural relevance of SPL's engagement and its lateralisation during sign language comprehension. SPL activation in hearing participants was observed in the right hemisphere before and bilaterally after the sign language course. Additionally, after the course hearing learners exhibited greater activation in the occipital cortex and left SPL than deaf signers. TMS applied to the right SPL decreased accuracy in both hearing learners and deaf signers. Stimulation of the left SPL decreased accuracy only in hearing learners. Our results suggest that right SPL might be involved in visuospatial attention while left SPL might support phonological decoding of signs in non-proficient signers.

Keywords: Sign language; Superior parietal lobule; TMS; Visuospatial attention; fMRI.

Fig. 1 –

Experimental design of the TMS study. A) Hearing and deaf participants performed a lexical task in sign language, requiring discrimination between signs and pseudosigns. Each run started with a fixation cross (15 s). The stimuli were ~2 s long and were followed by a fixation cross displayed for 3–5 s. Response time was counted from the stimulus onset until 1 s after its end. Pulses were administered at 400, 600, 800, 1000 and 1200 ms post-stimulus onset (5 Hz). B) TMS was delivered to three target sites–right SPL, left SPL and a control site–the occipital pole at 110% of the individual motor threshold.

Fig. 2 –

A) Hearing participants underwent five fMRI sessions performed in the ~2.5-month intervals: Time Points TP0-TP4, where TP0 was a pre-exposure scan, TP3 was a scan at the end of the course and TP4 a follow-up). Deaf signers participated only in a single fMRI session that was matched in time to TP3. Approximately six weeks after TP3 hearing and deaf participants underwent the TMS session. B) Behavioral results for the sign language lexical decision task. Differences in accuracy scores for hearing learners (HL) before and after the course and between HL and deaf participants are indicated with an asterisk. *p ≤ .001; error bars represent SD.

Fig. 3 –

A-C) Brain activations during lexical processing of sign language for each group (p < .05; FWEc). D) Results from the main effect of time point in hearing learners (p < .05; FWE); bar graphs of independently defined ROIs are shown to illustrate the time course of changes. Error bars represent SEM. EXP: explicit condition; IMP: implicit condition (gender discrimination).

Fig. 4 –

Brain activation differences during lexical processing of sign language between groups at p < .05; FWEc.

Fig. 5 –

Whole-brain interaction of group (HL and deaf) by condition (EXP and IMP) at TP3 at p < .05; FWEc; bar graphs of the independently defined ROI in the left SPL are shown to illustrate the obtained interaction, *p < .005, **p = .001, Bonferroni corrected. Error bars represent SEM.

Fig. 6 –

Accuracy results from the TMS experiment: percentage of correct responses in the Lexical Decision Task in hearing learners (after the course) and deaf signers during TMS/no TMS conditions, *p ≤ .05; **p ≤ .001, Bonferroni corrected. For brevity, only differences between TMS and no TMS conditions are indicated. Error bars represent SEM.