Transcranial magnetic stimulation during British Sign Language production reveals monitoring of discrete linguistic units in left superior parietal lobule

Abstract

Successful human hand and arm movements are typically carried out by combining visual, motoric, and proprioceptive information in planning, initiation, prediction, and control. The superior parietal lobule (SPL) has been argued to play a key role in integrating visual and motoric information particularly during grasping of objects and other such tasks which prioritise visual information. However, sign language production also engages SPL even though fluent signers do not visually track their hands or fixate on target locations. Does sign language production simply rely on the motoric/ proprioceptive processes engaged in visually guided action, or do the unique characteristics of signed languages change these processes? Fifteen fluent British Sign Language users named pictures while we administered transcranial magnetic stimulation (TMS) to left SPL, a control site, or no TMS.TMS to SPL had very specific effects: an increased rate of (sign-based) phonological substitution errors for complex two-handed signs (those requiring hand contact), but TMS did not slow or otherwise impair performance. Thus, TMS decreased the likelihood of detecting or correcting phonological errors during otherwise successful bimanual coordination, but it did not noticeably alter fine movement control. These findings confirm that for fluent signers SPL has adapted to monitor motor plans for discrete hand configurations retrieved from memory as well as more fine-grained aspects of visually guided actions.Some studies of reaching and grasping manipulate the availability of visual cues and/or their consistency with proprioceptive cues (e.g. Jackson and Husain, 2006;Reichenbach et al., 2014). The findings from this research suggest different action control systems (Chib et al., 2009) with proprioceptive-only systems distinct from combined sensory systems that take both types of information into account in movement correction. The tasks, however, are still explicitly visual in that specific visual properties of the target must determine the motor plan. To avoid this issue, Parkinson et al. (2010) asked blindfolded participants to make reaching hand movements to a button box near their waist, thus

Sign language experience has little effect on face and biomotion perception in bimodal bilinguals

“…While the bimodal bilinguals in Emmorey and McCullough’s 43 study did not show left-lateralized activity, they also did not show the same degree of right-lateralization observed in hearing non-signers, instead presenting a pattern of activity that fell somewhere in-between (see also 45 ). Overall, findings of lateralization in bimodal bilinguals are mixed; some studies have shown evidence of a right hemifield advantage for sign processing in bimodal bilinguals 46 that is consistent with anatomical 47 and functional 48 , 49 changes observed in the left hemisphere. However, D/deaf signers show a left hemisphere/right hemifield advantage for face 46 , 50 and motion processing 51 , 52 that has not been observed in bimodal bilinguals.…”

Sign and Spoken Language Processing Differences in the Brain: A Brief Review of Recent Research

“…Therefore, this suggests that the SPL is specifically involved in language processing beyond simply creating sign movements. 59 This parietal lobe recruitment and bilateral activation during SL production is a great disparity to that observed in SpL production.…”