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

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.