The neural circuits recruited for the production of signs and fingerspelled words

Abstract

Signing differs from typical non-linguistic hand actions because movements are not visually guided, finger movements are complex (particularly for fingerspelling), and signs are not produced as holistic gestures. We used positron emission tomography to investigate the neural circuits involved in the production of American Sign Language (ASL). Different types of signs (one-handed (articulated in neutral space), two-handed (neutral space), and one-handed body-anchored signs) were elicited by asking deaf native signers to produce sign translations of English words. Participants also fingerspelled (one-handed) printed English words. For the baseline task, participants indicated whether a word contained a descending letter. Fingerspelling engaged ipsilateral motor cortex and cerebellar cortex in contrast to both one-handed signs and the descender baseline task, which may reflect greater timing demands and complexity of handshape sequences required for fingerspelling. Greater activation in the visual word form area was also observed for fingerspelled words compared to one-handed signs. Body-anchored signs engaged bilateral superior parietal cortex to a greater extent than the descender baseline task and neutral space signs, reflecting the motor control and proprioceptive monitoring required to direct the hand toward a specific location on the body. Less activation in parts of the motor circuit was observed for two-handed signs compared to one-handed signs, possibly because, for half of the signs, handshape and movement goals were spread across the two limbs. Finally, the conjunction analysis comparing each sign type with the descender baseline task revealed common activation in the supramarginal gyrus bilaterally, which we interpret as reflecting phonological retrieval and encoding processes.

Evidence for shared conceptual representations for sign and speech

“…Regions containing greater representational structure for sign, than speech, were found in the bilateral occipital cortices, as well as in the left superior parietal lobule. This is consistent with the greater visual and body-space processing demands of sign language perception 29 and the growing evidence for superior parietal cortex involvement in sign perception and production 50 . As for speech, a subset of regions showing greater representational structure for sign than speech showed a significant fit with the semantic model, and this was driven by item-level encoding, consistent with visual sign form representations.…”

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

“…These findings from sign comprehension fit well with a PET study of sign production (Emmorey et al, 2016) showing increased in SPL activity for one-handed signs that contacted the body, compared to one-handed signs or two-handed signs produced in neutral space.…”

“…It may be tempting to conclude that for sign language production, for which visual guidance is not necessary and motor plans are retrieved from memory rather than being determined by the physical environment, the role of SPL has shifted to generation of phonologically-based forward models at the expense of finer control. However, in the present study (as well as Emmorey et al, 2016) SPL activity is prevalent for a subset of signs: those involving contact between the hand and body. These are cases where finer control is necessary to reach a target location, although some variation is still permitted.…”

“…However, for phonological feature errors there was a reliable interaction (Z 1 =2.60, p=.004; Z 2 =1.76, p=.039) indicating that TMS to SPL selectively increased the number of phonological feature errors while leaving the other two error types unaffected; see Figure 3 for overall response times and errors of different types by experimental condition. Because previous literature implies increased engagement of SPL in producing signs requiring coordination of hand/s with body locations (Emmorey et al, 2016;Parkinson et al, 2010;Striemer et al, 2011), we divided the signs into those produced with one or two hands, and those that involved handto-hand contact or no contact (there were insufficient numbers of signs produced with handto-body contact). Signs without contact showed no effects of TMS (both |Z|<1), but the number of phonological feature errors more than doubled under TMS to left SPL for twohanded signs with hand-to-hand contact (Z 1 =1.90, p=.023; Z 2 =1.98, p=.024); see Table 1.…”

“…If SPL is reliably engaged in retrieval of BSL signs or initiation of overt sign production, applying TMS should broadly affect both speed and accuracy: slowing response initiation relative to a control condition, and increasing the rate of naming errors. Because previous studies, both of sign language production and other types of actions, suggest greater SPL engagement during complex coordination or precise target-directed movement (Bernier and Grafton, 2010;Parkinson et al, 2010;Tagliabue and McIntyre, 2014;Emmorey et al, 2016), TMS effects on response times might be limited to signs requiring coordinated movement of both hands or precisely-defined place of articulation. However, if SPL is more engaged in generating internal plans for monitoring language-specific forms (Emmorey et al, 2004;2007;2014;, effects on response initiation should not occur.…”