Seeing our hand or a tool during visually-guided actions: different effects on the somatosensory and visual cortices

Abstract

The processing of proprioceptive information in the context of a conflict between visual and somatosensory feedbacks deteriorates motor performance. Previous studies have shown that seeing one's hand increases the weighting assigned to arm somatosensory inputs. In this light, we hypothesized that the sensory conflict, when tracing the contour of a shape with mirror-reversed vision, will be greater for participants who trace with a stylus seen in their hand (Hand group, n=17) than for participants who trace with the tip of rod without seen their hand (Tool group, n=15). Based on this hypothesis, we predicted that the tracing performance with mirror vision will be more deteriorated for the Hand group than for the Tool group, and we predicted a greater gating of somatosensory information for the Hand group to reduce the sensory conflict. The participants of both groups followed the outline of a shape in two visual conditions. Direct vision: the participants saw the hand or portion of a light 40 cm rod directly. Mirror Vision: the hand or the rod was seen through a mirror. We measured tracing performance using a digitizing tablet and the cortical activity with electroencephalography. Behavioral analyses revealed that the tracing performance of both groups was similarly impaired by mirror vision. However, contrasting the spectral content of the cortical oscillatory activity between the Mirror and Direct conditions, we observed that tracing with mirror vision resulted in significantly larger alpha (8-12 Hz) and beta (15-25 Hz) powers in the somatosensory cortex for participants of the Hand group. The somatosensory alpha and beta powers did not significantly differ between Mirror and Direct vision conditions for the Tool group. For both groups, tracing with mirror vision altered the activity of the visual cortex: decreased alpha power for the Hand group, decreased alpha and beta power for the Tool group. Overall, these results suggest that seeing the hand enhanced the sensory conflict when tracing with mirror vision and that the increase of alpha and beta powers in the somatosensory cortex served to reduce the weight assigned to somatosensory information. The increased activity of the visual cortex observed for both groups in the mirror vision condition suggests greater visual processing with increased task difficulty. Finally, the fact that the participants of the Tool group did not show better tracing performance than those of the Hand group suggests that tracing deterioration resulted from a sensorimotor conflict (as opposed to a visuo-proprioceptive conflict).

Guided by touch: Tactile Cues in Hand Movement Control

“…It has been suggested that the dynamic suppression of cortical somatosensory inputs observed in individuals performing goal-directed movements under conflicting visuosomatosensory feedback serves to partially resolve the sensory conflict 20,47,48 . This sensory gating was observed in the Tactile group, but not in the NoTactile group.…”

“…Proprioception plays a crucial role in motor control by providing the brain with information about the position, orientation, and movement of various body parts, including the fingers 49 . Previous studies exploiting the mirror paradigm reported gating over the somatosensory cortex even when participants traced a shape with the tip of a pen 20,48 . In this light, one might question why the tracing performance and the activity of the source-localised somatosensory cortex were unaffected by the mirror-reversed visual in the NoTactile group.…”

“…All participants gave a fully informed written consent to participate in the study. The number of participants (sample size) was determined by referring to previous studies using the mirror-reversed tracing paradigm 31,48 . The protocols and procedures adhered to the guidelines established in the 1964 Declaration of Helsinki and were approved by the CERSTAPS ethics committee.…”