Augmentation technology is a rapidly expanding field, and with it there is growing interest in how such devices interface with the body. When learning to control augmentation devices, one important sensory input is the tactile feedback received from where the device is worn on the body, described as intrinsic touch. We asked whether the brain gathers information from intrinsic tactile inputs to construct an internal representation of the device. We particularly wanted to determine whether the brain integrates intrinsic touch inputs with the somatosensory inputs from the biological fingers.
To investigate such changes in somatosensory processing, we are using a supernumerary robotic finger (the Third Thumb, Dani Clode Design). The Third Thumb is worn on the ulnar side of the hand and controlled by pressure sensors underneath the big toes. In our ongoing study, we are assessing changes to inter-finger sensory representations before and after a week of altered finger-synchronisation motor training: either due to extended Third Thumb training, or training to play the keyboard (n=50 in total). We are using fMRI to study the representational similarity patterns across the biological fingers and Third Thumb (via intrinsic touch) before and after training using a soft pneumatic actuator stimulation system. We are also using a psychophysics paradigm to explore changes in sensory integration, examining tactile temporal order judgements involving the biological fingers and the Third Thumb.
Our fMRI results so far suggest that Third Thumb training has the largest impact on the biological finger representation. Following training, we see a collapsing of the canonical finger representation, not seen to the same extent in our active control group, and replicating previous results obtained from an active movement paradigm1. We then wanted to investigate whether the intrinsic touch inputs from the Third Thumb are represented differently from merely stimulating the side of the hand where the Third Thumb is worn (the ‘palm’). Preliminary findings suggest there may be some small shifts in the way the somatosensory system mediates this intrinsic touch information compared to just palm stimulation after Third Thumb training. However, when considering the whole hand representation, it appears shifts in the Third Thumb and palm are likely tied to shifts in the finger representation.
Overall, these early findings suggest that the brain is more likely to change the representation of the biological body to account for the use of an additional limb, rather than creating an independent representation of the technology. If these results withstand, this could produce important considerations for how to safely integrate such technologies with our body in the future. The next steps for this work will be to use markerless tracking data to precisely investigate how co-usage of the Third Thumb with the biological fingers shapes their neural representation.