Functional MRI (fMRI) has shown that hand-selective regions in the brains of prosthesis users respond more strongly to images of prostheses. This response occurred regardless of prosthesis familiarity or type (cosmetic versus active artificial limbs) compared with controls, and scaled with usage. Resting-state fMRI also identified stronger functional connectivity between visual and sensorimotor areas in prosthesis users who used their prosthesis more. These findings suggest a categorical representation of artificial limbs in the brain, and adds to the wealth of literature regarding the plastic nature of our brains and compensation/re-appropriation mechanisms (Brain doi: 10.1093/brain/awy054).
By examining individuals with missing limbs, specifically hands, one can investigate to what extent the cortical area(s) belonging to the former limb is altered, compared with controls. It is unknown how much the compensatory mechanisms affect visual-hand selective areas, as opposed to traditional primary sensorimotor brain regions (cortices). In this study, the researchers – from University College London, University of Oxford, Radboud University and others – hypothesized that daily use of a prosthesis would scale cortical processing in hand-selective visual areas and increase communication within functional networks.
In the study, 32 participants (16 with congenital hand loss, 16 with hand loss due to amputation) were presented with familiar and unfamiliar images of prostheses while lying in an MR scanner – teasing apart experience and general categorization. The researchers also scanned 24 controls in a similar paradigm. The photos presented were either of the upper limb, man-made objects, the participant’s own prosthesis (controls viewed their own shoe), unfamiliar cosmetic prostheses and unfamiliar active prostheses. An active prosthesis (colloquially known as a hook) is one in which grip strength can be controlled mechanically or myoelectrically; while a cosmetic or “passive” prosthesis is not operational, but used to resemble the human hand.
The researchers also used a motor task to find and define participant-specific sensory and motor brain regions. They determined functional connectivity between sensorimotor hand regions and visual hand regions from a resting state scan. Functional connectivity between visual and motor cortices was identified between the participants’ missing hand area and visual hand-selective areas. The strength of this connectivity correlated with prosthesis usage, implying increased communication between these regions across the two separate networks.
In all hand-loss participants, activity in the visual-hand selective area was greater than seen in controls, in response to either active or cosmetic unfamiliar prostheses. Additionally, the more the participants used their prosthetic limb, the stronger the correlation to activity in visual hand-selective areas. This finding goes beyond a familiarity response, which is consistent with the fact that participants regularly change their prostheses.
This work shows that the use of artificial limbs affects cortical reorganization in two ways: by increasing communication across the functional network containing visual and sensorimotor hand-specific areas; and by increasing the processing of visual hand-selective regions in response to a prosthesis category, rather than just the participant’s own prosthesis. Most interestingly, these relationships increased with increased usage of the prosthetic limb.
The key finding is that daily prosthesis usage correlates with stronger activity in hand-selective visual areas when amputees were shown images of prostheses. Additionally, prosthesis usage shapes functional connectivity between visual and sensorimotor areas. These findings may find use in the development of highly advanced, cybernetic, prosthetic limbs, and also in rehabilitation following amputation.