Phantom digit somatotopy: a functional magnetic resonance imaging study in forearm amputees

Abstract

Forearm amputees often experience non-painful sensations in their phantom when the amputation stump is touched. Cutaneous stimulation of specific stump areas may be perceived as stimulation of specific phantom fingers (stump hand map). The neuronal basis of referred phantom limb sensations is unknown. We used functional magnetic resonance imaging to demonstrate a somatotopic map of the phantom fingers in the hand region of the primary somatosensory cortex after tactile stump stimulation. The location and extent of phantom finger activation in the primary somatosensory cortex corresponded well to the location of normal fingers in a reference population. Stimulation of the stump hand map resulted in an increased bilateral activation of the primary somatosensory cortex compared with stimulation of forearm regions outside the stump hand map. Increased activation was also seen in contralateral posterior parietal cortex and premotor cortex. Ipsilateral primary somatosensory cortex activation might represent a compensatory mechanism and activation of the non-primary fronto-parietal areas might correspond to awareness of the phantom limb, which is enhanced when experiencing the referred sensations. It is concluded that phantom sensation elicited by stimulation of stump hand map areas is associated with activation of finger-specific somatotopical representations in the primary somatosensory cortex. This suggests that the primary somatosensory cortex could be a neural substrate of non-painful phantom sensations. The stump hand map phenomenon might be useful in the development of prosthetic hand devices.

Neural correlates of evoked phantom limb sensations

“…For example, Björkman et al (2012) used functional magnetic resonance imaging (fMRI) during tactile stimulation of the residual limb and reported bilateral activation of the primary somatosensory cortex (SI), contralateral parietal and premotor cortices. However, they were not able to dissociate the neural activation induced by the stimulation of the residual limb from the percept of phantom sensations as they might have activated inputs from the residual limb to the brain region that formerly represented the amputated limb and they did not assess ratings of evoked phantom sensations.…”

Remodeling of cortical activity for motor control following upper limb loss

“…In a case-study report, it was shown that when observing actions outside of their motor repertoire, amputees recruit areas outside of the expected left parietofrontal system (attributed to the mirror neuron system), additionally areas including the right temporoparietal junction (attributed to the mentalizing system (Van Overwalle et al, 2009, Aziz-Zadeh et al, 2012)). Other studies have also shown motor activation expanded to the parietal areas which correlates with phantom sensations (Bjorkman et al, 2012). While none of our participants reported lingering phantom sensations, in this study activation is best aligned to concerns in motor planning beyond sensation.…”

Restoring tactile and proprioceptive sensation through a brain interface

“…Thus, the remapping of S1 does not necessarily reflect changed circuitry in S1. Fourth, some amputees develop distinct patches of skin on their limb stump, stimulation of which results in a percept projected to individual digits on the phantom hand and activation in the corresponding digit representation in S1 (Bjorkman et al, 2012). As might be expected, then, electrical stimulation of deafferented regions of S1 in human amputees evokes sensations on the phantom limb rather than on the invading body regions (Ojemann and Silbergeld, 1995; Woolsey et al, 1979).…”