Whose arm is it anyway? An fMRI case study of supernumerary phantom limb

Abstract

Under normal circumstances, information from a number of sources is combined to compute a unitary percept of the body. However, after pathology these influences may be perceived simultaneously, resulting in multiple dissociated conscious representations. In a recent paper, we described subject E.P., a right-handed female stroke patient with a right frontomesial lesion who sporadically experiences a supernumerary 'ghost' left arm that occupies the previous position of the real left arm after a delay of 60-90 s. We used a delayed response paradigm with functional MRI to examine the haemodynamic correlates of E.P.'s illusion. Comparison of periods of time during scanning when the ghost arm was present against when it was not revealed a single cluster (9 voxels, t = 5.11, P < 0.012 corrected for multiple comparisons) located on the right medial wall in the supplementary motor area ('SMA proper'). Our results suggest that areas traditionally classified as part of the motor system can influence the conscious perception of the body. We propose that, as a consequence of her injury, E.P. is aware of the position of the phantom limb in this 'action space' while also continuing to be aware of the true position of her real limb on the basis of afferent somatosensory information.

What is embodiment? A psychometric approach

“…It confirms the intuition that the external object becomes a substitute for, and a part of, the functional self. But it also suggests that plasticity is constrained by a principle of body constancy: the novel rubber hand is incorporated by functionally suppressing the existing hand, rather than by adding an additional 'supernumerary' hand (McGonigle et al, 2002). Furthermore, the participant's own hand is felt to disappear.…”

The bodily self: Insights from clinical and experimental research

“…1 and [42]). A woman with right fronto-mesial damage involving the supplementary motor area and the cingular gyrus and no hemiplegia or motor disorders reported the following peculiarity: whenever she moved her physical limb, a phantom seemed to occupy, after a few seconds, the place left by the real limb [43]. Thus, the existence of this supernumerary phantom specifically relied on movements of the real counterpart limb.…”

Phantoms in the Voice: A Neuropsychoanalytic Hypothesis on the Structure of the Unconscious

“…Accordingly, it is also thought that the sustained activation of secondary motor area neurons due to a right frontomesial lesion in patient E.P. (reported by McGonigle et al, 2002) is the cause of her intermittent experiences of a supernumerary "ghost" left arm in her so-called Figure 4. The efference copy model (Blakemore, Wolpert, & Frith, 1998;Georgieff & Jeannerod, 1998;Sperry, 1950;van Holst, 1954).…”