Figure 2. Missing hand maps are replicated during dissociated bimanual hand movements.

(A) In the bimanual task, individuals performed paired digit movements with both hands, resulting in reversed digit cycles between the hands. The colour code indicates the cycle lag as shown in (A). Symmetrical colour gradients across the two hemispheres would reveal the missing hand map as dependent on intact hand movements (B: case 1). Colour gradient reversal across hemispheres indicates dissociated hand representations (B: case 2), as was seen in both amputees and controls (C, D respectively, left two panels). Maps contralateral to the missing hand (left hand in controls) resembled the unimanual task maps (Figure 1), both qualitatively (C, D, right panel) and quantitatively, using the Dice overlap coefficient (E). The digit maps were adjusted for multiple comparisons using false discovery rate (FDR). Other annotations are as in Figure 1.

DOI: http://dx.doi.org/10.7554/eLife.15292.009

Figure 2—figure supplement 1. Over-representation of the intact hand in the missing hand territory.

To investigate whether the observed missing hand maps (Figure 1, Figure 4) were preserved despite reorganisation of displaced inputs (as reported previously [Makin et al., 2013b]), we quantified intact hand representation in the territory of the missing hand in the amputees participating in the current study. For this purpose, we employed an independent dataset, acquired using standard field neuroimaging (acquisition parameters were identical to those described for the standard field imaging in the materials and methods section, with the exception of the number of acquisition volumes). (A) Participants were cued to perform movements with their intact hand (simultaneous flexion and extension of all five digits), or dominant hand in controls (n = 15), as part of a block-design protocol, used to identify body-part representations (see [Makin et al., 2013b] for details of a similar protocol). This movement condition was contrasted against a baseline (rest) period and standardised (z) beta values during intact/dominant hand movements were extracted from the peak voxel within a postcentral gyrus (PCG) mask in the contralateral hemisphere. To identify the estimated position of the missing hand territory, this peak voxel was mirror flipped on the axial plane, and the standardised beta value for intact hand activity was extracted from this ROI in the ipsilateral hemisphere. (B) To assess asymmetry in intact hand activity across the two hemispheres, the following inter-hemispheric asymmetry index was calculated for each of the three amputees and 15 controls individually: (contralateral hemisphere – ipsilateral hemisphere) / (contralateral hemisphere + ipsilateral hemisphere). Higher values of this index indicate that the activity pattern for the intact hand was greater in the contralateral, compared to the ipsilateral, sensorimotor hand area, as found was in the control participants. Lower values in this index represent less asymmetry, resulting from (relatively) higher activity in the ipsilateral hand area during intact hand movements. Error bars indicate 95% confidence intervals. The above elbow and brachial plexus amputees showed significantly lower asymmetry than controls (one-tailed Crawford and Howell t-test [Corballis, 2009]; t₍₁₄₎ = −1.84, p = 0.04; and t₍₁₄₎ = −1.96, p = 0.04 respectively). The below elbow amputee showed a high asymmetry value, similar to controls (one-tailed Crawford and Howell t-test (Corballis, 2009); t₍₁₄₎ = 1.26, p = 0.11). This indicates that two of the tested amputees show over-representation of their intact hand in the missing hand territory, as previously reported (Makin et al., 2013b). These results suggest that the missing hand maps can be preserved while reorganisation of displaced inputs from the intact hand has taken place.

DOI: http://dx.doi.org/10.7554/eLife.15292.010