Sensorimotor tongue representation in individuals with unilateral upper limb amelia

The purpose of the present study was to examine the impact of the congenital absence of one hand on cortical organization of the sensorimotor cortex (S1/M1). We investigated the tongue representation in S1/M1 in nine participants with normally developed limbs, comprising the control group, and in eight persons with a congenitally completely missing hand (i.e. unilateral hand amelia). All participants were examined by fMRI while performing horizontal tongue movements. The significantly activated clusters covering S1/M1 in both hemispheres were analyzed with respect to the number and intensity of activated voxels, as well as the location of the activation. In the right-handed control group, the number of activated voxels was significantly higher in the left as compared to the right hemisphere demonstrating a clear left hemispheric motor dominance for horizontal tongue movements. In the amelic individuals, no such hemispheric lateralization effect was observed. The neural activation pattern underlying tongue movement, however, was enlarged and displaced in the hemisphere contralateral to the missing limb when compared to the respective motor non-dominant, right hemisphere of the control group participants. The present findings suggest that congenital absence of one hand leads to an appreciably altered topological organization of S1/M1 consisting of an enlargement of the tongue representation and a shift towards the "hand" area which, however, had never received any input from a hand.