Fig. 2.

Neuromechanics of the polydactyly hand. a Dedicated isometric interface to investigate the force capability of each finger in individuals with five- and six-fingered hands. The interface was used for the analyses presented in subplots (b–d). Subjects were initially asked to exert maximal force (MF) with a single finger. In a consecutive experiment subjects were asked to control 10%, 20%, or 30% of MF during 15 s long trials. Two six- and 13 five-fingered subjects carried out these experiments. b MF produced by individual fingers. The MF was similar for five- and six-fingered subjects for all regular fingers. c Force variability (standard deviation of the force) as a function of the magnitude of the produced force expressed in percentage of the maximal force. Error bars depict SEMs across subjects. d Enslaving shows the forces induced in other fingers when the subject is instructed to exert the maximal force in one finger. Enslaving between finger i and j was computed as $e_{ij}=\frac{F_j\left(i\right)}{{\mathrm{MF}}_j}$, where F_j(i) is the force produced by finger j when finger i was instructed to produce maximal force. MF_j depicts the maximal force of finger j. In the matrix plot the instructed finger is shown on the y axis, hence, each row shows the induced force relative to the maximal force of the corresponding finger, i.e. e_i. e Two-dimensional projection (multidimensional scaling, MDS) of fMRI activation in sensorimotor cortex during individual finger movements in subject P1 and the average across nine five-fingered control subjects (left). Colors depict different fingers as in (c). Ellipses show the standard error of the mean. The location of the activation cluster of the supernumerary finger is separate from the activation clusters of the other fingers. Selected voxels which were used for the MDS are shown on the right. f Mental representation of six-fingered hands. Blindfolded subjects pointed with the index finger of one hand to a cued location (first, second knuckle or tip) on the other hand. Pointing errors were similar in the two six- and 9 five-fingered subjects and similar for the supernumerary as for other fingers