Fig. 5. Comparison of muscular recruitment profiles induced by lateral and medial electrodes.

a Approximate positions of the medial electrodes used to obtain the results in (b) and underlying motoneuronal distributions. Electrode contacts are magnified for better visualization (scale factor = 2). b Mean muscular activations obtained with the medial electrodes at the same rostro-caudal levels than the lateral electrodes of Fig. 4c for the 2 monkeys implanted with the design-1 array (see “Methods”). DEL: deltoid, BIC: biceps brachii, TRI: triceps brachii, FCR: flexor carpi radialis, FDS: flexor digitorum superficialis, ECR: extensor carpi radialis, EDC: extensor digitorum communis, APB: abductor pollicis brevis. c Correlation coefficients between muscular recruitment profiles and motor nuclei distributions (see “Methods”) for lateral and medial electrodes. d Maximal mean muscular activations achieved with lateral or with medial electrodes. e Maximal selectivity indexes (see “Methods”) achieved with lateral or medial electrodes. In b–e data points (small squares/circles without borders) may be hidden by medians (large squares/circles with black borders). f Examples of muscular response latencies following stimulation from rostral medial or caudal medial electrodes. Top: muscular responses recorded in the flexor digitorum profundis of monkey Mk-Li. Bottom: muscular responses recorded in the abductor policis brevis of monkey Mk-Lo. Onsets of responses are indicated by the vertical dashed lines. g Statistical analysis of the differences in onset latencies between rostrally-induced responses and caudally-induced responses (two-sided unpaired t-tests, **p < 0.01; ***p < 0.001, see “Methods”). For each muscle, eight responses induced at amplitudes near motor threshold with one rostral (purple) and one caudal (yellow) electrodes were retained. Bottom histogram: distribution of the delay of rostrally-induced responses compared to caudally-induced responses.