Fig. 4.

Threshold force at recruitment and firing rate at steady state vary systematically with the direction of force at the wrist. a, Posture of the arm (side view) during the experiments. b, All unit recordings are aligned with force rise onset (marked by arrowhead below traces) and were recorded from one BI electrode during the same experiment. Three trials are shown for each direction; the presentation order was randomized during the experiment. As force direction changes from +1 to +3, and from 0 to −2, the force levels at recruitment become progressively higher, whereas firing rate at steady state decreases.c, Typical force trajectory with onset of force rise marked by the arrowhead. The vertical scale bar represents 20 N. d, Threshold force data plotted in y/x coordinates are fit by a line whose perpendicular (arrow) points in the best direction of the unit.