Figure 1.

Schema of hypothetical activity of different cells to generate adapted movements under force field. A, Left, Trajectories in the absence of force field are straight (upper row, black arrows). Firing rate of cells whose PD corresponds to these directions are marked by the length of the colored arrows (lower row). Center, Trajectory (red, upper row) deviates in the direction of the force field (orange arrows) as nonadapted cells fire as in the null-field (lower row). For simplicity, the viscous curl force field was represented as straight lines whose length varied to reflect the bell-shaped velocity profile. Note that the direction of the force field was always orthogonal to the direction of movement. Right, When monkeys effectively anticipate the force properties (orange) and generate a compensatory direction (purple), the hand takes an approximately straight path (black) when reaching to the target. B, Two possibilities: (1) adaptive changes in one subpopulation of cells in which activity increases for cells with PD near the LD (B1) or those with PD orthogonal to the LD (B2); and (2) adaptive changes in different subpopulations of cells (B3) in which activity increases for cells with PD that counters the force field and decreases with PD along the direction where force field assists movement. C, Behavioral after-effects. Hand paths corresponding to single trials (early and late force field trials and catch trials) from the same recording day for each feedback condition (counterclockwise, FFv; clockwise, FFnv). Deviations of hand paths in catch trials (blue) were mirror images of the deviations early in adaptation (red). D, Histograms of the initial directional deviations of all catch trials. The directional deviations are expressed in angles. Negative values denote directional deviations opposite to the force field direction. Initial directional deviations did not differ with and without VFB (t test, p > 0.10). The mean directional deviations of significant after-effects [i.e., those above 2 SD from the mean directional deviation (dashed vertical lines) of control trials] reflect the compensatory direction (FFv = −45 ± 2° and FFnv = −41 ± 2° for LD at 0° convention).