Extended Data Fig. 8 |. Interference and sequential learning of opposite curl fields applied to the same target.
a, Block design of the interference experiment. Same as in Fig. 4a except that two opposite curl fields were applied to the same reach target. b, Behavioural learning quantified by lateral hand deviation. Hand lateral deviation in both curl fields only slightly decreased during simultaneous learning (blue), and significantly reduced during sequential learning (red). One-sided rank-sum test: monkey U, ***P = 1.51 × 10−11; monkey V, ***P = 1.51 × 10−11. c, Preparatory neural states in the force-predictive TDR subspace. In blocks ii and iii (left panel), preparatory states of the two curl fields (triangles) were mixed together around the before-learning state (circle). Error-clamp states (diamonds) of most targets shifted from their corresponding before-learning states. These shifts did not show coherent patterns across targets or monkeys and were likely unrelated to learning. In the sequential-learning block (right panel), preparatory states of the two curl fields (triangles) gradually rotated opposite their curl field directions. The small arrow points to the before-learning state of the reach target that later had curl fields (trained target). Neural states for seven nearest targets are shown. d, Perpendicular hand force differences between error-clamp and before-learning trials (top panel), and the rotatory angle from before-learning to error-clamp neural states (bottom panel), did not show coherent patterns across targets or monkeys. Zero degree on the x axis, the trained target. Error bars, s.e.m. from resampling (100 repeats). e, Uniform shifts for learning two curl fields and the residual interference shift were defined in the same way as in Fig. 4e, f. The two field-specific uniform shifts were close to antiparallel (monkey U, dot product = − 0.79; monkey V, dot product = −0.64), and so we could visualize preparatory neural states in a 2D plane spanned by the field 1 uniform shift and the residual interference shift. In blocks ii and iii, preparatory states of the two curl fields (orange and pink) shifted away from the before-learning centroid (grey circle) along the residual interference axis, but they remained close to each other (Hotelling’s T2 test: monkey U, P = 0.66; monkey V, P = 0.98). During sequential learning, preparatory states of the two curl fields (green and purple) were separated by opposite uniform shifts (Hotelling’s T2 test: monkey U, P = 2.49 × 10−4; monkey V, P = 2.90 × 10−5). b–e, One session per monkey. Though just one session of this interference experiment was performed with each monkey, the results were consistent across monkeys and complimentary to findings when monkeys learned multiple fields sequentially, which supported the indexing hypothesis (Fig. 3).