Implicit adaptation's effect on sensorimotor and motor confidence.

Sensorimotor adaptation maintains movement accuracy by counteracting perturbations or miscalibrations. It can operate explicitly, by consciously adjusting motor plans to correct errors, or implicitly, by automatically recalibrating sensorimotor mappings without altering the motor plan. While explicit adaptation is known to reduce sensorimotor confidence-the perceived likelihood of successful action-it is unclear whether implicit adaptation similarly affects confidence in sensorimotor judgments or motor awareness (knowledge of one's own limb position). To investigate this, participants made reaching movements to visual targets without seeing their hand. Cursor feedback followed an "error-clamped" trajectory: its radial position matched the hand, but its angular direction was fixed and independent of actual hand direction, a manipulation participants were told to ignore. The clamp direction varied sinusoidally over trials (±10°; 12 cycles per session). Participants reported confidence by adjusting the size of an arc centered on the target or, in another task, centered on reported reach direction; larger arcs indicated lower confidence. Points were awarded when the arc encompassed the true reach direction, with fewer points for larger arcs, encouraging accurate and meaningful confidence reports. Fourier analysis of reach and report time series revealed a strong 12-cycle component in both, demonstrating robust implicit adaptation and corresponding changes in motor awareness. These findings indicate that although implicit adaptation operates unconsciously, the resulting mismatch between motor plans and proprioceptive signals can bias judgments of reach outcomes. However, confidence judgments were not consistently affected, suggesting that sensorimotor confidence and confidence in proprioceptive awareness may rely on partially distinct mechanisms.