Dopamine, motor imagery, and proprioception: a neurochemical probe into the perception-imagery debate

The relationship between mental imagery and perception is a central debate in cognitive neuroscience. This study provides a pharmacological causal test of motor imagery frameworks by examining how acute dopaminergic modulation affects motor imagery and proprioceptive processing. In a double-blind, placebo-controlled pre–post design, 42 healthy adults received branched-chain amino acids (BCAAs) to transiently reduce dopaminergic tone. Serum prolactin levels confirmed successful modulation: the BCAA group showed stable levels, while the placebo group exhibited the expected diurnal decline. Dopaminergic reduction impaired total motor imagery (total MIQ-3 performance) and selectively internal visual motor imagery, eliminating the practice-related improvement observed in the placebo group. In contrast, proprioceptive accuracy remained stable across constant error (CE) and variable error (VE), with Bayesian analyses showing anecdotal evidence for the null model (BF₁₀ < 1). For absolute error (AE), the evidence was weak to moderate in favor of the alternative (BF₁₀ = 2.61–3.96), indicating some support for a group difference but not strong enough to draw firm conclusions. Angle-dependent effects revealed greater undershooting at 50° than 30° in both groups, reflecting task difficulty rather than dopaminergic influence. This selective imagery impairment alongside stable proprioception challenges strong PM accounts that predict parallel modulation across domains and supports frameworks allowing functional dissociation between imagery and perceptual-motor processes. These findings highlight dopamine’s specific role in sustaining internal motor simulations, advancing theoretical debates on the relationship between imagery and perception.