Phasic arousal sharpens, rather than amplifies, goal-dependent feedback control
Voluntary movements tolerate variability that does not threaten the goal: when reaching toward a wide target the hand can drift sideways, whereas a narrow target demands tighter control. The nervous system enforces this through corrective feedback responses that scale with task demands, yet how this scaling is set remains unclear. Arousal is a candidate, but how it would act is contested: the classical view casts it as a global amplifier that raises neural gain uniformly, whereas a more recent account holds that it is selective, strengthening task-relevant signals while suppressing others. These accounts make opposite predictions for goal-dependent feedback control, which we tested causally. In 27 human participants (24 {+/-} 2.7 years old) reaching toward narrow or wide targets on a KINARM robot, a 4-second pulsed train of transcutaneous auricular vagus nerve stimulation (taVNS) evoked a phasic boost of arousal before each movement, confirmed by pupil dilation. On a subset of trials the robot applied a mechanical perturbation during the reach, and we measured the corrective muscle activity it evoked in the short- and long-latency reflex windows. The results favored the selective account. Phasic arousal sharpened corrections to the goal: it amplified the difference between narrow and wide responses in the long-latency window, the established readout of goal-dependent control, and suppressed the short-latency response, normally insensitive to the goal, specifically for the wide target, where deviations are harmless. Phasic arousal therefore does not energize the motor system indiscriminately but allocates corrective control to where it serves the task, extending the principle of selective gain, established in perception and memory, to the online control of movement.