Automatic adjustment of visuomotor readiness.

Participants initiated a reaching movement to a single target more rapidly than to an odd-color target among distractors when the two trial types were presented in separate blocks, reflecting differentiated states of sensorimotor readiness for a relatively easy (single target) versus harder (odd-color target) tasks. This pattern was eliminated when the two trial types were randomly mixed. Latencies for the easy single trials increased, and those for the harder odd-color trials decreased, showing homogenization. The faster movement initiation in the odd-color target task was accompanied by curved trajectories, directed toward a distractor initially but corrected in midflight. Two possible hypotheses could account for this differentiated adjustment in visuomotor readiness: (1) explicit knowledge of upcoming trial types and (2) implicit leaning derived from history of the very recent past, that is, repetition of the same type of trials. To distinguish between these two accounts, we included a third condition where the trial types were predictably alternated. Contrary to the explicit knowledge hypothesis, this also led to homogenization of initiation latencies, and curved trajectories. We conclude that visuomotor readiness is automatically adjusted by the recent experience of trial difficulty.