Discrete task-relevant features of an overt response, such as response location, are bound to, and retrieved by coincidentally occurring auditory stimuli. Here we studied whether continuous, task-irrelevant response features like force or response duration also become bound to, and retrieved by such stimuli. In two experiments we asked participants to carry out a pinch which produced a certain auditory effect in a prime part of each trial. In a subsequent probe part, tones served as imperative stimuli which either repeated or changed as compared to the effect tone in the prime. We conjectured that the repetition of tones should result in more similar responses in terms of force output and duration as compared to tone changes. Most parameters did not show notable indications for such similarity increases, including peak force or area under force curve, though the correlation between response durations in prime and probe was higher when tones repeated rather than changed from prime to probe. We discuss these results regarding perceptual discriminability and deployment of attention to different nominally task-irrelevant aspects of pinch responses.

Previous research indicates that quick, repetitive actions (pinches, taps, button presses) are executed with smaller force when followed by predictable and salient action effects (tones, light flashes). It has been suggested that successive actions become gradually softer until an optimum is reached, which presumably reflects a balance between the ability to maintain a high probability of action success, and the reduction of exerted force to conserve energy. In the present experiments, we investigated whether this action-effect-related motor adaptation appeared when the arrival of the action effect was unpredictable. Young adult participants produced evenly spaced pinches (Experiment 1) or taps (Experiment 2), which resulted in a tone in 50% of the trials. The presence of the tone effect varied randomly from trial to trial, leading to action sequences with various tone-elicitation patterns. We have found that pinches and taps preceded by sequences of tone-eliciting actions were softer than actions preceded by sequences of toneabsent trials. In the case of pinches, actions were also modulated on the fly, with the current action being softer and briefer when a tone was elicited. Our results demonstrate that action effects can modulate subsequent and ongoing actions even when the arrival of these effects is unpredictable. Public Significance StatementThis study shows that salient action effects are used to optimize action execution when the action is performed repeatedly. This type of-potentially automatic-motor optimization allows one to quickly adapt to interactions offered by novel devices and tools.