Repetitive finger force production in predictable environments.

Previous research suggests that removal of visual feedback of force output in a sequential force production task results in a continuous escalation of the force magnitudes produced. Central predictive mechanisms involving reafference result in self-generated forces being perceived as weaker, thus leading to a systematic over-production of force. While this effect has been widely examined with respect to sensation of self-produced stimuli, its role in the sequential production of discrete forces has not been extensively studied. The objective of the present experiment was to further investigate the force escalation effect by examining the sequential force production task in two predictable environments where force targets progressively changed between 8 and 16 N. In one environment target force magnitudes continually increased at a predictable rate, while in the second target force magnitudes decreased at the same rate. Eight healthy participants produced a series of repetitive pinch grip forces in time to a metronome at a frequency of 2 Hz. Visual feedback of force output was removed 10 s in to the trial, with participants performing continued responses for the remaining 20 s. Following removal of visual feedback participants rapidly accrued errors in force magnitude, but slopes of the peak force series did not differ from those of the target series. Errors in magnitude were bi-directional, showing dependence on the increasing or decreasing environment in which the task was performed. The observed escalation and de-escalation effect can both be attributed to central predictive mechanisms and the force salience effect in reafference.