Force and Position Control in Humans – The Role of Augmented Feedback

The overall goal of this experiment was to investigate the influence of the interpretation of feedback on motor behavior, as well as how the interpretation and inhibitory motor cortical activity can be assessed. This approach can help to answer key questions in several fields such as sports, science, psychology, rehabilitation, and basic science. The main advantage of this technique is that it excludes any feedback related effect such as color or gain, so the effects can be clearly attributed to the interpretation of the feedback.

Begin by seating the subject in a comfortable chair. Next, shave the subject's skin over the first dorsal interosseous muscle, or FDI, and the abductor pollicis brevis, or APB, of the right hand. Use sandpaper to slightly abraid the area and disinfect with propanol.

Then attach the self-adhesive EMG electrodes to the skin over the muscle bellies of the FDI and APB. Place an additional reference electrode on the olecranon of the same arm. Use this bipolar configuration of surface EMG electrodes in order to measure electro physiological responses elicited by trans cranial magnetic stimulation, or TMS, and muscular activation produced by the subject.

Next, set up TMS by using a figure of eight coil attached to a TMS stimulator in order to stimulate the contralateral motacortical hand area. Use a mapping procedure to find the optimal position of the coil relative to the scalp for eliciting motor evoked potentials, or MEPs, in the FDI muscle. To do this, place the coils zero point five centimeters interior to the vertex and over the mid line with the handle pointing at 45 degrees anti-clockwise relative to the sagitaal plane in order to induce a posterior anterior flow of current in the center of the coil.

At first, choose a small stimulation intensity to get the subjects accustomed to the magnetic pulses. Then increase the stimulation intensity in small steps, such as two to three percent maximum stimulator output. Move the coil in the frontal rostral and media lateral direction in order to find the optimal hot spot, or area where the greatest MEP can be observed for stimulating the FDI muscle.

After locating the FDI hot spot, determine the resting motor threshold as the minimum intensity required to evoke MEP peak to peak amplitudes in the EMG larger than fifty microvolts in three out of five consecutive trials. Inspect the size of the MEPs displayed online on the computer screen. Finally, decrease the stimulation intensity of the TMS machine in steps by two percent MSO until the MEP can no longer be observed and an EMG suppression of the ongoing muscle activity becomes apparent.

For feedback presentation, visually display the force and the position feedback on a computer placed one meter in front of the subject. In each of the three conditions, present a target line that either corresponds to 30%of the subject's individual maximal voluntary force or the finger angle of the index finger at 30%maximally voluntary contraction, referred to as MVC on the computer screen. Instruct the subject to match the target line as closely as possible.

Next, execute a sustained contraction using the fatiguing motor task. Instruct the subjects to match the target line of 30%MVC for as long as possible with the line corresponding to the applied force or with the position of their finger corresponding to a force level of 30%MVC. Ask the subjects to hold the contractions until task failure, which for the fF group is defined as the point where the subjects are no longer able to maintain the contraction inside a five percent window of the target force over a period of five seconds.

Finally, on a separate day from the fatiguing experiments, conduct the TMS experiment by asking the subjects to match the target line of 30%MVC and only hold the contraction as long as the TMS stimulation lasts. In this procedure, subjects were instructed in a way that they believed in half of their trials to have received position feedback and in the other half of the trials to have received force feedback, but in fact they were tricked in half of their trials, as the pF group always received position feedback and the fF group always received force feedback. Regardless of group, the subjects displayed an increase in the EMG activity from the start to the end of the contraction.

Furthermore, in the pF and the fF groups, the stimulation with TMS resulted in a greater suppression during the position controlled tasks compared to the force controlled task. Once mastered, this technique can be done in one and a half hours if performed properly. Following this procedure, additional measurements such as stimulation can be applied in order to answer additional research questions like which cortical mechanisms contribute to the results.

After watching this video, you should have good understanding of how the interpretation of feedback can be used to study the influence of feedback on motor behavior.