Time-division multiplexing for myoelectric closed-loop control using electrotactile feedback

Abstract

BackgroundRestoring sensory feedback in myoelectric prostheses is still an open challenge. Closing the loop might lead to a more effective utilization and better integration of these systems into the body scheme of the user. Electrotactile stimulation can be employed to transmit the feedback information to the user, but it represents a strong interference to the recording of the myoelectric signals that are used for control. Time-division multiplexing (TDM) can be applied to avoid this interference by performing the stimulation and recording in dedicated, non-overlapping time windows.MethodsA closed-loop compensatory tracking task with myocontrol and electrotactile stimulation was used to investigate how the duration of the feedback window (FW) influences the ability to perceive the feedback information and react with an appropriate control action. Nine subjects performed eight trials with continuous recording and contralateral feedback (CONT-CLT) and TDM with ispilateral stimulation and recording using the FW of 40 ms (TDM40), 100 ms (TDM100) and 300 ms (TDM300). The tracking quality was evaluated by comparing the reference and generated trajectories using cross-correlation coefficient (CCCOEF), time delay, root mean square tracking error, and the amount of overshoot.ResultsThe control performance in CONT-CLT was the best in all the outcome measures. The overall worst performance was obtained using TDM with the shortest FW (TDM40). There was no significant difference between TDM100 and TDM300, and the quality of tracking in these two conditions was high (CCCOEF ~ 0.95). The results demonstrated that FW duration is indeed an important parameter in TDM, which appears to have an optimal value. Among the tested cases, the FW duration of 100 ms seems to be the best trade-off between the quality of perception and a limited command update rate.ConclusionsThis study represents the first systematic evaluation of a TDM-based approach for closing the loop using electrotactile feedback in myoelectric systems. The overall conclusion is that TDM is a feasible and attractive method for closed-loop myocontrol, since it is easy to implement (software-only solution), has limited impact on the performance when using proper FW duration, and might decrease habituation due to burst-like stimulation delivery.Electronic supplementary materialThe online version of this article (doi:10.1186/1743-0003-11-138) contains supplementary material, which is available to authorized users.

Task-Specific Somatosensory Feedback via Cortical Stimulation in Humans

“…Secondly, we chose to use a sufficiently long ITI in order to potentially pull out ECoG recording data between stimulations. This time-division multiplexing (TDM) of the stimulation and recording periods could be used in a closed-loop BCI to allow for tactile feedback without obscuring all neural recordings of motor intention with a stimulation artifact [29]. After removing the stimulation artifact from the ECoG recording we believe that our ITI still leaves enough time for meaningful motor decoding based on our previous experiments which generally use 200 ms windows for decoding.…”

Multichannel Electrotactile Feedback With Spatial and Mixed Coding for Closed-Loop Control of Grasping Force in Hand Prostheses

“…In the reinforced learning phase the levels were randomly presented by delivering the stimulation at a specific electrode pad (SPA) or at a specific pad group and frequency (MIX) for 1 second, after which the subjects were asked to guess the presented level, by reporting a number from 1 to 15. The experimenter then provided verbal feedback about the correct answer, by saying "correct" if the subject successfully guessed the level or by saying "incorrect" and reporting the correct number (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15). Each level was presented 3 times in total (3 x 15 = 45 trials).…”

“…Also, the electrical stimulation represents a strong interference to the recording of the myoelectric signals. These factors need to be considered for practical applications, e.g., the parameters should remain below discomfort threshold and the methods to avoid [13], [14] and/or suppress [15] stimulation artefacts need to be implemented. The most obvious variable to feedback to the user is the grasping force, since it cannot be directly assessed through vision and it critically determines the grasp outcome.…”

Non-Invasive, Temporally Discrete Feedback of Object Contact and Release Improves Grasp Control of Closed-Loop Myoelectric Transradial Prostheses

“…eliciting phantom sensations) can shorten the learning process [1]. Among the non-invasive techniques, vibro-or electro-tactile feedback have been widely investigated in the past as they do not require surgery, their high acceptance by the users, low power consumption, small dimensions and compatibility with the EMG signal [11] [12][13] [14].…”