Mechanotactile Sensory Feedback Improves Embodiment of a Prosthetic Hand During Active Use

Abstract

There have been several advancements in the field of myoelectric prostheses to improve dexterity and restore hand grasp patterns for persons with upper limb loss, including robust control strategies, novel sensory feedback, and multifunction prosthetic terminal devices. Although these advancements have shown to improve prosthesis performance, a key element that may further improve acceptance is often overlooked. Embodiment, which encompasses the feeling of owning, controlling and locating the device without the need to constantly look at it, has been shown to be affected by sensory feedback. However, the specific aspects of embodiment that are influenced are not clearly understood, particularly when a prosthesis is actively controlled. In this work, we used a sensorized simulated prosthesis in able-bodied participants to investigate the contribution of sensory feedback, active motor control, and the combination of both to the components of embodiment; using a common methodology in the literature, namely the rubber hand illusion (RHI). Our results indicate that (1) the sensorized simulated prosthesis may be embodied by able-bodied users in a similar fashion as prosthetic devices embodied by persons with upper limb amputation, and (2) mechanotactile sensory feedback might not only be useful for improving certain aspects of embodiment, i.e., ownership and location, but also may have a modulating effect on other aspects, namely sense of agency, when provided asynchronously during active motor control tasks. This work may allow us to further investigate and manipulate factors contributing to the complex phenomenon of embodiment in relation to active motor control of a device, enabling future study of more precise quantitative measures of embodiment that do not rely as much on subjective perception.

Keywords: electromyography; embodiment; motor learning; prosthetics; rubber hand illusion; sensory feedback; simulated prosthesis.

FIGURE 1

Overview of experimental protocol. (A) A representative randomization sequence for the Passive Prosthesis Test. There were 4 feedback conditions tested: Synchronous Brushing (SB), Asynchronous Brushing (AB), Synchronous Tapping (ST), and Asynchronous Tapping (AT). A single trial block consisted of the experimenter applying one of the feedback conditions, followed by the participant performing the drift test, and then filling out the questionnaire (Q). Participants were randomized into one of ten predetermined randomization sequences, each consisting of 2 repetitions of each of the 4 conditions, presented in a random order for a total of 8 trial blocks. (B) A representative randomization sequence for the Active Prosthesis Test. For this test, the participant actively controlled the prosthesis in 3 feedback conditions: Synchronous Tapping (ST), Asynchronous Tapping (AT), and no feedback (Nil), for which the tactors were turned off. A trial block consisted of the participant actively using the prosthesis for grasp activities with one of the feedback conditions, followed by an assessment of proprioceptive drift, and then filling out the questionnaire. Participants were randomized to one of ten randomized sequences, consisting of 2 repetitions of each of the 3 conditions, presented in a random order for a total of 6 trial blocks.

FIGURE 2

Experimental setup for Passive Prosthesis Test. (A) Brushing feedback condition. The brushing stimulation was administered by the experimenter to the prosthetic hand and to the participant’s hand with two paintbrushes. (B) Tapping feedback condition. Three mechanotactile tactors placed on the thumb, index and middle fingers of the participant delivered a mechanotactile stimulus when the experimenter applied pressure to the corresponding sensors on the fingertips of the prosthetic hand.

FIGURE 3

Passive Prosthesis Test: Questionnaire results for synchronous and asynchronous brushing. The questionnaire included these 10 statements, presented randomly. Statements 1–5 were used to describe aspects of the embodiment phenomena (Longo et al., 2008). Subjects indicated their response on a visual analog scale ranging from strongly disagree to strongly agree. Red points indicate mean responses for synchronous brushing condition and blue points indicate mean responses for asynchronous brushing condition. Bars extending from these points indicate standard error of the mean (SEM) response. Horizontal black lines indicate statistically significant tendency to evoke affirmative responses (p < 0.05).

FIGURE 4

Passive Prosthesis Test: Mean proprioceptive drifts toward the prosthetic hand for each of the conditions. Error bars indicate standard error of the mean. Black horizontal lines indicate statistical significance at p < 0.05. Red blocks indicate synchronous feedback, and blue blocks indicate asynchronous feedback.

FIGURE 5

Passive Prosthesis Test: Questionnaire results for synchronous feedback conditions. Participants’ responses on statements 1–4 for synchronous brushing were higher than their corresponding responses for synchronous tapping. No statistically significant difference was found between both feedback types. Red points indicate mean responses for synchronous brushing condition and red with black outline dots indicate mean responses for synchronous tapping condition. Bars extending from these points indicate standard error of the mean (SEM) response.

FIGURE 6

Passive Prosthesis Test: Questionnaire results for synchronous and asynchronous tapping feedback conditions. Subjects indicated their response on a visual analog scale ranging from strongly disagree to strongly agree. Red with black outline points indicate mean responses for synchronous tapping condition and blue with black outline points indicate mean responses for asynchronous tapping condition. Bars extending from these points indicate standard error of the mean (SEM) response.

FIGURE 7

Active Prosthesis Test: Questionnaire results for controlling a prosthesis while receiving three feedback conditions. Participants’ responses on statements 1–5 for synchronous tapping were higher than responses for no feedback condition. Red with black outline points indicate mean responses for synchronous tapping condition, blue with black outline dots indicate mean responses for asynchronous tapping condition, and green dots indicate mean responses for no feedback condition. Bars extending from these points indicate standard error of the mean (SEM) response. Horizontal black line indicates statistically significant tendency to evoke affirmative responses (p < 0.05).

FIGURE 8

Active Prosthesis Test: Mean proprioceptive drifts toward the prosthetic hand after actively controlling it while receiving three types of feedback. Error bars indicate standard error of the mean. Red with black outline bar indicates synchronous tapping feedback, blue with black outline bar indicates asynchronous tapping feedback, and green bar indicates no feedback condition.