Subjects
Six subjects with transradial amputation participated in this study [Table I], which was approved by the Northwestern University Institutional Review Board; all experiments were performed in accordance with relevant guidelines and regulations, and all subjects provided informed consent before starting the study.
Table I. Transradial Amputee Subject Demographics
Subject ID
|
Sex
|
Age
|
Side of Amputation
|
Years since Amputation
|
Cause of Amputation
|
Home Prosthesis
|
Familiarity with Myoelectric Control
|
TR1
|
M
|
71
|
R
|
32
|
Trauma
|
Passive
|
Familiar from participation in research studies
|
TR2
|
M
|
33
|
L
|
5
|
Trauma
|
Myoelectric, multiarticulate hand
|
Daily user of myoelectric pattern recognition, 5 years
|
TR3
|
M
|
28
|
R
|
10
|
Trauma
|
Body-powered
|
Familiar from participation in research studies
|
TR4
|
M
|
56
|
R
|
40
|
Trauma
|
Myoelectric, multiarticulate hand
|
Daily user of two-site myoelectric control, 5+ years
|
TR5
|
F
|
60
|
R
|
6
|
Cancer
|
Passive
|
Familiar from participation in research studies
|
TR6
|
M
|
65
|
L
|
6
|
Trauma
|
Body-powered
|
Previous myoelectric pattern recognition user
|
Experimental Setup
Subjects sat in front of a computer monitor displaying a virtual arm. A Biometrics twin-axis electrogoniometer was attached to the upper and lower arm to measure the elbow flexion angle. Goniometer signals were low-pass filtered at 5 Hz with a 2nd order Butterworth filter. Two Delsys Bagnoli electromyographic (EMG) sensors measured EMG signals from wrist flexor and extensor sites on the residual limb [Fig. 1a]. The electrode placement was determined via voluntary muscle contraction and palpation (similar to the method used to place electrodes when controlling a myoelectric prosthesis), and the reference electrode was placed over the olecranon or on the clavicle. EMG signals were high-pass filtered at 0.1 Hz, positive-rectified, and low-pass filtered at 5 Hz using a 2nd order Butterworth filter. Data were acquired at 1000 Hz and downsampled to 100 Hz after filtering.
Subjects controlled a virtual two-link arm using the goniometer to dictate proximal link position, and the EMG sensors to dictate distal link velocity [Fig. 1b]. The virtual arm started in a neutral position on the screen and targets appears around the screen in four fixed positions [Fig. 1c]. Specifics for the control of the arm and the positioning of elements on the screen are the same as in our previous study27; however, the task was mirrored horizontally for left-side amputee subjects to align the movement of the virtual arm with the subject’s arm.
Fig 1. Center-Out Reaching Experiment Setup for a subject with left-side amputation. (a) Subject holds their arm in a relaxed posture at their side. Attached to the subject’s residual limb, a goniometer (green) measures elbow angle, and EMG sensors (blue) measure EMG amplitude. (b) Subjects perform center-out reaches with a virtual limb (black); goniometer angle controls the angle of the proximal link (or elbow, green), and the EMG amplitude controls the speed of the distal link (or wrist, blue). Subjects started with the limb endpoint in the home circle, and a grey ball would appear above a target; each target could only be reached with a single limb configuration (dashed grey). When the limb endpoint left the home circle, the ball began to drop, centering on the target after 0.5 s, signifying the end of the trial. (c) Distal link speed is used for frequency-modulated audio feedback, with higher speed corresponding to higher frequency. This audio feedback was played through headphones worn by the subject, providing wrist speed feedback. Figure adapted from Earley et al., 202126.
Subjects controlled the virtual arm to perform ballistic center-out reaches. With the cursor in the home circle [red hollow circle, Fig. 1c], a ball (grey filled circle) would appear above one target. The ball would drop and align with the center of the target 0.5 seconds after the arm left the home circle. Subjects were instructed to reach towards the target, stopping when the ball reached the target29.
Familiarization
To learn to control the virtual arm, subjects began each visit with a familiarization session. During this session, subjects were provided time to understand the controller through unstructured exploration, untimed target reaches, and a structured protocol comprising 32 training center-out reaches. The first 16 trials had a specified reaching order (four sets of 4 reaches towards each target), and the second 16 trials had a balanced and randomized reaching order (4 reaches total towards each target) [Fig. 2a]. No artificial feedback was provided during this session.
During the first visit, subjects only completed the Familiarization session. During the next two visits, subjects additionally completed a feedback protocol and a no feedback protocol in balanced randomized order. During the feedback protocol, subjects wore a pair of noise-canceling headphones (Bose QuietComfort 35 II) which played frequency-modulated tones determined by the speed of the distal link, where the pitch would increase by one octave for every multiple of 60 °/s. During the no feedback protocol, subjects wore the noise-canceling headphones, but no sound was played.
Fig 2. Transradial amputee Experimental Protocol. After one separate familiarization session, subjects completed the experimental protocol twice – once with and once without audio feedback. The order of the feedback and no feedback sessions was randomized across subjects. (a) The structured protocol for familiarization involved a total of 32 reaches: four sets of 4 reaches towards each target, and 16 reaches towards targets in balanced random order. (b) The steady-state block involved a total of 100 reaches: four sets of 15 reaches towards each target, and 40 reaches towards targets in balanced random order. The order of same- or different-target groupings was randomized across subjects and consistent between subject visits. (c) The Perturbation block started with 12 reaches towards targets in random order. After these baseline trials, subjects did cycles of 8-10 reaches towards targets in random order, followed by either 8 reaches towards the same target, or 8 reaches towards targets in balanced random order. The order of these cycles was randomized across subjects and consistent between subject visits. Reaches towards different targets with a dashed border indicate that balanced randomization was not enforced, and the number of reaches towards targets could differ from one another. Figure adapted from Earley et al., 202126.
Steady-State Block
To test trial-by-trial adaptation to self-generated errors, subjects completed two repetitions of 100 center-out reaches, each separated into one set of 60 and one set of 40 reaches [Fig. 2b]. The order of these sets was randomized across subjects using balanced block randomization. Subjects were allowed a short break between sets.
During the set of 60 trials, subjects completed four sets of 10 reaches towards each target. During the set of 40 trials, subjects reached towards targets in a balanced and randomized order. After each set, expanding window optimization separated initial trials from steady-state trials for post-experiment analysis30.
Two quantities were extracted from this trial-by-trial analysis. Adaptation rate was defined as the proportion of error from one trial that was corrected for in the following trial. Bias was defined as the amount of error which elicited no correction on average. This analysis was performed separately on the angular errors of both the elbow and the wrist, and was analyzed using a linear mixed effects model investigating main and interaction effects of the target set (same targets or different targets) and the feedback. Subjects were coded as random variables, and p-values were adjusted using Holm-Bonferroni corrections.
A second stochastic signal processing approach was used to filter inherent motor control noise and provide unbiased estimates of true adaptation behavior30–32. This analysis provided outcomes for the internal model adaptation rate and the control noise; both were analyzed using the same linear mixed effects model as described above.
Perturbation Block
To test the speed of adaptation to external perturbations to the control system, subjects completed two iterations of the Perturbation block comprising 12 practice trials followed by 8 sets of perturbation trials. During each set, subjects started by making 8–10 unperturbed reaches towards random targets. The control system was then perturbed by doubling the EMG gain, which increased the speed of the distal link and made accurate and precise control more difficult. Subjects then made 8 reaches with the perturbed dynamics. These sets of 8 reaches fell into two categories: towards the same target, or towards different targets. Each category was tested in 4 sets of the perturbation trials [Fig. 2c]. The order of these sets was determined randomly.
Perturbation adaptation of the Euclidean distance between the cursor and the target was estimated using an exponential decay model which fit a gain (\(\alpha\)), decay rate (\(\lambda\)), and baseline error (\({\epsilon }_{\infty }\)) to the perturbation trial data33–35.
A hierarchical nonlinear mixed effects model described in a previous publication was intended to analyze data from the perturbation block27. However, this method was not viable due to the variability of reaches; thus, an exponential decay function was fit separately for each subject, for each condition, and the coefficients from these models were compared36.
Statistical Analysis
Statistical analyses were performed using R-4.0.5. During all statistical tests, Holm-Bonferroni correction were made for the number of terms in each model. Deidentified raw data and code for statistical analysis are publicly available on The Open Science Framework37.