This ground breaking study unravels the visions of sensory feedback in prosthetics.
Present findings may reshape our understanding of the interface between biology and technology. The research team endeavored to design an interface that acted as a conduit between the prosthetic and the brain.
Traditional leg prostheses don’t provide sensory feedback to above-knee amputees. Their usage lead to decreased confidence, reduced walking speed, heightened mental and physical fatigue.
This lack of feedback can also intensify phantom limb pain. Although prosthetic advancements are significant, the exploration of sensory feedback in leg prostheses remains limited.
Prior studies were focused on hand prostheses, revealing that sensory feedback can reduce phantom limb pain.
Probing the Biological-Tech Overlap
Neural prosthetics are devices designed to communicate directly with the human nervous system. They aim to either restore or amplify lost or waning functionalities.
Despite the evident technological advancements in the field, a tangible gap has persisted.
While prosthetic limbs have made significant advances in mimicking movement, can they simulate genuine sensations like the touch of a raindrop or the warmth of a hand?
Methods and Approach
A study included two transfemoral amputees. The SensArs neuroprosthetic device provided the stimulation through the tibial nerve. The SensArs device was integrated with foot and knee sensors connected to neural stimulation electrodes connected to the tibial nerve.
For this comprehensive research, the authors used variety of assessment methods to examine the potential benefits of sensory feedback mechanism.
This study assessed the present neural prosthetic models, pinpointing their inherent drawbacks. But theoretical analysis wasn’t the climax.
The study escalated its efforts, conducting experiments with a set of interfaces crafted to connect prosthetic limbs with residual nerve endings in amputees. The summary of methods are presented below:
Walking Speed and Confidence Assessment
- Participants walked in a figure-eight outside a defined rectangular area on sandy terrain.
- Experiments conducted with and without sensory feedback.
- Walking distance was recorded. Participants also rated their confidence on a scale from 0 to 10 after each walk.
Mental Effort Assessment
- Dual-task paradigm: a walking task (primary) and an auditory oddball task (secondary).
- The oddball task involved headphones presenting three tones in random order. Participants had to silently count specific tones while ignoring others.
- EEG data captured participants’ attention to specific tones. The P300 amplitude indicates mental resource allocation to target tones.
Pain Treatment
- Participants underwent pain treatment after electrode implantation.
- Two stimulation types were tested: frequency-invariant and frequency-variant.
- Pain treatment aimed to elicit sensations spatially close to the painful area in the phantom foot.
EEG Signal Processing
- EEG data were collected, processed, and analysed offline to extract relevant information.
- The aim was to examine the modulation of P300 ERP amplitude.
Auditory oddball task
The auditory odd-ball task was used as a secondary task during walking. The portable EEG SMARTING was used for EEG data recording.
The auditory oddball task played from the smartphone, using Neurobs Presentation app. The main purpose of the task was to assess how hard is walking with or without the sensory feedback.
Neuroprosthesis Overview. The participant wears a system incorporating a sensorized insole beneath the foot (1), linked with electronics positioned at the ankle, alongside a bespoke lower-limb prosthesis fitted with a microprocessor-controlled knee that houses an integrated knee encoder (1). As the participant traverses an outdoor terrain in a figure-eight trajectory, data from both insoles (with an insole also fitted on the healthy leg) and the knee encoder is relayed in real-time via Bluetooth to an external controller (2). This information is subsequently transformed into neural stimulation predicated on sensation mapping and intensity modulation (3). The ensuing neural stimulation (4) is channeled through neural implants (5), evoking immediate sensations in the phantom foot and leg as the participant walks.Main Findings
The results showed increased walking speed and elevated confidence. Also, sensory feedback led to reduced mental and physical fatigue.
Patient’s outdoor walking speed and confidence levels also increased with sensory feedback. Participants reported less phantom limb pain with the feedback.
Mental effort assessment during walking
The Auditory odd-ball task indicated less mental effort when sensory feedback was present. This was evident through increased P300 amplitude on target tones during sensory feedback.
Physical fatigue assessment
The oxygen uptake served to investigate the impact of neuroprosthesis on physical fatigue. Results indicated more efficient gait and reduced oxygen consumption with feedback.
Finally, low-frequency neural stimulation was effective in reducing phantom limb pain.
Assessment of Walking Speed, Confidence, and Cognitive Engagement.a, Comparative walking speeds of participants 1 and 2 over sandy terrain, with and without sensory feedback across two 6-minute sessions. Results derived from six 1-minute trials per condition. Statistical analysis indicated significant variations.
b, Post-session confidence levels in the prosthesis were collected, with 12 reports from two experimental sessions. Analysis revealed heightened confidence with sensory feedback.
c, A depiction of voltage distribution on the scalp during the P300 window in response to target sounds across stimulation conditions.
d & e, Comparison of Event-Related Potentials (ERPs) during walking, with distinctions in target and non-target sounds and P300 amplitude variations based on auditory cues. Analysis showed pronounced cortical reactions to target sounds during sensory feedback.
Various statistical analyses were performed, confirming significant findings. Box plots showcase median values, quartile ranges, and statistical significance denoted by asterisks.
Restoring touch sensation
Patients confirmed that they naturally felt the sensations on amputated leg. The feelings included touch, pressure and vibration sensations.
Sensory feedback reduces pain
This study also observed significant pain reduction, exceeding 80%, among participants. While the exact reasons for such pain suppression need further investigation, one possibility is the triggering of beneficial neuroplastic changes in the brain due to sensory feedback.
Unravelling Implications and Path Forward
The results from this research aren’t just ground-breaking; they’re transformative.
For individuals relying on prosthetics, the dream of sensing through artificial limbs got closer to reality. These revelations promise richer sensory experiences and an era where prosthetics won’t be about mobility but holistic integration.
Participants displayed more confidence and reduced mental effort when using sensory feedback. This could help address issues related to the high abandonment rates of prostheses.
Microprocessor-controlled knees can increase the walking speed of participants by 8% compared to passive devices. This increase is even more significant with sensory feedback, leading to over a 10% improvement. This enhanced performance might result from participants exerting more force with both limbs.
Long-term effects of above-knee amputations include increased cardiovascular risks. Improvements in walking economy could counter these risks. A study found reduced metabolic rates when walking, implying greater gait efficiency.
These results state the potential of sensory feedback in enhancing the energy economy of amputee walking.
Yet, challenges loom. The tactile sensation breakthrough, while promising, remains in early stages. Rigorous refinement and in-depth calibration are crucial to its widespread acceptance and safety.
Meanwhile, the dawn of cognitive prostheses mandates a thorough ethical examination. Enhancing human cognition, while thrilling, calls for robust moral dialogue.
Conclusions
The current work is a proof-of-concept trial. Study demonstrated the benefits of restoring sensory feedback to leg amputees through intraneural stimulation.
Further research, involving more extended studies with larger participant groups and advancements like implantable devices, is needed to solidify the findings.
The aim is to use this technology to substantially enhance the health and quality of life of amputees.
Original publication source: https://www.nature.com/articles/s41591-019-0567-3
