Sensory feedback has long been recognized for its pivotal role in accomplishing everyday tasks. Consider the challenges encountered when attempting to ignite a match with fingers impaired by cold — the sensory feedback is compromised, complicating a seemingly ordinary task. However, this example only skims the surface of the intricate role somatosensory feedback plays in human motor functions.

Sensory feedback, while important, is a segment of a broader system governing human movement. For instance, the impact of cold on fingers affects both sensory perception and fine motor control. Over extended periods, adaptation to such sensory deficits is observed as individuals come to rely more on alternative sensory channels, such as vision. The well-known case of Ian Waterman, who lost his touch and proprioception, yet managed object manipulation after extensive training, exemplifies this adaptability.

Engineer fitting a prosthetic arm. Source: ThisIsEngineering/Pexels

Sensory feedback in prosthetic research and development

In recent years, sensory feedback in prosthetics has garnered considerable attention. A surge in research has presented both invasive and non-invasive solutions, reflecting the heightened interest from prosthetic manufacturing entities. This burgeoning interest is reminiscent of the enthusiasm observed in the 1970s and 1980s. However, despite the proliferation of research during that period, none of the proposed solutions transitioned to clinical use. Some posit that technological limitations of the era hindered clinical application. Yet, an examination of contemporary studies reveals a diverse range of conclusions regarding feedback efficacy, casting ambiguity on the tangible benefits of feedback.

Prosthesis users frequently prioritize the restoration of sensory capabilities via artificial feedback. Surveys indicate dissatisfaction with prostheses in high dexterity tasks due to the absence of sensory feedback. Sensory input plays a pivotal role in typical human motor control. Even though research on feedback systems dates back to the 1950s, the commercial sector lacks artificial sensory feedback implementation. The integration of feedback into prosthesis control is not solely a technological challenge but also hinges on a deeper understanding of feedback's role within the system.

Human hand functions and myoelectric prostheses

The human hand is an integral tool for various tasks, from stable grasping and intricate manipulation to serving as a medium for social interactions. Its efficiency arises from a dense network of motor pathways (feedforward) and sensory pathways (feedback) linking the hand and the brain. Individuals with congenital or acquired hand loss experience significant disruptions in these capabilities.

Myoelectric prostheses. Source: Defense Visual Information Distribution Service

Myoelectric prostheses aim to address these deficiencies, mimicking both the form and functionality of the human hand. By converting muscle signals, like the contraction of wrist muscles, into prosthetic hand movements, they maintain an intuitive relationship between muscle activity and resultant motion. However, despite advanced multi-degree of freedom in these prostheses restoring motor function, they lack an effective method for sensory feedback.

Techniques to provide sensory feedback

Research has delved into various feedback methods, both invasive and non-invasive. For a feedback-enabled prosthesis, it is imperative to integrate sensors detecting proprioceptive (joint angles) and exteroceptive (grasping force) inputs. This data, obtained in real-time, is converted into stimulation profiles delivered to post-amputation sensory motor structures. In non-invasive feedback techniques, stimulation is administered to the residual limb's skin using electrical currents, vibrations or force applicators. Studies often relay grasping force as feedback to users, as it is challenging to deduce purely from visual cues, especially after an object has been firmly grasped. Various studies have explored feedback mechanisms for this purpose, employing spatial, parameter or mixed coding to convey force magnitude.

Ambiguities in feedback benefits

Technologically, the process of relaying sensor data back to the prosthesis user appears straightforward. However, literature on feedback benefits remains inconsistent. Studies have exhibited benefits only under specific conditions, such as sensory deprivation. While some research indicates clear advantages from non-invasive feedback, others reveal feedback's negligible impact on performance. These disparities highlight the need for more comprehensive studies to better understand the value and potential of sensory feedback in prosthetic use.

The impact and integration of sensory feedback

Three primary factors are often overlooked in sensory feedback prosthetic research, despite their potential significance in determining the utility of feedback. These factors revolve around human motor control concepts, emphasizing anticipation and learning.

Balancing simplicity and complexity

While research shows that users can adapt to repetitive tasks even without feedback, it is often posited that artificial feedback becomes especially beneficial when handling intricate tasks that demand precision.

The evolution of feedforward control

Over time, repeated usage helps users internalize an anticipatory model of prosthesis control. As this proficiency grows, the dependence on external feedback may decrease. However, understanding the relationship between feedforward control and external feedback is crucial for prosthetic design.

Deciphering feedback signals

The efficacy of feedback also hinges on its comprehensibility. Introducing feedback incrementally and ensuring it is easily interpretable can make a world of difference for the user.

Exploring the vibrotactile feedback scheme (VFS)

VFS represents a groundbreaking approach, amalgamating vibratory cues with mixed coding to convey vital information such as contact initiation, active prosthesis mode and force levels. Its significance lies in its adaptability to complex prosthetic designs.

The relationship between the utility of feedback and task intricacies is clear. When engaged in basic tasks, prosthetic users can often rely on existing senses like sight and hearing. However, feedback's importance escalates during tasks demanding precision and nuanced control, like delicately switching between prosthetic functionalities.

As users gain experience, feedback becomes a tool that complements their honed skills. For some tasks, feedback's significance might wane as users' control precision improves, but its impact remains profound in tasks that demand delicate control and quick adjustments.

Perception and subjectivity of feedback

Feedback is not only about functional improvement. For many users, it adds a layer of intuition and comfort, enhancing their connection with the prosthesis. However, integrating feedback means ensuring it is not just effective but also intuitive and pleasant.

Feedback’s role in prosthetics is intricate and influenced by numerous factors. While basic tasks may not require explicit feedback, complex tasks underscore their indispensability. The challenge is to synchronize feedback with users' adaptability curves, ensuring that as they master the prosthesis, feedback remains a complementary tool and not a redundant one. This dynamic suggests a gradual transition from simpler feedback mechanisms to sophisticated, nuanced ones in line with the advancements in prosthetic designs.

In closing

While the human body possesses an inherent adaptability, mastering control even in the absence of feedback, there is potential in integrating effective and intuitive sensory feedback into next-gen prosthetics. The increasing focus on the vibrotactile feedback scheme and the deeper exploration of the interplay between feedback and task intricacies offer promising avenues for the future. As prosthetic technology continues to evolve, ensuring a seamless melding of human adaptability with technological advancements will be the linchpin in creating devices that truly emulate and even enhance the human experience. It is a quest not just for functionality but for harmony, comfort and intuitive connection.

Author Byline

Jody Dascalu is a freelance writer in the technology and engineering niche. She studied in Canada and earned a Bachelor of Engineering. As an avid reader, she enjoys researching upcoming technologies and is an expert on a variety of topics.