Multiparametric non-linear TENS modulation to integrate intuitive sensory feedback

Abstract

Objective. Transcutaneous Electrical Nerve Stimulation (TENS) has been recently introduced in neurorehabilitation and neuroprosthetics as a promising, non-invasive sensory feedback restoration alternative to implantable neurostimulation. Yet, the adopted stimulation paradigms are typically based on single-parameter modulations (e.g., pulse amplitude PA, pulse-width PW or pulse frequency PF). They elicit artificial sensations characterized by a low intensity resolution (e.g., few perceived levels), low naturalness and intuitiveness, hindering the acceptance of this technology. To address these issues, we designed novel multiparametric stimulation paradigms, featuring the simultaneous modulation of multiple parameters, and implemented them in real-time tests of performance when exploited as artificial sensory inputs. 
Approach. We initially investigated the contribution of PW and PF variations to the perceived sensation magnitude through discrimination tests. Then, we designed three multiparametric stimulation paradigms comparing them with a standard PW linear modulation in terms of evoked sensation naturalness and intensity. The most performant paradigms were then implemented in real-time in a Virtual Reality - TENS platform to assess their ability to provide intuitive somatosensory feedback in a functional task. 
Main results. Our study highlighted a strong negative correlation between perceived naturalness and intensity: less intense sensations are usually deemed as more like natural touch. In addition, we observed that PF and PW changes have a different weight on the perceived sensation intensity. As a result, we adapted the Activation Charge Rate (ACR) equation, proposed for implantable neurostimulation to predict the perceived intensity while co-modulating the PF and charge per pulse, to TENS (ACRT). ACRT allowed to design different multiparametric TENS paradigms with the same absolute perceived intensity. Although not reported as more natural, the multiparametric paradigm, based on sinusoidal PF modulation, resulted being more intuitive and subconsciously integrated than the standard linear one. This allowed subjects to achieve a faster and more accurate functional performance. 
Significance. Our findings suggest that TENS-based, multiparametric neurostimulation, despite not consciously perceived naturally, can provide integrated and more intuitive somatosensory information, as functionally proved. This could be exploited to design novel encoding strategies able to improve the performance of non-invasive sensory feedback technologies.

Wearable neuroprosthesis improves mobility and reduces pain in neuropathic participants

“…3B-C-D ). The protocol used to define the charge thresholds similar to the one proposed in (23, 40) can be found in Methods, Electrical sensory feedback characterization .…”

“…Near-natural tactile sensation has been achieved only through invasive neural technologies both at the peripheral level (22,25,67), and at the central level (68)(69)(70). While restoring natural touch information with non-invasive electrical stimulation remains an unresolved challenge (27,29,30,35,71), these paresthetic sensations can still be subconsciously integrated in the sensory motor scheme (40). Despite not eliciting natural sensation, non-invasive stimulation technologies, still have many advantages, namely that they do not require a surgical procedure, avoiding extensive post-surgery care and complications (35).…”

Neuromorphic hardware for somatosensory neuroprostheses

“…The lack of a bio-signal to objectively quantify the error between the evoked and target sensations makes the development of efficient algorithms very challenging. Perceived naturalness could be influenced by past or present experience, and could be modified with the modulation of stimulation parameters and its related perceived intensity 153 . To this aim, a detailed evaluation is needed, combining both subjective (e.g., questionnaires, free description, psychophysics) and objective measures (e.g., reaction times, neural recordings, performance metrics, learning).…”

A Sensory Feedback Neural Stimulator Prototype for Both Implantable and Wearable Applications

“…So, for external stimulation (i.e., TENS), it is mandatory to have a higher voltage compliance, which is achieved by adding the third board. Additionally, the quantity of stimulation channels required for external applications is less than that required for PNS; as reported in the literature, TENS was already used for sensory feedback with a number of stimulating channels ranging between 2 and 4 [40][41][42][43]. For this reason, the third board was developed with a lower number of active channels.…”

“…Regarding the wearable version, it is necessary to add the third board to increase the voltage compliance of the system, making it suitable for TENS, and maintaining control of the current stimulation via the current mirrors. All this, however, resulted in a reduction in the stimulation channels from 30 to 8, which are still suitable for TENS [40][41][42][43]. To summarize, the stimulator is a modular system, composed of two or three boards based on the application scenario.…”