Evoked Somatosensory Feedback for Closed-Loop Control of Prosthetic Hand

Somatosensory feedback, such as tactile and proprioceptive feedback, is essential to our daily sensorimotor tasks. A lack of sensory information limits meaningful human-machine interactions. Different somatosensory feedback strategies have been developed in recent years. Non-invasive sensory substitutional approaches often evoke sensations that are unintuitive, requiring extensive sensory training. Alternatively, invasive neural stimulation can elicit intuitive percepts that are interpretable readily by prosthetic hand users; however, the invasive nature of the procedure limits wide clinical applications. To overcome these issues, we developed a multimodal sensory feedback approach that delivers tactile and proprioceptive information non-invasively. We used a skin-surface nerve stimulation array to target afferent fibers in the peripheral nerves, which can elicit intuitive tactile feedback at the fingertips. We used a vibrotactile array to deliver proprioceptive percepts encoding kinematic information of prosthetic joints. First, we evaluated whether the peripheral nerve stimulation technique could be used for the recognition of object properties. Evoked tactile sensations were modulated using forces recorded by a sensorized prosthesis not actively controlled by the users. We demonstrated that the elicited tactile sensation at the fingertips can enable recognition of object shape and surface topology. Second, we evaluated how evoked tactile feedback can be integrated into the functional utility of a prosthetic hand. We quantified the benefits of tactile feedback under different myoelectric control strategies, when participants performed an object manipulation task. We showed an improved task success rate and reduced muscle activation effort when tactile feedback was provided. Finally, we investigated whether multimodal (tactile and proprioceptive) feedback can enable the recognition of more complex object properties during active control of a prosthetics hand. We found that integrated tactile and proprioceptive feedback allowed for simultaneous recognition of multiple object properties (size and stiffness) in individuals with and without an arm amputation. Overall, this work demonstrates that artificially evoked somatosensory feedback can be utilized effectively to improve the closed-loop control of prostheses. These outcomes highlight the critical role of somatosensory feedback during human-machine interactions, which can enhance functional utility of prosthetic devices and promote user experience and confidence.Doctor of Philosoph

Sensory Feedback for Upper-Limb Prostheses:Opportunities and Barriers

The addition of sensory feedback to upper-limb prostheses has been shown to improve control, increase embodiment, and reduce phantom limb pain. However, most commercial prostheses do not incorporate sensory feedback due to several factors. This paper focuses on the major challenges of a lack of deep understanding of user needs, the unavailability of tailored, realistic outcome measures and the segregation between research on control and sensory feedback. The use of methods such as the Person-Based Approach and co-creation can improve the design and testing process. Stronger collaboration between researchers can integrate different prostheses research areas to accelerate the translation process

The temporal pattern of impulses in primary afferents analogously encodes touch and hearing information

An open question in neuroscience is the contribution of temporal relations between individual impulses in primary afferents in conveying sensory information. We investigated this question in touch and hearing, while looking for any shared coding scheme. In both systems, we artificially induced temporally diverse afferent impulse trains and probed the evoked perceptions in human subjects using psychophysical techniques. First, we investigated whether the temporal structure of a fixed number of impulses conveys information about the magnitude of tactile intensity. We found that clustering the impulses into periodic bursts elicited graded increases of intensity as a function of burst impulse count, even though fewer afferents were recruited throughout the longer bursts. The interval between successive bursts of peripheral neural activity (the burst-gap) has been demonstrated in our lab to be the most prominent temporal feature for coding skin vibration frequency, as opposed to either spike rate or periodicity. Given the similarities between tactile and auditory systems, second, we explored the auditory system for an equivalent neural coding strategy. By using brief acoustic pulses, we showed that the burst-gap is a shared temporal code for pitch perception between the modalities. Following this evidence of parallels in temporal frequency processing, we next assessed the perceptual frequency equivalence between the two modalities using auditory and tactile pulse stimuli of simple and complex temporal features in cross-sensory frequency discrimination experiments. Identical temporal stimulation patterns in tactile and auditory afferents produced equivalent perceived frequencies, suggesting an analogous temporal frequency computation mechanism. The new insights into encoding tactile intensity through clustering of fixed charge electric pulses into bursts suggest a novel approach to convey varying contact forces to neural interface users, requiring no modulation of either stimulation current or base pulse frequency. Increasing control of the temporal patterning of pulses in cochlear implant users might improve pitch perception and speech comprehension. The perceptual correspondence between touch and hearing not only suggests the possibility of establishing cross-modal comparison standards for robust psychophysical investigations, but also supports the plausibility of cross-sensory substitution devices

Optimally-calibrated non-invasive feedback improves amputees' metabolic consumption, balance and walking confidence

Objective.Lower-limb amputees suffer from a variety of health problems, including higher metabolic consumption and low mobility. These conditions are linked to the lack of a natural sensory feedback (SF) from their prosthetic device, which forces them to adopt compensatory walking strategies that increase fatigue. Recently, both invasive (i.e. requiring a surgery) and non-invasive approaches have been able to provide artificial sensations via neurostimulation, inducing multiple functional and cognitive benefits. Implants helped to improve patient mobility and significantly reduce their metabolic consumption. A wearable, non-invasive alterative that provides similar useful health benefits, would eliminate the surgery related risks and costs thereby increasing the accessibility and the spreading of such neurotechnologies.Approach.Here, we present a non-invasive SF system exploiting an optimally-calibrated (just noticeable difference-based) electro-cutaneous stimulation to encode intensity-modulated foot-ground and knee angle information personalized to the user's just noticeable perceptual threshold. This device was holistically evaluated in three transfemoral amputees by examination of metabolic consumption while walking outdoors, walking over different inclinations on a treadmill indoors, and balance maintenance in reaction to unexpected perturbation on a treadmill indoors. We then collected spatio-temporal parameters (i.e. gait dynamic and kinematics), and self-reported prosthesis confidence while the patients were walking with and without the SF.Main results.This non-invasive SF system, encoding different distinctly perceived levels of tactile and knee flexion information, successfully enabled subjects to decrease metabolic consumption while walking and increase prosthesis confidence. Remarkably, more physiological walking strategies and increased stability in response to external perturbations were observed while walking with the SF.Significance.The health benefits observed with the use of this non-invasive device, previously only observed exploiting invasive technologies, takes an important step towards the development of a practical, non-invasive alternative to restoring SF in leg amputees

Mapping quantal touch using 7 Tesla functional magnetic resonance imaging and single-unit intraneural microstimulation.

Using ultra-high field 7 Tesla (7T) functional magnetic resonance imaging (fMRI), we map the cortical and perceptual responses elicited by intraneural microstimulation (INMS) of single mechanoreceptive afferent units in the median nerve, in humans. Activations are compared to those produced by applying vibrotactile stimulation to the unit's receptive field, and unit-type perceptual reports are analyzed. We show that INMS and vibrotactile stimulation engage overlapping areas within the topographically appropriate digit representation in the primary somatosensory cortex. Additional brain regions in bilateral secondary somatosensory cortex, premotor cortex, primary motor cortex, insula and posterior parietal cortex, as well as in contralateral prefrontal cortex are also shown to be activated in response to INMS. The combination of INMS and 7T fMRI opens up an unprecedented opportunity to bridge the gap between first-order mechanoreceptive afferent input codes and their spatial, dynamic and perceptual representations in human cortex

Encoding tactile frequency and intensity information in the temporal pattern of afferent nerve impulses

Using our hands to interact with the world around us produces complex vibrations travelling across the skin. These complex waves are transduced by tactile afferent neurons whose impulse patterns convey information about the external world. A major question in this field is how important the timing of these afferent impulses is in shaping perception. We have the means to investigate this question by artificially inducing impulse patterns using brief mechanical and electrical stimuli, allowing us to study the neural coding of vibrotactile sensory information. Our lab has used this to show that when mechanical pulses evoked impulse trains grouped into periodic bursts, perceived frequency corresponded to the duration of the silent inter-burst gap interval, rather than the periodicity or the mean impulse rate. In this thesis, we induced controlled impulse trains, while measuring the perceptual responses of human subjects using psychophysical methods to assess the dimensions of frequency and intensity. As electrical stimulation has broad utility in prosthetic applications, we first verified that the same perceived frequency as predicted by the burst gap was elicited with electrical pulses in subjects within the low frequency flutter range. We then tested whether this same coding mechanism also applied outside the flutter frequency range by conducting further experiments with higher pulse rates. We found that burst gap coding correctly predicted perceived frequencies above flutter, suggesting a generalised temporal processing strategy that operates on tactile afferent inputs spanning a broad range of frequencies. Next, we investigated perceived intensity where stimulus pulse rate was varied without changes in afferent population recruitment or in perceived frequency by using bursts of pulsatile stimuli. Increasing the number of pulses within a burst caused a significant increase in perceived intensity when electrical stimulation was used. Mechanical pulses with the same burst groupings did not produce a comparable intensity increase, possibly due to minimal variations in the population firing rate. These new insights into the encoding of tactile information through temporal patterning in peripheral impulse patterns may allow the multiplexing of frequency and intensity sensations with a fixed stimulation amplitude for use in neural interfaces to deliver sensory feedback information

Estudo neurofisiológico da discriminação de distância em humanos

Tactile width discrimination processing has been extensively studied in rodents and has demonstrated multiple relevant basic mechanisms. Despite this relevance, the number of studies of width discrimination in humans has been scarce. During the present dissertation, neurophysiological correlates of width discrimination were analyzed through electroencephalography recordings in participants performing a width discrimination task in active or passive modes. Analysis of power in the delta, theta, alpha, beta, and gamma frequency bands revealed differences in the power for different frequency bands and electrodes recorded. Active width discrimination processing was characterized by an increase in the power of delta, theta and gamma frequency bands in electrodes F3 and F4, and an increase in power in the gamma frequency band in T4. Passive tactile width processing was characterized by an increase in the power of delta in electrodes Fp1 and T4, and an increase in gamma frequency band in Tp10. Altogether these results suggest that active and passive tactile width discrimination processing are characterized by an asymmetrical network involving prefrontal, frontal and temporal electrodes, in delta, theta, and gamma frequency bands.O estudo do processamento tátil de distâncias encontra-se bastante desenvolvido em roedores, tendo sido útil para a demonstração de múltiplos mecanismos básicos relevantes. Apesar desta relevância, o estudo da discriminação de distâncias em humanos é ainda bastante reduzido. Durante a presente dissertação foram analisados os correlatos neurofisiológicos, através do registo de eletroencefalografia, em participantes que realizavam uma tarefa de discriminação de distância em modo ativo ou passivo. A análise da potência das bandas de frequências delta, teta, alfa, beta e gama revelou diferenças na potência do sinal para diferentes bandas de frequências e elétrodos. O processamento ativo era caracterizado por um aumento da potência nas bandas de frequências delta, teta e gama nos elétrodos F3, F4; e um aumento da atividade na banda de frequência gama no elétrodo T4. O processamento passivo era caracterizado por um aumento da potência de delta nos elétrodos Fp1 e T4 e um aumento da potência de gama em Tp10. No seu conjunto, estes resultados sugerem que o processamento ativo e passivo da distância são caraterizados por uma rede assimétrica envolvendo elétrodos pré-frontais, frontais e temporais nas bandas de frequência delta, teta e gama.Mestrado em Biomedicina Molecula

Sensory and cognitive factors in multi-digit touch, and its integration with vision

Every tactile sensation – an itch, a kiss, a hug, a pen gripped between fingers, a soft fabric brushing against the skin – is experienced in relation to the body. Normally, they occur somewhere on the body’s surface – they have spatiality. This sense of spatiality is what allows us to perceive a partner’s caress in terms of its changing location on the skin, its movement direction, speed, and extent. How this spatiality arises and how it is experienced is a thriving research topic, compelled by growing interest in the nature of tactile experiences from product design to brain-machine interfaces. The present thesis adds to this flourishing area of research by examining the unified spatial quality of touch. How does distinct spatial information converge from separate areas of the body surface to give rise to our normal unified experience of touch? After explaining the importance of this question in Chapter 1, a novel paradigm to tackle this problem will be presented, whereby participants are asked to estimate the average direction of two stimuli that are simultaneously moved across two different fingerpads. This paradigm is a laboratory analogue of the more ecological task of representing the overall movement of an object held between multiple fingers. An EEG study in Chapter 2 will reveal a brain mechanism that could facilitate such aggregated perception. Next, by characterising participants’ performance not just in terms of error rates, but by considering perceptual sensitivity, bias, precision, and signal weighting, a series of psychophysical experiments will show that this aggregation ability differs for within- and between-hand perception (Chapter 3), is independent from somatotopically-defined circuitry (Chapter 4) and arises after proprioceptive input about hand posture is accounted for (Chapter 5). Finally, inspired by the demand for integrated tactile and visual experience in virtual reality and the potential of tactile interface to aid navigation, Chapter 6 will examine the contribution of tactile spatiality on visual spatial experience. Ultimately, the present thesis will reveal sensory factors that limit precise representation of concurrently occurring dynamic tactile events. It will point to cognitive strategies the brain may employ to overcome those limitations to tactually perceive coherent objects. As such, this thesis advances somatosensory research beyond merely examining the selectivity to and discrimination between experienced tactile inputs, to considering the unified experience of touch despite distinct stimulus elements. The findings also have practical implications for the design of functional tactile interfaces