Combined spatial and frequency encoding for electrotactile feedback of myoelectric signals

Electrotactile stimulation has been commonly used in human–machine interfaces to provide feedback to the user, thereby closing the control loop and improving performance. The encoding approach, which defines the mapping of the feedback information into stimulation profiles, is a critical component of an electrotactile interface. Ideally, the encoding will provide a high-fidelity representation of the feedback variable while being easy to perceive and interpret by the subject. In the present study, we performed a closed-loop experiment wherein discrete and continuous coding schemes are combined to exploit the benefits of both techniques. Subjects performed a muscle activation-matching task relying solely on electrotactile feedback representing the generated myoelectric signal (EMG). In particular, we investigated the performance of two different coding schemes (spatial and spatial combined with frequency) at two feedback resolutions (low: 3 and high: 5 intervals). In both schemes, the stimulation electrodes were placed circumferentially around the upper arm. The magnitude of the normalized EMG was divided into intervals, and each electrode was associated with one interval. When the generated EMG entered one of the intervals, the associated electrode started stimulating. In the combined encoding, the additional frequency modulation of the active electrode also indicated the momentary magnitude of the signal within the interval. The results showed that combined coding decreased the undershooting rate, variability and absolute deviation when the resolution was low but not when the resolution was high, where it actually worsened the performance. This demonstrates that combined coding can improve the effectiveness of EMG feedback, but that this effect is limited by the intrinsic variability of myoelectric control. Our findings, therefore, provide important insights as well as elucidate limitations of the information encoding methods when using electrotactile stimulation to convey a feedback signal characterized by high variability (EMG biofeedback)

Design And Evaluation Of Sensory Stimulation Patterns For Enhancing Spatial Awareness

Spatial awareness refers to our ability to develop knowledge about the surrounding environment and our relation with it. It is essential for people of all ages and constantly used in everyday activities. Typically, sensory and motor systems work cooperatively to acquire a complete and detailed representation of the external world. However, there are several health conditions that may affect the integrity of spatial awareness, including autism spectrum disorder (ASD) as well as injury or neurological conditions (e.g., amputation, spinal cord lesions, or stroke). In these cases, the lack of spatial awareness is associated with severe impairments in everyday activities. This work is about novel audio and tactile stimulation patterns for enhancing spatial awareness, with a specific focus on attention and manipulation deficits. Spatial awareness can be increased non-visually, through acoustical stimulation, for sources of interest that are distal from the body. Specifically, in the first part, I studied ways to augment the human sense of hearing with spatially filtered soundscapes that can improve speech reception thresholds in a noisy environment. The sensory augmentation pattern was tested on autistic children who exhibit abnormal performance on auditory-based tasks. In particular, they show significantly poorer speech reception in noise relative to their neurotypical peers. Using a novel spatial filtering, speech reception in children with ASD significantly increased compared to their typical hearing condition. This finding is important because this filtering technique was previously used only on neurotypical and hearing-impaired subjects. The results indicate that artificially modulating sounds perception may positively impact daily social activities that require communication in noisy environments, not only in sensory-disabled persons but also in children with attentional deficits. Spatial awareness can also be increased non-visually, using tactile stimulation, which is intimately linked to contact and manipulation tasks, for sources of interest that are proximal to the body. In the second part, I designed and assessed different tactile stimulation patterns that can be used to carry gestural information in the spatial and frequency domain, with applications in prosthetics for amputees and rehabilitation of post-stroke patients. Amputees and post-stroke patients suffer from impairments of both motor and somatosensory functions. While the functional recovery of upper extremities is one of the primary goals of rehabilitation programs, somatosensory deficits are often overlooked. Sensory substitution systems, providing tactile feedback, might facilitate manipulation capability and improve dexterity during grasping movements. As a first step, I evaluated the ability to determine the number of concurrent stimuli composing electrotactile feedback in healthy subjects. This study showed that temporal coding, and not total energy or duration, modulated the accuracy in numerosity judgment using electrotactile stimulation. As a second step, considering the body of evidence showing the modulation of vision on tactile perception, I investigated how the human\u2019s enumeration abilityof tactile stimuli in a sensory substitution context could be influenced by vision. Results showed that vision, even when non-informative about the tactile stimulation, improves tactile numerosity judgment. The last part considered the link between tactile feedback and hand gesture. Normally, accurate integration of feedforward motor commands and sensory feedback is crucial for refined hand movements (sensory-motor loop). In the last study, I proposed a novel concept to close the sensory-motor loop. In particular, I provided the users with online feedback about the control signal generated by their muscle activation (i.e., myoelectric signal), allowing them to achieve an effective closed-loop system. More precisely, I assessed the extent to which frequency modulation of tactile stimulation can enhance the control of muscle activation reducing its variability, trying to find the most successful mapping between the generated myoelectric signal by the hand and its representation across the tactile array. Results showed that the addition of frequency modulation facilitates myoelectric control but only under specific feedback conditions. Summarizing, this thesis proposed and assessed new methods to artificially alter signals out of the body (acoustic feedback) and on the body (tactile feedback) aimed at increasing spatial awareness in healthy subjects and persons who lack sensory or attentional integrity. The results of this thesis can be interesting for engineers, psychologists, and rehabilitators. Having shown that spatial sound filtering techniques are beneficial for persons with ASD means that this technology may be used to support learning at school, behavioral therapy, and social interaction, which is typically lacking in ASD. Then, the proposed tactile patterns might be associated with occupational therapy to favor the motor recovery of the hand in post-stroke patients with severe impairment of hand sensitivity and mild or absent hand strength deficit. Moreover, these tactile patterns might offer one possible solution for closing the loop in the context of myoelectric prosthesis control, leading to more capable prosthetic limbs. The cross-modal findings can be applied in clinical rehabilitation. For instance, additional visual feedback might be used along with tactile stimulation to improve somatosensory performance in case of brain damage patients or to reduce pain intensity in chronic pain patients

Vibrotactile feedback to control the amount of weight shift during walking - A first step towards better control of an exoskeleton for spinal cord injury subjects

People with Spinal Cord Injury do not only lack the ability to control their muscles, but also miss the sensory information from below the level of their lesion. Therefore, it may become difficult for them to perceive the state of the body during walking, which is however often used to control wearable exoskeletons. In the present study the possibilities of providing vibrotactile feedback about the Center of Mass (CoM) during walking were investigated. The results showed that healthy subjects could successfully interpret the provided vibrotactile cues and change their walking pattern accordingly. Vibrotactile stimulation was either provided in a concurrent (over the complete CoM movement) or terminal (only when the desired CoM displacement was reached) way. The latter led to a better accuracy and can be easily implemented in a wearable exoskeleton where a certain amount of CoM displacement is needed to initiate stepping