EVALUATION COMMITTEE

The Faculty Council has appointed the following adjudication committee to evaluate the thesis and the associated lecture: 

• Prof. Kianoush Nazarpour, Reader, University of Edinburgh
• Dr. Raoul M. Bongers, Associate Professor, University of Groningen
• Dr. Sabata Gervasio, HST, Aalborg University, Denmark (Chairman).

Moderator:
Dr. Jakob Lund Dideriksen, HST, Aalborg University

HOW TO PARTICIPATE

The Ph.D. Defense is organized as a hybrid event you can participate digitally via Zoom or physical presence. 

Meeting ID: 662 2431 0476

ABSTRACT

The human central nervous system continuously receives and processes information from thousands of peripheral sensors to obtain an estimate from which future movements can be planned. State estimation can be improved by the fusion of multiple sources of sensory information. Thus, before reaching a decision, our brains combine and integrate information from different natural senses using a weighted average of the sensory estimates, where the weight assigned to each estimate reflects its reliability. Notably, the sensory integration occurs so effortlessly that we may not be aware it is happening. The Lack of one natural sensory modality, however, might impair sensory integration, leading to an overreliance on residual sensory signals.  For this reason, interacting with an artificial system through a human-machine interface is challenging if the interface does not provide sensory information about the state of the machine to the user. One example of this problem is the use of upper-limb myoelectric prostheses, where the absence of sensation in the upper-limb prosthesis may constrain the functional potential of these devices. Addressing this challenge has been a highly active field of study during the last 5 decades and plenty of invasive and non-invasive approaches have been developed to allow the activity of once insensate prosthesis to be felt. According to the role of sensory integration to enhance our perceptual accuracy, one question of interest is whether supplementary feedback is integrated with other sensory information in a natural manner.  In this regard, it has been already shown that the supplementary feedback provided through interneural stimulation is integrated with residual natural senses in the state estimation when both natural and artificial signals were available. However, invasive approaches involve complicated hardware and specialized surgical procedures. As an alternative to invasive feedback, several noninvasive methods have been proposed by providing feedback as sensory substitution which convey information to the prosthesis user through delivering electrical or mechanical stimulation to the surface of the skin of the residual limb. Although the efficiency of sensory substitution feedback has been shown to improve control performance in some conditions, it is still unclear how such feedback is exploited by the nervous system in the state estimation process and whether multisensory integration is feasible even though the information is conveyed through a non-invasive feedback channel.

In this Ph.D. project, we aimed to develop a framework to investigate whether the nervous system processes and integrates the non-invasive electrotactile feedback with natural sensory modalities during a multisensory task. This framework was then exploited to introduce a new approach for evaluating feedback efficiency by comparing different configurations for sensory feedback systems focusing on quantifying the role of the supplementary feedback in the state estimation process of the nervous system. 

To this end, we introduced a small unnoticeable mismatch between the two natural and artificial modalities. Using psychophysical tools, we ensured the induced bias is adjusted at a level below the subject’s conscious perception (Study I). Next, we validated the framework to quantify the degree to which the nervous system exploits the non-invasive electrical stimulation in a natural way in the state estimation process. (Study II). In the next step, we exploited the sensory integration principle to compare three different configurations for non-invasive electrotactile feedback by modulating amplitude or frequency individually, or both parameters simultaneously in an isometric grasp force matching task (Study III). Here, the underlying assumption is that the higher the weight assigned to artificial feedback by the nervous system, the more efficient the encoding scheme will be.

The main results of this Ph.D. project suggest that sensory substitution does not preclude sensory integration. In particular, the nervous system adopts electrotactile stimulation as a highly reliable source of information, processed subconsciously, that can improve the precision in the sensory estimation of the motor state during the multisensory task. Moreover, we found that the perceptual ambiguity was decreased further by the integration of redundant information provided by single-channel electrotactile multimodal modulation.

The findings of the present work could improve our understanding of the role of sensory substitution feedback in the state estimation process which has important implications for the design of supplementary feedback in bidirectional human-machine interfaces.