ETD

• balance
• haptic feedback
• postural control
• pressure-based feedback
• sensory augmentation
• vibrotactile feedback

Postural stability during upright standing is a fundamental motor function that relies on the integration of multisensory information, including visual, vestibular, and somatosensory inputs. Alterations or degradation of these sensory channels can impair balance control and increase the risk of falls. In response to this challenge, wearable sensory augmentation systems have been increasingly investigated as a means of providing supplementary feedback to support postural control. Among the various feedback modalities explored in the literature, vibrotactile stimulation is the most commonly adopted approach for balance-related applications. However, alternative haptic modalities, such as pressure-based mechanotactile feedback, may offer advantages in terms of intuitiveness, perceptual consistency, and user acceptance.
The present thesis describes a non-clinical validation study aimed at evaluating a wearable device delivering pressure-based mechanotactile feedback for standing balance assistance. The primary objective of the study was to assess the intuitiveness and efficacy of pressure-based stimulation and to compare its performance with vibrotactile feedback. A secondary objective was to investigate the influence of different actuator mounting configurations on stimulus perception, functional performance, and balance-related outcomes. The study was conducted on able-bodied participants to establish a controlled baseline evaluation of the proposed stimulation system, independent of confounding clinical factors.
Sixteen able-bodied participants were recruited and randomly assigned to one of two experimental groups, each corresponding to a different actuator mounting configuration: a cross configuration or a square configuration. Each participant experienced only one mounting configuration but was exposed to both feedback modalities, pressure-based mechanotactile stimulation and vibrotactile stimulation. The order of presentation of the two feedback modalities was randomized across participants to minimize learning and order effects. Throughout all experimental sessions, participants wore noise-cancelling headphones playing white noise to eliminate auditory cues generated by the device and to prevent perceptual bias related to sound.
The experimental protocol was designed to evaluate complementary aspects of the stimulation system and was divided into three main parts: fundamental perceptual tests, functional intuitiveness tests, and static balance assessments. In addition to objective performance measures, subjective evaluations were collected through post-experimental questionnaires to capture participants’ perceptions of comfort, naturalness, and usability.
The first part of the protocol consisted of fundamental tests assessing stimulus perception. These tests aimed to quantify participants’ ability to detect and discriminate the provided haptic stimuli for both feedback modalities. Sensory threshold tests were conducted to determine minimum and maximum detection thresholds at each actuator location. Just-noticeable difference (JND) tests were performed to assess participants’ sensitivity to changes in stimulus intensity. These perceptual measures provided insight into the resolution and consistency of each feedback modality across actuator locations and mounting configurations.
The second part of the protocol focused on functional tests designed to assess intuitiveness and ease of interpretation of the haptic feedback during task execution. These tests required participants to actively interpret directional and spatio-temporal stimulation cues. The Labyrinth Test evaluated directional accuracy, reaction time, and task failures under both single-task and dual-task conditions involving a concurrent cognitive game. The Clock Test assessed participants’ ability to correctly identify stimulation directions mapped to discrete spatial locations. Additionally, a Pattern Test was conducted to qualitatively evaluate participants’ ability to follow imposed spatio-temporal stimulation patterns using trunk movements.
The third part of the protocol comprised static balance assessments aimed at evaluating the influence of the stimulation system on postural control under stationary conditions. Participants performed a series of standing balance tasks under different visual and cognitive conditions, including eyes open, eyes open with a concurrent reading task, and eyes closed. These tests were conducted with no feedback, pressure-based feedback, and vibrotactile feedback to allow comparison across feedback conditions.
All collected data were processed and analyzed offline using custom-developed scripts implemented in Python. Quantitative analyses were performed to compare outcomes across feedback modalities, actuator locations, and mounting configurations. Statistical significance was assessed at a p-value of 0.05. Perceptual measures, including detection thresholds and JNDs, were compared using repeated measures analyses of variance. Functional performance metrics, such as accuracy and reaction time, were similarly analyzed to assess the effects of feedback modality, actuator configuration, and task condition. Postural sway during static balance tests was quantified by computing 95% confidence sway ellipses from trunk sway angles derived from inertial measurement unit data, with ellipse area used as an indicator of postural stability.
The results of the fundamental perceptual tests revealed modality-dependent differences in stimulus perception. Detection thresholds for pressure-based mechanotactile stimulation were comparable across all tested actuator locations, indicating a homogeneous perception of pressure stimuli. In contrast, vibrotactile stimulation exhibited location-dependent thresholds, suggesting greater sensitivity to actuator placement. JND analysis showed consistently lower JND values for vibrotactile feedback across all locations, indicating a higher sensitivity to changes in stimulus intensity for vibration compared to pressure-based stimulation.
Functional test results demonstrated systematic differences between feedback modalities and actuator configurations. In the Labyrinth Test, directional accuracy was consistently higher when the cross mounting configuration was used, independent of feedback modality. Within the same experimental conditions, pressure-based feedback yielded higher accuracy than vibrotactile feedback. Performance was also affected by cognitive load, with accuracy generally decreasing in dual-task conditions compared to single-task conditions. Reaction time analysis indicated that responses to vibrotactile stimulation were consistently slower than those to pressure-based stimulation, suggesting that vibratory cues required longer processing time and were less intuitive to interpret.
In the Clock Test, pressure-based mechanotactile feedback resulted in higher directional identification accuracy than vibrotactile feedback across actuator configurations.
In contrast to the perceptual and functional outcomes, static balance assessments did not reveal significant differences between feedback modalities or actuator configurations. Postural sway measures were comparable across no-feedback, pressure-based feedback, and vibrotactile feedback conditions under the tested visual and cognitive conditions. This suggests that, within the scope of the present experimental protocol and in an able-bodied population, neither feedback modality produced a measurable improvement in static standing balance.
Subjective evaluations indicated that both actuator mounting configurations were perceived as comfortable, natural, and intuitive by participants. Both pressure-based and vibrotactile feedback were reported as pleasant and non-painful, with only minor perceived differences between modalities and configurations. These findings support the overall usability and acceptability of the stimulation system.
The main limitation of the proposed study is that the experiments were conducted exclusively on able-bodied participants, which may limit generalizability to clinical populations. Furthermore, the balance assessments focused on static tasks, and the duration of exposure to the feedback was limited, precluding evaluation of learning effects or long-term adaptation.
In conclusion, this thesis provides a systematic non-clinical evaluation of a pressure-based mechanotactile feedback system for balance-related applications. While vibrotactile stimulation demonstrated superior intensity discrimination, pressure-based feedback showed more consistent perception across locations and superior intuitiveness in functional tasks. Although no significant effects were observed in static balance performance, the results suggest that pressure-based mechanotactile feedback represents a promising and user-accepted alternative to vibrotactile stimulation. These findings provide a foundation for future studies involving dynamic balance tasks, extended training periods, and validation in clinical populations.