A considerable amount of our everyday tactile experience requires interactions between textured surfaces and our fingertips. Such interactions elicit complex vibrations on our skin surface, which are encoded by the mechanosensitive afferents and conveyed to the brain where the perception of the textures emerges seemingly effortlessly. Intuitively, a fundamental question that may be asked is: “what features of the vibration stimuli are behaviourally relevant and what are the neural signatures of these features?” The goal of this thesis is to investigate these questions, which we have done using a combination of theoretical and experimental approaches. Our theoretical approach (in Chapter 2) has been to create an ideal Bayesian perceptual observer that utilizes all the information available in a spike-rate based neural code and makes optimal inferences regarding the amplitude and the frequency of vibration stimuli. Our experimental approach has been to estimate the performance of human participants in vibrotactile detection (in Chapter 3), and in amplitude and frequency discrimination (in Chapter 4) tasks by using psychophysical procedures. The results of these approaches suggest that the human perceptual observer, i.e. the human nervous system, probably uses a rate code to represent vibrotactile amplitude, but a non-rate code, such as a spike timing code, to represent vibrotactile frequency. Additionally, we conclude that humans are capable of inferring and separately perceiving the amplitude and frequency of vibrotactile stimuli; however, depending on experimental tasks, humans might also rely on a feature that combines the amplitude and frequency of vibrotactile stimuli.

Doctor of Philosophy (PhD)

Sensory adaptation is an important aspect of perception. A seemingly non-beneficial consequence of adaptation is that it produces perceptual illusions. For instance, following focal adaptation, the perceived separation between stimuli straddling the adapted attribute or region is often exaggerated. This type of illusion, known as perceptual repulsion, is both a consequence of and a clue to the brain’s coding strategies and how they are influenced by recent sensory events. Adaptation-induced perceptual repulsion has been well documented in vision (e.g. the tilt aftereffect) and to a lesser extent in audition, but rarely studied in touch. The present thesis investigated the effects of adaptation on tactile spatial perception using a combination of human psychophysics and computational modeling. In a two-interval forced choice task, participants compared the perceived separation between two point-stimuli applied on the forearms successively. The point of subjective equality was extracted as a measure of perceived two-point distance. We showed that tactile spatial perception is subject to an adaptation-induced repulsion illusion: vibrotactile adaptation focally reduced tactile sensitivity and significantly increased the perceived distance between points straddling the adapted skin site (Chapter 2). This repulsion illusion, however, was not observed when the intervening skin was desensitized with topical anesthesia instead of vibrotactile adaptation, suggesting that peripheral desensitization alone is insufficient to induce the illusion (Chapter 3). With Bayesian perceptual modeling, we showed that the illusion was consistent with the hypothesis that the brain decodes tactile spatial input without awareness of the adaptation state in the nervous system (Chapter 4). Together, the empirical and theoretical work furthers the understanding of dynamic tactile spatial coding as the somatosensory system adapts to the sensory environment. Its main findings are consistent with the adaptation- induced repulsion illusions reported in vision and audition, suggesting that perception in different sensory modalities shares common processing features and computational principles.

Thesis

Doctor of Philosophy (PhD)

Sensory adaptation can shape how we perceive the world. In this thesis, we showed that the perception of space in touch is pliable and subject to the influence of adaptation. Psychophysical testing in human participants showed that vibratory adaptation induced an illusion that expanded the perceived distance between stimuli on the skin. This illusion provides clues into how information about space in touch is normally processed and interpreted by the brain. In addition, we developed a computational model that used a powerful statistical framework – Bayesian inference – to probe touch on a theoretical basis. To the best of our knowledge, the present thesis provides the first combined psychophysical and computational study on the effects of adaptation on tactile spatial perception. Our findings suggest that touch shares…

Persons with visual impairments often rely on navigational electronic aids, which typically employ speech commands for guidance through novel routes. However, navigational speech commands may interfere with the perception of acoustically rich environmental information, resulting in potentially detrimental effects. We investigated the sense of touch as a means to convey navigational commands instead. The somatotopic representation of the body surface within the central nervous system makes spatial information intuitive to our skin, suggesting that the tactile channel should be equivalent to, if not better than, the auditory channel at processing directional commands. Additionally, based on Wickens’ Multiple resource theory, the tactile channel should mitigate the sensory load in the auditory channel in travelers with visual impairments. We tested the ability of blind users to process directional commands conveyed via a tactile navigational belt. 14 blind participants were tested with the tactile belt under conditions of either low or high acoustic sensory load, simulating different outdoor environments. For comparison, the same participants were tested also with a conventional auditory device. Consistent with previous studies, we found navigation with the tactile belt to be less efficient than navigation with the auditory aid in the absence of environmental sounds. However, we found also – for the first time, to our knowledge – that tactile performance was less compromised under conditions of high acoustic sensory load. These results will help to inform the further investigation and development of tactile displays to benefit blind travelers.

Thesis

Master of Science (MSc)

The study of tactile illusions like visual illusions can reveal the brain's processing strategies. A famous tactile illusion is the cutaneous rabbit illusion. Fundamental to this illusion is the perceptual length contraction phenomenon: two taps that occur in rapid succession on the forearm are perceived as occurring closer together than they were physically placed. Our lab previously proposed a Bayesian probabilistic model that views perception as a compromise between expectation (prior experience) and sensation (likelihood of sensorineural data given hypothesized tap locations). The model proposes a low-speed prior, an expectation based on experience that objects tend to be stationary or to move slowly on the skin. When the sensation of space is unclear (e.g., taps are weak), the model predicts that expectation will strongly influence perception. Consistent with this prediction, our lab previously showed that the use of weaker taps causes more pronounced perceptual length contraction. Here we report psychophysical tests on 64 participants, which confirmed this finding. Our study also used stimulus sequences consisting of a weak and a strong tap, for which the Bayesian model predicts an asymmetric perceptual length contraction, such that the weaker tap location will be perceived to shift more than the stronger tap. The experimental results confirmed this prediction, providing further support for our Bayesian probabilistic model as an explanation for perceptual length contraction. However, our results revealed a discrepancy in the data at the smaller SOAs, which showed less length contraction than predicted. We hypothesized that participants might overestimate the smaller SOAs, an effect our lab defines as time dilation. Accordingly, in a second study we investigated the effects of varying SOA and lengths on perceived SOA. The model predicts more pronounced time dilation at smaller SOAs and larger lengths. The psychophysical data from 37 participants confirmed the trends predicted by the model.

Thesis

Master of Science (MSc)

The earliest measures of tactile spatial acuity reflect the ability of human observers to localize and discriminate the simplest of stimuli: single or double point (punctate) indentations. Because punctate stimuli cover an extremely small area, they typically only activate a few peripheral afferents at a time. Therefore, many researchers have used single-point localization and two-point discrimination thresholds to probe the density of innervation at different body sites. This thesis explores the relationship between peripheral properties and the spatial perception of tactile point stimuli. In chapter 2, we simulate the neural responses of primary afferents to single and double points, capturing many realistic properties of the periphery: innervation density, the shape and size of receptive fields, and interactions between two-point stimuli. Furthermore, we model optimal performance in localization and discrimination tasks given these afferent responses, and compare it to human performance. We find that human performance is well below optimal, suggesting that humans do not make use of all the information present at the level of the primary afferents. Nevertheless, many human performance trends, resulting from peripheral properties, are predicted by our computational analysis. Using empirical methods, in Chapter 3, we further investigate one of these trends: surround suppression (the suppression of two-point responses relative to that of a single point) is thought to provide a magnitude cue during two-point discrimination (2PD), resulting in elevated performance even at zero separation between two points. We demonstrate that human observers do indeed show elevated 2PD performance at zero separation on a variety of tested body-sites; an alternative task involving orientation discrimination, however, does not show this same trend and is therefore unlikely to be contaminated by the same magnitude cue. In Chapter 4 we review and test a Bayesian model of two-point trajectory estimation that replicates a famous perceptual length contraction illusion. We provide evidence in support of the model: stimuli that give rise to poor spatial acuity also give rise to a stronger length contraction illusion. Overall, the three studies covered in this thesis elucidate many of the peripheral and stimulus properties that shape our perception of tactile point stimuli.

Thesis

Doctor of Philosophy (PhD)

This thesis details an in-depth investigation into the physiological and computational factors involved in the perception of tactile point stimuli. Touch is an often overlooked and under-appreciated sense, however its importance in daily functioning is unquestionable: touch feedback allows us to safely and efficiently interact with the environment, while fine touch allows us to detect and discriminate textures and patterns. The perception of point stimuli is a fundamental aspect of texture and pattern discrimination, with applications as broad as clinical testing and sensory substitution (i.e. Braille).

In recent years, many studies have reported that the tactile spatial acuity of blind participants is enhanced relative to that of sighted participants, but it is unclear what factors drive this enhancement. In the series of three psychophysics studies (of tactile spatial acuity) presented in this thesis, we attempted to tease apart two hypotheses explaining tactile spatial acuity enhancement in the blind: visual deprivation and tactile experience. To measure tactile spatial acuity in these studies, we used a grating orientation task. In the first study (Chapter 2), we found that blind participants outperformed sighted participants, but only on body parts where tactile experience is presumably greater in blind than in sighted participants (i.e., fingertips, not lips); we found additionally that blind participants’ tactile acuity correlated with their Braille reading behaviour (e.g., style, frequency of reading). In the second study (Chapter 3), we found that visual deprivation of sighted participants for periods up to 110 minutes did not enhance their sense of touch. In the third study (Chapter 4), we found that extensive training on a tactile task can substantially improve sighted participants’ sense of touch. The findings from our three studies thus provide consistent support for the hypothesis that tactile experience, but not visual deprivation, drives tactile spatial acuity enhancement in the blind.

Doctor of Philosophy (PhD)

Measurement of human tactile spatial acuity – the ability to perceive the fine spatial structure of surfaces contacting our fingertips – provides a valuable tool for probing both the peripheral and central nervous system. However, measures of tactile spatial acuity have long been plagued by a prodigious amount of variability present between individuals in their sense of touch. Previously proposed sources of variability include sex, and age; here we propose a novel source of variability – fingertip size. Building upon anatomical research, we hypothesize that mechanoreceptors are more sparsely distributed in larger fingers. In this thesis, I provide empirical and theoretical support for the hypothesis that fingertip growth from childhood into adulthood sets up an apparent sex difference in human tactile spatial acuity during young adulthood (Chapter 2), and also predicts changes in acuity more strongly than does age over development (Chapter 3). To further understand how fingertip size could limit an individual's tactile spatial acuity, we develop an ideal observer model using neurophysiological data collected by other labs (Chapter 4). In summary, this research provides support for a novel source of variability in the sense of touch: one that parsimoniously explains an apparent sex difference, and helps clarify the source of changes in tactile spatial acuity occurring with age during childhood.

Doctor of Philosophy (PhD)

Holistic processing has been deemed a crucial part of human face processing. There are three tasks that are indexes of holistic processing and each is used by many researchers for the purposes of demonstrating that either their participants have intact holistic processing or that holistic processing is impaired or missing. The tasks that demonstrate holistic processing are the face inversion, composite face, and the whole-part tasks. In this dissertation, I evaluate the hypothesis that holistic processing is important for face identification. A secondary hypothesis that is evaluated is whether the three indexes of holistic processing are related and whether they are tapping the same underlying process. Chapter 2 tests the first hypothesis in a large group of young adults and shows that the composite face effect (an index of holistic processing) is not related to accuracy on two identification tasks. Chapter 3 tested both hypotheses and showed that none of the holistic indexes are related to one another and they are unrelated to face identification accuracy. In Chapter 4, a large group of older adults are tested on the composite face task and a face identification task, similar to Experiment 2 from Chapter 2. Unlike the results for young adults, older adults show a significant positive correlation between the composite face effect and identification accuracy even though older adults perform worse on the identification task.

Doctor of Philosophy (PhD)

In recent years, computational neuroscientists have suggested that human behaviour, including perception, occurs in a manner consistent with Bayesian inference. According to the Bayesian ideal observer model, the observer combines cues from multiple sensory streams as a weighted average based on each cue’s reliability. Most cue-combination research has focused on integration of cues between sensory modalities or within the visual modality. Cue combination within the tactile modality has been relatively rarely studied, and it is still not known whether cues from individual digits combine optimally. In this thesis, we use the ideal observer model to determine whether cues from three different digits are combined optimally. We predicted that cues from multiple digits would be combined according to the optimal cue combination model. To test our hypothesis, we devised a two-interval forced choice (2IFC) task where participants had to discriminate the distal/proximal location of a 1-mm thick edge across the fingerpad(s) of the index (D2), middle (D3), and ring (D4) fingers. We used a Bayesian adaptive method, the ψ method, to compute participants’ psychometric functions for single-digit (D2, D3, and D4) and multiple-digit (D23, D24, D34, and D234) conditions. We determined the stimulus level ∆x, the distance (mm) between the distal and proximal stimuli locations, at 76% correct probability. This distance corresponds to a sensitivity index d'=1 and is the σ value of the participant’s stimulus measurement distribution. We then used the single-digit σ values to predict optimal cue combination for the multiple-digits combinations. We did not observer optimal cue-combination between the digits in this study. We outline potential implications the results of this experimental have on determining how the nervous system combines cues between digits, focusing on theoretical and experimental updates to the experiment that might result in the observation of optimal cue combination between digits.

Thesis

Master of Science (MSc)