Body representation disorders have a key role in the characterization of obesity. So far, the literature consistently pointed to a negative attitudinal body image. Conversely, after reviewing the pertinent literature, it emerges that more incoherent results have been reported for the self-perceived body size. Chapter 2 tries to clarify this issue by adopting a more innovative theoretical framework (i.e., the implicit/explicit model; Longo, 2015). For the first time, we probed the implicit representation underlying position sense in obesity, reporting a similar representation to healthy weight participants. Importantly, this result shows that not all components of body representation are affected by obesity. Chapter 3 addresses another aspect of body representation that has been neglected in obesity, namely bodily self-consciousness. The Rubber Hand Illusion has been traditionally used to investigate the mechanisms underlying body awareness. Our results show that individuals with obesity have comparable subjective experience of the illusion, while the effect of the illusion on self-location is reduced. This dissociation can be interpreted as the result of a preserved visuo-tactile integration and an altered visuo-proprioceptive integration in obesity. However, in Chapter 4 we reported that individuals with obesity have a reduced temporal resolution of visuo-tactile integration, meaning that they integrated stimuli over an extended range of asynchronies than healthy weight participants. In fact, this evidence predicts that in the RHI individuals with obesity might perceive more synchronously the asynchronous stimulation, showing a greater effect of the illusion also in this condition. Nevertheless, we failed to show this pattern of results in our study with an interval of asynchronous stimulation of 1000 ms (usually adopted in the RHI paradigm). We hypothesized that smaller time-lags, which are inside the temporal binding window of individuals with obesity and outside the temporal binding widow of healthy weight participants, might not be perceived by individuals with obesity but detected by healthy weight individuals. Accordingly, a dissimilar susceptibility to the illusion should be observed. Chapter 5 investigates this issue by adopting a modified version of the RHI that enables a parametrical modulation of the timing of the stimulation. However, we could not replicate the RHI even in healthy weight participants. The possible methodological reasons for this failure are discussed. Overall, this work tries to fill some gaps in the previous literature about body representation in obesity. Moreover, our findings provide an important clue about the possible cognitive mechanisms involved in body representation disorders in obesity. However, many questions still need an answer: due to the complexity of the domain a comprehensive knowledge of the topic might be challenging. A deep understanding of obesity is fundamental to develop multidisciplinary and efficacious rehabilitative protocols. Indeed, better treatments would significantly ameliorate individuals’ well-being but also contribute to reduce the huge health costs related to obesity comorbidities.

Efficient navigation of our social world depends on the generation, interpretation and combination of social signals within different sensory systems. However, the influence of adult aging on cross-modal integration of emotional stimuli remains poorly understood. Therefore, the aim of this PhD thesis is to understand the integration of visual and auditory cues in social situations and how this is associated with other factors important for successful social interaction such as recognising emotions or understanding the mental states of others. A series of eight experiments were designed to compare the performance of younger and older adults on tasks related to multisensory integration and social cognition. Results suggest that older adults are significantly less accurate at correctly identifying emotions from one modality (faces or voices alone) but perform as well as younger adults on tasks where congruent auditory and visual emotional information are presented concurrently. Therefore, older adults appear to benefit from congruent multisensory information. In contrast, older adults are poorer than younger adults at detecting incongruency from different sensory modalities involved in decoding cues to deception, sarcasm or masking of emotions. It was also found that age differences in the processing of relevant and irrelevant visual and auditory social information might be related to changes in gaze behaviour. A further study demonstrated that the changes in behaviour and social interaction often reported in patients post-stroke might relate to problems in integrating the cross-modal social information. The pattern of findings is discussed in relation to social, emotional, neuropsychological and cognitive theories.

To make sense of our unpredictable world, humans use sensory information streaming through billions of peripheral neurons. Uncertainty and ambiguity plague each sensory stream, yet remarkably our perception of the world is seamless, robust and often optimal in the sense of minimising perceptual variability. Moreover, humans have a remarkable capacity for dexterous manipulation. Initiation of precise motor actions under uncertainty requires awareness of not only the statistics of our environment but also the reliability of our sensory and motor apparatus. What happens when our sensory and motor systems are disrupted? Upper-limb amputees tted with a state-of-the-art prostheses must learn to both control and make sense of their robotic replacement limb. Tactile feedback is not a standard feature of these open-loop limbs, fundamentally limiting the degree of rehabilitation. This thesis introduces a modular closed-loop upper-limb prosthesis, a modified Touch Bionics ilimb hand with a custom-built linear vibrotactile feedback array. To understand the utility of the feedback system in the presence of multisensory and sensorimotor influences, three fundamental open questions were addressed: (i) What are the mechanisms by which subjects compute sensory uncertainty? (ii) Do subjects integrate an artificial modality with visual feedback as a function of sensory uncertainty? (iii) What are the influences of open-loop and closed-loop uncertainty on prosthesis control? To optimally handle uncertainty in the environment people must acquire estimates of the mean and uncertainty of sensory cues over time. A novel visual tracking experiment was developed in order to explore the processes by which people acquire these statistical estimators. Subjects were required to simultaneously report their evolving estimate of the mean and uncertainty of visual stimuli over time. This revealed that subjects could accumulate noisy evidence over the course of a trial to form an optimal continuous estimate of the mean, hindered only by natural kinematic constraints. Although subjects had explicit access to a measure of their continuous objective uncertainty, acquired from sensory information available within a trial, this was limited by a conservative margin for error. In the Bayesian framework, sensory evidence (from multiple sensory cues) and prior beliefs (knowledge of the statistics of sensory cues) are combined to form a posterior estimate of the state of the world. Multiple studies have revealed that humans behave as optimal Bayesian observers when making binary decisions in forced-choice tasks. In this thesis these results were extended to a continuous spatial localisation task. Subjects could rapidly accumulate evidence presented via vibrotactile feedback (an artificial modality ), and integrate it with visual feedback. The weight attributed to each sensory modality was chosen so as to minimise the overall objective uncertainty. Since subjects were able to combine multiple sources of sensory information with respect to their sensory uncertainties, it was hypothesised that vibrotactile feedback would benefit prosthesis wearers in the presence of either sensory or motor uncertainty. The closed-loop prosthesis served as a novel manipulandum to examine the role of feed-forward and feed-back mechanisms for prosthesis control, known to be required for successful object manipulation in healthy humans. Subjects formed economical grasps in idealised (noise-free) conditions and this was maintained even when visual, tactile and both sources of feedback were removed. However, when uncertainty was introduced into the hand controller, performance degraded significantly in the absence of visual or tactile feedback. These results reveal the complementary nature of feed-forward and feed-back processes in simulated prosthesis wearers, and highlight the importance of tactile feedback for control of a prosthesis.

Olfactory sensing is ubiquitous across animals and important for survival. Yet, its characteristics, mechanisms, and functions in humans remain not well understood. In this dissertation, I present four studies on human olfactory perception. Study I investigates the impact of short-term exposures to an odorant on long-term olfactory learning and habituation, while Study II examines human ability to localize smells; Study III probes visual-olfactory integration of object representations, and Study IV explores the role of olfaction in sensing nutrients. Several conclusions are drawn from these studies. First, brief intermittent exposures to even a barely detectable odorant lead to long-term incremental odorant-specific habituation. Second, humans localize smells based on gradient cues between the nostrils. Third, there is a within-hemispheric advantage in the integration of visual-olfactory object representations. Fourth, olfaction partakes in nutrient-sensing and facilitates the detection of food. Some broader implications of our findings are discussed.

Spatial navigation is essential for everyday function. It is successfully achieved by combining internally generated information – such vestibular and self-motion cues (known as path integration) – with external sources of information such as visual landmarks. These multiple sources and sensory domains are often associated with uncertainty and can provide conflicting information. The key to successful navigation is therefore how to integrate information from these internal and external sources in the best way. Healthy younger adults do this in a statistically optimal fashion by considering the perceived reliability of a cue during integration, consistent with the rules of Bayesian integration. However, the precise impact of ageing on the component senses of path integration and integration of such self-motion with external information is currently unclear. Given that impaired spatial ability is a common problem associated with ageing and is often a primary indicator of Alzheimer’s disease, this thesis asks whether age-related navigational impairments are related to fundamental deficits in the components of path integration and/or inadequate integration of spatial cues. Part 1 focussed on how ageing impacts the vestibular, kinaesthetic and visual components of path integration during linear navigation in the real world. Using path reproduction, distance estimation and depth perception tasks, I found that older adults showed no performance deficits in conditions that replicated those of everyday walking when visual and self-motion cues were present. However, they were impaired when relying on vestibular information alone. My results suggest that older adults are especially vulnerable to sensory deprivation but that weaker sensory domains can be compensated for by other sensory information, potentially by integrating different spatial cues in a Bayesian fashion: where the impact of unreliable/diminished senses can be minimised. Part 2 developed the conclusions of Part 1 by testing younger and older adults’ integration of visual landmarks and self-motion information during a simple homing task. I investigated the hypothesis that the integration of spatial information from multiple sensory domains is driven by Bayesian principles and that old age may affect the efficiency and elasticity of reliability-driven integration. Younger and older participants navigated to a previously visited location using self-motion and/or visual information. In some trials there was a conflict of information, which revealed the relative influence of self-motion and visual landmarks on behaviour. Findings revealed that both younger and older adults integrated visual and self-motion information to improve accuracy and precision, but older adults did not place as much influence on visual information as would have been optimal. This may have been the result of increased noise in the underlying spatial representations of older adults. Furthermore, older adults did not effectively re-weight visual and self-motion cues in line with the changing reliability of visual information, suggesting diminished plasticity in the underlying spatial representations. However, further development of the testing paradigm would strengthen support for these findings. Together, the findings of Part 2 suggest that increased neural noise and the suboptimal weighting of spatial cues might contribute to the common problems with navigation experienced by many older adults. This thesis provides original evidence for age-related changes to multisensory integration of spatial cues. Path integration abilities are relatively preserved when older adults navigate linear paths in the real world, despite loss of vestibular function. However, navigation is affected by old age when the task becomes more complex. Multisensory integration of spatial cues is partially preserved but it is not fully efficient. I offer evidence that the navigational impairments common to old age are related to fundamental deficits in the components of path integration, task complexity, and suboptimal integration of spatial cues. Crucially however, path integration is preserved sufficiently in older adults that they are able to navigate in small scale with relative success.

abstract: Understanding where our bodies are in space is imperative for motor control, particularly for actions such as goal-directed reaching. Multisensory integration is crucial for reducing uncertainty in arm position estimates. This dissertation examines time and frequency-domain correlates of visual-proprioceptive integration during an arm-position maintenance task. Neural recordings were obtained from two different cortical areas as non-human primates performed a center-out reaching task in a virtual reality environment. Following a reach, animals maintained the end-point position of their arm under unimodal (proprioception only) and bimodal (proprioception and vision) conditions. In both areas, time domain and multi-taper spectral analysis methods were used to quantify changes in the spiking, local field potential (LFP), and spike-field coherence during arm-position maintenance. In both areas, individual neurons were classified based on the spectrum of their spiking patterns. A large proportion of cells in the SPL that exhibited sensory condition-specific oscillatory spiking in the beta (13-30Hz) frequency band. Cells in the IPL typically had a more diverse mix of oscillatory and refractory spiking patterns during the task in response to changing sensory condition. Contrary to the assumptions made in many modelling studies, none of the cells exhibited Poisson-spiking statistics in SPL or IPL. Evoked LFPs in both areas exhibited greater effects of target location than visual condition, though the evoked responses in the preferred reach direction were generally suppressed in the bimodal condition relative to the unimodal condition. Significant effects of target location on evoked responses were observed during the movement period of the task well. In the frequency domain, LFP power in both cortical areas was enhanced in the beta band during the position estimation epoch of the task, indicating that LFP beta oscillations may be important for maintaining the ongoing state. This was particularly evident at the population level, with clear increase in alpha and beta power. Differences in spectral power between conditions also became apparent at the population level, with power during bimodal trials being suppressed relative to unimodal. The spike-field coherence showed confounding results in both the SPL and IPL, with no clear correlation between incidence of beta oscillations and significant beta coherence. / Dissertation/Thesis / Doctoral Dissertation Biomedical Engineering 2017

京都大学 / 0048 / 新制・課程博士 / 博士(文学) / 甲第20828号 / 文博第758号 / 新制||文||655(附属図書館) / 京都大学大学院文学研究科行動文化学専攻 / (主査)教授 蘆田 宏, 教授 板倉 昭二, 教授 Anderson James Russell, 准教授 ALTMANN Christian / 学位規則第4条第1項該当 / Doctor of Letters / Kyoto University / DFAM

The physical properties of the distal stimuli activating our senses are often correlated in nature; it would therefore be advantageous to exploit such correlations to better process sensory information. Stimulus correlations can be contingent and readily available to the senses (like the temporal correlation between mouth movements and vocal sounds in speech), or can be the results of the statistical co-occurrence of certain stimulus properties that can be learnt over time (like the relation between the frequency of acoustic resonance and the size of the resonator). Over the last century, a large body of research on multisensory processing has demonstrated the existence of compatibility effects between individual features of stimuli from different sensory modalities. Such compatibility effects, termed crossmodal correspondences, possibly reflect the internalization of the natural correlation between stimulus properties. The present dissertation assesses the effects of crossmodal correspondences on multisensory processing and reports a series of experiments demonstrating that crossmodal correspondences influence the processing rate of sensory information, distort perceptual experiences and lead to stronger multisensory integration. Moreover, a final experiment investigating the effects of contingent signals’ correlation on multisensory processing demonstrates the key role of temporal correlation in inferring whether two signals have a common physical cause or not (i.e., the correspondence problem). A Bayesian framework is proposed to interpret the present results whereby stimulus correlations, represented on the prior distribution of expected crossmodal co-occurrence, operate as cues to solve the correspondence problem.

The research presented in this thesis was designed to investigate whether and how the temporal synchrony and spatial congruence of non-visual cues with visual targets could work together to improve the discrimination and identification of visual targets in neurologically-healthy adult humans. The speed and accuracy of participants’ responses were compared following the presence or absence of temporally synchronous and/or spatially congruent or incongruent auditory, vibrotactile, and audiotactile cues in the context of dynamic visual search and rapidly-masked visual target identification. The understanding of the effects of auditory, vibrotactile, and audiotactile cues derived from these laboratory-based tasks was then applied to an air traffic control simulation involving the detection and resolution of potential conflicts (represented as visual targets amidst dynamic and cluttered visual stimuli). The results of the experiments reported in this thesis demonstrate that, in the laboratory-based setting, temporally synchronous and spatially informative non-visual cues both gave rise to significant improvements in participants’ performance, and the combination of temporal and spatial cuing gave rise to additional improvements in visual target identification performance. In the real-world setting, however, only the temporally synchronous unimodal auditory and bimodal audiotactile cues gave rise to a consistent facilitation of participants’ visual target detection performance. The mechanisms and accounts proposed to explain the effects of spatial and temporal cuing, namely multisensory integration and attention, are examined and discussed with respect to the observed improvements in participants’ visual target identification performance.

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