Autoencoding sensory substitution

Tens of millions of people live blind, and their number is ever increasing. Visual-to-auditory sensory substitution (SS) encompasses a family of cheap, generic solutions to assist the visually impaired by conveying visual information through sound. The required SS training is lengthy: months of effort is necessary to reach a practical level of adaptation. There are two reasons for the tedious training process: the elongated substituting audio signal, and the disregard for the compressive characteristics of the human hearing system. To overcome these obstacles, we developed a novel class of SS methods, by training deep recurrent autoencoders for image-to-sound conversion. We successfully trained deep learning models on different datasets to execute visual-to-auditory stimulus conversion. By constraining the visual space, we demonstrated the viability of shortened substituting audio signals, while proposing mechanisms, such as the integration of computational hearing models, to optimally convey visual features in the substituting stimulus as perceptually discernible auditory components. We tested our approach in two separate cases. In the first experiment, the author went blindfolded for 5 days, while performing SS training on hand posture discrimination. The second experiment assessed the accuracy of reaching movements towards objects on a table. In both test cases, above-chance-level accuracy was attained after a few hours of training. Our novel SS architecture broadens the horizon of rehabilitation methods engineered for the visually impaired. Further improvements on the proposed model shall yield hastened rehabilitation of the blind and a wider adaptation of SS devices as a consequence

The temporal pattern of impulses in primary afferents analogously encodes touch and hearing information

An open question in neuroscience is the contribution of temporal relations between individual impulses in primary afferents in conveying sensory information. We investigated this question in touch and hearing, while looking for any shared coding scheme. In both systems, we artificially induced temporally diverse afferent impulse trains and probed the evoked perceptions in human subjects using psychophysical techniques. First, we investigated whether the temporal structure of a fixed number of impulses conveys information about the magnitude of tactile intensity. We found that clustering the impulses into periodic bursts elicited graded increases of intensity as a function of burst impulse count, even though fewer afferents were recruited throughout the longer bursts. The interval between successive bursts of peripheral neural activity (the burst-gap) has been demonstrated in our lab to be the most prominent temporal feature for coding skin vibration frequency, as opposed to either spike rate or periodicity. Given the similarities between tactile and auditory systems, second, we explored the auditory system for an equivalent neural coding strategy. By using brief acoustic pulses, we showed that the burst-gap is a shared temporal code for pitch perception between the modalities. Following this evidence of parallels in temporal frequency processing, we next assessed the perceptual frequency equivalence between the two modalities using auditory and tactile pulse stimuli of simple and complex temporal features in cross-sensory frequency discrimination experiments. Identical temporal stimulation patterns in tactile and auditory afferents produced equivalent perceived frequencies, suggesting an analogous temporal frequency computation mechanism. The new insights into encoding tactile intensity through clustering of fixed charge electric pulses into bursts suggest a novel approach to convey varying contact forces to neural interface users, requiring no modulation of either stimulation current or base pulse frequency. Increasing control of the temporal patterning of pulses in cochlear implant users might improve pitch perception and speech comprehension. The perceptual correspondence between touch and hearing not only suggests the possibility of establishing cross-modal comparison standards for robust psychophysical investigations, but also supports the plausibility of cross-sensory substitution devices

How touch and hearing influence visual processing in sensory substitution, synaesthesia and cross-modal correspondences

Sensory substitution devices (SSDs) systematically turn visual dimensions into patterns of tactile or auditory stimulation. After training, a user of these devices learns to translate these audio or tactile sensations back into a mental visual picture. Most previous SSDs translate greyscale images using intuitive cross-sensory mappings to help users learn the devices. However more recent SSDs have started to incorporate additional colour dimensions such as saturation and hue. Chapter two examines how previous SSDs have translated the complexities of colour into hearing or touch. The chapter explores if colour is useful for SSD users, how SSD and veridical colour perception differ and how optimal cross-sensory mappings might be considered. After long-term training, some blind users of SSDs report visual sensations from tactile or auditory stimulation. A related phenomena is that of synaesthesia, a condition where stimulation of one modality (i.e. touch) produces an automatic, consistent and vivid sensation in another modality (i.e. vision). Tactile-visual synaesthesia is an extremely rare variant that can shed light on how the tactile-visual system is altered when touch can elicit visual sensations. Chapter three reports a series of investigations on the tactile discrimination abilities and phenomenology of tactile-vision synaesthetes, alongside questionnaire data from synaesthetes unavailable for testing. Chapter four introduces a new SSD to test if the presentation of colour information in sensory substitution affects object and colour discrimination. Chapter five presents experiments on intuitive auditory-colour mappings across a wide variety of sounds. These findings are used to predict the reported colour hallucinations resulting from LSD use while listening to these sounds. Chapter six uses a new sensory substitution device designed to test the utility of these intuitive sound-colour links for visual processing. These findings are discussed with reference to how cross-sensory links, LSD and synaesthesia can inform optimal SSD design for visual processing

Perceptual Strategies and Neuronal Underpinnings underlying Pattern Recognition through Visual and Tactile Sensory Modalities in Rats

The aim of my PhD project was to investigate multisensory perception and multimodal recognition abilities in the rat, to better understand the underlying perceptual strategies and neuronal mechanisms. I have chosen to carry out this project on the laboratory rat, for two reasons. First, the rat is a flexible and highly accessible experimental model, where it is possible to combine state-of-the-art neurophysiological approaches (such as multi-electrode neuronal recordings) with behavioral investigation of perception and (more in general) cognition. Second, extensive research concerning multimodal integration has already been conducted in this species, both at the neurophysiological and behavioral level. My thesis work has been organized in two projects: a psychophysical assessment of object categorization abilities in rats, and a neurophysiological study of neuronal tuning in the primary visual cortex of anaesthetized rats. In both experiments, unisensory (visual and tactile) and multisensory (visuo-tactile) stimulation has been used for training and testing, depending on the task. The first project has required development of a new experimental rig for the study of object categorization in rat, using solid objects, so as to be able to assess their recognition abilities under different modalities: vision, touch and both together. The second project involved an electrophysiological study of rat primary visual cortex, during visual, tactile and visuo-tactile stimulation, with the aim of understanding whether any interaction between these modalities exists, in an area that is mainly deputed to one of them. The results of both of the studies are still preliminary, but they already offer some interesting insights on the defining features of these abilities

An object's smell in the multisensory brain : how our senses interact during olfactory object processing

Object perception is a remarkable and fundamental cognitive ability that allows us to interpret and interact with the world we are living in. In our everyday life, we constantly perceive objects–mostly without being aware of it and through several senses at the same time. Although it might seem that object perception is accomplished without any effort, the underlying neural mechanisms are anything but simple. How we perceive objects in the world surrounding us is the result of a complex interplay of our senses. The aim of the present thesis was to explore, by means of functional magnetic resonance imaging, how our senses interact when we perceive an object’s smell in a multisensory setting where the amount of sensory stimulation increases, as well as in a unisensory setting where we perceive an object’s smell in isolation. In Study I, we sought to determine whether and how multisensory object information influences the processing of olfactory object information in the posterior piriform cortex (PPC), a region linked to olfactory object encoding. In Study II, we then expanded our search for integration effects during multisensory object perception to the whole brain because previous research has demonstrated that multisensory integration is accomplished by a network of early sensory cortices and higher-order multisensory integration sites. We specifically aimed at determining whether there exist cortical regions that process multisensory object information independent of from which senses and from how many senses the information arises. In Study III, we then sought to unveil how our senses interact during olfactory object perception in a unisensory setting. Other previous studies have shown that even in such unisensory settings, olfactory object processing is not exclusively accomplished by regions within the olfactory system but instead engages a more widespread network of brain regions, such as regions belonging to the visual system. We aimed at determining what this visual engagement represents. That is, whether areas of the brain that are principally concerned with processing visual object information also hold neural representations of olfactory object information, and if so, whether these representations are similar for smells and pictures of the same objects. In Study I we demonstrated that assisting inputs from our senses of vision and hearing increase the processing of olfactory object information in the PPC, and that the more assisting input we receive the more the processing is enhanced. As this enhancement occurred only for matching inputs, it likely reflects integration of multisensory object information. Study II provided evidence for convergence of multisensory object information in form of a non-linear response enhancement in the inferior parietal cortex: activation increased for bimodal compared to unimodal stimulation, and increased even further for trimodal compared to bimodal stimulation. As this multisensory response enhancement occurred independent of the congruency of the incoming signals, it likely reflects a process of relating the incoming sensory information streams to each other. Finally, Study III revealed that regions of the ventral visual object stream are engaged in recognition of an object’s smell and represent olfactory object information in form of distinct neural activation patterns. While the visual system encodes information about both visual and olfactory objects, it appears to keep information from the two sensory modalities separate by representing smells and pictures of objects differently. Taken together, the studies included in this thesis reveal that olfactory object perception is a multisensory process that engages a widespread network of early sensory as well higher-order cortical regions, even if we do not encounter ourselves in a multisensory setting but exclusively perceive an object’s smell

Estudo neurofisiológico da discriminação de distância em humanos

Tactile width discrimination processing has been extensively studied in rodents and has demonstrated multiple relevant basic mechanisms. Despite this relevance, the number of studies of width discrimination in humans has been scarce. During the present dissertation, neurophysiological correlates of width discrimination were analyzed through electroencephalography recordings in participants performing a width discrimination task in active or passive modes. Analysis of power in the delta, theta, alpha, beta, and gamma frequency bands revealed differences in the power for different frequency bands and electrodes recorded. Active width discrimination processing was characterized by an increase in the power of delta, theta and gamma frequency bands in electrodes F3 and F4, and an increase in power in the gamma frequency band in T4. Passive tactile width processing was characterized by an increase in the power of delta in electrodes Fp1 and T4, and an increase in gamma frequency band in Tp10. Altogether these results suggest that active and passive tactile width discrimination processing are characterized by an asymmetrical network involving prefrontal, frontal and temporal electrodes, in delta, theta, and gamma frequency bands.O estudo do processamento tátil de distâncias encontra-se bastante desenvolvido em roedores, tendo sido útil para a demonstração de múltiplos mecanismos básicos relevantes. Apesar desta relevância, o estudo da discriminação de distâncias em humanos é ainda bastante reduzido. Durante a presente dissertação foram analisados os correlatos neurofisiológicos, através do registo de eletroencefalografia, em participantes que realizavam uma tarefa de discriminação de distância em modo ativo ou passivo. A análise da potência das bandas de frequências delta, teta, alfa, beta e gama revelou diferenças na potência do sinal para diferentes bandas de frequências e elétrodos. O processamento ativo era caracterizado por um aumento da potência nas bandas de frequências delta, teta e gama nos elétrodos F3, F4; e um aumento da atividade na banda de frequência gama no elétrodo T4. O processamento passivo era caracterizado por um aumento da potência de delta nos elétrodos Fp1 e T4 e um aumento da potência de gama em Tp10. No seu conjunto, estes resultados sugerem que o processamento ativo e passivo da distância são caraterizados por uma rede assimétrica envolvendo elétrodos pré-frontais, frontais e temporais nas bandas de frequência delta, teta e gama.Mestrado em Biomedicina Molecula