Background. Tripping is a common factor in falls and a typical safety strategy to avoid tripping on steps or stairs is to increase foot clearance over the step edge. In the present study we asked whether the perceived height of a step could be increased using a visual illusion and whether this would lead to the adoption of a safer stepping strategy, in terms of greater foot clearance over the step edge. The study also addressed the controversial question of whether motor actions are dissociated from visual perception. Methodology/Principal Findings. 21 young, healthy subjects perceived the step to be higher in a configuration of the horizontal-vertical illusion compared to a reverse configuration (p = 0.01). During a simple stepping task, maximum toe elevation changed by an amount corresponding to the size of the visual illusion (p<0.001). Linear regression analyses showed highly significant associations between perceived step height and maximum toe elevation for all conditions. Conclusions/Significance. The perceived height of a step can be manipulated using a simple visual illusion, leading to the adoption of a safer stepping strategy in terms of greater foot clearance over a step edge. In addition, the strong link found between perception of a visual illusion and visuomotor action provides additional support to the view that the original, controversial proposal by Goodale and Milner (1992) of two separate and distinct visual streams for perception and visuomotor action should be re-evaluated. / College of Optometrists

There are often discrepancies in how a product is perceived in different representation media employed in typical product development processes. The first goal of this research project was to determine how visual illusions influence a designer's perception of a product across three representations: industrial design sketches, computer aided design (CAD) models, and physical prototypes (FDM rapid prototyping). A visualization experiment was conducted in which participants were asked to report how they perceived the shape and size of certain features, representing two types of illusions across the three model representations. Their statements were analyzed to identify the trends of how these two illusions affect overall appearance, categorized by representation type and the users' backgrounds (i.e., specialization and years of experience). The participants included students and professionals with various levels of engineering and industrial design experience. The analysis shows that there are differences in how designers see models depending on the representation media, and to some degree depending on the participants' professional background. The second goal was to explore the process of identifying such illusions automatically during the design process. In this regard, a discussion on how to implement the results from the visualization experiment is presented. Emphasis is on the potential development of a tool in CAD systems that would identify illusory effects and subsequently suggest potential design solutions. The possibility of using spectral analysis (fast Fourier transform) for an automated shape recognition capability in CAD systems is discussed. / Master of Science

Yes / Many visual effects are believed to be processed at several functional and anatomical levels of cortical processing. Determining if and how the levels contribute differentially to these effects is a leading problem in visual perception and visual neuroscience. We review and analyze a combination of extant psychophysical findings in the context of neurophysiological and brain-imaging results. Specifically using findings relating to visual illusions, crowding, and masking as exemplary cases, we develop a theoretical rationale for showing how relative levels of cortical processing contributing to these effects can already be deduced from the psychophysically determined functions relating respectively the illusory, crowding and masking strengths to the contrast of the illusion inducers, of the flankers producing the crowding, and of the mask. The wider implications of this rationale show how it can help to settle or clarify theoretical and interpretive inconsistencies and how it can further psychophysical, brain-recording and brain-imaging research geared to explore the relative functional and cortical levels at which conscious and unconscious processing of visual information occur. Our approach also allows us to make some specific predictions for future studies, whose results will provide empirical tests of its validity.

Dans notre vie quotidienne, nous recueillons et intégrons constamment un grand nombre d’informations sensorielles (Calvert & Thesen, 2004). Tout au long de nos activités perceptives, les connaissances que nous avons sur l’environnement sont continuellement "récupérées" en mémoire. Le cadre de la cognition incarnée et située proposent que les processus cognitifs (i.e. processus mnésiques, processus langagiers) sont ancrés dans les mêmes systèmes sensorimoteurs que ceux engagés dans les processus perceptivo-moteurs (Glenberg, 1997 ; Slotnick, 2004 ; Pecher & Zwaan, 2005).La mémoire contient des traces sensori-motrices encodées lors des multiples expériences de l’individu dans son environnement (Versace, Labeye, Badard, & Rose, 2009). De nombreux travaux en psychologie cognitive et en neurosciences démontrent que les connaissances sont construites et (re)émergent à partir de l’activation des systèmes neuronaux typiquement associés aux mécanismes perceptivo-moteurs. Le contenu et le fonctionnement de notre mémoire sont intrinsèquement liés à nos activités sensori-motrices passées et présentes. Pour être efficace, les connaissances impliquées dans nos activités cognitives doivent être étroitement liées à la situation présente. Cette capacité à s’adapter à des situations spécifiques ne serait pas possible à moins que les connaissances, y compris les connaissances conceptuelles, soientissues de la réactivation de traces mnésiques d’expériences passées (Barsalou, 2008 ; Versace et al., 2014). Réciproquement, les activités sensori-motrices sont totalement dépendantes des traces mnésiques d’expériences sensori-motrices passées. Ainsi, la différence entre perception et mémoire réside dans le fait que, dans le premier cas, les propriétés sont perceptivement présentes, tandis que, dans le deuxième cas, celles-ci sont absentes mais réactivées.Ce travail de thèse avait pour objectif d’étudier les liens entre mémoire et perception et, plus précisément, d’apporter des arguments en faveur de la similarité entre les processus mnésiques et perceptifs qui résultent de l’activation de composants de même nature sensorimotrice.Nous avons testé l’hypothèse selon laquelle des effets perceptifs devraient pouvoir être obtenus avec des composants réactivés en mémoire. Pour cela, nous avons utilisé des effets perceptifs - tels que l’effet de masquage ou les biais de jugement perceptif - afin d’explorer la possibilité de répliquer ce type d’effets avec l’intervention des dimensions mnésiques. / In everyday life, each of us is constantly processing perceptual input from the environment, we collect and then integrate numerous items of sensory information (Calvert & Thesen, 2004). Alongside these perceptual activities, knowledge related to our environment is continually "recovered" from memory. Embodied cognition and grounded cognition theories suggest that cognitive processes (e.g., memory processes, language processes) are grounded in the same sensory-motor systems as those used in perceptual and motor processes (Glenberg, 1997 ; Slotnick, 2004 ; Pecher & Zwaan, 2005).Memory is composed of sensorimotor traces encoded during the several experiences of an individual in his environment (Versace et al., 2009). A large number of studies in cognitive psychology and neurosciences demonstrated that knowledge is constructed and (re)emerged from the activation of neural systems typically associated with perceptual-motor mechanisms. The contents and the functioning of our memory are intrinsically linked to our past and present sensorimotor activities. To be effective, knowledge involved in our cognitive activities must be closely linked to the actual situation. This ability to adapt to specific situations would not be possible unless knowledge, including conceptual knowledge, is derived from the reactivation of memory traces of past experiences (Barsalou, 2008 ; Versace et al., 2014). Conversely, sensorimotor activities are totally dependent on memory traces of past sensorimotor experiences. Thus, the difference between perception and memory is that, in the former, properties are perceptually present, whereas, in the latter, they are absent but reactivated. This PhD research focused on the link between memory and perception and, more precisely, aims to provide arguments in favor of the similarity of memory and perceptual processes that result from the activation of components of same sensorimotor nature. We tested the hypothesis that perceptual effects should be observed with reactivated components in memory. We used well-known perceptual effects (such as masking effect or perceptual bias invisual illusion) to investigate the possibility to replicate these effects by replacing the sensorial present components by reactivated components in memory.

Processing of visual stimuli in the vertebrate retina is complex and diverse. The retinal output to the higher centres of the nervous system, mediated by ganglion cells, consists of several different channels. Neurons in these channels can have very distinct response properties, which originate in different retinal pathways. In this work, the retinal origins and possible functional implications of the segregation of visual pathways will be investigated with a detailed, biologically realistic computational model of the retina. This investigation will focus on the two main retino-cortical pathways in the mammalian retina, the parvocellular and magnocellular systems, which are crucial for conscious visual perception. These pathways differ in two important aspects. The parvocellular system has a high spatial, but low temporal resolution. Conversely, the magnocellular system has a high temporal fidelity, spatial sampling however is less dense than for parvocellular cells. Additionally, the responses of magnocellular ganglion cells can show pronounced nonlinearities, while the parvocellular system is essentially linear. The origin of magnocellular nonlinearities is unknown and will be investigated in the first part of this work. As their main source, the results suggest specific properties of the photoreceptor response and a specialised amacrine cell circuit in the inner retina. The results further show that their effect combines in a multiplicative way. The model is then used to examine the influence of nonlinearities on the responses of ganglion cells in the presence of involuntary fixational eye movements. Two different stimulus conditions will be considered: visual hyperacuity and motion induced illusions. In both cases, it is possible to directly compare properties of the ganglion cell population response with psychophysical data, which allows for an analysis of the influence of different components of the retinal circuitry. The simulation results suggest an important role for nonlinearities in the magnocellular stream for visual perception in both cases. First, it will be shown how nonlinearities, triggered by fixational eye movements, can strongly enhance the spatial precision of magnocellular ganglion cells. As a result, their performance in a hyperacuity task can be equal to or even surpass that of the parvocellular system. Second, the simulations imply that the origin of some of the illusory percepts elicited by fixational eye movements could be traced back to the nonlinear properties of magnocellular ganglion cells. As these activity patterns strongly differ from those in the parvocellular system, it appears that the magnocellular system can strongly dominate visual perception in certain conditions. Taken together, the results of this theoretical study suggest that retinal nonlinearities may be important for and strongly influence visual perception. The model makes several experimentally verifiable predictions to further test and quantify these findings. Furthermore, models investigating higher visual processing stages may benefit from this work, which could provide the basis to produce realistic afferent input.

Yes / Falls on stairs are a common and dangerous problem for older people. This series of studies evaluated whether or not selected changes to the appearance of stairs could make them safer for older people to negotiate. Objectives: To determine the effect of (1) a step edge highlighter and its position and (2) an optimised horizontal–vertical (H–V) visual illusion placed on a step riser on gait safety during stair descent and ascent. Design: A series of studies using a repeated measures, laboratory-based design, investigating gait control and safety in independently mobile older people. Setting: The University of Bradford Vision and Mobility Laboratory. Participants: Fit and healthy older people aged 60 years of age or more, independently mobile, reasonably active and with normal healthy eyes and corrected vision. Interventions: A step edge highlighter in a variety of offsets from the stair edge and an optimised H–V visual illusion placed on the stair riser. The H–V illusion was provided on a staircase by horizontal step edge highlighters on the tread edges and vertical stripes on the step risers. Main outcome measures: Gait parameters that are important for safe stepping in ascent and descent, particularly toe clearance during stair ascent and heel clearance during stair descent. Results: The step edge highlighter increased the precision of heel clearance during stepping and its positioning relative to the tread edge determined the extent of heel clearance over the tread edge. Positioning the highlighter away from the tread edge, as is not uncommonly provided by friction strips, decreased heel clearance significantly and led to greater heel scuffs. Although psychophysics experiments suggested that higher spatial frequencies of the H–V illusion might provide greater toe clearance on stair ascent, gait trials showed similar increased toe clearances for all spatial frequencies. When a 12 cycle per step spatial frequency H–V illusion was used, toe clearance increases of approximately 1 cm (17.5%) occurred without any accompanying changes in other important gait parameters or stability measures. Conclusions: High-contrast tread edge highlighters present on steps and stairs and positioned flush with the edge of the tread or as near to this as possible should improve stair descent safety in older people. A H–V illusion positioned on the riser of a raised surface/walkway (e.g. kerbs) and/or the top and/or bottom of a stairway is likely to increase foot clearance over the associated step/stair edge, and appears not to lead to any decrement in postural stability. Thus, their use is likely to reduce trip risk and hence improve stair ascent safety. The effect of the step and stair modifications should be assessed in older people with visual impairment. The only other remaining assessment that could be made would be to assess fall prevalence on steps and stairs, perhaps in public buildings, with and without these modifications. / National Institute for Health Research, Public Health Research programme. PHR programme as project number 10/3009/06