Perceptual rivalry refers to visual phenomena that are characterised by alternations between different percepts, despite an unchanging sensory input. Two common types of perceptual rivalry are (i) reversible figures — two-dimensional stable images that when viewed, are perceived to switch between different interpretations, and (ii) binocular rivalry — the alternations in image dominance resulting from the presentation of conflicting stimuli, one to each eye. Several investigators have suggested that these rivalling phenomena are mediated by similar neural mechanisms. Such a view, however, has not only been inadequately substantiated, but has also yet to be assessed in the context of a directly testable neurophysiological model. Miller and Pettigrew have proposed a novel, high-level interhemispheric switch (IHS) explanatory model of binocular rivalry. This model conceptualises the perceptual alternations as being mediated by alternations between one hemisphere’s selected image and the other hemisphere’s selected (rival) image. To assess their hypothesis, caloric vestibular stimulation (CVS) was used. CVS is a simple, inexpensive, and non-invasive brain stimulation technique that unilaterally activates high-level attentional areas. In accordance with the IHS model, CVS was found to significantly affect predominance (the duration that one image is perceived relative to the other, within a given viewing period) during conventional binocular rivalry with horizontal/vertical gratings and with orthogonal oblique gratings. The present thesis therefore aims, through the use of CVS, to extend this IHS model of binocular rivalry to reversible-figure alternations and propose a general IHS model of perceptual rivalry. Chapter 1 provides a detailed literature review of the field within the context of comparing both perceptual rivalries. In Chapter 2, investigations are presented on two different reversible figures — the perspective-reversing Necker cube and the figure–ground reversing Rubin’s vase–faces illusion. In these experiments, CVS was found to significantly change observers’ predominance compared to their baseline predominance. These results demonstrate that interhemispheric switching also mediates the alternations of these visual phenomena, in addition to binocular rivalry, thereby extending the IHS model to one of perceptual rivalry in general. Moreover, the findings are interpreted in a cognitive neuroscience context, including a novel proposal of a forebrain framework for the IHS model. Chapter 3 presents CVS experiments that address the issue of percept–to–hemisphere selection and the reproducibility of CVS effects, following Miller’s initial work on two types of conventional binocular rivalry. In planned analyses, significant predominance changes were not found in horizontal/vertical rivalry, oblique rivalry and Necker-cube rivalry. In post-hoc analyses that accounted for study-design differences between Miller’s original experiments and the present experiment, CVS was again not shown to induce significant predominance changes in any of the rivalry types. Assessment of directional predominance changes following CVS appeared to suggest an arbitrary selection of percept–to–hemisphere in all rivalry types, although no firm conclusions could be drawn from the obtained data on this issue. Nevertheless, the experiments further extend upon Miller’s earlier work by examining the inter- and intra-individual reproducibility of CVS-induced effects on predominance. Such reproducibility was found to be low and potential reasons for this are discussed. The experiments in Chapter 4 examine a type of binocular rivalry in which dichoptic presentation of Díaz-Caneja stimuli yields rivalry among four different stable images: half-field rivalry between the images presented to the eyes, and coherence rivalry in which aspects of each eye’s presented image are perceptually regrouped into rivalling coherent images. Each of these rivalries was found to occur for about half the given viewing time. Furthermore, CVS significantly shifted the predominance of perceived coherent images (coherence rivalry) but not half-field images (eye rivalry). This finding suggests that coherence rivalry (like conventional rivalry according to previous experiments) is mediated by interhemispheric switching at a high level, while eye rivalry is mediated by intrahemispheric mechanisms, most likely at a low level. In addition, it is proposed that Díaz-Caneja stimuli induce ‘meta-rivalry’ whereby these discrete high- and low-level competitive processes themselves rival for visual consciousness. The current thesis thus presents a novel meta-rivalry model of multistable binocular rivalry. It also presents the first direct evidence that interhemispheric switching mediates reversible-figure alternations, thereby supporting a generalised IHS model of perceptual rivalry. It is argued that both models provide a parsimonious exploratory framework within which specific predictions can be made and readily tested. Finally, the findings of all experiments in the current thesis are summarised.

The vestibular system has more connections with and influence on higher cortical centers than previously thought. These interactions with higher cortical centers and the phenomena that they elicit require a structural intact cerebral cortex. To date, little is known about the role and influence of the vestibular system on one’s body experience. In this study we show that caloric vestibular stimulation (CVS) in healthy participants has an effect on the perceptive component of one’s body experience. After CVS all participants showed a statistically significant difference of thigh width estimation. In contrast to previous studies, which demonstrated an influence of CVS on higher cortical centers with an intact cerebral cortex both the cognitive and affective component of body experience were not effected by the CVS. Our results demonstrate the influence of the vestibular system on body perception and emphasize its role in modulating different perceptive-qualities which contributes to our body experience. We found that CVS has a limited influence on one’s conscious state, thought process and higher cortical functions.

The vestibular system has more connections with and influence on higher cortical centers than previously thought. These interactions with higher cortical centers and the phenomena that they elicit require a structural intact cerebral cortex. To date, little is known about the role and influence of the vestibular system on one’s body experience. In this study we show that caloric vestibular stimulation (CVS) in healthy participants has an effect on the perceptive component of one’s body experience. After CVS all participants showed a statistically significant difference of thigh width estimation. In contrast to previous studies, which demonstrated an influence of CVS on higher cortical centers with an intact cerebral cortex both the cognitive and affective component of body experience were not effected by the CVS. Our results demonstrate the influence of the vestibular system on body perception and emphasize its role in modulating different perceptive-qualities which contributes to our body experience. We found that CVS has a limited influence on one’s conscious state, thought process and higher cortical functions.

Ce travail a pour but de décrire les relations réciproques entre prise de perspective et informations vestibulaires. Une étude chez des patients avec un déficit vestibulaire bilatéral ancien et des sujets contrôles a montré que l’ancrage du soi sur le corps et la simulation implicite de la perspective visuo-spatiale d’autrui étaient similaires chez les deux groupes. Ainsi, une perte vestibulaire ancienne n’entraînerait pas de conflits multisensoriels, connus pour évoquer un sentiment de perspective désincarnée chez des patients avec des déficits vestibulaires aigus. Une étude chez des volontaires sains a combiné des stimulations vestibulaires naturelles sur fauteuil rotatoire à des tâches de prise de perspective dans un environnement virtuel embarqué. Les temps de prise de perspective étaient modulés en fonction de la direction de la rotation. Cette influence n’était pas présente pour la rotation mentale d’objets 3D. La contribution vestibulaire canalaire modulerait donc spécifiquement les rotations mentales du point de vue. Enfin, les modulations cognitives du traitement des informations vestibulaires ont été analysées par l’enregistrement de potentiels évoqués myogéniques vestibulaires sur les muscles sternocléidomastoïdiens et trapèzes. L’amplitude des potentiels évoqués était significativement modulée par l’angle séparant le point de vue du participant et celui d’un avatar distant. Nos travaux théoriques et les résultats de cette série d’expériences démontrent la contribution des informations vestibulaires à la prise de perspective visuo-spatiale. / This thesis aims at describing the reciprocal relations between perspective taking and the vestibular system. A study in patients with bilateral vestibular deficits and controls showed that the anchoring of the self to the body and implicit visuo-spatial perspective taking were similar in both groups. Our negative findings offer insight into the multisensory mechanisms of embodiment: only acute peripheral vestibular disorders and neurological disorders in vestibular brain areas may evoke disembodied experiences. A second study, combined natural vestibular stimulation on a rotatory chair with virtual reality to test how vestibular signals are processed to simulate the view point of a distant avatar. While they were rotated, participants tossed a ball to a virtual character from the view point of a distant avatar. Our results showed that participants were faster when their physical body rotated in the same direction as the mental rotation needed to take the avatar's viewpoint. Altogether, these data indicate that vestibular signals have a direction-specific influence on visuo-spatial perspective taking, but not a general effect on mental imagery. Finally, cognitive modulations of vestibular information processing were analyzed by recording vestibular-evoked myogenic potentials on the sternocleidomastoid and trapeze muscles. The amplitude of evoked potentials was significantly modulated by the angle separating the participant’s viewpoint to that of a distant avatar. To conclude, our theoretical work, together with results from this series of experiments, demonstrate the contribution of vestibular information to visuo-spatial perspective taking.