20 research outputs found

    Spatial analysis in the human cerebral cortex : behavioural and functional magnetic resonance studies of spatial transformations in visual perception and imagery

    Get PDF
    Die vorliegende Dissertation berichtet ĂŒber eine Serie von Verhaltens- und funktionellen Bildgebungsstudien zu experimentalpsychologischen Paradigmata, die eine rĂ€umliche Analyse und Koordinatentransformation von Material der visuellen Wahrnehmung oder Vorstellung beinhalten. Nach einer EinfĂŒhrung in die Prinzipien und Techniken der funktionellen Kernspintomographie, der hier benutzten Methode fĂŒr die Messung von GehirnaktivitĂ€t, werden die Versuche einer Replikation des berĂŒhmten Stratton'schen Umkehrbrillen-Experiments dargestellt (Kapitel 1). Unsere vier Probanden zeigten zwar eine zĂŒgige Anpassung der visuomotorischen Funktionen an die neue visuelle Umwelt, berichteten aber, anders als Stratton, nicht, daß sie die Welt nach einigen Tagen mit der Umkehrbrille wieder normal sĂ€hen. Diese Persistenz des umgekehrten Bildes wurde durch eine psychphysische Testbatterie bestĂ€tigt. Des weiteren ergaben die funktionellen Kernspinmessungen, daß sich die kortikale retinotope Organisation im Verlaufe des Experiments nicht geĂ€ndert hat. Da sich also Strattons Haupteffekt, das Aufrechtsehen durch die Umkehrbrille nach einwöchiger Adaptation, nicht replizieren ließ, werden andere Möglichkeiten der Interpretation der verschiedenen Umkehrexperimente der letzten hundert Jahre vorgeschlagen. Dieses Ergebnis einer funktionellen Anpassung ohne grĂ¶ĂŸere Änderungen der visuellen Wahrnehmung (und ohne VerĂ€nderungen der ReprĂ€sentation der Netzhautareale in der Sehrinde) fĂŒhrte zu der Hypothese, daß die erforderlichen Transformationen auf einer höheren Stufe der kortikalen Hierarchie der visuellen Verarbeitung erfolgen. Zur Testung dieser Hypothese wurde eine funktionelle Kernspinstudie des Umkehrlesens durchgefĂŒhrt (Kapitel 2). Hierbei lasen die Probanden Wörter und SĂ€tze in Spiegelschrift oder auf dem Kopf. Der neuronale Mechanismus der rĂ€umlichen Transformationen, die zur BewĂ€ltigung dieser Aufgabe nötig sind, konnte in bestimmten Regionen des Parietallappens, die zwischen den Probanden sehr konstant waren, lokalisiert werden. Weiterhin fand sich eine Koaktivierung okzipitootemporaler Objekterkennungs-Areale. Die SpezifizitĂ€t der parietalen Aktivierungsfoci wurde durch ein Kontrollexperiment bestĂ€tigt, in welchem das kortikale System fĂŒr rĂ€umliche Transformationen von den Netzwerken der allgemeinen visuellen Aufmerksamkeit und der Augenbewegungskontrolle unterschieden werden konnte. In einem weiteren Experiment wurden die rĂ€umlichen Funktionen des Parietallappens unter dem Vorzeichen der visuellen Vorstellung untersucht. Als Paradigma wurde der "mental clock" - Test verwendet, bei welchem die Probanden die Winkel der Zeiger zweier Uhren vergleichen mĂŒssen, deren Zeiten nur akustisch vorgegeben werden (Kapitel 3). Diese Aufgabe erfordert die Generierung eines entsprechenden Vorstellungsbildes und dessen rĂ€umliche Analyse, stellt also sowohl ein kontrolliertes Vorstellungs-Paradigma als auch einen Test rĂ€umlicher Funktionen dar, der nicht auf visuell prĂ€sentiertem Material beruht. Das parietale Aktivierungsmuster, das der Analyse der Winkel dieser vorgestellten Uhren zugeschrieben werden konnte, entsprach weitgehend demjenigen, das mit der rĂ€umlichen Transformation von Buchstaben verbunden war. Es handelt sich also wahrscheinlich um ein kortikales System fĂŒr rĂ€umliche Analyse und Koordinatentransformationen, das nicht auf eine visuelle Stimulation angewiesen ist, sondern auch bei bloßer visueller Vorstellung aktiv werden kann. Die vorgelegten Resultate werden im Kontext neuerer neuropsychologischer Befunde zu Defiziten rĂ€umlicher Analyse und Vorstellung bei LĂ€sionen des Parietallappens diskutiert (Kapitel 4). Auch die methodologischen Probleme der kognitiven Subtraktion, die in unseren Studien teilweise benutzt wurde, werden behandelt. Dabei wird erlĂ€utert, inwiefern diese fĂŒr die Beurteilung der vorgelegten Studien nur von untergeordneter Bedeutung sind. Nichtsdestoweniger schlagen wir Modifikationen der experimentellen Paradigmata im Sinne des parametrischen Designs und des "event-related functional magnetic resonance imaging" vor, die bei zukĂŒnftigen Studien einen vollstĂ€ndigen Verzicht auf die kognitive Subtraktion ermöglichen dĂŒrften. Schließlich wird die Bedeutung der vorgelegten Ergebnisse fĂŒr die Erforschung der AnpassungsfĂ€higkeit des menschlichen Gehirns und des VerhĂ€ltnisses von Vorstellung und visueller Wahrnehmung dargelegt

    Change blindness: eradication of gestalt strategies

    Get PDF
    Arrays of eight, texture-defined rectangles were used as stimuli in a one-shot change blindness (CB) task where there was a 50% chance that one rectangle would change orientation between two successive presentations separated by an interval. CB was eliminated by cueing the target rectangle in the first stimulus, reduced by cueing in the interval and unaffected by cueing in the second presentation. This supports the idea that a representation was formed that persisted through the interval before being 'overwritten' by the second presentation (Landman et al, 2003 Vision Research 43149–164]. Another possibility is that participants used some kind of grouping or Gestalt strategy. To test this we changed the spatial position of the rectangles in the second presentation by shifting them along imaginary spokes (by ±1 degree) emanating from the central fixation point. There was no significant difference seen in performance between this and the standard task [F(1,4)=2.565, p=0.185]. This may suggest two things: (i) Gestalt grouping is not used as a strategy in these tasks, and (ii) it gives further weight to the argument that objects may be stored and retrieved from a pre-attentional store during this task

    Body-ownership and visual perception

    Get PDF
    The idea that our body plays an important role in visual perception has a long history in storytelling and philosophy. Some ideas are very intuitive. For example, few will disagree with the notion that smaller people perceive the world to be bigger, and vice versa, that larger people perceive the world to be smaller. In contrast, more controversial ideas regarding the role of our body in visual perception have been debated by philosophers. According to these philosophers, the very nature of visual perception lies in the fact that we have a body that moves in space. Since George Berkeley (1685 – 1753) first formalized such a fundamental role for our body in visual perception, different philosophical theories have branched out to account for the latest scientific findings. Although experimental psychologists and neuroscientists have long neglected these intriguing philosophical accounts, the recent development of body-ownership illusions allowed for a more rigorous investigation of the supposed link between our body and visual perception. In body-ownership illusions, research participants experience an artificial body (or body-part) to be their own. These illusions allow for the dissociation between the subjective experience that your body belongs to you (i.e. body-ownership) and the mere visual impression of your body from a first-person perspective. The studies that comprise this thesis use different ownership illusions to investigate the role of body-ownership in visual perception, and the role of visual perception in body-ownership, with visual perception being an umbrella-term for both visuospatial perception and visual awareness. In Study I and Study II we investigated the mechanisms by which body-size influences the perceived size of the world, by having participants experience ownership of different sized (and sometimes invisible) bodies. Our results show that this own-body-size effect does not rely on visual information per se, but instead, on the recalibration between visual and tactile information that updates the representation of external space. In Study III we combine the rubber hand illusion with binocular rivalry to show that body-ownership promotes visual awareness of a fake hand. And in Study IV we combine the rubber hand illusion with continuous flash suppression to show that ownership can be induced in the absence of visual awareness. Such unconscious ownership calls for a reevaluation of the standard definition of body-ownership. Taken together, these studies illuminate the intricate relationship between body-ownership and visual perception. In addition to the scientific research fields of visuospatial perception, visual awareness, and body-ownership, these results are valuable to the philosophical debate on the nature of visual perception and might provide future applications in clinical psychology

    Over my fake body: body ownership illusions for studying the multisensory basis of own-body perception

    Get PDF
    Which is my body and how do I distinguish it from the bodies of others, or from objects in the surrounding environment? The perception of our own body and more particularly our sense of body ownership is taken for granted. Nevertheless, experimental findings from body ownership illusions (BOIs), show that under specific multisensory conditions, we can experience artificial body parts or fake bodies as our own body parts or body, respectively. The aim of the present paper is to discuss how and why BOIs are induced. We review several experimental findings concerning the spatial, temporal, and semantic principles of crossmodal stimuli that have been applied to induce BOIs. On the basis of these principles, we discuss theoretical approaches concerning the underlying mechanism of BOIs. We propose a conceptualization based on Bayesian causal inference for addressing how our nervous system could infer whether an object belongs to our own body, using multisensory, sensorimotor, and semantic information, and we discuss how this can account for several experimental findings. Finally, we point to neural network models as an implementational framework within which the computational problem behind BOIs could be addressed in the future

    Optic flow, egocentric direction and walking

    Get PDF
    This research explored two aspects of visually guided walking (1) what is the role of optic flow in the recalibration of misperceived direction while walking, and (2) how does a change in perceived direction map onto a change in walking direction. Data from five studies investigating adaptation to displaced direction (by prism glasses) suggested the following. First, optic flow is important in the recalibration of perceived direction. Further, processing optic flow is attentionally demanding, such that when cognitive load is increased, recalibration decreases. The results also demonstrated that the timecourse of recalibration changed as a function of the presence, or absence, of optic flow. With regards to the relationship between egocentric direction and walking direction, we demonstrated that a change in visual straight ahead could be mapped onto a change in target-heading error. We found that this relationship held when we unpacked the data according to the direction of displacement to which observers were exposed. The important relationship between visually perceived direction and walking direction was also highlighted in a patient study, using patients whose perception of direction was endogenously shifted after a right hemisphere stroke. Taken together, the results of this thesis help to highlight the role of optic flow in the recalibration of perceived direction, and the role of perceived direction in the visual guidance of walking. It is argued that optic flow promotes rapid recalibration of visual direction, and that change in perceived visual straight ahead can be mapped onto a changed in walking direction
    corecore