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Seeing and perceiving: Phenomenology and physiological mechanisms


Logothetis,  NK
Department Physiology of Cognitive Processes, Max Planck Institute for Biological Cybernetics, Max Planck Society;

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Logothetis, N. (2003). Seeing and perceiving: Phenomenology and physiological mechanisms. Talk presented at 26th European Conference on Visual Perception. Paris, France.

Although the brain mechanisms underlying perceptual organisation have long been a central quest in vision research, they still remain poorly understood and continue to be a topic of intensive research and debate. Over the past ten years records have been made of the activity of cells in the visual cortex of monkeys trained to report what they perceive when viewing perceptually rivalrous stimuli, which are known to instigate a continuous reorganisation of perception. In any studied area, only a fraction of neurons was found to respond in a manner that reliably reflects shifts in perception. This small number of neurons is distributed over the entire visual pathway, rather than being part of a single area in the brain. Of the areas studied, the inferior temporal (IT) cortex of the temporal lobe was found to have the highest number of perception-related cells. Notably, a tight relationship between IT activity and the animal's perception was clearly evident also in experiments in which monkeys were trained to look for and identify familiar targets embedded in natural scenes. During exploration, neuronal activation began shortly before effective targets were fixated, but only if the target was the goal of the next fixation. I briefly summarise these results and continue with the description of rivalry experiments in which both local field potentials and multiple unit activity were measured with multiple electrodes, placed over more than one visual area. Of interest is the study of covariation of activity within and between various occipito-parietal areas under different stimulus and perceptual conditions. Analysis of data collected in such experiments, revealed significant coupling between distant sites both in non-stimulated and in stimulated conditions. While these patterns were consistent and robust, stimulus-specific differences were subtle. During rivalry, covariation patterns were significantly diminished, and in some cases completely disappeared. These findings suggest that the coherence in the response of visual neurons during rivalry may be related to system's stability rather than to the perceptual state of the animal. Finally, psychophysical experiments are described which examine the relation of the process of stabilisation of ambiguous percepts with perceptual memory.