Highly accurate retinotopic maps of the physiological blind spot in human visual cortex

The physiological blind spot is a naturally occurring scotoma corresponding with the optic disc in the retina of each eye. Even during monocular viewing, observers are usually oblivious to the scotoma, in part because the visual system extrapolates information from the surrounding area. Unfortunately, studying this visual field region with neuroimaging has proven difficult, as it occupies only a small part of retinotopic cortex. Here we used functional magnetic resonance imaging and a novel data-driven method for mapping the retinotopic organization in and around the blind spot representation in V1. Our approach allowed for highly accurate reconstructions of the extent of an observer's blind spot, and out-performed conventional model-based analyses. This method opens exciting opportunities to study the plasticity of receptive fields after visual field loss, and our data add to evidence suggesting that the neural circuitry responsible for impressions of perceptual completion across the physiological blind spot most likely involves regions of extrastriate cortex - beyond V1.

Illusory sound texture reveals multi-second statistical completion in auditory scene analysis

Sound sources in the world are experienced as stable even when intermittently obscured, implying perceptual completion mechanisms that “fill in” missing sensory information. We demonstrate a filling-in phenomenon in which the brain extrapolates the statistics of background sounds (textures) over periods of several seconds when they are interrupted by another sound, producing vivid percepts of illusory texture. The effect differs from previously described completion effects in that 1) the extrapolated sound must be defined statistically given the stochastic nature of texture, and 2) the effect lasts much longer, enabling introspection and facilitating assessment of the underlying representation. Illusory texture biases subsequent texture statistic estimates indistinguishably from actual texture, suggesting that it is represented similarly to actual texture. The illusion appears to represent an inference about whether the background is likely to continue during concurrent sounds, providing a stable statistical representation of the ongoing environment despite unstable sensory evidence.

Illusory sound texture reveals multi-second statistical completion in auditory scene analysis

Sound sources in the world are experienced as stable even when intermittently obscured, implying perceptual completion mechanisms that “fill in” missing sensory information. We demonstrate a filling-in phenomenon in which the brain extrapolates the statistics of background sounds (textures) over periods of several seconds when they are interrupted by another sound, producing vivid percepts of illusory texture. The effect differs from previously described completion effects in that 1) the extrapolated sound must be defined statistically given the stochastic nature of texture, and 2) in lasting much longer, enabling introspection and facilitating assessment of the underlying representation. Illusory texture appeared to be integrated into texture statistic estimates indistinguishably from actual texture, suggesting that it is represented similarly to actual texture. The illusion appears to represent an inference about whether the background is likely to continue during concurrent sounds, providing a stable representation of the environment despite unstable sensory evidence.

Neuroimaging Findings on Amodal Completion: A Review

Amodal completion is the phenomenon of perceiving completed objects even though physically they are partially occluded. In this review, we provide an extensive overview of the results obtained from a variety of neuroimaging studies on the neural correlates of amodal completion. We discuss whether low-level and high-level cortical areas are implicated in amodal completion; provide an overview of how amodal completion unfolds over time while dissociating feedforward, recurrent, and feedback processes; and discuss how amodal completion is represented at the neuronal level. The involvement of low-level visual areas such as V1 and V2 is not yet clear, while several high-level structures such as the lateral occipital complex and fusiform face area seem invariant to occlusion of objects and faces, respectively, and several motor areas seem to code for object permanence. The variety of results on the timing of amodal completion hints to a mixture of feedforward, recurrent, and feedback processes. We discuss whether the invisible parts of the occluded object are represented as if they were visible, contrary to a high-level representation. While plenty of questions on amodal completion remain, this review presents an overview of the neuroimaging findings reported to date, summarizes several insights from computational models, and connects research of other perceptual completion processes such as modal completion. In all, it is suggested that amodal completion is the solution to deal with various types of incomplete retinal information, and highly depends on stimulus complexity and saliency, and therefore also give rise to a variety of observed neural patterns.

The Illusory Flow and Passage of Time within Consciousness: A Multidisciplinary Analysis

Flow and passage of time puzzles were analyzed by first clarifying their roles in the current multidisciplinary understanding of time in consciousness. All terms ( flow, passage, happening, becoming) are carefully defined. Flow and passage are defined differently, the former involving the psychological aspects of time and the latter involving the evolving universe and associated new cerebral events. The concept of the flow of time (FOT) is deconstructed into two levels: (a) a lower level ― a perceptual dynamic flux, or happening, or flow of events (not time); and (b) an upper level ― a cognitive view of past/present/future in which the observer seems to move from one to the other. With increasing evidence that all perception is a discrete continuity provided by illusory perceptual completion, the lower-level FOT is essentially the result of perceptual completion. The brain conflates the expression flow (passage, for some) of time with experiences of perceptual completion. However, this is an illusory percept. Converging evidence on the upper-level FOT reveals it as a false cognition that has the illusory percept of object persistence as its prerequisite. To research this argument, an experiment that temporarily removes the experience of the lower-level FOT might be conducted. The claustrum of the brain (arguably the center of consciousness) should be intermittently stimulated to create a scenario of discrete observations (involving all the senses) with long interstimulus intervals of non-consciousness and thereby no perceptual completion. Without perceptual completion, there should be no subjective experience of the lower-level FOT.

Geometric Illusions in the Human Face and Body

Clothing and cosmetic makeup take advantage of visual illusions so as to make the human body and face look more attractive. This chapter lists such real-life geometric illusions and reviews studies that psychophysically measured them. These illusions include the Müller-Lyer illusion, the Helmholtz illusion, the Delboeuf illusion, the “bicolor” illusion, the “shape echo” illusion, and perceptual completion. Puzzling characteristics of these bodily illusions, which can be called “biological illusions,” are discussed. The ways in which geometric illusions in the human face and body differ from classical geometric illusions consisting of simple lines are also discussed, and the concept of “biological motion” as a separate field is proposed.