Change blindness: eradication of gestalt strategies

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

A neural field model for color perception unifying assimilation and contrast

37 pages, 17 figures, 3 ancillary filesInternational audienceWe propose a neural field model of color perception in context, for the visual area V1 in the cortex. This model reconciles into a common framework two opposing perceptual phenomena, simultaneous contrast and chromatic assimilation. Previous works showed that they act simultaneously, and can produce larger shifts in color matching when acting in synergy with a spatial pattern. At some point in an image,the color perceptually seems more similar to that of the adjacent locations, while being more dissimilar from that of remote neighbors. The influence of neighbors hence reverses its nature above some characteristic scale. Our model fully exploits the balance between attraction and repulsion in color space, combined at small or large scales in physical space. For that purpose we rely on the opponent color theory introduced by Hering, and suppose a hypercolumnar structure coding for colors. At some neural mass, the pointwise influence of neighbors is spatially integrated to obtain the final effect that we call a color sensation. Alongside this neural field model, we describe the search for a color match in asymmetric matching experiments as a mathematical projector. We validate it by fitting the parameters of the model to data from (Monnier and Shevell, 2004) and (Monnier, 2008) and our own data. All the results show that we are able to explain the nonlinear behavior of the observed shifts along one or two dimensions in color space, which cannot be done using a simple linear model

TOWARDS A COMPUTATIONAL MODEL OF RETINAL STRUCTURE AND BEHAVIOR

Human vision is our most important sensory system, allowing us to perceive our surroundings. It is an extremely complex process that starts with light entering the eye and ends inside of the brain, with most of its mechanisms still to be explained. When we observe a scene, the optics of the eye focus an image on the retina, where light signals are processed and sent all the way to the visual cortex of the brain, enabling our visual sensation. The progress of retinal research, especially on the topography of photoreceptors, is often tied to the progress of retinal imaging systems. The latest adaptive optics techniques have been essential for the study of the photoreceptors and their spatial characteristics, leading to discoveries that challenge the existing theories on color sensation. The organization of the retina is associated with various perceptive phenomena, some of them are straightforward and strictly related to visual performance like visual acuity or contrast sensitivity, but some of them are more difficult to analyze and test and can be related to the submosaics of the three classes of cone photoreceptors, like how the huge interpersonal differences between the ratio of different cone classes result in negligible differences in color sensation, suggesting the presence of compensation mechanisms in some stage of the visual system. In this dissertation will be discussed and addressed issues regarding the spatial organization of the photoreceptors in the human retina. A computational model has been developed, organized into a modular pipeline of extensible methods each simulating a different stage of visual processing. It does so by creating a model of spatial distribution of cones inside of a retina, then applying descriptive statistics for each photoreceptor to contribute to the creation of a graphical representation, based on a behavioral model that determines the absorption of photoreceptors. These apparent color stimuli are reconstructed in a representation of the observed scene. The model allows the testing of different parameters regulating the photoreceptor's topography, in order to formulate hypothesis on the perceptual differences arising from variations in spatial organization

Abstract expressionist paintings reveal the neural systems involved in processing color and luminance: an fMRI study

In this study, we use paintings that canonically and/or visually belong to the Abstract Expressionist movement to study how the brain processes different forms of color and luminance. Subjects viewed 240 unique images in the experiment in a block design. All of the paintings used in the experiment were painted by the artists in color. Half of the 240 images were three styles of painting in their original (saturated) forms, and the other half were desaturated versions of the same paintings. In our analysis, we compared saturated Abstract Expressionist paintings to their desaturated counterparts - both as a group and within the Gesture and Color Field styles. We also compared the desaturated Gesture paintings to their saturated counterparts because many of the Gesture painters created work with little to no color. Through these contrasts, we show that different regions in the brain process color and luminance. Notably, we show that (1) the brain processes color in the Gesture and Color Field paintings differently (2) color is capable of producing significant activation in the amygdala and (3) the color and luminance in the Gesture paintings produce strikingly different patterns of significant activation

The Bayesian-Laplacian Brain

We discuss here what we feel could be an improvement in future discussions of the brain acting as a Bayesian-Laplacian system, by distinguishing between two classes of priors on which the brain's inferential systems operate. In one category are biological priors (β priors) and in the other artefactual ones (α priors). We argue that β priors are inherited or acquired very rapidly after birth and are much more resistant to varying experiences than α priors which, being continuously acquired at various stages throughout post-natal life, are much more accommodating of, and hospitable to, new experiences. Consequently, the posteriors generated from the two sets of priors are likewise different, being more constrained (i.e., precise) for β than for α priors

Quasi-Modal Encounters Of The Third Kind: The Filling-In Of Visual Detail

Although Pessoa et al. imply that many aspects of the filling-in debate may be displaced by a regard for active vision, they remain loyal to naive neural reductionist explanations of certain pieces of psychophysical evidence. Alternative interpretations are provided for two specific examples and a new category of filling-in (of visual detail) is proposed

A Physiological and Psychometric Evaluation of Human Subconscious Visual Response and Its Application in Health Promoting Lighting.

Subconscious vision is a recent focus of the vision science community, brought on by the discovery of a previously unknown photoreceptor in the retina dedicated to driving non-image-forming responses, intrinsically photosensitive retinal ganglion cells (ipRGCs). In addition to accepting inputs from rod and cone photoreceptors, ipRGCs contain their own photopigment, melanopsin, and are considered true photoreceptors. ipRGCs drive various non-image-forming photoresponses, including circadian photoentrainment, melatonin suppression, and pupil constriction. In order to understand more about ipRGC function in humans, we studied its sensitivity to light stimuli in the evening and day. First, we measured the sensitivity threshold of melatonin suppression at night. Using a protocol that enhances data precision, we have found the threshold for human melatonin suppression to be two orders of magnitude lower than previously reported. This finding has far-reaching implications since there is mounting evidence that nocturnal activation of the circadian system can be harmful. Paradoxically, ipRGCs are understimulated during the day. Optimizing daytime non-image-forming photostimulation has health benefits, such as increased alertness, faster reaction times, better sleep quality, and treatment of depression. In order to enhance ipRGC excitation, we aimed to circumvent adaptation (i.e. desensitization) of the photoresponse by using flickering instead of steady light. We find that properly timed flickering light enhances pupillary light reflex significantly when compared to steady light with 9-fold more energy density. Employing our findings, a new form of LED light is proposed to enhance subconscious visual responses at a typical indoor illuminance level. Using the silent substitution technique, a melanopsin-selective flicker is introduced into the light. A linear optimization algorithm is used to maximize the contrast of the subconscious, melanopsin-based response function while keeping conscious, cone-driven responses to the pulsing light fixed. Additional boundary conditions utilizing test color samples as an environmental mimic are introduced to limit the amount of perceived color change in a simulated environment. Two examples of lights are given to illustrate potential applications for general illumination and therapeutic purposes. For the lighting and electronics industry, we hope our study of subconscious-stimulative thresholds at night will better inform their design guidelines for health conscious products.PhDMacromolecular Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/133226/1/garenv_1.pd

An investigation into the spectral characteristics of metacontrast phenomena and of adaption effects in human vision

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