Afterimages and the breathing light illusion

Asymmetric Brightness Effects With Dark Versus Light Glare-Like Stimuli

The glare effect is a brightness illusion that has captured the attention of the vision community since its discovery. However, its photometrical reversal, which we refer to here as photometrical reversed glare (PRG) stimuli, remained relatively unexplored. We presented three experiments that sought to examine the perceived brightness of a target area surrounded by luminance gradients in PRG stimuli and compare them with conventional glare effect configurations. Experiment 1 measured the brightness of the central target area of PRG stimuli through an adjustment task; the results showed that the target appeared brighter than similar, comparative areas not surrounded by luminance gradients. This finding was unexpected given the recent report that PRG stimuli cause pupil dilation. Meanwhile, Experiments 2 and 3 implemented a rating task to further test the findings in Experiment 1. Again, the study found a robust brightening illusion in the target area of PRG stimuli in a wide range of target and background luminance. The results are discussed in comparison with the brightness enhancement of the glare effect.

Functional symmetry of the primary visual pathway evidenced by steady-state visual evoked potentials

The primary visual pathway exhibits a symmetrical anatomical structure, initially arising from the left and right retinas, passing through the lateral geniculate nucleus, and finally projecting to the left and right primary visual cortices. However, to our knowledge, studies based on scalp EEG have not provided adequate evidence of the functional symmetry of the primary visual pathway, as the usual visual ERP is often related to other higher-level brain areas. Steady-state visual evoked potentials (SSVEPs) can be considered as the direct response of the primary visual pathway to a repetitive stimulus, with a very limited correlation with responses of higher-level brain areas. Therefore, SSVEPs can be used to evaluate the functional symmetry of the primary visual pathway. In this study, we draw a comparison among the powers and distributions of SSVEPs of different frequencies when the left or right eye alone is stimulated, and when both the eyes are stimulated together. Our results indicate that the primary visual pathway is almost symmetrical in generating SSVEPs from either eye and that there is some functional interaction between the left and right primary visual pathways.

Visual Illusions: An Interesting Tool to Investigate Developmental Dyslexia and Autism Spectrum Disorder

A visual illusion refers to a percept that is different in some aspect from the physical stimulus. Illusions are a powerful non-invasive tool for understanding the neurobiology of vision, telling us, indirectly, how the brain processes visual stimuli. There are some neurodevelopmental disorders characterized by visual deficits. Surprisingly, just a few studies investigated illusory perception in clinical populations. Our aim is to review the literature supporting a possible role for visual illusions in helping us understand the visual deficits in developmental dyslexia and autism spectrum disorder. Future studies could develop new tools - based on visual illusions - to identify an early risk for neurodevelopmental disorders.

Illusory object motion in the centre of a radial pattern: The Pursuit-Pursuing illusion

A circular object placed in the centre of a radial pattern consisting of thin sectors was found to cause a robust motion illusion. During eye-movement pursuit of a moving target, the presently described stimulus produced illusory background-object motion in the same direction as that of the eye movement. In addition, the display induced illusory stationary perception of a moving object against the whole display motion. In seven experiments, the characteristics of the illusion were examined in terms of luminance relationships and figural characteristics of the radial pattern. Some potential explanations for these findings are discussed.

Measuring the Breathing Light Illusion by means of induced simultaneous contrast

By blurring the margins of a surface, both its brightness and the perceived contrast against a superimposed figure with sharp boundaries increase. Also, if one approaches a blurred white spot on a grey background, this spot will appear wider and brighter: this phenomenon is known as the Breathing Light Illusion (BLI) (Gori and Stubbs, 2006 Perception 35 1573-1577). We studied the increment of the achromatic contrast of a grey sharp-boundary disk when it was superimposed on the BLI. This augmentation of the perceived contrast in the dynamic presentation of the BLI was significantly stronger than the effect that Agostini and Galmonte (2002a Psychonomic Bulletin and Review 9 264-269) obtained in static presentation. Our study leads to an indirect quantification of the BLI. Two control experiments showed that the increment of the achromatic contrast depends on the blurred spot and is independent of the dynamic increment in angular size. These results argue for a causal relationship between the increase in brightness due to the BLI and the darkening of the superimposed disk.

A new motion illusion based on competition between two kinds of motion processing units: the accordion grating

Parametric psychophysical investigations are reported for two related illusory effects that occur when viewing an elementary square-wave grating while making "back and forth" head movements along the projection line. Observers report a non-rigid distortion of the pattern, including: (i) an expansion in a direction perpendicular to the stripes, and (ii) a perceived curvature of the stripes. We investigated these two phenomena independently. The first depends on the classical physiological aperture problem that confronts early cells in the vision system. Interactions between ambiguous and unambiguous motion signals, generated at line interiors and line ends, respectively, can explain why the perceived expansion occurs only in directions perpendicular to the stripes. A simple model is presented and successfully tested by a nulling psychophysical experiment with four subjects. The experiment varies key stimulus attributes that generate ambiguous and unambiguous motion signals. Regarding the illusory curvature, a differential geometry model of the optics of our display, which identifies a non-classical three-dimensional (3D) aperture problem, is proposed (Yazdanbakhsh & Gori, 2011). We tested that model by implementing its closed form prediction of distortion to design displays for a second psychophysical experiment that also uses a nulling technique. Results from four subjects allow the quantification of the degree of perceived curvature as a function of speed, distance and stimulus type (blurred vs. unblurred grating) and are compatible with the predictions of the model.