In visual perception, an optical illusion is an illusion caused by the visual system and characterized by a visual percept that arguably appears to differ from reality. Illusions come in a wide variety; their categorization is difficult because the underlying cause is often not clear but a classification proposed by Richard Gregory is useful as an orientation. According to that, there are three main classes: physical, physiological, and cognitive illusions, and in each class there are four kinds: Ambiguities, distortions, paradoxes, and fictions. A classical example for a physical distortion would be the apparent bending of a stick half immerged in water; an example for a physiological paradox is the motion aftereffect. An example for a physiological fiction is an afterimage. Three typical cognitive distortions are the Ponzo, Poggendorff, and Müller-Lyer illusion. Physical illusions are caused by the physical environment, e.g. by the optical properties of water. Physiological illusions arise in the eye or the visual pathway, e.g. from the effects of excessive stimulation of a specific receptor type. Cognitive visual illusions are the result of unconscious inferences and are perhaps those most widely known.
Color constancy is an example of subjective constancy and a feature of the human color perception system which ensures that the perceived color of objects remains relatively constant under varying illumination conditions. A green apple for instance looks green to us at midday, when the main illumination is white sunlight, and also at sunset, when the main illumination is red. This helps us identify objects.
Darkness is defined as a lack of illumination, an absence of visible light, or a surface that absorbs light, such as a black one.
A grid illusion is any kind of grid that deceives a person's vision. The two most common types of grid illusions are the Hermann grid illusion and the scintillating grid illusion.
The Cornsweet illusion, also known as the Craik–O'Brien–Cornsweet illusion or the Craik–Cornsweet illusion, is an optical illusion that was described in detail by Tom Cornsweet in the late 1960s. Kenneth Craik and Vivian O'Brien had made earlier observations in a similar vein.
The receptive field, or sensory space, is a delimited medium where some physiological stimuli can evoke a sensory neuronal response in specific organisms.
A contrast effect is the enhancement or diminishment, relative to normal, of perception, cognition or related performance as a result of successive or simultaneous exposure to a stimulus of lesser or greater value in the same dimension.
The McCollough effect is a phenomenon of human visual perception in which colorless gratings appear colored contingent on the orientation of the gratings. It is an aftereffect requiring a period of induction to produce it. For example, if someone alternately looks at a red horizontal grating and a green vertical grating for a few minutes, a black-and-white horizontal grating will then look greenish and a black-and-white vertical grating will then look pinkish. The effect is remarkable because, although it diminishes rapidly with repeated testing, it has been reported to last up to 2.8 months when exposure to testing is limited.
The Ehrenstein illusion is an optical illusion of brightness or colour perception. The visual phenomena was studied by the German psychologist Walter H. Ehrenstein (1899–1961) who originally wanted to modify the theory behind the Hermann grid illusion. In the discovery of the optical illusion, Ehrenstein found that grating patterns of straight lines that stop at a certain point appear to have a brighter centre, compared to the background.
In mathematics, physics, and engineering, spatial frequency is a characteristic of any structure that is periodic across position in space. The spatial frequency is a measure of how often sinusoidal components of the structure repeat per unit of distance.
In neurobiology, lateral inhibition is the capacity of an excited neuron to reduce the activity of its neighbors. Lateral inhibition disables the spreading of action potentials from excited neurons to neighboring neurons in the lateral direction. This creates a contrast in stimulation that allows increased sensory perception. It is also referred to as lateral antagonism and occurs primarily in visual processes, but also in tactile, auditory, and even olfactory processing. Cells that utilize lateral inhibition appear primarily in the cerebral cortex and thalamus and make up lateral inhibitory networks (LINs). Artificial lateral inhibition has been incorporated into artificial sensory systems, such as vision chips, hearing systems, and optical mice. An often under-appreciated point is that although lateral inhibition is visualised in a spatial sense, it is also thought to exist in what is known as "lateral inhibition across abstract dimensions." This refers to lateral inhibition between neurons that are not adjacent in a spatial sense, but in terms of modality of stimulus. This phenomenon is thought to aid in colour discrimination.
Visual neuroscience is a branch of neuroscience that focuses on the visual system of the human body, mainly located in the brain's visual cortex. The main goal of visual neuroscience is to understand how neural activity results in visual perception, as well as behaviors dependent on vision. In the past, visual neuroscience has focused primarily on how the brain responds to light rays projected from static images and onto the retina. While this provides a reasonable explanation for the visual perception of a static image, it does not provide an accurate explanation for how we perceive the world as it really is, an ever-changing, and ever-moving 3-D environment. The topics summarized below are representative of this area, but far from exhaustive. To be less topic specific, one can see this textbook for the computational link between neural activities and visual perception and behavior: "Understanding vision: theory, models, and data", published by Oxford University Press 2014.
Contrast is the difference in luminance or colour that makes an object visible on a background of different luminance or color. The human visual system is more sensitive to contrast than to absolute luminance; we can perceive the world similarly regardless of the huge changes in illumination over the day or from place to place. The maximum contrast of an image is the contrast ratio or dynamic range. Images with a contrast ratio close to their medium's maximum possible contrast ratio experience a conservation of contrast, wherein any increase in contrast in some parts of the image must necessarily result in a decrease in contrast elsewhere. Brightening an image will increase contrast in dark areas but decrease contrast in bright areas, while darkening the image will have the opposite effect. Bleach bypass destroys contrast in both the darkest and brightest parts of an image while enhancing luminance contrast in areas of intermediate brightness.
In vision, filling-in phenomena are those responsible for the completion of missing information across the physiological blind spot, and across natural and artificial scotomata. There is also evidence for similar mechanisms of completion in normal visual analysis. Classical demonstrations of perceptual filling-in involve filling in at the blind spot in monocular vision, and images stabilized on the retina either by means of special lenses, or under certain conditions of steady fixation. For example, naturally in monocular vision at the physiological blind spot, the percept is not a hole in the visual field, but the content is “filled-in” based on information from the surrounding visual field. When a textured stimulus is presented centered on but extending beyond the region of the blind spot, a continuous texture is perceived. This partially inferred percept is paradoxically considered more reliable than a percept based on external input..
The Chubb illusion is an optical illusion or error in visual perception in which the apparent contrast of an object varies substantially to most viewers depending on its relative contrast to the field on which it is displayed. These visual illusions are of particular interest to researchers because they may provide valuable insights in regard to the workings of human visual systems.
The frequency-doubling illusion is an apparent doubling of spatial frequency when a sinusoidal grating is modulated rapidly in temporal counterphase. Recently, it has been proposed that the illusion arises from a spatially nonlinear ganglion cell class. The contrast threshold values needed for perceiving this physiological effect are used in frequency doubling technology perimetry for the detection of even early phases of glaucoma. A more recent study's results argue against the hypothesis that spatially nonlinear retinal ganglion cells are the physiological substrate of the frequency-doubling illusion. A cortical pathway of temporal phase discrimination may be the principal cause of the illusion, whereas spatial phase information is retained.
The watercolor illusion, also referred to as the water-color effect, is an optical illusion in which a white area takes on a pale tint of a thin, bright, intensely colored polygon surrounding it if the coloured polygon is itself surrounded by a thin, darker border. The inner and outer borders of watercolor illusion objects often are of complementary colours. The watercolor illusion is best when the inner and outer contours have chromaticities in opposite directions in color space. The most common complementary pair is orange and purple. The watercolor illusion is dependent on the combination of luminance and color contrast of the contour lines in order to have the color spreading effect occur.
A phantom contour is a type of illusory contour. Most illusory contours are seen in still images, such as the Kanizsa triangle and the Ehrenstein illusion. A phantom contour, however, is perceived in the presence of moving or flickering images with contrast reversal. The rapid, continuous alternation between opposing, but correlated, adjacent images creates the perception of a contour that is not physically present in the still images. Quaid et al. have also authored a PhD thesis on the phantom contour illusion and its spatiotemporal limits which maps out limits and proposes mechanisms for its perception centering around magnocellularly driven visual area MT.
Due to the effect of a spatial context or temporal context, the perceived orientation of a test line or grating pattern can appear tilted away from its physical orientation. The tilt illusion (TI) is the phenomenon that the perceived orientation of a test line or grating is altered by the presence of surrounding lines or grating with a different orientation. And the tilt aftereffect (TAE) is the phenomenon that the perceived orientation is changed after prolonged inspection of another oriented line or grating.
Russell L. De Valois was an American scientist recognized for his pioneering research on spatial and color vision.
