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Checker-Shadow Illusion Explained

✔ https://StudyForce.com ✔ https://Biology-Forums.com ✔ Ask questions here: https://Biology-Forums.com/index.php?... Follow us: ▶ Facebook:   / studyforceps   ▶ Instagram:   / studyforceonline   ▶ Twitter:   / studyforceps   Here's our explanation of the Adelson's Checker-Shadow Illusion. Explanation: The low-level constancy mechanism is referred to as Hering-type lightness contrast, after German physiologist Ewald Hering (1834-1918). Hering emphasised the explanatory power of localized retinal processes affecting the neurons, retinal ganglion cells, which send signals from the eye to the brain. The retinal ganglion cells receive electrical impulses from the photoreceptive rods and cones contained in the outer retina and send corresponding signals to the brain, producing a measure of the intensity of light hitting the eye (the luminance). For each retinal ganglion cell there is a corresponding receptive field, which may be thought as the small region of rods and cones which can affect the given cell’s firing rate. Cells that are close together will have overlapping receptive fields. Two kinds of retinal ganglion cells exist: an ON-centre cell is excited by a central bright spot in its field but is inhibited by a bright surround; an OFF-centre cell has the opposite arrangement. The process of turning the retinal stimulus into a neural image is spatial filtering; the inhibitory mechanism operates across the 2-D space of the receptive field to effectively remove the image data wherever there is uniform luminance. Of course, we are not consciously aware of any such blind spots; this could be explained by the well-attested perceptual phenomenon of ‘filling-in’ which takes place in the primary and secondary visual cortices (De Weerd 2010). These retinal and early cortical processes provide an underlying mechanism for lightness constancy—our visual system is less sensitive to gradual changes in luminance than it is to sudden local contrasts, and uniform regions are ‘filled in’. However, this does not fully explain Adelson’s Checker-Shadow. Other lightness illusions have shown that a simple localized centre-surround model is inadequate (Gilchrist 1979), and depth cues and the presence of transparent overlays can dramatically alter perceived lightness without any significant changes to the light information received at the eye (Gilchrist 1977; Hochberg and Beck 1954). These results have motivated an alternative, high-level approach. The high-level explanation of lightness constancy is associated with Hermann von Helmholtz (1821-1894), a German physician, physicist and philosopher of science. As a physicist, Helmholtz believed that the eye was an imperfect optical instrument and that the brain must combine raw sensory input and previous experience to arrive at a ‘best guess’ as to what is out in the world. Given his emphasis on the purpose of the visual system in relation to the natural environment, it is interesting to note that Helmholtz was a contemporary of Darwin and spoke approvingly of evolutionary theory (Helmholtz 1873/1865). According to Helmholtz (1867), perception is the result of ‘unconscious inferences’. Lightness constancy, in particular, is achieved by the visual system inferring and then discounting the illuminant. While there is a shadow depicted as falling across the checkerboard, there isn't really a shadow there—for Adelson's Checker-Shadow illusion is just a picture of a shadow. And this is what causes the illusion. Adelson’s Checker-Shadow Illusion is therefore the result of a failure of our inductive inferences to line up with the world. Using premises based on sensory evidence, perceived lighting conditions, and previous experiences, our visual system arrives at a false conclusion.