The Daily Omnivore

PJ B Hancock and C Foster investigate the double face illusion, where the eyes and mouth are duplicated. On an identification task for briefly (80ms) presented faces, there are strong individual differences: doubling has little effect for the majority, but inhibits recognition for about a quarter of participants. A second experiment shows that some participants are unable to detect face doubling at this speed, while others are 100% correct at 50 ms.

Unlike the Thatcher illusion, a doubled face is still obvious when inverted, but it is less unsettling to look at and a third study found that participants were about 35 ms faster to decide that a face has been doubled when it is inverted. A final experiment tested visual search for normal and doubled faces; neither pops out from the other and the search time per item is again about 35 ms longer for double faces.

Researcher Mark Changizi of Rensselaer Polytechnic Institute in New York has a more imaginative take on optical illusions, saying that they are due to a neural lag which most humans experience while awake. When light hits the retina, about one-tenth of a second goes by before the brain translates the signal into a visual perception of the world. Scientists have known of the lag, yet they have debated how humans compensate, with some proposing that our motor system somehow modifies our movements to offset the delay.

Changizi asserts that the human visual system has evolved to compensate for neural delays by generating images of what will occur one-tenth of a second into the future. This foresight enables humans to react to events in the present, enabling humans to perform reflexive acts like catching a fly ball and to maneuver smoothly through a crowd. Illusions occur when our brains attempt to perceive the future, and those perceptions don’t match reality.

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A subtractive color model explains the mixing of paints, dyes, inks, and natural colorants to create a full range of colors, each caused by subtracting (that is, absorbing) some wavelengths of light and reflecting the others. The color that a surface displays depends on which colors of the electromagnetic spectrum are reflected by it and therefore made visible.

Subtractive color systems start with light, presumably white light. Next, colored inks, paints, or filters between the viewer and the light source subtract wavelengths from the light, giving it color. If the incident light is other than white, our visual mechanisms are able to compensate well, but not perfectly, often giving a flawed impression of the ‘true’ color of the surface. Conversely, additive color systems start without light (black). Light sources of various wavelengths combine to make a color. Often, three primary colors are combined to stimulate humans’ trichromatic color vision, sensed by the three types of cone cells in the eye, giving an apparently full range.

Additive color describes the situation where color is created by mixing the visible light emitted from differently colored light sources. This is in contrast to subtractive colors where light is removed from various part of the visible spectrum to create colors. Computer monitors and televisions are the most common form of additive light, while subtractive color is used in paints and pigments and color filters. The additive reproduction process usually uses red, green and blue light to produce the other colors. Combining one of these additive primary colors with another in equal amounts produces the additive secondary colors cyan, magenta, and yellow. The colored pixels in displays do not overlap on the screen, but when viewed from a sufficient distance, the light from the pixels diffuses to overlap on the retina.

Results obtained when mixing additive colors are often counterintuitive for people accustomed to the more everyday subtractive color system of pigments, dyes, inks and other substances which present color to the eye by reflection rather than emission. For example, in subtractive color systems green is a combination of yellow and blue; in additive color, red + green = yellow and no simple combination will yield green. Additive color is a result of the way the eye detects color, and is not a property of light. There is a vast difference between yellow light, with a wavelength of approximately 580 nm, and a mixture of red and green light. However, both stimulate our eyes in a similar manner, so we do not detect that difference.

An isometric illusion (also called an ambiguous figure or inside/outside illusion) is a type of optical illusion, specifically one due to multistable perception.

In general, any shape built entirely of same-length (i.e., isometric) lines that does not clearly indicate relative direction between its components will evoke such a perceptual ‘flip-flopping.’ The Necker Cube is a famous example of an isometric illusion.