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Chromostereopsis is a visual
illusion An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people. Illusions may o ...
whereby the impression of depth is conveyed in
two-dimensional In mathematics, a plane is a Euclidean (flat), two-dimensional surface that extends indefinitely. A plane is the two-dimensional analogue of a point (zero dimensions), a line (one dimension) and three-dimensional space. Planes can arise as s ...
color images, usually of red–blue or red–green colors, but can also be perceived with red–grey or blue–grey images. Such
illusion An illusion is a distortion of the senses, which can reveal how the mind normally organizes and interprets sensory stimulation. Although illusions distort the human perception of reality, they are generally shared by most people. Illusions may o ...
s have been reported for over a century and have generally been attributed to some form of
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...
.
Chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...
results from the differential
refraction In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
of light depending on its
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
, causing some light rays to
converge Converge may refer to: * Converge (band), American hardcore punk band * Converge (Baptist denomination), American national evangelical Baptist body * Limit (mathematics) * Converge ICT, internet service provider in the Philippines *CONVERGE CFD s ...
before others in the eye (longitudinal chromatic aberration or LCA) and/or to be located on non-corresponding locations of the two eyes during binocular viewing (transverse chromatic aberration or TCA). Chromostereopsis is usually observed using a target with red and blue bars and an achromatic background. Positive chromostereopsis is exhibited when the red bars are perceived in front of the blue and negative chromostereopsis is exhibited when the red bars are perceived behind the blue. Several models have been proposed to explain this effect which is often attributed to longitudinal and/or transverse chromatic aberrations. However, some work attributes most of the stereoptic effect to transverse chromatic aberrations in combination with cortical factors. It has been proposed that chromostereopsis could have evolutionary implications in the development of eyespots in certain butterfly species. The perceived differences in color's optical power span about 2
diopter A dioptre (British spelling) or diopter (American spelling) is a unit of measurement with dimension of reciprocal length, equivalent to one reciprocal metre, 1 dioptre = 1 m−1. It is normally used to express the optical power of a lens or cur ...
(Blue: −1.5, Red +0.5). The effect can appear much more pronounced when suitable images are viewed while wearing eyeglasses needed to correct myopia, with the effect almost completely disappearing when the glasses are removed.


History

It is commonly found in
stained-glass Stained glass is coloured glass as a material or works created from it. Throughout its thousand-year history, the term has been applied almost exclusively to the windows of churches and other significant religious buildings. Although tradition ...
, historically artists have been aware of this effect, using it to generate advancing or receding perspectives within the images. Over two centuries ago, the effect of color depth perception was first noted by
Goethe Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as treat ...
in his Farbenlehre (
Theory of Colours ''Theory of Colours'' (german: Zur Farbenlehre, links=no) is a book by Johann Wolfgang von Goethe about the poet's views on the nature of colours and how these are perceived by humans. It was published in German in 1810 and in English in 1840. ...
) in which he recognized blue as a receding color and yellow/red as a protruding color. He argued that, "like we see the high sky, the far away mountains, as blue, in the same way a blue field seems to recede…(also) One can stare at a perfectly yellow/red field, then the color seems to pierce into the organ". This phenomenon, now referred to as chromostereopsis, or the stereoptic effect, explains the visual science behind this color depth effect, and has many implications for art, media,
evolution Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...
, as well as our daily lives in how we perceive colors and objects. Although Goethe did not propose any scientific reasoning behind his observations, in the late 1860s Bruecke and Donders first suggested that the chromostereoptic effect was due to accommodative awareness, given that ocular
optics Optics is the branch of physics that studies the behaviour and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behaviour of visible, ultraviole ...
are not achromatic and red objects require more accommodation to be focused on the
retina The retina (from la, rete "net") is the innermost, light-sensitive layer of tissue of the eye of most vertebrates and some molluscs. The optics of the eye create a focused two-dimensional image of the visual world on the retina, which then ...
. This notion of accommodation could then be translated into perception of distance. However, what Donders and Bruecke originally missed in their theory is the necessity of
binocular Binocular may refer to: Science and technology * Binocular vision, seeing with two eyes * Binoculars, a telescopic tool * Binocular microscope, binocular viewing of objects through a single objective lens Other uses * Binocular (horse), a thoroug ...
observation to produce chromostereopsis. Later, veering off from accommodative awareness, Bruecke proposed that chromatic aberration, along with the temporal off-axis effect of the pupil, can explain the chromostereoptic effect. It is this hypothesis that still forms the basis for our present day understanding of chromostereopsis. Over the years, art analysis has provided ample evidence of the chromostereoptic effect, but until about thirty years ago little was known about the neurological, anatomical and/or physiological explanation behind the phenomena. For example, in 1958 Dutch art historian De Wilde noted that in analyzing
cubist Cubism is an early-20th-century avant-garde art movement that revolutionized European painting and sculpture, and inspired related movements in music, literature and architecture. In Cubist artwork, objects are analyzed, broken up and reassemble ...
painter
Leo Gestel Leo Gestel (11 November 1881, Woerden – 26 November 1941, Hilversum) was a Dutch painter. His father Willem Gestel was also an artist. Leo Gestel experimented with cubism, expressionism, futurism and postimpressionism. Along with Piet Mondrian a ...
's painting "The Poet Rensburg", instead of using conventional graded depth cues, "If you put violet next to yellow or green next to orange, the violet and the green retreat. In general, the warm colours come forward, and the cool colours retreat". In this sense, the chromostereoptic effect gives shapes plasticity and allows for depth perception through color manipulation.


Binocular nature of chromostereopsis

The binocular nature of the chromostereopsis was discovered by Bruecke and arises due to the position of the
fovea Fovea () (Latin for "pit"; plural foveae ) is a term in anatomy. It refers to a pit or depression in a structure. Human anatomy *Fovea centralis of the retina * Fovea buccalis or Dimple * Fovea of the femoral head * Trochlear fovea of the fr ...
relative to the optical axis. The fovea is located temporally to the
optical axis An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light propagat ...
and as a result, the
visual axis In architecture, sightlines are a particularly important consideration in the design of civic structures, such as a stage, arena, or monument. They determine the configuration of such items as theater and stadium design, road junction layout an ...
passes through the
cornea The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Along with the anterior chamber and lens, the cornea refracts light, accounting for approximately two-thirds of the eye's total optical power ...
with a nasal
horizontal eccentricity Horizontal eccentricity refers to the horizontal axis, measured in degrees, along the visual field. The blind spot extends from an eccentricity ''d''1 to eccentricity ''d''2 in temporal direction from the fovea. The size of the blind spot can be c ...
, meaning that the average ray bound for the fovea must undergo
prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...
atic deviation and is thus subject to chromatic
dispersion Dispersion may refer to: Economics and finance *Dispersion (finance), a measure for the statistical distribution of portfolio returns *Price dispersion, a variation in prices across sellers of the same item *Wage dispersion, the amount of variatio ...
. The prismatic deviation is in opposite directions in each eye, resulting in opposite color shifts that lead to a shift in stereoptic depth between red and blue objects. The eccentric foveal receptive system, along with the
Stiles–Crawford effect The Stiles–Crawford effect (subdivided into the Stiles–Crawford effect of the first and second kind) is a property of the human eye that refers to the directional sensitivity of the cone photoreceptors. The Stiles–Crawford effect of the firs ...
, work in opposite directions of one another and roughly cancel out, offering another explanation to why subjects may show color stereoscopy "against the rule" (a reversal of the expected results). Image that may show four different layers of depth. From near to far: red, yellow, green and blue.


Reversal effect

Evidence for the stereoptic effect is often quite easy to see. For example, when red and blue are viewed side by side on a dark surrounding, most people will view the red as "floating" in front of the blue. However, this is not true for everyone, as some people see the opposite and others no effect at all. This is the same effect that both
Goethe Johann Wolfgang von Goethe (28 August 1749 – 22 March 1832) was a German poet, playwright, novelist, scientist, statesman, theatre director, and critic. His works include plays, poetry, literature, and aesthetic criticism, as well as treat ...
and De Wilde had indicated in their observations. While a majority of people will view red as "floating" in front of blue, others experience a reversal of the effect in which they see blue floating in front of the red, or no depth effect at all. While this reversal may appear to discredit chromostereopsis, it does not and instead, as originally proposed by
Einthoven Willem Einthoven (21 May 1860 – 29 September 1927) was a Dutch doctor and physiologist. He invented the first practical electrocardiograph (ECG or EKG) in 1895 and received the Nobel Prize in Physiology or Medicine in 1924 for it ("for the dis ...
, can be explained by an increase in the effect and subsequent reversal via blocking of the eccentric position of the pupil with respect to the optical axis. The diverse nature of the chromostereoptic effect is because the color depth effect is closely intertwined with both perceptual and optical factors. In other words, neither the optical nor the
perceptual Perception () is the organization, identification, and interpretation of sense, sensory information in order to represent and understand the presented information or environment. All perception involves signals that go through the nervous ...
factors can be taken in isolation to explain chromostereopsis. This multifactorial component of chromostereopsis offers one explanation of the reversal of the effect in different people given the same visual cues. Another interesting reversal effect was observed in 1928 by
Verhoeff Verhoeff is a surname. Notable people with the surname include: * Frederick Herman Verhoeff (1874–1968), American ophthalmic surgeon who developed Verhoeff's stain * Hendrik Verhoeff (c. 1645–1680), Dutch silversmith and assassin * Jacobus Verh ...
in which the red bars were perceived as farther away and the blue bars as protruding when the bars are paired on a white background instead of a black background. Verhoeff proposed that this paradoxical reversal can be understood in terms of the pupil's
luminance Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls withi ...
contours (see:
Illusory Contours Illusory contours or subjective contours are visual illusions that evoke the perception of an edge without a luminance or color change across that edge. Illusory brightness and depth ordering often accompany illusory contours. Friedrich Schumann i ...
). The pupil has lines of constant luminance efficiency, with each subsequent line marking a 25% decrease in efficiency. Around 1998, Winn and co-workers confirmed Verhoeff's interpretation of this reversal using experiments on different colored backgrounds. Other research has also suggested that border contrast changes could lead to color depth reversal with the switch from black to white backgrounds. In 1933, Stiles and Crawford discovered that the light sensitivity of the fovea differs significantly for rays entering the eye through the center of the pupil versus rays entering from its
peripheral A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by the ...
regions. They observed that the usual "intensity multiplied by
aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An opt ...
" rule did not apply in foveal vision and that rays entering the eye via peripheral regions of the pupil were less efficient by roughly a factor of five. This effect is now known as the
Stiles–Crawford effect The Stiles–Crawford effect (subdivided into the Stiles–Crawford effect of the first and second kind) is a property of the human eye that refers to the directional sensitivity of the cone photoreceptors. The Stiles–Crawford effect of the firs ...
and also has implications for the reverse chromostereoptic effect.


Theory

In 1885,
Einthoven Willem Einthoven (21 May 1860 – 29 September 1927) was a Dutch doctor and physiologist. He invented the first practical electrocardiograph (ECG or EKG) in 1895 and received the Nobel Prize in Physiology or Medicine in 1924 for it ("for the dis ...
proposed a
theory A theory is a rational type of abstract thinking about a phenomenon, or the results of such thinking. The process of contemplative and rational thinking is often associated with such processes as observational study or research. Theories may be s ...
which states: "The phenomenon (chromostereopsis) is due to chromatic difference of
magnification Magnification is the process of enlarging the apparent size, not physical size, of something. This enlargement is quantified by a calculated number also called "magnification". When this number is less than one, it refers to a reduction in siz ...
, for since, for example, blue rays are refracted more than red rays by the ocular media, their foci not only lie at different levels (chromatic aberration) but make different angles with the optic axis, and will thus stimulate disparate points. It follows that individuals with temporally eccentric
pupils The pupil is a black hole located in the center of the iris of the eye that allows light to strike the retina.Cassin, B. and Solomon, S. (1990) ''Dictionary of Eye Terminology''. Gainesville, Florida: Triad Publishing Company. It appears black ...
see red in front of blue, while with nasally
eccentric Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...
pupils the relief is reversed." Einthoven first explained chromatic aberration in the eye, which means that the eyes will not
focus Focus, or its plural form foci may refer to: Arts * Focus or Focus Festival, former name of the Adelaide Fringe arts festival in South Australia Film *''Focus'', a 1962 TV film starring James Whitmore * ''Focus'' (2001 film), a 2001 film based ...
all the colors at the same time. Depending on the
wavelength In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats. It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
, the focal point in the eyes varies. He concluded that the reason why people see red in front of blue is because light with different wavelengths project onto different parts of the retina. When the vision is binocular, a disparity is created, which causes depth perception. Since red is focused temporally, it appears to be in front. However, under monocular vision, this phenomenon is not observed. However, Bruecke objected to Einthoven's theory on grounds that not all people see red as closer than blue. Einthoven explained that this negative chromostereopsis is probably due to eccentrically positioned pupils because shifting the pupil can change the position of where light wavelengths focus in the eye. Negative chromostereopsis was further studied by Allen and Rubin who suggested that changing the angle between the pupillary center and visual axis can change the direction of chromostereopsis. If the pupillary center is located temporal to the visual axis, red will appear closer. The reverse effect is observed when the pupillary center is nasal to the visual axis.


Stiles–Crawford effect

Research has attempted to extend the basis for the traditional chromostereoptic theory, including work done by Stiles and Crawford. In 1933, Stiles and Crawford accidentally discovered that the
light sensitivity Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicity ...
differed for rays entering through center versus those entering from
peripheral A peripheral or peripheral device is an auxiliary device used to put information into and get information out of a computer. The term ''peripheral device'' refers to all hardware components that are attached to a computer and are controlled by the ...
regions of the eye. The efficiency of the rays is less when the rays enter via the peripheral region because the shape of the
cone cells Cone cells, or cones, are photoreceptor cells in the retinas of vertebrate eyes including the human eye. They respond differently to light of different wavelengths, and the combination of their responses is responsible for color vision. Cones ...
that collect the incident quanta are different from cone receptors in the center of the eye. This effect can cause both positive and negative chromostereopsis depending on the position of the pupil. If the pupil is centered on optical axis, it causes positive chromostereopsis. However, if the pupil is significantly off-center from the optical axis, negative chromostereopsis will ensue. Because most people have a point of maximum luminous efficiency that is off-center, the
Stiles–Crawford effect The Stiles–Crawford effect (subdivided into the Stiles–Crawford effect of the first and second kind) is a property of the human eye that refers to the directional sensitivity of the cone photoreceptors. The Stiles–Crawford effect of the firs ...
s generally will have antagonistic chromostereoptic effects. Therefore, instead of seeing red in front of blue, blue will be seen in front of red and the effect will be reversed. The Stiles–Crawford effect also explains why positive chromostereopsis is decreased when illumination is lowered. At lower illumination, the dilation of pupil increases the pupillary peripheral region and therefore increases the magnitude of the Stiles–Crawford effect.


Chromatic aberration

Stereoptic depth perception obtained from two
dimensional In physics and mathematics, the dimension of a mathematical space (or object) is informally defined as the minimum number of coordinates needed to specify any point within it. Thus, a line has a dimension of one (1D) because only one coordi ...
red and blue or red and green images is believed to be caused primarily by optical
chromatic aberrations In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...
. Chromatic aberrations are defined as types of optical
distortions In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signa ...
that occur as a consequence of
refracting In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
properties of the eye. However, other pticalfactors, image characteristics, and perceptual factors also play a role in color depth effects under natural viewing conditions. Additionally, texture properties of the stimulus can also play a role.
Newton Newton most commonly refers to: * Isaac Newton (1642–1726/1727), English scientist * Newton (unit), SI unit of force named after Isaac Newton Newton may also refer to: Arts and entertainment * ''Newton'' (film), a 2017 Indian film * Newton ( ...
first demonstrated the presence of chromatic aberration in the human eye in 1670. He observed that isolated incident light rays directed at an opaque card held close to the eye strike the refracting surfaces of the eye obliquely and are therefore strongly
refracted In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
. Because the indices of refraction (see:
Refractive Index In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...
) vary inversely with wavelength, blue rays (short wavelength) will be refracted more than red rays (long wavelength). This phenomenon is called
chromatic dispersion In optics, and by analogy other branches of physics dealing with wave propagation, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to ...
and has important implications for the optical performance of the eye, including the
stereoptic Stereopsis () is the component of depth perception retrieved through binocular vision. Stereopsis is not the only contributor to depth perception, but it is a major one. Binocular vision happens because each eye receives a different image becaus ...
effect. For example, Newton noted that such chromatic dispersion causes the edges of a white object to be tinged with color. Modern accounts of chromatic aberrations divide ocular chromatic aberrations into two main categories; longitudinal chromatic aberration (LCA), and transverse chromatic aberration (TCA).


Longitudinal chromatic aberration

LCA is defined as the "variation of the eye's focusing power for different wavelengths". This chromatic difference varies from about 400 nm to 700 nm across the visible spectrum. In LCA, the refracting properties of the eye cause light rays of shorter wavelengths, such as blue, to converge before longer wavelength colors.


Transverse chromatic aberration

TCA is defined as the variation in angle between the refracted chief rays for different wavelengths.
Chief ray In optics a ray is an idealized geometrical model of light, obtained by choosing a curve that is perpendicular to the ''wavefronts'' of the actual light, and that points in the direction of energy flow. Rays are used to model the propagation o ...
s, in this case, refer to rays from a point source that passes through the center of the pupil. Unlike LCA, TCA depends on object location in the
visual field The visual field is the "spatial array of visual sensations available to observation in introspectionist psychological experiments". Or simply, visual field can be defined as the entire area that can be seen when an eye is fixed straight at a point ...
and pupil position within the eye. Object location determines the angle of incidence of the selected rays. By
Snell's Law Snell's law (also known as Snell–Descartes law and ibn-Sahl law and the law of refraction) is a formula used to describe the relationship between the angles of incidence and refraction, when referring to light or other waves passing through ...
of Refraction, this incidence angle subsequently determines the amount of
chromatic dispersion In optics, and by analogy other branches of physics dealing with wave propagation, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to ...
and thus location of the retinal images for different wavelengths of light. In TCA, different wavelengths of light are displaced in non-corresponding retinal positions of each eye during binocular viewing. The chromostereoptic effect is generally attributed to the interocular difference in TCA. Color-induced depth effects due to TCA can only be perceived in images containing achromatic information and a single non-achromatic color. The amplitude of the perceived depth in an image due to the stereoptic effect can be predicted from the amount of induced TCA. In other words, as the
pupillary distance Pupillary distance (PD) or interpupillary distance (IPD) is the distance measured in millimeters between the centers of the pupils of the eyes. This measurement is different from person to person and also depends on whether they are looking at near ...
from the foveal achromatic axis is increased, perceived depth also increases.


Implications of chromatic aberrations

Longitudinal and transverse chromatic aberrations work together to affect retinal image quality. Additionally, pupil displacement from the
visual axis In architecture, sightlines are a particularly important consideration in the design of civic structures, such as a stage, arena, or monument. They determine the configuration of such items as theater and stadium design, road junction layout an ...
is critical for determining the
magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...
of the aberration under natural viewing conditions. In chromostereopsis, if the pupils of the two eyes are displaced temporally from the visual axis, then blue rays from a point source will intersect the retinae on the nasal side of red rays from the same source. This induced ocular disparity makes blue rays appear to come from a more distant source than red rays.


Evolutionary significance

Chromostereopsis may also have
evolutionary Evolution is change in the heredity, heritable Phenotypic trait, characteristics of biological populations over successive generations. These characteristics are the Gene expression, expressions of genes, which are passed on from parent to ...
implications for
predator Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill th ...
s and
prey Predation is a biological interaction where one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill the ...
, giving it historical and practical significance. Possible evidence for the evolutionary significance of chromostereopsis is given in the fact that the fovea has developed in the
lateral Lateral is a geometric term of location which may refer to: Healthcare *Lateral (anatomy), an anatomical direction *Lateral cricoarytenoid muscle *Lateral release (surgery), a surgical procedure on the side of a kneecap Phonetics *Lateral cons ...
eyes of hunted animals to have a very large
angle In Euclidean geometry, an angle is the figure formed by two Ray (geometry), rays, called the ''Side (plane geometry), sides'' of the angle, sharing a common endpoint, called the ''vertex (geometry), vertex'' of the angle. Angles formed by two ...
between the
optical axis An optical axis is a line along which there is some degree of rotational symmetry in an optical system such as a camera lens, microscope or telescopic sight. The optical axis is an imaginary line that defines the path along which light propagat ...
and
visual axis In architecture, sightlines are a particularly important consideration in the design of civic structures, such as a stage, arena, or monument. They determine the configuration of such items as theater and stadium design, road junction layout an ...
to attain at least some binocular field of view. For these hunted animals, their eyes serve to detect predatory animals, which explains their lateral position in order to give them a full panoramic field of view. In contrast, this observed foveal development is opposite in predators and in primates. Predators and primates depend primarily on
binocular vision In biology, binocular vision is a type of vision in which an animal has two eyes capable of facing the same direction to perceive a single three-dimensional image of its surroundings. Binocular vision does not typically refer to vision where an ...
, and therefore their eyes developed to be frontal in position. The angle between their optical and visual axis, therefore, can be reduced to almost negligible values, down about five degrees in humans).
Butterflies Butterflies are insects in the macrolepidopteran clade Rhopalocera from the Order (biology), order Lepidoptera, which also includes moths. Adult butterflies have large, often brightly coloured wings, and conspicuous, fluttering flight. The ...
may also have taken evolutionary advantage of chromostereopsis in developing distinctive "eye" patterns, which are presented on their wings. These eyespots can appear as being forward or receding in depth based on their color pattern, producing an effect of protruding or receding eyes, respectively.
Natural selection Natural selection is the differential survival and reproduction of individuals due to differences in phenotype. It is a key mechanism of evolution, the change in the heritable traits characteristic of a population over generations. Charle ...
may have developed these color and texture schemas because it produces the illusion of protruding or receding eyes of much larger organisms than the actual butterfly, keeping potential predators at bay. Yet another evolutionary example of chromostereopsis comes from
cuttlefish Cuttlefish or cuttles are marine molluscs of the order Sepiida. They belong to the class Cephalopoda which also includes squid, octopuses, and nautiluses. Cuttlefish have a unique internal shell, the cuttlebone, which is used for control of ...
. It has been suggested that cuttlefish estimate the distance of prey via
stereopsis Stereopsis () is the component of depth perception retrieved through binocular vision. Stereopsis is not the only contributor to depth perception, but it is a major one. Binocular vision happens because each eye receives a different image becaus ...
. Additional evidence suggests that their choice of
camouflage Camouflage is the use of any combination of materials, coloration, or illumination for concealment, either by making animals or objects hard to see, or by disguising them as something else. Examples include the leopard's spotted coat, the ...
is also sensitive to visual depth based on color-induced depth effects.


Methods of testing

Many different methods of testing have been employed to view the effects of chromostereopsis on
depth perception Depth perception is the ability to perceive distance to objects in the world using the visual system and visual perception. It is a major factor in perceiving the world in three dimensions. Depth perception happens primarily due to stereopsis an ...
in humans. Technological progress has allowed for accurate, efficient, and more conclusive testing, in relation to the past, where individuals would merely observe the occurrence. In one method, twenty-five control subjects were tested using color-based depth effects through the use of five different colored pairs of squares. The different colors were blue, red, green,
cyan Cyan () is the color between green and blue on the visible spectrum of light. It is evoked by light with a predominant wavelength between 490 and 520 nm, between the wavelengths of green and blue. In the subtractive color system, or CMYK color ...
and yellow. Subjects were placed in a dark room and the colored square
stimuli A stimulus is something that causes a physiological response. It may refer to: *Stimulation **Stimulus (physiology), something external that influences an activity **Stimulus (psychology), a concept in behaviorism and perception *Stimulus (economi ...
were presented for 400
milliseconds A millisecond (from ''milli-'' and second; symbol: ms) is a unit of time in the International System of Units (SI) equal to one thousandth (0.001 or 10−3 or 1/1000) of a second and to 1000 microseconds. A unit of 10 milliseconds may be called ...
each, and during this time the subjects were asked to attend to either the right or left square (evenly counterbalanced across subjects). Using a
joystick A joystick, sometimes called a flight stick, is an input device consisting of a stick that pivots on a base and reports its angle or direction to the device it is controlling. A joystick, also known as the control column, is the principal cont ...
, the subject indicated whether the square was behind, in front of, or in the same plane as its pair. According to the theory, the longer the wavelength of the color, the closer it should be perceived by the observer for positive chromostereopsis. Having a longer wavelength than the other colors, red should appear closest. To enhance this effect, subjects put on
blazed grating A blazed grating – also called echelette grating (from French ''échelle'' = ladder) – is a special type of diffraction grating. It is optimized to achieve maximum grating efficiency in a given diffraction order. For this purpose, maximu ...
High Definition ChromaDepth® C3D™ 3D-lenses
glasses Glasses, also known as eyeglasses or spectacles, are vision eyewear, with lenses (clear or tinted) mounted in a frame that holds them in front of a person's eyes, typically utilizing a bridge over the nose and hinged arms (known as temples or ...
, which contain a
prism Prism usually refers to: * Prism (optics), a transparent optical component with flat surfaces that refract light * Prism (geometry), a kind of polyhedron Prism may also refer to: Science and mathematics * Prism (geology), a type of sedimentary ...
structure to
refract In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
the
light Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
to an
angle In Euclidean geometry, an angle is the figure formed by two Ray (geometry), rays, called the ''Side (plane geometry), sides'' of the angle, sharing a common endpoint, called the ''vertex (geometry), vertex'' of the angle. Angles formed by two ...
of approximately 1° and were tested again. The use of
electrodes An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
to test brain activity is another, relatively new way to test for chromostereopsis. This form of testing utilizes
EEG Electroencephalography (EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex ...
recordings of visual-
evoked potentials An evoked potential or evoked response is an electrical potential in a specific pattern recorded from a specific part of the nervous system, especially the brain, of a human or other animals following presentation of a stimulus such as a light fla ...
through the use of electrodes. In one experiment, subjects were shown different stimuli in regard to color-contrast and were asked questions about its depth, as before. The electrodes attached to the subjects subsequently collected data while the experiment occurred. Another more routinely used technique tests the subject's extent of
chromatic aberration In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wave ...
. In one such experiment, slits placed before the subject's eyes measured the
chromatic dispersion In optics, and by analogy other branches of physics dealing with wave propagation, dispersion is the phenomenon in which the phase velocity of a wave depends on its frequency; sometimes the term chromatic dispersion is used for specificity to ...
of the eyes as a function of the separation of the slits. Prisms in front of the eyes determined the separation of the visual and null axes. The product of these separate measurements predicted the apparent depth expected with full-pupil
stereoscopy Stereoscopy (also called stereoscopics, or stereo imaging) is a technique for creating or enhancing the illusion of depth in an image by means of stereopsis for binocular vision. The word ''stereoscopy'' derives . Any stereoscopic image is ...
. Agreement was good with expected results, supplying additional evidence that chromostereopsis depends on chromatic dispersion. Other experimental techniques can be used to test for reverse chromostereopsis, an occurrence seen by a minority of the population. The direction of chromostereopsis can be reversed by moving both artificial pupils in a nasal direction or temporal direction with respect to the centers of the natural pupils. Moving the artificial pupils nasally induces blue-in-front-of-red stereopsis and moving them temporally has the opposite effect. This is because moving the pupil changes the position of the optic axis, but not the
visual axis In architecture, sightlines are a particularly important consideration in the design of civic structures, such as a stage, arena, or monument. They determine the configuration of such items as theater and stadium design, road junction layout an ...
, thus changing the sign of transverse chromatic aberration. Therefore, changes in the magnitude and sign of transverse chromatic aberration brought about by changing the lateral distance between small artificial pupils are accompanied by equivalent changes in chromostereopsis.


Research

While a lot of physiological mechanisms that cause chromostereopsis have been discovered and researched, there are still unanswered questions. For example, many researchers believe that chromostereopsis is caused by combination of multiple factors. Because of this, some research has attempted to investigate how the different
luminescence Luminescence is spontaneous emission of light by a substance not resulting from heat; or "cold light". It is thus a form of cold-body radiation. It can be caused by chemical reactions, electrical energy, subatomic motions or stress on a cryst ...
of backgrounds and different luminescence of red and blue color affect the chromostereoptic effect. Additionally, previous studies have taken a psychophysical approach to studying chromostereopsis in order to document it as a perceptual effect and observe its optic mechanisms, but no studies had examined the neurophysiological basis of chromostereopsis. A neurophysiological study by Cauquil et al. describes V1 and V2 color-preferring cells as coding local image characteristics (such as binocular disparity) and surface properties of a 3D scene, respectively. The study conducted by Cauquil et al. indicates, based on
electrode An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials de ...
stimulation results, that both
dorsal Dorsal (from Latin ''dorsum'' ‘back’) may refer to: * Dorsal (anatomy), an anatomical term of location referring to the back or upper side of an organism or parts of an organism * Dorsal, positioned on top of an aircraft's fuselage * Dorsal co ...
and
ventral Standard anatomical terms of location are used to unambiguously describe the anatomy of animals, including humans. The terms, typically derived from Latin or Greek language, Greek roots, describe something in its standard anatomical position. Th ...
pathways in the brain are involved in chromostereoptic processing. This study also concluded that chromostereopsis starts in the early stages of visual cortical processing, first in the occipito- parietal region of the brain, followed by a second step in the right parietal area and temporal lobes. Additionally, activity was found to be greater in the
right hemisphere The lateralization of brain function is the tendency for some neural functions or cognitive processes to be specialized to one side of the brain or the other. The median longitudinal fissure separates the human brain into two distinct cerebra ...
, which is dominant for 3D cortical processing, indicating that chromostereopsis is a task-dependent, top-down effect. Overall, chromostereopsis involves cortical areas that underlie depth processing for both
monocular A monocular is a compact refracting telescope used to magnify images of distant objects, typically using an optical prism to ensure an erect image, instead of using relay lenses like most telescopic sights. The volume and weight of a monocul ...
and
binocular Binocular may refer to: Science and technology * Binocular vision, seeing with two eyes * Binoculars, a telescopic tool * Binocular microscope, binocular viewing of objects through a single objective lens Other uses * Binocular (horse), a thoroug ...
cues.


References

{{Stereoscopy, state=expanded Optical illusions