Chromostereopsis

Chromostereopsis is a visual illusion whereby the impression of depth is conveyed in two-dimensional color images, usually of red-blue or red-green colors, but can also be perceived with red-grey or blue-grey images. Such illusions have been reported for over a century and have generally been attributed to some form of chromatic aberration. Chromostereopsis is a visual illusion whereby the impression of depth is conveyed in two-dimensional color images, usually of red-blue or red-green colors, but can also be perceived with red-grey or blue-grey images. Such illusions have been reported for over a century and have generally been attributed to some form of chromatic aberration. Chromatic aberration results from the differential refraction of light depending on its wavelength, causing some light rays to converge 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, recent 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. Additionally, it is posited that some stained-glass artists may have been aware of this effect, using it to generate advancing or receding, sometimes referred to as 'warm' and 'cold', color images. Over two centuries ago, the effect of color depth perception was first noted by Goethe in his Farbenlehre (Theory of Colours) 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, 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 are not achromatic and red objects require more accommodation to be focused on the retina. 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 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 painter Leo Gestel'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. The binocular nature of the chromostereopsis was discovered by Bruecke and arises due to the position of the fovea relative to the optical axis. The fovea is located temporally to the optical axis and as a result, the visual axis passes through the cornea with a nasal horizontal eccentricity, meaning that the average ray bound for the fovea must undergo prismatic deviation and is thus subject to chromatic dispersion. 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, 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). 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 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, 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 factors can be taken in insolation 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.