There has been some debate over the degree to which simultaneous contrast is a physiological process caused by the connections of neurons in the visual cortex, or whether it is a psychological effect. Both appear to have some effect. A possible source of the effect are neurons in the V4 area that have inhibitory connections to neighboring cells. The most likely evolutionary rationale for this effect is that it enhances edges in the visual field, thus facilitating the recognition of shapes and objects. Successive contrast |alt=|192x192pxThis is a different concept from contrast, which by itself refers to one object's difference in color and luminance compared to its surroundings or background. Successive contrast Successive contrast occurs when the perception of currently viewed stimuli is modulated by previously viewed stimuli.  Hermann von Helmholtz: Handbuch der physiologischen Optik section II § 24: ‘Genauer unterscheidet Chevreul die hierher gehörigen Erscheinungen unter dem Namen des simultanen Contrastes von denjenigen, wo zwei Farben nach einander auf derselben Netzhautstelle erscheinen, welche er mit dem Namen des successiven Contrastes belegt.’ In the example below you can use the scrollbar to quickly swap the red and green disks for two orange disks. Staring at the dot in the centre of one of the top two coloured disks and then looking at the dot in the centre of the corresponding lower disk makes the two lower disks briefly appear to have different colours, though in reality their colour is identical. Metacontrast and paracontrast  Metacontrast and paracontrast  involve both time and space. When one half of a circle is lit for 10 milliseconds (ms), it is at its maximal intensity. If the other half is displayed at the same time (but 20–50 ms later), there is a mutual inhibition: the left side is darkened by the right half (metacontrast), and the center may be completely obliterated. At the same time, there is a slight darkening of the right side due to the first stimulus (paracontrast)."Eye, human". Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD. References Chevreul, Michel Eugène (1839). De la loi du contraste simultané des couleurs et de l'assortiment des objets colorés .- traduit en anglais par Charles Martel comme Les principes d’harmonie et de contraste des couleurs (1854)
There has been some debate over the degree to which simultaneous contrast is a physiological process caused by the connections of neurons in the visual cortex, or whether it is a psychological effect. Both appear to have some effect. A possible source of the effect are neurons in the V4 area that have inhibitory connections to neighboring cells. The most likely evolutionary rationale for this effect is that it enhances edges in the visual field, thus facilitating the recognition of shapes and objects. Successive contrast Successive contrast occurs when the perception of currently viewed stimuli is modulated by previously viewed stimuli. Chevreul, Michel Eugène (1839). De la loi du contraste simultané des couleurs et de l'assortiment des objets colorés – traduit en anglais par Charles Martel comme Les principes d’harmonie et de contraste des couleurs (1854) Hermann von Helmholtz: Handbuch der physiologischen Optik section II § 24: ‘Genauer unterscheidet Chevreul die hierher gehörigen Erscheinungen unter dem Namen des simultanen Contrastes von denjenigen, wo zwei Farben nach einander auf derselben Netzhautstelle erscheinen, welche er mit dem Namen des successiven Contrastes belegt.’ In the example below you can use the scrollbar to quickly swap the red and green disks for two orange disks. Staring at the dot in the centre of one of the top two coloured disks and then looking at the dot in the centre of the corresponding lower disk makes the two lower disks briefly appear to have different colours, though in reality their colour is identical. Metacontrast and paracontrast 296x296px| Metacontrast and paracontrast |alt=Metacontrast and paracontrast involve both time and space. When one half of a circle is lit for 10 milliseconds (ms), it is at its maximal intensity. If the other half is displayed at the same time (but 20–50 ms later), there is a mutual inhibition: the left side is darkened by the right half (metacontrast), and the center may be completely obliterated. At the same time, there is a slight darkening of the right side due to the first stimulus (paracontrast)."Eye, human". Encyclopædia Britannica. 2008. Encyclopædia Britannica 2006 Ultimate Reference Suite DVD. References 
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Successive contrast |alt=|192x192pxThis is a different concept from contrast, which by itself refers to one object's difference in color and luminance compared to its surroundings or background.
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Chevreul, Michel Eugène (1839). De la loi du contraste simultané des couleurs et de l'assortiment des objets colorés – traduit en anglais par Charles Martel comme Les principes d’harmonie et de contraste des couleurs (1854)
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296x296px|
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Chevreul, Michel Eugène (1839). De la loi du contraste simultané des couleurs et de l'assortiment des objets colorés .- traduit en anglais par Charles Martel comme Les principes d’harmonie et de contraste des couleurs (1854)