Color assimilation is somewhat considered the opposite of the color contrast: the color is perceived in the direction of the hue of the surrounding color, whereas in color contrast the perceived color tends to be the complementary color. Assimilation can be experienced also in grayscale stimuli and it is called lightness assimilation or brightness assimilation. In this case a gray is perceived lighter if it is close to a light object and darker if it is close to a darker object which is, again, the opposite of the simultaneous contrast.
According to a naive physicalism, the physical stimulus should present a direct relationship with its mental correlate. Visual illusions (e.g. [1, 2, 3]) indicate, however, that physical manipulations of a stimulus do not directly determine the perceptual experience . The existence of visual illusions has been already reported by ancient philosophers and they were commonly considered counterintuitive singularities , which demonstrate the active work of the sensorial system involved in the stimulus processing [6, 7]. It was proved that also non-human animals can experience visual illusions [8, 9]. An important family of this illusion is the brightness illusion and this also shows a similar counterpart in colored stimuli. It is often assumed that perceiving a surface as a source of light depends just on its physical radiant emission. However, the Persian natural philosopher Ibn Al-Haytham (circa 965–1040 AD), known as Alhazen, stressed the subjective nature of color sensation and argued that color appearance was partly due to a mental process in his description of the simultaneous contrast .
Simultaneous contrast can be described as follows: a gray target surrounded by a bright inducer that appears darker than its physical value. If the same gray target will be surrounded by a dark inducer, the results will be the opposite: the target will be perceived lighter than its physical value. The simultaneous contrast can be enhanced by blurring the boundaries of the inducer [11, 12, 13, 14]. The simultaneous contrast is observed also in color stimuli where the color of the target tends to the hue of the opposite color of the inducer.
Possible Explanations for Assimilation
One approach to explain the assimilation is that bottom-up, peripheral mechanisms are sufficient to produce this percept. Assimilation could be indeed produced by the fact that the retinal input can be imagined as a blurry image; consequently, if the visual system will consider the retinal input of a display like the one in Figs. 1 or 2, the final percept will be in the direction of assimilation. However, considering how important is the cortical process in the final percept, it is hard to believe that at least in typically developed individuals the cortex will simply “accept” the retinal signal without any post processing in order to provide the final percept. Other proposals that consider the assimilation exclusively a bottom-up process are based on local averaging of luminance within large neurons’ receptive fields. The receptive fields are small in the fovea and larger in the periphery and they also increase their size going up in the cortical hierarchy . Specifically, two possible mechanisms have been proposed: a neuronal spatial integration  or neural weighted averages , suggesting that the primary anatomical site for assimilation could be spatially close but still outside V1. Both mechanisms may result in assimilation when the physical stimulus is a similar pattern to the one showed in Figs. 1 and 2. However, some predictions based on this explanation fell short in front of the lab tests. For example, DeValois and DeValois  suggested that stronger assimilation should be found for color in comparison with grayscale stimuli, because of the lack of lateral inhibition in chromatic receptive fields. However, the results by de Weert and Spillmann  showed that it was not the case.
Another approach that is not necessarily opposite to the bottom-up interpretation suggests that assimilation is primarily generated by more central mechanisms of visual processing, such as figure–ground segmentation [23, 24] and observer expertise . It is also important to note that assimilation received interest based on the White illusion . It has been suggested that assimilation depends on the existence of T-junctions that produce a perception of figure–ground segregation [27, 28]. T-junctions seem, for example, to affect also illusory motion . However, this explanation was not supported by the lab test, which demonstrated that assimilation effects can also be seen in versions of White’s display where T-junctions have been completely removed . More recently, Soranzo et al.  supported the central mechanism explanation for assimilation by testing that in stroboscopic conditions. In 2010 Rude  proposed an intriguing computational neural model that includes the effect of the top-down attentional control in explaining the assimilation effect.
In summary the assimilation is an interesting phenomenon that is still searching for a convincing explanation that can keep all the experimental results under the same theoretical umbrella. However, several important steps were done in order to explain how the brain interprets these patterns.
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