Encyclopedia of Color Science and Technology

2016 Edition
| Editors: Ming Ronnier Luo


Reference work entry
DOI: https://doi.org/10.1007/978-1-4419-8071-7_272



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 [4]. The existence of visual illusions has been already reported by ancient philosophers and they were commonly considered counterintuitive singularities [5], 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 [10].

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.

Assimilation Description

The brightness assimilation can be considered the opposite of the simultaneous contrast because under specific conditions a gray target will appear lighter when bordered by a brighter inducer and vice versa. The color assimilation is a similar phenomenon in which a colored target tends to be perceived as similar in hue to the inducer color. Figure 1 shows a typical color assimilation display: the yellow background on the left is perceived reddish, while the yellow background on the right is perceived bluish; however, the two yellows are exactly the same hue. Figure 2 shows brightness assimilation: the gray background on the left is perceived lighter, while the gray background on the right is commonly perceived darker; however, the two gray values have the exact same luminance. von Bezold [15] probably described first this effect in 1874. Based on the seminal observations by von Bezold, it seems that reducing the inducers’ size and increasing their number (which implies increasing their spatial frequency and density together) will lead to a shift from simultaneous contrast to assimilation. Consequently, it seems clear that the physical variables responsible for this shift are the spatial frequency and the density of the inducers. It is known, for example, that those two variables also affected the size perception [16]. Recently, it was demonstrated that even the perception of beauty is influenced by the spatial frequency [17]. However, the questions that remain open are: at which level of the brain processing are those variables interpreted in this specific way? And how does the visual system transform those variables in the final percept?
Assimilation, Fig. 1

The color assimilation display: the yellow background on the left is perceived reddish while the yellow background on the right is perceived bluish. The two yellows are physically the exact the same hue

Assimilation, Fig. 2

The brightness assimilation display: the gray background on the left is perceived lighter while the gray background on the right is commonly perceived darker The two gray values present the exact same luminance

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 [18]. Specifically, two possible mechanisms have been proposed: a neuronal spatial integration [19] or neural weighted averages [20], 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 [21] 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 [22] 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 [25]. It is also important to note that assimilation received interest based on the White illusion [26]. 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 [29]. 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 [30]. More recently, Soranzo et al. [31] supported the central mechanism explanation for assimilation by testing that in stroboscopic conditions. In 2010 Rude [32] 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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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.Department of Human and Social SciencesUniversity of BergamoBergamoItaly
  2. 2.Developmental Neuropsychology UnitScientific Institute “E. Medea”, Bosisio PariniLeccoItaly