What Is Colour Constancy And How Is It Achieved In The Brain - Essay Example

? Colour constancy Colour constancy refers to a segment of the human visual perception mechanism, which allows individuals to distinguish colour under a wide array of light conditions, while at the same time seeing some consistency in that particular colour (Foster, 2011). This is, therefore, a phenomenon whereby the colour of an object remains invariable, irrespective of variations in the light spectrum to which the object is subjected, and hence variation of the light reflected from the item towards the observer. It is imperative to note that light intensity emitted by any wavelength and that, which is reflected from a surface changes constantly based on the illuminant or type of light, in which it is observed (Gilchrist et al., 1999). Colour constancy as an element of humans’ colour perception mechanism was unveiled in the later part of the 20th century. Discovery of this phenomenon was actually by a photographer; a breakthrough possibly attributable to the fact that photography regularly requires both sufficient light and a very high colour awareness. Following the discovery, many investigative studies have attempted to explicate how individuals perceive colours, as well as, how distortion of colour perception can occur. This manuscript seeks to give an insightful analysis of theories proposed to explain colour constancy, while trying to identify the most suitable (Michael, 1995). Under ordinary visual conditions, we usually perceive a particular surface’s colour as being the same. This occurs even if light intensity from one part of the surface to another, thus causing variation in shade. It is rather indubitable that, the world would be an extremely confusing place in the event of surface colours changing with the slightest change in light intensity or wavelength composition. This is because we would be virtually incapable of categorizing colour-affiliated properties in a similar manner (Hansen et al., 2006). Additionally, colours in nature would cease acting like efficient signalling mechanisms thus partially inhibiting biological processes to some extent. There appears to be a consensus among many colour constancy researchers, on the notion that the human brain largely supports this phenomenon. This is because the brain has the capacity of discounting the influence of the recurrent variation, in wavelength composition, of light that gets reflected from a given surface (Lee, 1986). The colour stability, as observed, allows an individual’s brain to acquire knowledge regarding the attributes of that particular surface. This occurs irrespective of the constant changes discerned by the eyes from the surface. Colour constancy is part of yet another large phenomenon referred to as subjective constancy (Kraft & Brainard, 1999). This is a phenomenon adopted by the human brain to enable individuals perceive certain objects even in varying situations. Subjective constancy makes sure that people can easily recognize objects, thus assisting with understanding of the world, a crucial aspect for safety. For instance, an individual’s ability to recognize specific shapes may help him or her in avoiding hazards, and the capability to compensate for space while observing a view can also be vital. In addition, subjective constancy makes it possible for individuals to recognize and connect thematic elements. An excellent illustration of the significance of subjective constancy is when people identify a given art work owing to its familiar and distinctive features. The extensive description of subjective constancy and its importance simply seeks to emphasize the eyes’ and brain’s role even in colour constancy. This is best explained by the actual perception process, which asserts that colour constancy utilizes input from different retinal cone cells. These cones usually depict sensitivity to changing light wavelengths. Further, the cells’ collective data is transmitted to the brain, where it is processed to establish the colours that an individual is viewing. This gives the implication that colours are greatly influenced by the available light wavelengths, as well as, surrounding colours, thus explaining why an object’s colour could look completely different, based on the item placed adjacent to it. Numerous examples used to illustrate colour tricks and colour constancy utilize a colour grid referred to as a Mondrian. The grid comprises of several squares, which an experimenter uses while manipulating levels of light with the intention of examining how research subjects perceive colours in these squares (Wandell, 1995). For instance, an orange square could seem red under a certain light wavelength, while squares bearing a similar colour could appear dissimilar, based on the colours borne by squares around. A real life example is that of a banana, which appears yellow under sunny conditions, candle, as well as, fluorescent light. In each of the mentioned conditions, light shed on the banana varies significantly, and despite the fact that a banana reflects varying wavelength compositions under diverse circumstances, human minds still attempt to establish the banana’s essence. This is because our yellow perception ingrained in the mind, asserts that a banana reflects a great degree of yellow, although based on the source of light source, extra green or orange colour may actually get to our eyes. The colour constancy concept is also easy to understand through examination of photography and its constituents. For instance, our eyes have the capacity to interpret an assortment of colours under different light conditions, while cameras cannot do the same. It is due to this technical limitation that cameras usually have white balance settings, which are selected properly on the basis of the available type of light. This capability is delegated to human capacity to comprehend colour, thus ensuring digital images are accurate in regard to colour processing. It is simply astounding that human brains are capable of discerning an object’s colour under varying types of light. With everything under consideration, colour perception is crucial in individuals’ daily lives, thus placing people suffering from colour blindness at a disadvantage. This is because they lack the capacity to appropriately distinguish objects around them. Since the discovery of colour constancy, different researchers have put forth several theories, trying to explain where processes governing this phenomenon take place. The most prevalent theories assert that colour constancy occurs in retinal cells, cortical cells or both, all in relation to the brain. In consideration of the retina theory, it is imperative to first understand the retina’s structural composition. This light sensitive eye membrane comprises of photoreceptors, that is, cones and rods. These light receptors usually detect light, hence initiating transmission of messages to an individual’s brain. Colour contrast and light adaptation are the principal phenomena, which explain constancy of colour at the retinal point. With regard to adaptation of an individual to light, Kraft, Maloney & Brainard (2002) put forth the notion that adjustment mechanics in the retina are as a result of colour constancy. This implies that the visual mechanism adapts its sensitivity to intensity of lighting. For example, in surroundings high levels of red light, retinal L-cones usually adapt better than the S- and M-cones. Adaptation causes sensitivity reduction in retinal L-cones, as compared to S- and M-cones, thus causing colour constancy. According to (Werner, & Walraven, 1982) colour contrast, is yet another significant aspect encouraging colour constancy. This is because, when the type of light varies, the light wavelength from an object’s surface, as well as, that of the object’s background also changes. However, the ratio of their total spectral reflection remains constant. Consequently, the proportion of retinal cone responses also stays roughly unchanged. This means that the programmed colour of the object’s surface does not vary, thus achieving colour constancy (Li, & Gilchrist, 1999). Another concept explaining colour constancy, and one showing how constancy is achieved in the brain is the cortical theory. Cortical colour processing of information begins with the photoreceptors in the eyes (Hannah, Smithson & Qasim, 2004). The photoreceptors then transport information through the ganglion cells via the optic nerve, thus making the optic radiation flow to the lateral geniculate nucleus (LGN). The LGN further projects the photoreceptors to the striate cortex which is generally known as area V1. The area which surrounds area V1 is referred to as the extra-striate cortex and it comprises of many areas governing interpretation of visual information these include areas; V2, V3, V4, and V5. However, most of the studies revolving around the cortical theory of colour constancy seem to focus on area V4, as the location in the brain where colour constancy takes place. Adelson, (1999), proposed three cortical phases of processing colour, which occur in the human brain. The first stage is generally concerned with wavelength detection and this takes place in the V1 and V2 areas, while the second phase constitutes operations dealing with automatic colour constancy located in area V4. The third colour processing stage occurs in the inferior temporal, as well as, the brain’s frontal cortex. This third stage principally interprets information concerning colours on objects. In further support of this notion, Brainard (2003) made the assertion that, there are numerous accounts of people whose colour constancy systems have failed owing to brain injuries affecting the mentioned brain parts. However, the damage to these parts of the brain does not necessarily have to be as a result of an injury. This is because some colour blind people also show limited development of colour constancy controlling brains parts. Cortex cells damage or underdevelopment may explain why some people cannot be able to distinguish colours or even identify the colour of a certain object. Gilchrist et al. (1999) primarily support the cortical theory of colour constancy by further explicating the role of area V4. This is achieved by recording responses of V4 colour neurons to colour stimuli. In their experiment, these authors studied anesthetized monkeys, in order to examine their sensitivity to colour hence chromaticity tuning, during waking. The researchers illuminated a banana with varying light wavelengths, and observed the monkeys’ reaction when they woke up and saw the banana. They found out that the majority of the monkeys’ colour selective cells, in the ventral part of V4 area modulated their arousal or physiological activity rate (Shevell & Kingdom, 2008). This experiment clearly showed the psychophysical colour constancy effects, because the monkeys could notice the colour of a banana immediately but they did not respond to the diverse wavelengths. In fact, the monkeys were selective as shown by their capacity to choose the ripe banana, by successfully distinguishing it from a green banana. This experiment was repeated with human beings and the inferences were the same as those obtained while studying the V4 area of anesthetized animals (Rutherford & Brainard, 2002). Contrastingly, (2003) demonstrated that, V1 neurons in the monkeys known as macaques got strongly influenced by chromatic spatial integration. This was an aspect evident from the monkeys’ reactions towards colour contrast and consequently to colour constancy signals. Additional support for the cortical theory contribution to colour constancy is apparent from studies conducted by (Hansen, Gegenfurtner, Walter and Olkkonen 2006; Olkkonen, Hansen and Gegenfurtner 2008; Ling and Hurlbert, 2008). In all these studies the authors investigated colour memory in monkeys. The studies indicated that, the knowledge of an object's actual colour can be affected by a person’s perceived colour of the same object. An experiment to prove that memory actually affects an individual’s perception of an object’s colour was conducted by Hansen et al. (2006). The experiment utilized natural fruit items in investigating colour memory. The research subjects were supposed to adjust the fruit items colour until the object appeared to be gray. Results of the experimental study showed that, the participants perceived objects as being gray, when the object got adjusted in the direction opposite from the perceived or known typical colour. This implies that subjects perceived the fruits to be in their typical colour, while in actual sense they were achromatic. Therefore, the results explicitly showed that people tend to perceive natural fruit items in their typical colour, via object-colour knowledge, or memory, rather than actual observation. This is a clear indicator that colour memory has immense influence of colour constancy. Another study conducted by Gegenfurtner et al. (2008), also utilizing natural fruit items concluded that the strength of colour memory largely depends on the degree of naturalness of the stimulus. The perception or memory for colour was found to be the largest influence among most natural stimuli, when the item is in its typical colour. However, the colour perception decreased with diminishing stimulus realism. Even though colour memory plays a huge role in attaining colour constancy, Hurlbert et al. (2008), found out that colour perception/memory was unsatisfactory especially in situations where there is no change in illumination. In such events, the degree, as well as, direction of colour constancy shifted depending on an individual’s memory. This gives the implication that if an individual’s memory is higher, then colour constancy is equally high and the converse is also true. Up to this point, the theories suggested to explain colour constancy largely focus on retinal and cortical processes. For this reason, there is a high possibility that both of these processes are linked to colour constancy, this time not singly, but at the same time. The reason scholars suggest this concept, in the attempt to elucidate the colour constancy phenomenon, is uncertainty associated with the process responsible for colour construction. As a result, researchers like Moutoussis and Zeki (2000), proposed that the Retinex-theory best explains colour constancy, by combining the roles of both the cortex and the retina. The first phase of the combined mechanism involves chromatic spatial incorporation, while the second stage takes place during convergence of input from a person’s two eyes, most probably in the V4 area. Overall, the process is involved in generation of colour perception hence colour constancy. The other potential constancy explanatory concept is that of colour theory, which is generally presumed to form the human construct in colours. This theory is founded on the notion that, people need to have knowledge of defining colours, categorizing them in an orderly manner, in addition to adjusting and relating them to each other, in order to develop other colours. The colour theory endeavours to bond together facts held by individuals, in order to provide a common ground while analyzing and using colours. The early theories of colour were not as perfect. However they explained how the brain perceives colour. For instance, it can be deduced that colour is a perception, or a response from the brain after a person has seen a particular object. Therefore, it is justifiable to say that just like artificial flavours bring to mind alike smell in real foods or, just like artificial sugar arouses people’s sense of sweetness, this is the same way that diverse combinations of light are perceived to be the same "colour", thus explaining constancy. Constancy perception can be an inconsequential issue if variations in illumination are caused by small disparities in the spectral allotment of light that reaches an individual’s eye. However, the opposite of this phenomenon is true and therefore, if an object colour depended solely on the light that is reflected to or from it, the shifts created can be dramatic (Adelson, 1999). In conclusion, the comprehension of human colour constancy has progressed considerably from the time it was discovered in the late 20th century. It is evident that, the theoretical prerequisites for constancy have been defined in a better manner; and the nature of visual findings clarified. Additionally, the assortments of experimental techniques have also increased significantly. Novel invariant properties of objects, as well as, a wide array of neural visual interpretation have also been identified. From the analysis it is apparent that no single theory exhaustively explains colour constancy. 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