Colour vision is a secondary property of objects of perception, which means that if there is no viewer there is no colour – only radiance. Our perception of any one ‘pixel’ within an extended optical image is shaped by a ‘spatial gestalt’ that integrates the spectral composition of surrounding pixels rather then representing each pixel independently. It is also shaped by a ‘temporal gestalt’ that integrates stimulus changes over time, including hue, saturation, and brightness. More interestingly, visual perception is also shaped by a ‘logical gestalt’ that constrains visual perception to within a logically coherent domain shaped by our understanding of reality. This latter claim is supported by Edward Land’s retinex experiments in which he showed that colour perceptions were not fully determined by the spectral reflectance of objects but were constrained by object recognition within a background context.  Another good example is Adelson’s checkerboard shadow  illusion ( These psychological observations are strong evidence that colour perceptions arise from top down processes, or mental constructs. Evolution equipped us with the capacity to detect spectral contrast since that is highly adaptive for object recognition in a world where shadows confound achromatic edge recognition. A primate ancestor viewing a tiger illuminated in shadow may not have detected the predator if the retinal image was processed as shades of grey. The successful evolution of humans was largely dependant on our unusual capacity (amongst mammals) for spectral contrast detection. That is not the same thing as colour consciousness, and somewhat counter-intuitively, evolutionary pressure drove spectral contrast detection prior to and probably independently of our later mental trick of coding spectral perceptions as colour categories. The ‘idea’ of colour supervened on spectral contrast patterns.

Common language references to colour, such as ‘the grass is green,’ are linguistic constructs that imply correspondence with reality. This a priori assumption is called naive realism, and to deny it is to render communication inefficient, but  scientific enquiry and logical discourse frequently find it necessary to deny it, since it is often demonstrably false.

Sometimes a common language reference is appropriated by a discipline and redefined as a ‘term of craft,’ a process that refines linguistic reference so it coheres more closely with advancing knowledge. In the 19th century Thomas Young, James Clerk Maxwell, and Hermann von Helmholz all contributed to a physiologically advanced theory of colour perception called trichromacy. This theory challenged the naive assumption of ‘colour realism’ – the determination of colour perception by inherent object properties outside the observer. That idea was replaced with the idea that because observers can only perceive objects through contrast between their three spectral processing streams, the observer themself is producing ‘colours’ out of  spectral responses to light energy. Linguistic reference to colour is now somewhat like a term of craft in that it denotes a more metaphysically nuanced idea than naive realism.  The metaphysical status of colour perceptions was updated from ‘realism’ to ‘physicalism,’ which is the view that colour is the end result of an unbroken chain of bottom up processes, even if those processes are physiologically complex and vary from one observer to another. 

But there was still a problem.  Bottom up processes enable simultaneous contrast, hue matching, and categorical distinctions, but bottom up processing does not account for Edward Land’s findings or explain some the infamous controversy of the coloured dress (   

Once the perception of colour evolved we developed cultural uses for it. Anthropological comparisons between colour naming in different cultures were studied by Berlin and Kay, and revealed a relationship between colour vocabulary and culture. Colour theory can be understood from a variety of perspectives – semiotics, aesthetics, psychology, physiology, physics, engineering, and art – to name the most obvious. Since practitioners in the various disciplines sometimes mean different things by ‘colour’ those who approach the subject from a medical perspective need to be rather careful how they express their opinions, since given our limited linguistic resources, universally true statements are not even possible.

The button below will download a pdf copy of an article describing the correct use of the Ishihara plate test


Amblyopia involves functional suppression of the parvocellular signal at the lateral geniculate nucleus but the koniocellular signal, as measured by the C test, is preserved. The article below describes the use of the C test in amblyopia

Littlewood R. Normal Tritan Discrimination in Amblyopia Suggests Preservation of Koniocellular Function. International Journal of Ophthalmology & Visual Science 2018;3(3):43-46. 

The button below will download a pdf copy of an essay entitled “The history and physiology of human tritan perception.” This discussed the evolution of human colour vision in a narrative style that non scientists may find accessible. 

The button below will open a you tube video link to the 2021 Perth Eye Foundation lecture. In this talk I attempt to clarify some metaphysical confusion, briefly discuss the evolution of human colour vision, and provide arguments in favour for B/Y instead of R/G colour testing in acquired visual disorders.  

Clinical colour testing for tritan contrast is important in any acquired colour vision disorder. Of the tritan contrast plates presently available only the C Test isolates the koniocellular signal from the parvocellular signal sufficiently to provide a dominantly tritan response. It also provides an ordinal score, which is useful in monitoring acquired colour vision defects. To learn more about the C Test or download a pdf visit the C Test page on this website via the menu link above.

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