Most people know that strawberries are red, lemons are yellow and grass is green. And practically all adults can correctly identify the name of a colour when visually presented with it (allowing for some disagreements based on retinal pigment gene variants – more on that in a later post – and, yes, your wife is right, it’s green, not blue – just accept it).
The ability to recognise colours and remember which one goes with which object seems so trivial that it is hard to appreciate how specialised a skill it is – one that requires a lot of practice and which involves dedicated brain circuits. Children are able to visually discriminate between colours from a very young age and will readily separate objects by colour. However, they learn the names of colours with great difficulty, usually starting around the age of 3. At this stage, they will still frequently misidentify quite dissimilar colours, like red and blue. As they learn more colour names, they will only mix up ones that are more similar, like red and pink. They also have trouble picking out appropriately from inappropriately coloured objects – like picking which banana is the correct colour if shown one blue and one yellow one.
Part of the difficulty with colour is that it is completely unisensory and unlinked to any other information. Colour can’t be cross-checked with another sense in the way that form or texture can, for example. (This may be one reason why colour is such a common part of the “extra” perception in synaesthesia – it can be added to a sound or an odour without conflicting with the primary sensory information). In a way it’s remarkable that we can learn to so accurately categorise light of different wavelengths into specific colours, without any external reference point – after all, those of us without perfect pitch (the vast majority of the population) are not able to do that for musical notes of different frequency.
A rare condition called colour agnosia (or lack of knowledge of colours) sheds some light on how the brain categorises colours and how typical colours come to be attached to objects in our minds. As with other types of agnosia (including prosopagnosia, the lack of knowledge of faces), colour agnosia is characterised by normal processing of sensory information but an inability to categorise and assimilate this information – in essence, people with this condition have lost the concept of colour. For example, they will typically be perfectly able to separate objects by colour but be unable to name the colours, to pick out an example of a particular colour or to group distinct colours into related categories – hues of red, for example. They may, for example, be unable to remember what colour their car is – not just to name it, but also to pick it out of a colour palette. They do not readily incorporate colour into their “schema” of objects – though they may know, semantically, that the word grass and the word green are associated they will not associate the concept of green with the concept of grass.
This condition is all the more amazing for how specific it is – naming and knowledge of other types of stimuli or categories is typically unimpaired and the overall neuropsychological profile is unremarkable. Until a few years ago, only acquired cases of colour agnosia were known – caused by damage to a specific region of the brain, the left occipito-temporal region. This region is in the “ventral stream” of the visual system, where colour information is processed. It sits at a higher level in the hierarchy of processing than regions such as V4, where lesions cause the inability to perceive colour at all.
In 2007, Edward de Haan and colleagues reported a case of developmental colour agnosia. This was a man who showed all the classic deficits of colour agnosia but who claimed to have always had the condition. While he was referred to a neurology clinic following a stroke, this affected an area not involved in colour processing (in the cerebellum). He had otherwise no history of neurological insult or other abnormalities on an MRI scan.
Interestingly, this patient reported that his mother and daughter had the same problem. A follow-up study by the same authors confirmed this – both mother and daughter performed very poorly on selective colour knowledge tasks, while they were perfectly able to distinguish different colours. Now, I know what you’re thinking and you’re right – that might just mean that they never learned their colours in this family. Apart from the inherent implausibility of that idea (given that learning colours is also a part of early formal education) and the fact that the subjects had a full colour term vocabulary, the observation that the subject’s other daughter performed normally on all tasks argues strongly against that explanation. This suggests instead a genetic cause of this developmental form of colour agnosia. The authors speculate that it might involve the wiring of the colour knowledge area in the visual ventral stream.
This has yet to be tested, but if true, would be another example of a situation where altered wiring is thought to explain highly specific differences in the subjective representation of perceptual parameters.
van Zandvoort MJ, Nijboer TC, & de Haan E (2007). Developmental colour agnosia. Cortex; a journal devoted to the study of the nervous system and behavior, 43 (6), 750-7 PMID: 17710826
Nijboer TC, van Zandvoort MJ, & de Haan EH (2007). A familial factor in the development of colour agnosia. Neuropsychologia, 45 (8), 1961-5 PMID: 17337019