Overview of colour vision deficiency
The first scientific paper on abnormal colour vision was published over 200 years ago in 1794 when John Dalton, the distinguished physicist who pioneered atomic theory, described the effects of his own colour vision deficiency (refs 3, 4).
Whilst Dalton gave a detailed and fascinating account of his experiences with colour, it was not realised at the time that abnormal colour vision was quite common and might present practical problems.
Wilson (ref 5) was the first to report, in 1855, that abnormal colour vision occurred quite often. He was Professor of Chemistry at the University of Edinburgh and noticed that a number of his chemistry students had problems seeing the colour changes in chemical titration. Using a colour vision test he constructed himself, he found that over 5% of his all-male class of chemistry students had defective colour vision. We now know that 8% of males (and 0.4% of females) in Caucasian societies have inherited abnormal colour vision. The prevalence in non-Caucasian societies is often less, especially those that are still hunter-gatherer societies or only one or two generations from that kind of society.
Wilson immediately realised that abnormal colour could be a problem for train drivers who, if they had abnormal colour vision, may not recognise railway signal lights. Colour vision requirements for train drivers were introduced in the 1880s. At about the same time it was realised that colour vision was important for seafarers who had to recognise the colours of lighthouse signals and harbour lights and the red and green port and starboard lights on ships. Colour vision requirements were introduced for seafarers in the 19th century. They were introduced for aviation in 1919. There are also colour vision requirements for many occupations within the armed forces.
First reports of the problem of abnormal colour vision in medicine
The first report of the difficulties caused by colour vision deficiency in the practice of medicine was published quite some time ago, in 1881. Little (ref 35) reported on the difficulties a doctor with abnormal colour vision had with use of the ophthalmoscope. A few years later, in 1883, Jeffries, a physician for a US railway company, (ref 6) reported another doctor who had difficulties with clinical observations due to his deficiency. In 1885 Wilson (ref 5) reported on a doctor who also had difficulties with clinical observations and with colour changes in chemistry.
In 1933, Tocantis and Jones (ref 53) published a careful study of nine medical students out of a class of 70 who made many errors in observing stained bacteria, in titrations, and naming colours seen through with the spectroscope.
In 1951 Ahlenstiel (ref 13), a physician with a red/green deficiency, wrote from his own experience about the clinical signs a doctor should be aware of in his clinical work.
In 1971, Olson (ref 38) found 26 students with colour vision deficiency among 400 studying histology. He observed that they were having difficulties with tissue stains but that they gained help from the use of coloured filters.
In 1980 Voke (ref 56) in her general report on colour vision deficiency in a wide range of occupations reported that doctors and other medical personnel who had abnormal colour vision had similar difficulties. She made the significant comment that the colours that are important to clinical diagnosis are the very colours that are confused by people with abnormal colour vision.
Between 1979 and 1994 four doctors with abnormal colour vision, Seaton (ref 44), Logan (ref 36), Spalding (ref 45), and Currier (ref 23) wrote personal accounts of their own experiences in everyday life and in medical practice. They all made very similar comments and three of them recommended screening of medical students for abnormal colour vision.
Quantitative studies of colour blindness and medicine
Poole et al (ref 39) show that histologists with colour vision deficiency make errors interpreting histopathology slides. This finding has led to debate among histologists about the suitability of those with a deficiency for this kind of work and about their staining methods (ref 22, 28, 42); and Reiss et al. (ref 39) show that the those with a deficiency can fail to see fresh blood in body products.
Two bulletins from the Institute for Safe Medication Practices discuss in some detail aspects of colour coding for medication (ref 30, 31).
Spalding (ref 47) sent a questionnaire to 40 doctors with colour vision deficiency, recruited through a letter in the medical press. The aim was to determine their range of difficulties in work and their attitude to these difficulties. 38 were general practitioners. They were tested for the type and severity of their deficiency either by a visit to their surgery or by attending a university department of optometry and visual science. With anonymity they felt free to describe their difficulties and their ways of coping with them. Those with a moderate or severe deficiency tended to report more difficulties but there were 11 doctors with either a moderate or severe deficiency who said they experienced very few or no difficulties. A further investigation was carried out with 23 of these doctors. They were visited and shown photographs of common clinical signs of disease or normality. They were asked to name the colours of the signs, give their confidence in their answers, and to outline the signs on transparent acetates. Their reponses were compared with those of a control group of 28 doctors with normal colour vision matched for age, sex and occupation. The group of doctors with abnormal colour vision made many errors and had lower confidence in naming the colours compared with the doctors with normal colour vision. The results of these studies are given in four articles by Campbell, Spalding, Mir, and others (refs 17, 18, 19, 20).
An unpublished questionnaire study by Burca (refs 15, 16) of 155 medical students with colour vision deficiency showed that they reported many difficulties in their clinical studies and failed to receive adequate advice about how to cope with them or about their career decisions.
Visual search
One of the difficulties that colour vision deficiency often causes is the inability to find small objects when the background is irregularly patterned or dappled.
Even people with a mild colour vision deficiency perform less well in this than those with normal colour vision when searching for objects that are marked out by colour (ref 7, 9).
Scanning is the way we search a scene or an object for significant features, and colour is often the attribute that draws attention or marks out the target feature. Visual search can done unconsciously (the target object attracts attention) or as a deliberate process (look for a particular characteristic of the target).
Colour is often the characteristic that helps search. Doctors with a colour vision deficiency may have problems noticing critical clinical signs that require scanning when colour is the distinguishing characteristic, as it often is.
Studies of natural scenes, searching for berries against green leaves for example, have confirmed these reports of inability to detect small objects: colour vision deficient individuals consistently perform less well than normal observers in this task even when the deficiency is mild (ref 9). This kind of searching is very similar to that of a doctor searching for a rash or other lesions on a patient. And to quote one doctor with a colour vision deficiency: “the nurse points out the invisible dots” (ref 47).
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