Metropsis: cambridge colour test

The Cambridge Colour Test provides a rapid means of screening subjects for colour vision deficiencies; but it also can be used to examine in more detail the changes in colour discrimination that occur as a result of congenital or acquired conditions. It allows the investigator to monitor quantitatively over time the progression or remission of disease. Many drugs affect colour vision and the pharmacologist will find the test well suited to monitoring the short-term or long-term course of such side-effects.

The test determines discrimination ellipses in colour deficient subjects by probing chromatic sensitivity along the colour confusion lines. Ellipses measured in individuals with even slightly anomalous colour vision are characteristically orientated and enlarged (see references).

Easy to use

The procedure is easy to use for both the investigator and the subject. It uses the familiar Landolt C stimulus, defined by the two test colours that are to be discriminated, on an achromatic background.

The Cambridge Colour test uses the proven concept of introducing spatial and luminance noise into the stimulus, which is composed of grouped circles randomly varying in diameter and having no spatial structure. The Landolt C is therefore defined by chromaticity alone, ensuring that the subject's responses are not due to luminance or spatial cues in the stimulus, and thus avoiding the necessity for a preliminary procedure to find isoluminance for the test colours.

The test is conceptually very simple to explain to the subject, who responds to the orientation of the Landolt C using the 4-button infra-red response box. The chromaticity of the components of the C is varied along the protan, deutan and tritan lines (or other chromatic axes if desired) using a standard descending psychophysical staircase procedure.

The importance of stimulus resolution

The 8 bit output resolution of conventional computer graphics systems offer too coarse a resolution for measuring the limits of human colour discrimination. The stimuli for all Metropsis tests are produced by a dedicated DVP-14, which provides 14 bit colour and luminance control and is calibrated using a ColorCAL. Cambridge Colour Test can therefore provide a finely graded measurement of discrimination and allows small changes to be monitored over time or in different experimental conditions.

Proven

The Cambridge Colour Test is an established method and has been widely used for both clinical and basic research including an extensive normative data set. (see references).

Results

Results are saved in ASCII format and presented graphically as discrimination ellipses in CIELUV or CIE(x,y) colour space. The results shown are typical of a subject with normal colour vision; in deficient subjects the discrimination ellipses are significantly extended in the protan, deutan or tritan chromaticity directions.

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