Unique Eye Test
Unique Clinician Testing Method
C Test Design
Two Tritan Borders
About Us C Test
The C test is a spiral bound book containing pseudoisochromatic plates for tritan contrast measurement, arranged in equal difficulty steps from 1 to 10. It enables an ordinal score based on the last correctly identified plate (C score). It should be administered under bright artificial daylight illumination (ideally 800 Lux) to minimise the effect of blue absorption in the human lens.
The normal value in the absence of nuclear cataract is 9, and lower scores are highly suggestive of retinal or optic nerve pathology. A test for both eyes can be completed in less than 90 seconds in most naive observers. To avoid false negative results in the second eye plates 9 and 10 are provided in right and left eye variations.
About The Designers
About Us C Test
Conscious visual perception is mostly constructed from the output of the ‘red’ and ‘green’ cones of the retina. The red/ green signal travels to the visual cortex in ‘parvocellular’ nerves that account for nearly 90% of the visual information from the retina, including the foveal signal. The ‘blue’ cones are mainly distributed around the fovea, and the blue/yellow signal travels to the visual cortex in ‘koniocellular’ nerves that account for less than 8% of the visual information. These signals remain separate in the visual cortex, where parallel signal processing occurs. The output from the visual cortex includes a ‘dorsal stream’ that contributes to an ‘egocentric visual map’ in which objects are positioned as ‘nested volumes.’ It is not known how visual consciousness is generated, but it probably arises beyond this level. When coloured objects appear in visual consciousness they do so with all their secondary properties, including colour, fully integrated into the perception. It is likely that the koniocellular (blue/yellow) signal is blended with the parvocellular signal after a ‘nested volume’ has been positioned into our egocentric visual map.’
Clinical colour testing for blue/yellow (tritan) contrast relies on object location by the parvocellular system, but if the purpose of the test is to minimise the red/green contrast in order to test tritan contrast then it is important to avoid providing clues that would enable an observer to recognise the target using their parvocellular signal. The C test is designed to avoid straight lines, parallel lines, vertical and horizontal borders, and red/green contrast, since those are all features to which the parvocellular system is especially well tuned. It provides two tritan borders within a ring, with an average gap of 2 degrees, corresponding to the maximum density of blue cones in the retina. To further frustrate the parvocellular system the subject is instructed not to stare directly at the page but to move their gaze around the foreground circle in order to encounter the gap dynamically. When the gap is identified it will always be adjacent to one of the surrounding letters, from A to H, allowing a response without touching the page.
Older subjects with nuclear scleroses have less tritan contrast on the retina than young or pseudophakic subjects. To avoid a false positive response due to nuclear sclerosis it is recommended that 800 Lux of artificial daylight illumination be used. Under those viewing conditions all normal observers should detect at least plate 9. If an older subject fails plate 9 they should have a slit lamp examination to exclude blue light absorption by their lens as the cause. A LOCS III grading for nuclear colour (NC) of 4 or more may lead to a reduced score, but NC scores of up 3 do not reduce the C score.
The normal data for the test is available at
(Littlewood R & Hyde F. The “C test” for tritan discrimination. Color research and application 2018;43(1): 58-64. https://doi.org/10.1002/col.22155).
Amblyopia involves suppression of the parvocellular signal at the lateral geniculate nucleus and above, but the koniocellular signal is spared. The C test has been found to be normal in amblyopic eyes with acuity as low as 6/36. A case series confirming this finding is available on line at: http://www.sciencepublishinggroup.com/journal/paperinfo?journalid=230&doi=10.11648/j.ijovs.20180303.12
(Littlewood R. Normal Tritan Discrimination in Amblyopia Suggests Preservation of Koniocellular Function. International Journal of Ophthalmology & Visual Science 2018;3(3):43-46).
Optic neuropathies are often tested with the Ishihara plates. Those plates do not provide an ordinal score, merely a finite probability of pass or fail each plate, but they almost all have a similar level of difficulty from a colorimetric perspective. In contrast the C test generates an ordinal score, and the author and his colleagues have collected many cases in which the C score became abnormal before any other clinically available test in eyes later proven to have toxic or inflammatory optic neuropathies. Future publications are expected.
Littlewood R & Hyde F. The “C test” for tritan discrimination. Color research and application 2018;43(1):58-64. https://doi.org/10.1002/col.22155
Littlewood R. Normal Tritan Discrimination in Amblyopia Suggests Preservation of Koniocellular Function. International Journal of Ophthalmology & Visual Science 2018;3(3):43-46.