28 May 2019
Could colour blindness be affecting the results of your study?
With colour vision deficiency affecting an estimated 300 million people worldwide, we consider how colour blindness might affect participants’ performance on cognitive tests, and what measures can be taken to minimise the impact on study results.
What is colour blindness?
Colour blindness, or colour vision deficiency, is a heterogeneous condition characterised by a decreased ability to perceive colours. It is rare that those diagnosed with a colour vision deficiency cannot observe colour at all: most just have difficulty seeing the full spectrum of colours.
What causes colour blindness?
Humans have three types of photopigment (light-absorbing molecules), which are each sensitive to different wavelengths of light and are located in corresponding cone cells in the retina of the eye: short, medium and long-wavelength cones detect blue, green and red respectively. The full spectrum of colours perceivable by humans is produced by mixing these three cone colours in differing proportions, because colour perception is a consequence of unequal stimulation of the three cone types. Thus, normal vision is described as ‘trichromatic’.
A colour vision deficiency occurs when there are abnormalities in cone cells, causing them to not respond correctly to the variations in wavelengths of light. These abnormalities can occur in any of the three cone types. Red-green colour blindness is a collective term for abnormality in either the red or the green cones, and blue-yellow colour blindness occurs because of abnormality in blue cones.
Generally, colour blindness is inherited, although it can also be acquired by physical or chemical damage to the eye, optic nerve or vision-processing brain areas.
What are the different types of colour blindness?
There are three categories of colour blindness:
1. Anomalous trichromatism
Anomalous trichromatism a mild form of colour blindness, caused by an abnormality in the function of one type of cone cell whereby its peak sensitivity is shifted, so people are able to see the full spectrum of colours but with the wavelengths in different proportions to normal trichromats. Protanomaly occurs when red cones function abnormally, deuteranomaly is the result of green cones functioning abnormally, and tritanomaly is abnormal functioning of blue cones.
Dichromatism is caused by the complete lack of function of one type of cone cell, either due to a mutation that causes it to fail, or an absence of that cone cell type altogether. The full spectrum of colours perceivable by somebody diagnosed with dichromatism can be achieved by mixing two of the base colours. Protanopia is the consequence of a lack of functioning red cones, deuteranopia is a lack of green cones, and tritanopia a lack of blue cones.
Monochromatism is caused by abnormality or failure of two or more cone cell types, so people with monochromacy do not experience a colour spectrum at all. Cone monochromacy occurs when two of the cone cell types are absent or not functional, so everything is perceived as a shade of the remaining cone cell type. A person diagnosed with achromatopsia has no functional cone cell types, so they see the world in black, white and grey.
Here is an example of how our DMS stimuli may appear to people with various types of colour vision deficiency.
What are the key considerations for colour blindness in cognitive testing?
Colours are often a feature of cognitive tasks, so colour blindness could have an impact on participants’ performance. For example, colour images have been shown to have a positive influence on memory performance versus black and white images, so people with colour vision deficiencies may perform worse on memory tasks which use colour than someone with intact vision, although their actual memory capabilities are equivalent. This could result in masking or inflation of true group differences.
Colour vision deficiency is particularly important to consider in populations that are likely to have:
- a diagnosis of diabetes, glaucoma, age-related macular degeneration, multiple sclerosis, Alzheimer’s disease, or Parkinson’s disease.
- medications such as ethambutol, chloroquine, hydroxychloroquine, digoxin, phenytoin, carbamazepine, tamoxifen, sildenafil, vardenafil or tadalafil.
- been exposed to organic solvents, including carbon disulphide, perchloroethylene, n-hexane and styrene.
- a history of alcoholism and/or cocaine use, which have both been linked to blue-yellow colour vision impairment,.
- a high number of males, as red-green colour vision deficiencies are much more common in males than females.
What are the recommendations for colour blindness in CANTAB testing?
This blog post is intended to be a resource for general information and guidance. Due to the heterogeneity in colour vision deficiency, it is ultimately the researcher’s responsibility to determine whether tasks are suitable for their participants. To support with this, here are our recommendations when using CANTAB:
- Tasks where colour is not a variable, and therefore performance should be unaffected for participants with colour vision deficiencies, are MTT, SST, ERT, EBT and VRM.
- Tasks where performance is dependent on participants’ ability to distinguish particular pairs of colours from one another, and therefore are not advised for those with a colour vision deficiency that affects the colours important to each task, include CGT, SSP, RTI, OTS, and SOC.
- Tasks where colour is a feature, but the ability to distinguish colours from one another is not a requirement, are as follows: MOT, RVP, PAL, DMS, PRM, SWM and IED. People with colour vision deficiency should be able to complete these tasks, but their performance may be affected. We would recommend either screening for colour blindness relevant to each task before testing, or noting any colour vision deficiency in performance observations.
For more information on what colours appear in each task, please see below or contact our team.
Which colours are used in CANTAB Connect task variants?
We have put together a guideline of the colours used in the CANTAB task. All tasks use a black background, so participants must be able to distinguish all other featured colours from black.
If you are concerned about colour blindness, or interested in finding out more, please refer to the following groups for further information:
If you have questions regarding CANTAB tasks and colour blindness, please contact our customer support team here.
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Tags : colour
Millie Lowther, Operational Scientist