17 January 2023

How CANTAB can be used to overcome challenges in assessing cognition in people with intellectual disabilities

Operational scientist Eleanor Rist explores how CANTAB™ assessments can be beneficial for studies involving people with intellectual disabilities.

Intellectual disabilities, often referred to as learning disabilities in the UK, are generally defined as having an IQ of less than 70 and having significant impairment in social or adaptive functioning1. Intellectual disabilities have a wide variety of causes which can largely be grouped by time of onset:


  • Genetic causes, e.g. Down’s syndrome, Fragile X syndrome, Prader-Willi syndrome.
  • Maternal infection.
  • Exposure to environmental toxins in utero, e.g. Foetal Alcohol Spectrum disorders.


  • Cerebral Palsy.


  • Infection (e.g. meningitis).

During cognitive assessments, it is vital that participants can perform at their true ability so that results are accurate and valid. This is arguably particularly important in studies involving intellectually impaired populations, where within-group comparisons are more frequent than between-group comparisons, for example, measuring cognitive decline in people with Down’s syndrome to predict onset of Alzheimer’s disease. In examples like these, a valid baseline needs to be established so an individual’s progress or decline can be charted. There are unique challenges that need to be considered when assessing people with intellectual disabilities so the process can be more accessible to them and to ensure they are performing at their true ability.

Key considerations for cognitive testing in intellectually disabled populations

Task non-comprehension: Arguably the biggest challenge is ensuring that participants understand how to complete a task. Depending on the severity of impairment, task non-comprehension is an outcome in itself, but it is important to give participants every opportunity to complete a task if they’re able to.

Task non-completion: Due to issues with attention and frustration, intellectually disabled people are more likely to have their attention wane during cognitive testing, as they could potentially find it boring. Feelings of task non-comprehension or subjective assessment of low performance coupled with boredom can lead to frustration and non-compliance. Digital tasks such as CANTAB™ may feel more ‘game-like’ than traditional pen-and-paper tests and so may be less tedious to participants.

Variability in cognitive impairment: The degree and nature of cognitive impairment in these populations is highly variable and variation exists between populations, between individuals within populations, and even within individuals. This variability poses a challenge in cognitive assessment as some tests are insufficiently sensitive to these different extents of impairment. CANTAB™ tasks are well-suited to overcome this challenge due to the scaling difficulty that is at the core of many tasks. The Paired Associates Learning task (PAL), which is extensively used and well-validated in intellectually impaired populations such as Down’s syndrome, features scaling difficulty. This aspect of PAL and other CANTAB tasks allows for the minimisation of floor effects which can often be observed when assessing these populations.


The innate variability of intellectual disability between populations and between individuals means that the ultimate responsibility falls on researchers to determine whether a testing protocol is suitable for their participants. This list is intended to provide guidance for doing so.

Remote testing is not suitable

It is imperative that cognitive assessment of intellectually disabled people is carried out in-person, with a rater in the room. Ideally, the rater should have prior experience working with intellectually impaired populations, which may be beneficial in providing the most supportive environment for testing. Whilst this may seem to limit the scope of some studies, it has myriad benefits for improving participant completion, as well as the accuracy of data collected.

The ultimate benefit to carrying out in-person testing is task comprehension. The rater can physically show participants how to complete the task, which can be more effective than telling them. This format also allows for more informal learning sessions, allowing participants to attempt a task unassessed as many times as is needed for them to understand the instructions. Remote assessment would only allow for one practice round before assessment, which could mean that only participants with mild impairments would be able to perform to their true ability. In this way, in-person assessment helps to avoid floor effects.

In addition, the rater can make notes on participant performance throughout. Despite repeated opportunities for explanation and practice, it is inevitable that some participants simply won’t understand how to complete a task. Sometimes in these cases the participant will make correct responses by chance and therefore provide invalid data. This can only be picked up by a rater in the room. In-person testing means that participant non-comprehension is a valid outcome in its own right, not a by-product of inaccessible assessment delivery. As well as notes on participant non-comprehension, raters can monitor participant attention and frustration and allow participants to take breaks as and when they’re needed, instead of conforming to a prescribed schedule.

When and where to test?

It is important to consider the location of testing. Being introduced to a new environment and being expected to complete cognitive tests could be stressful for some participants, making them less likely to enjoy the testing process and perform to their true ability. If it is possible, testing participants in their homes (e.g. Startin et al. 20162) or schools if assessing paediatric populations (e.g. Danielsson et al. 20113), can help reduce this stress. CANTAB™ tests on iPads are portable by nature, making this an easier option. If testing at home is not possible, it’s advised to let participants adjust to the environment they’ll be tested in, possibly by visiting before the assessment session.

Individuals in some populations, such as adults with Down’s syndrome and prodromal Alzheimer’s, can be affected by sundowning. It could be advisable to ask a participant or a carer (if applicable) at what time of day they tend to feel their best, and consistently testing at this time. It’s worth noting that the effects of time of day can be highly variable between populations and individuals.

Tasks and batteries

CANTAB™ tasks vary in both length and complexity, and it’s advisable when testing intellectually impaired populations to keep batteries as short as possible and to use shorter, simpler tests. For example, there is a precedent in Down’s syndrome research2, 4 for assessing attention using standard deviation of reaction time from the Reaction Time (RTI) task, instead of using the Rapid Visual Information Processing (RVP) task, which is over twice as long. When assessing populations with mild intellectual impairments, the shorter 1-task variant of RVP is another possibility.

CANTAB™ language is simple and clear by design and the symbol and visual display of CANTAB™ tasks helps to reduce language dependency. In some cases, if needed, language can be simplified. For example, Sinai (2016) changed wording in the Intra-Extra Dimensional Shift task (IED) from “correct” to “right”5.

Tasks that allow for mid-test failure, like PAL, are particularly useful in intellectually impaired populations as they reduce floor effects and allow participants to stop a test mid-way through if they grow frustrated.

Cambridge Cognition recommends the following tasks for use in assessment of individuals with Down’s syndrome, which may translate to other intellectually impaired populations:

Motor Screening Task (MOT): 2 minutes
Paired Associates Learning (PAL): 8 minutes
Reaction Time (RTI): 3 minutes
Multitasking Test (MTT): 8 minutes

Key takeaways:

  • Intellectually disabled people should be assessed with a rater in the room, not remotely
  • Extensive learning sessions may be necessary to give participants a fair chance to perform at their true ability
  • Consider how the location and time of day of testing could be altered to improve accessibility
  • Adjust for participant fatigue and frustration by choosing shorter tasks and providing as many breaks as are required

If you have questions regarding CANTAB™ tests and their use in intellectually impaired populations, please contact our customer support team.


1. National Institute for Health and Care Excellence

2. Startin, C. M., Hamburg, S., Hithersay, R., Davies, A., Rodger, E., Aggarwal, N., … & Strydom, A. (2016). The LonDownS adult cognitive assessment to study cognitive abilities and decline in Down syndrome. Wellcome open research, 1.

3. Danielsson, H., Henry, L., Messer, D., & Rönnberg, J. (2012). Strengths and weaknesses in executive functioning in children with intellectual disability. Research in developmental disabilities, 33(2), 600-607.

4. D’Souza, H., Mason, L., Mok, K. Y., Startin, C. M., Hamburg, S., Hithersay, R., … & Thomas, M. S. (2020). Differential associations of apolipoprotein E ε4 genotype with attentional abilities across the life span of individuals with Down syndrome. JAMA network open, 3(9), e2018221-e2018221.

5. Sinai, A., Hassiotis, A., Rantell, K., & Strydom, A. (2016). Assessing specific cognitive deficits associated with dementia in older adults with Down syndrome: Use and validity of the Arizona Cognitive Test Battery (ACTB). PloS one, 11(5), e0153917.

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Eleanor Rist

Operational Scientist

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