An estimated 2% of the world population has an intellectual disability (ID) that could have lifelong effects on academic learning, personal functioning, and independence. Researchers have attempted to develop various treatments and therapies for this population with inconsistent results and data. “There are challenges in finding reliable, valid, and functional tests for this population,” explains David Hessl, PhD. “Researchers have been picking and choosing various tests, making it hard to compare results across labs and studies.” To standardize testing for patients with ID in neurologic or psychiatric conditions on scalable, psychometrically supported, and broadly accepted endpoints, Dr. Hessl, lead author Rebecca Shields, and colleagues sought to validate whether the NIH Toolbox Cognitive Battery (NIHTB-CB) could fill the void in testing.

Analyzing the Tests

The NIHTB-CB is an iPad-based battery of brief memory, executive function, processing speed, and language tests that could provide standardized, objective, and scalable results for clinical trials and research focused on people with ID. The battery includes seven tests:

  1. Dimensional Change Card Sort (DCCS)
  2. Flanker Inhibitory Control and Attention (Flanker)
  3. List Sorting Working Memory (LS)
  4. Pattern Comparison Processing Speed (PC)
  5. Picture Sequence Memory (PSM)
  6. Picture Vocabulary (PVT)
  7. Oral Reading Recognition (OR)

Dr. Hessl and colleagues’ study, published in Neurology, was an extension of a much smaller pilot study to refine and validate the NIHTB-CB in standardized administrative guidelines, with the goal of examining its potential use for outcome measures in future clinical trials and other research. The investigators focused on the performance of the NIHTB-CB in children and young adults with Down syndrome (DS) and fragile X syndrome (FXS), and other causes of ID (OID). Participants completed initial neuropsychological testing using the NIHTB-CB and recompleted the same tests a month later to examine test reliability. To improve reliability and validity for participants with ID, the team developed a manual of standardized accommodations and other procedures regarding the testing environment.

Taking a Closer Look

To understand whether the NIHTB-CB was sensitive to syndrome-specific cognitive profiles, the study team conducted a two-way mixed model analysis. There was an effect of group on NIHTB-CB z scores, along with an effect of test on z scores. Scores suggested that participants with FXS did not perform as well on the DCCS test as participants with OID. Similarly, participants with FXS performed worse on the Flanker test than did participants with OID or DS. These results supported the team’s hypothesis of executive function impairment in participants with FXS. “Group comparisons demonstrated that the NIHTB-CB is sensitive to substantial executive function deficits among individuals with ID in that all three groups performed relatively poorly on Flanker and DCCS,” emphasizes Dr. Hessl.

The study team examined the relative feasibility of different NIHTB-CB tests across groups (Table). Data indicated that participant feasibility was similar to the normative sample (typically developing children aged 3 to 15), with DCCS, PC, and LS having lower feasibility and Flanker, PSM, PVT, and OR having similar or higher feasibility rates than the normative sample. “We found that the majority of people with a mental age of at least 4 years could provide valid test results, with all tests being feasible for almost all participants with a mental age of 6 or older,” notes Dr. Hessl.

Future Use

Dr. Hessl acknowledges that, so far, the NIHTB-CB has been used almost exclusively for research. However, he believes that as the research evidence base grows, the battery might be used by clinical providers such as neuropsychologists, special educators, or school psychologists to track the developmental progress of their patients and students.

“Many people assume that children and adults with ID are not testable or that test results are not reliable or representative,” says Dr. Hessl. “This really is not true in most cases, so long as care is taken to design and validate appropriate tests and accommodations targeted for their level of ability.” NIHTB-CB developers continue to refine and improve the system. A key focus of these improvements is refining the tests to be feasible and valid for lower-functioning or younger individuals, even as low as a mental age of 1.