Cognitive controls and mental retardation

Inhibition deficits in individuals with intellectual disability: a meta-regression analysis

BACKGROUND

Individuals with intellectual disabilities (ID) are characterised by inhibition deficits; however, the magnitude of these deficits is still subject to debate. This meta-analytic study therefore has two aims: first to assess the magnitude of inhibition deficits in ID, and second to investigate inhibition type, age, IQ and the presence/absence of comorbid problems as potential moderators of effect sizes.

METHOD

Twenty-eight effect sizes comparing ID and age matched normal controls on inhibition tasks were included in a random effects meta-regression. Moderators were age, IQ, inhibition type and presence/absence of comorbid disorder.

RESULTS

The analysis showed a medium to large inhibition deficit in ID. Inhibition type significantly moderated effect size, whereas age and comorbid disorder did not. IQ significantly moderated effect size indicating increasing effect size with decreasing IQ, but only in studies that included a sample of ID participants with mean IQ > 70. The analysis indicated comparable deficits in behavioural inhibition and interference control, but no significant deficits in cognitive inhibition and motivational inhibition.

CONCLUSIONS

These results indicate that ID is characterised by a medium to large inhibition deficit in individuals with ID. ID seems not to be characterised by deficits in cognitive and motivational inhibition, which might indicate that distinct processes underlie distinct inhibition capacities.

Alternative to the Stroop Color-Word Test for illiterate individuals

An alternative to the Stroop Color-Word Test (SCWT), denominated the Colored Numbers Test (CNT), was developed to evaluate the selective attention of illiterate individuals. A total of 30 volunteers with basic education (control group) and 30 illiterate volunteers (experimental group) performed the SCWT and the CNT. Volunteers had to name the color of the rectangles in the CNT neutral condition, and in the critical condition they had to either name the color of the numbers or, when the numbers were black, read the numbers. An interference index (II) was calculated for both tests by subtracting the time taken to complete the task in the neutral condition from the time taken to complete the task in the critical condition. The control group showed an II of 14.9 s in the SCWT and of 19.1 s in the CNT, and the experimental group, which practically presented no interference in the SCWT (II = 0.2 s), showed an II of 18.7 s in the CNT. These findings suggest that the CNT can be used to evaluate selective attention. Further work should confirm its validity. Its advantage over the SCWT is that it does not depend on the ability to read words, being then suitable for illiterate individuals.