Introduction: Among the executive functions affected in attention deficit hyperactivity disorder (ADHD), inhibitory control is one of the primary areas of impairment, characterized by components that include response inhibition and interference control. Determining the impaired inhibitory control components will be useful in the differential diagnosis and treatment of ADHD. The present study aimed to investigate response inhibition and interference control abilities of adults with ADHD.
Methods: The study included 42 adults diagnosed with ADHD and 43 healthy controls. The stop-signal task (SST) and Stroop test were used for assessing the response inhibition and interference control, respectively. Multivariate analysis of covariance was used for comparing the ADHD and healthy control groups in terms of their SST and Stroop test scores, wherein the age and education level of the participants were taken as covariables. The relationship between SST and Stroop Test and Barratt Impulsiveness Scale-11 (BIS-11) was tested by Pearson correlation analysis. Mann-Whitney U test was used for comparing the test scores between those who were administered with psychostimulants among the adults with ADHD and those who were not.
Results: Response inhibition was observed to be impaired in adults with ADHD compared with the healthy controls, whereas no difference regarding interference control was observed. As per the Barratt Impulsiveness Scale-11 (BIS-11), a weak and moderately negative relationship was found between the stop signal delay and the attentional, motor, non-planning scores, and total scores and a weak positive relationship was found between the stop-signal reaction time and the attentional, motor, non-planning scores, and total scores. A significant improvement was observed in the response inhibition skills of the adults with ADHD who had received methylphenidate treatment compared to those who had not, and the former also showed lower impulsivity levels as measured by the BIS-11.
Conclusions: It should be noted that response inhibition and interference control, which are considered under the umbrella of inhibitory control, may exhibit different characteristics in adult individuals diagnosed with ADHD and this is important for differential diagnosis. An improvement was observed in the response inhibition of adults with ADHD caused by psychostimulant treatment, which was associated with positive outcomes that were also noticeable by the patients. Understanding the underlying neurophysiological mechanisms of the condition would further facilitate the development of appropriate treatments.
Keywords: Attention deficit hyperactivity disorder; impulsivity; inhibitory control; interference control; response inhibition.
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