Intra-Individual Variability Among Children with ADHD on a Working Memory Task: An Ex-Gaussian Approach

Abstract

Intra-individual variability on a computer-based working memory task was examined among 25 children/adolescents with ADHD and 24 typically developing peers. Participants completed the Visual Serial Addition Task (VSAT) and reaction time data were fit to an ex-Gaussian distribution. ADHD participants demonstrated significantly more variable performance than controls, and effects of working memory load were observed. Event rate, however, had no influence on group differences in performance. Follow-up correlations revealed associations between VSAT performance and ADHD symptomatology. This study supports intra-individual variability as a hallmark feature of ADHD beyond the domain of response inhibition and reinforces the need to consider variability in ADHD more broadly.

Keywords:

• ADHD
• Working memory
• Reaction time
• Variability

The authors thank the participants and their parents, and Mark Cochran, Ph.D., Barbara McGee, Caitlin Dunning, Ph.D., Carlos Cortes, M.D., and Catherine Fassbender, Ph.D., for assistance with data collection. The authors report no competing interests.

Funding for this study was provided by the National Institutes of Mental Health, National Institute of Health (R01 MH066310) and University of Maryland School of Medicine Intramural Awards. This work was also supported by the University of Maryland General Clinical Research Center Grant M01 RR 16500, General Clinical Research Centers Program, National Center for Research Resources (NCRR), NIH.

Notes

¹The omission errors variable was positively skewed for both the ADHD and control groups (z-scores for skewness ranged from 1.15 to 4.54 across the six experimental sessions). A log transformation, log₁₀(x + 1.0), was used to normalize the distribution, and no z-score remained significantly different from zero after the transformation.