Introduction: Theta-Beta Ratio (TBR) has been claimed as a biomarker to diagnose Attention Deficit and Hyperactivity Disorder (ADHD). However, the effectiveness of this index to differentiate between different groups of disorders is still under discussion. The primary purpose was to determine to what extent active TRB can differentiate between children with ADHD and specific Learning Disorder (sLD) as the most common comorbid disorder.
Methods: Two groups of school-aged children with sLD (N=15) and ADHD (N=15) were diagnosed through a process of clinical interview and observation. Electroencephalography (EEG) was recorded in both groups during active condition. The implemented cognitive task was the visual continuous performance task (VCPT). TBR in sites of CZ and Fz, and cognitive measures of VCPT were calculated in the aforementioned groups.
Results: There was no significant differences in cognitive measures (containing Commission, Omission, Reaction Time, and Variability of Reaction Times) shown in two matched groups of children with sLD and ADHD. According to TBR, two groups demonstrated no significant results in comparison. 
Conclusion: TBR cannot be considered as a reliable biomarker to differentiate between those groups of psychological disorders that contain primary cognitive deficits and require the allocation of attention and working memory loads.