Methylphenidate effects on focused and selective attention processing in children with ADHD

Attention-deficit/hyperactivity disorder (ADHD) is the most prevalent psychiatric disorder of childhood. Despite extensive research on ADHD, two critical issues remain unresolved. First, the precise nature of the attentional processing deficit remains largely unknown. Second, although methylphenidate continues to be the most common intervention for ADHD, the mechanisms through which this medication exerts its effectiveness are not completely understood. The present study was designed to examine both the attentional processing deficit in children with ADHD and the effects of methylphenidate on attentional processing.;Performance on focused and selective attention tasks were compared in 20 children with a DSM-III-R diagnosis of ADHD and 20 children without ADHD. Medication effects were studied using a double blind, placebo-controlled design involving a lower (0.26mg/kg) and a higher (0.56 mg/kg) dose of methylphenidate. Event-related potentials were recorded and analyzed in the focused attention task only. On the focused attention task, the results revealed that the ADHD group off medication had lower levels of attention, higher impulsivity, and shorter reaction times to targets than children without ADHD. For these behavioural measure of focused attention, lower doses of methylphenidate normalized performance of the ADHD group and higher doses of methylphenidate did not improve performance further. The ERP data showed that off medication, the N2 latencies were shorter in the ADHD group than in the children without ADHD. The latency of this component became longer in the ADHD group on higher doses of methylphenidate only. In contrast, the P3 latency was longer in children with ADHD off medication but was reduced (normalized) on both lower and higher doses of methylphenidate.;For the selective attention task, specific attentional processing deficits were not found and methylphenidate was not found to affect performance.;The combined performance and ERP data for the CPT were used to develop an attentional processing model. This model suggests that children with ADHD are faster in earlier aspects of attentional processing (stimulus evaluation and response decision), which was reflected in their greater impulsivity and in their faster reaction times and slower in post-decisional attentional processes, which creates greater inattentiveness. Methylphenidate acts to normalize these specific attentional processes, but they are different effects for different dose levels. Higher doses normalize both early stimulus evaluation and response decision processes as well as later post-decisional attentional processes, whereas lower doses normalize only the later post-decisional attention processes.