Normal attention orienting but abnormal stimulus alerting and conflict effect in combined subtype of ADHD

Recent pharmacological studies in animals and neuroimaging studies in normal humans suggest that the spatial and nonspatial cues in tasks measuring reflexive attention may be modulated by different neurotransmitter systems. The efficiency with which attention is oriented to explicit spatial cues may be altered by manipulating levels of brain acetylcholine, whereas reactions to nonspatial cues may be influenced by altering brain noradrenaline levels but not acetylcholine levels. In humans, however, previous attention studies have implicated dopamine when either explicit or implicit cueing is used. Some of the differences between animal and human work may be due to inadequate testing of nonspatial cues. To remedy this, we tested adult humans with ADHD that were primarily inattentive (ADHD/I) or combined inattentive/hyperactive (ADHD/C) and controls with the Attention Network Task that assesses both reflexive and voluntary attention and explicitly tests nonspatial cueing. Our results showed that spatial orienting in both subtypes was no different than controls. However, ADHD/C but not ADHD/I subjects had significantly slowed response times to nonspatial cues and cues with spatial conflict. Stimulant medication in a subset of ADHD/C subjects reduced these deficits to control levels. Based on these results, we conclude that ADHD/C subjects orient the focus of their attention normally but are impaired in their reactions both to abrupt visual cues and those that contain conflicting spatial cues.