Ruth Williams, in Nature Reviews Neuroscience, points out an article by Drouin et al in The Journal of Neurophysiology that suggests a potential brain mechanism for the action of methylphenidate (Ritalin) in relieving attention deficit hyperactivity disorder (ADHD). ADHD sufferers essentially "pay attention to too many things." They find it difficult to filter out unwanted distractions and focus one issue at a time. Behavioral experiments with rats have shown that Ritalin improves behavior in a sustained attention task, just as it does in humans. Drouin et al found that administering Ritalin to rats (at a dose equivalent to that used in human therapy) caused an increase in the levels of the neurotransmitter noradrenaline in the rat's sensory cortex and reduced a long-latency phase of the brain's response to sensory stimuli. This latter effect may be important in filtering out sensory noise.