AGE-RELATED DIFFERENCES IN DISTRACTIBILITY AND RESPONSE TO METHYLPHENIDATE IN MONKEYS

Increased susceptibility to distraction is a symptom of normal aging a nd several clinical syndromes, including Alzheimer's disease and atten tion deficit disorders. In the present study, aged and young adult mac aques were well-trained to perform an automated delayed matching-to-sa mple (DMTS) task which assesses both attention and short-term memory. On 19% of all trials. a task-relevant distracting stimulus was present ed during either the initial 1 or 3 s of delay intervals (early onset) or the final 1 or 3 s of delay intervals (late onset). In aged monkey s, both early and late onset distracters lasting 1 or 3 s impaired del ayed recall on trials with the shortest delay intervals, but did not a ffect accuracy on trials with long delay intervals. In contrast, young adult monkeys were impaired only by the presence of an early onset di stracter lasting 3 s. Impairment was selective for only those trials w ith the shortest delay intervals. Late onset distracters were relative ly ineffective in producing distractibility in young adult animals. Me thylphenidate (MPH; 0.005-1.0 mg/kg) failed to reduce distractibility in aged monkeys, producing locomotor abnormalities and hypophagia at d oses ranging from 0.25 to 1.0 mg/kg. In young adult monkeys, however, distractibility was significantly attenuated by administration of the 0.125 mg/kg dose. Habituation to the distracting stimulus (under salin e conditions) was assessed throughout the study and was not evident at any time point of testing. These data indicate that attention and rec all after brief delays are impaired following exposure to a task-relev ant distracting stimulus in both aged and young adult monkeys, hut tha t aged monkeys are more susceptible to distraction and do not receive significant benefit from MPH administration.