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PhD Thesis - McKetin, R

Cognitive functioning and psychological morbidity among illicit amphetamine users
McKetin R (1998)

Abstract:
This thesis investigates whether amphetamine use is associated with impaired cognitive and psychological functioning. The first study examined cognitive functioning among amphetamine users. Results showed that amphetamine dependence was associated with poorer performance on all indices of the Wechsler Memory Scale - Revised (WMS-R), whereas a history of heavy use related specifically to poor performance on visual memory tasks. Dependent amphetamine users performed a half to one standard deviation below non-drug-using controls (95% CI = 2.6 to 14.4 points). The second study used event-related potentials (ERPs) to further investigate the nature of cognitive impairment in dependent amphetamine users. ERPs were recorded during Hansen and Hillyard's (1983) location-easy - pitch-difficult multidimensional auditory selective attention task (SAT). Dependent amphetamine users showed reduced early attention-related negativity to relevant stimuli and slower reaction time. These results suggested impaired selection of relevant stimuli for further processing and poor maintenance of the attentional trace to the sensory representation of the relevant stimulus. The third study examined how amphetamine intoxication affected attention-related processes in healthy volunteers using the procedure from the second study. Amphetamine (10 mg and 20 mg d-amphetamine) improved performance on the SAT (decreasing reaction time and increasing hit rate), decreased late attention-related negativity to pitch-irrelevant stimuli, and increased the P3 wave to target stimuli. These changes suggest d-amphetamine reduced conscious processing of irrelevant stimuli, and improved recognition of relevant target stimuli. The fourth and fifth studies in this thesis examined the prevalence and severity of a range of psychiatric symptoms among amphetamine users. Dependent amphetamine users showed high levels of psychological distress, including several cardinal symptoms of affective disorders, symptoms of amphetamine psychosis and elevated levels of psychosis "proneness". In summary, amphetamine dependence was associated with impaired memory and attention, poor ability to select and preferentially process relevant stimuli, increased psychosis-proneness and general psychiatric disturbance that was not restricted to symptoms of amphetamine psychosis. Acute amphetamine intoxication in healthy subjects improved performance on the auditory selective attention task and did not appear to increase conscious processing of, or response to, irrelevant stimuli.

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