AUDITORY AND VISUAL P300 EVENT-RELATED POTENTIALS ARE NOT ALTERED IN MEDICALLY AND PSYCHIATRICALLY NORMAL CHRONIC MARIJUANA USERS

Attempts to use Event Related Potentials, particularly the cognitive o r P300 evoked potential, as measures of CNS effects of THC use have be en infrequent and have produced inconsistent results. We published a p ilot study in which psychiatric patient THC users had significantly pr olonged auditory P300 latencies and reduced amplitudes as contrasted w ith non-users. Because psychiatric diagnoses and medication effects co uld not be controlled, we repeated the study with medically and psychi atrically normal subjects selected with extremely stringent exclusion criteria and screening procedures. P300 latency differences between TH C users and controls were not detected. Using all subjects, THC users displayed reduced auditory and visual P300 amplitudes. However, when a ge differences between THC users and controls were removed, all signif icant P300 amplitude differences were removed as well. The contaminati ng effect of using psychiatric patients in THC research is discussed a nd the importance of using carefully screened normal subjects in studi es of neurophysiological abuse drug effects is stressed.