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Effect of Smoking Marijuana on Short-term Memory Processing

J. Polich, M.L. Hufner

MARIJUANA AND COGNITION

Cannabis (marijuana) is the most extensively used illicit drug in the Western world. However, the psychological and medical problems attributed to its use vary considerably. Although some studies suggest that short-term use of marijuana inhibits attentional and memory processes, the approaches used in these studies are limited because they cannot provide information on the neural effects of marijuana in terms of where and when such impairment occurs. Moreover, the long-term effects of marijuana use are relatively unknown, especially with respect to the neurocognitive operations underlying mental activity. Indeed, impaired attentional/mnestic processing associated with long-term use of marijuana would have serious implications for learning, memory, and even ordinary tasks such as driving an automobile. Thus, understanding the implications of short- and long-term use of marijuana is a necessary initial step in assessing how its use may contribute to declines in human cognition, especially in light of current social pressures to legalize marijuana for medicinal and other uses.

Short-term memory scanning can be assessed by having subjects store a set of memory stimuli by repeating the stimuli silently and then presenting the subjects a probe stimulus in the absence of the original memory set. Response time to the probe stimulus increases linearly as a function of the size of the memory set; probe stimuli from the memorized set and probe stimuli not in the memorized set have the same slope. These findings suggest that a serial and exhaustive scan of the stored items is used to determine whether or not the probe stimulus is a member of the target memory set. More important, proficiency in memory scanning is influenced not onlyby stimulus and task variables but also by population factors, because the baseline and slope of the response time curve increase if the subjects are cognitively impaired. Such findings imply that the rate of stimulus processing is influenced by how rapidly subjects can use short-term memory functions when stimulus items are scanned in a serial, exhaustive fashion.

LONG-TERM SMOKING OF MARIJUANA AND SHORT-TERM MEMORY

A study was designed to assess how long-term smoking of marijuana affects short-term memory scanning when letters are used as the items in the memory set. Young adult male university students who indicated by self-report that they smoked marijuana (1) once or fewer times per month (light users) or (2) 3 or more times per week (heavy users) for at least the past 6 months were paid to participate in the study. Each group had 12 participants. All participants abstained from using marijuana and alcohol for at least 24 hours before the study, were free of neurologic or psychiatric disorders, and had normal or corrected-to-normal vision. The groups were matched for age and academic level. The size of the memory set used was 2, 4, or 6 items, and it was always varied for each subject. The dependent variables were response time to the probe stimulus and error rate.

Response time for both the light and heavy smokers increased at a similar rate as a function of the size of the memory set, as is usually found for this task, but the light smokers responded about 75 ms more quickly than did the heavy smokers (Fig. 1). The groups did not differ in error rate, so that only the baseline or access time for short-term memory scanning appeared to be affected by long-term smoking of marijuana. These results suggest that long-term smoking of marijuana impairs short-term memory performance, even in successful university undergraduates who were unaware of any cognitive deficits. Thus, long-term smoking of marijuana in young adults appears to affect cognitive processes on a long-term basis.

PUBLICATIONS

Almasy, L., Porjesz, B., Blangero, J., Chorlian, D.B., O'Connor, S.J., Kuperman, S., Rohrbaugh, J., Bauer, L.O., Reich, T., Polich, J., Begleiter, H. Heritability of event-related brain potentials in families with a history of alcoholism. Am. J. Med. Genet. 88:383, 1999.

Bennington, J.Y., Polich, J. Comparison of P300 from passive and active tasks for auditory and visual stimuli. Int. J. Psychophysiol., in press.

Comerchero, M.D., Polich, J. P3a, perceptual distinctiveness, and stimulus modality. Brain Res. Cogn. Brain Res. 7:41, 1998.

Comerchero, M.D., Polich, J. P3a and P3b from typical auditory and visual stimuli. Clin. Neurophysiol. 110:24, 1999.

Covington, J.W., Geisler, M.W., Polich, J., Murphy, C. Normal aging and odor intensity effects on the olfactory event-related potential. Int. J. Psychophysiol. 32:205, 1999.

Hoffman, L.D., Friedmann, A., Saltman, P., Polich, J. Neuroelectric assessment of nutrient intake. Int. J. Psychophysiol. 32:93, 1999.

Hoffman, L.D., Polich, J. EEG, ERPs, and food consumption. Biol. Psychol. 48:139, 1998.

Hoffman, L.D., Polich, J. P300, handedness, and corpus callosal size: Gender, modality, and task. Int. J. Psychophysiol. 31:163, 1998.

Ilan, A.B., Polich, J. P300 and response time from a manual Stroop task. Clin. Neurophysiol. 110:367, 1999.

Ilan, A.B., Polich, J. Tobacco smoking and memory scanning: Behavioral and event-related potential effects. Nicotine Tobacco Res., in press.

Katayama, J., Polich, J. Auditory and visual P300 topography from a 3 stimulus paradigm. Clin. Neurophysiol. 110:463, 1999.

Lin, E., Polich, J. P300 habituation patterns: Individual differences and ultradian rhythms. Perceptual Motor Skills 88:1111, 1999.

Morgan, C.D., Geisler, M.W., Covington, J.W., Polich, J., Murphy, C. Olfactory P300 in young and older adults. Psychophysiology 36:281, 1999.

Nordin, S., Quiñonez, C., Morgan, C.D., Geisler, M.W., Polich, J., Murphy, C. Olfactory event-related potentials in young and elderly adults: Evaluation of tracking task vs eyes open/closed recording. Chem. Senses, in press.

Ochoa, C.J., Polich, J. P300 and blink instructions. Clin. Neurophysiol., in press.

Polich, J. Cognitive electrophysiology. In: Encyclopedia of Psychology. Kazdin, A.E. (Ed.). American Psychological Association, Washington DC, in press.

Polich, J. EEG, ERPs, and aging: Data and measurement issues. In: Cognitive Changes Due to Aging and Fatigue as Revealed in the Electrical Brain Activity. Bundesanstalt für Arbeitsschutz und Arbeitsmedizin, Dortmund, Germany, 1999, p. 146.

Polich, J. P300 in clinical applications. In: Electroencephalography: Basic Principles, Clinical Applications and Related Fields, 4th ed. Niedermeyer, E., Lopes da Silva, F. (Eds.). Urban Schwarzenberg, Baltimore, 1999, p. 1073.

Polich, J., Bloom, F.E. P300, alcoholism heritability, and stimulus modality. Alcohol 17:149, 1999.

Polich, J., Pitzer, A. P300 in early Alzheimer's disease: Oddball task difficulty and modality effects. Clin. Neurophysiol., in press.

Ravden, D., Polich, J. Habituation of P300 from visual stimuli. Int. J. Psychophysiol. 30:359, 1998.

Ravden, D., Polich, J. On P300 measurement stability: Habituation, intra-trial block variation, and ultradian rhythms. Biol. Psychol., in press.

Rodríguez-Holguín, S., Porjesz, B., Chorlian, D.B., Polich, J., Begleiter, H. Visual P3a in male alcoholics and controls. Alcohol. Clin. Exp. Res. 23:582, 1999.

Rodríguez-Holguín, S., Porjesz, B., Chorlian, D.B., Polich, J., Begleiter, H. Visual P3a in male subjects at high risk for alcoholism. Biol. Psychiatry 15:281, 1999.

Spencer, K.M., Polich, J. Poststimulus EEG spectral analysis and P300: Attention, task, and probability. Psychophysiology 36:220, 1999.