Early onset marijuana use is associated with learning inefficiencies

Abstract

Objective: Verbal memory difficulties are the most widely reported and persistent cognitive deficit associated with early onset marijuana use. Yet, it is not known what memory stages are most impaired in those with early marijuana use.

Method: Forty-eight young adults, aged 18-25, who used marijuana at least once per week and 48 matched nonusing controls (CON) completed the California Verbal Learning Test, Second Edition (CVLT-II). Marijuana users were stratified by age of initial use: early onset users (EMJ), who started using marijuana at or before age 16 (n = 27), and late onset marijuana user group (LMJ), who started using marijuana after age 16 (n = 21). Outcome variables included trial immediate recall, total learning, clustering strategies (semantic clustering, serial clustering, ratio of semantic to serial clustering, and total number of strategies used), delayed recall, and percent retention.

Results: Learning improved with repetition, with no group effect on the learning slope. EMJ learned fewer words overall than LMJ or CON. There was no difference between LMJ and CON in total number of words learned. Reduced overall learning mediated the effect on reduced delayed recall among EMJ, but not CON or LMJ. Learning improved with greater use of semantic versus serial encoding, but this did not vary between groups. EMJ was not related to delayed recall after adjusting for encoding.

Conclusions: Young adults reporting early onset marijuana use had learning weaknesses, which accounted for the association between early onset marijuana use and delayed recall. No amnestic effect of marijuana use was observed. (PsycINFO Database Record

Figure 1

Differences in Learning Across Five Learning Trials among Controls, Late Onset Marijuana Users and Early Onset Marijuana Users Note. Groups were significantly different in overall learning (EMJ < LMJ, CON; LMJ = CON). Recall improved significantly and similarly over time across all groups. Groups were significantly different in delayed recall (EMJ < LMJ, CON; LMJ = CON), but similar in percent retention.

Figure 2

Differences in Use of Learning Strategies among Controls, Late Onset Marijuana Users and Early Onset Marijuana Users Note. There was a significant difference in total strategy use (EMJ < CON; EMJ = LMJ; LMJ = CON). There was not an overall group difference in use of semantic strategies, but the pairwise comparison between EMJ and CON was significant (EMJ < CON; EMJ = LMJ; LMJ = CON). There were no group differences in use of serial strategies.

Figure 3

Total Learning Mediates the Relationship between Early Onset Marijuana Use, but not Late Onset Marijuana Use, and Delayed Recall (Controlling for Age and Gender)