An extended battery of neuropsychological tests was administered to three groups of college seniors (lysergic acid diethylamide [LSD]/mescaline users; marihuana/hashish users; and controls) who were matched on predrug usage intellectual and personality dimensions. The study was replicated one year later. In the combined-years' analyses, the three groups showed statistically significant differences only on the Trail Making Test: LSD/mescaline users performed within normal limits but significantly worse than either of the other two groups. Since the three groups also differed significantly in the extent of their alcohol usage, a covariance analysis was carried out that indicated that this variable did not account for the LSD/mescaline group's performance on the Trail Making Test. Inference about possible organic dysfunction cannot be drawn from these findings, but prospective neuropsychological testing might prove useful.
Culver CM, King FW. Neuropsychological Assessment of Undergraduate Marihuana and LSD Users. Arch Gen Psychiatry. 1974;31(5):707-711. doi:10.1001/archpsyc.1974.01760170093015