Krebs and Johansen raise some questions concerning the conclusions of our article “Cognition in Novice Ecstasy Users With Minimal Exposure to Other Drugs.”1 We interpret the absence of a retest effect on a verbal memory task in Ecstasy users as a decrease in verbal memory. This is based on the fact that the Ecstasy-naive subjects showed an increase in performance between the initial and follow-up examinations, while the Ecstasy users did not. The memory performance in our control group increased, with 4 words on total immediate recall and more than half a word on delayed recall at the second measurement occasion. However, Krebs and Johansen state that retest effects disappear within 1 month, a statement based on a review of Hawkins et al.2 In this review, 1 study is mentioned, in which no significant practice effects were seen in 30 normal subjects who were retested with a parallel version after 1 month.3 Another reviewed study in 19 undergraduates did find retest effects with a parallel form after 2 to 13 days.4 We find these 2 small studies meager evidence to conclude that retest effects cannot exist after 1 month. There are other studies that do show retest effects after more than 1 month.5-8 For example, 1 study in 38 healthy subjects reported an increase of 2.8 words on immediate recall and 0.8 word on delayed recall after an interval of 4 months.7 Perhaps the most compelling evidence that retest effects are the norm comes from a population-based study in 776 subjects that shows an increase of 4 words on immediate recall and 1 word on delayed recall after an interval of 3 years.8 (Moreover, most of the tests we used showed retest effects at follow-up [Table 2 of our article1]). The absence of improved performance in the Ecstasy-using group does not imply that Ecstasy users lack a practice effect. They probably also gained “test wiseness” and knew what they could expect. However, our hypothetic conclusion is that this practice effect could not fully compensate for the decreased memory functioning due to the Ecstasy use.
Schilt T, de Win MM, Koeter M, et al. No Evidence of Decrease in Cognitive Function in Users of Low-Dose Ecstasy—Reply. Arch Gen Psychiatry. 2008;65(2):236–237. doi:10.1001/archgenpsychiatry.2007.35
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