These findings by Ranganathan and D’Souza (2006) indicate that, besides a decrease in correct recall, cannabis is also associated with an increase in incorrect recall (Ranganathan & D’Souza, 2006; Vredeveldt, Charman, den Blanken, & Hooydonk, 2018). To determine whether cannabis has an effect on the formation of false memories, Riba et al. (2015) applied an adjusted version of the Deese/Roediger-McDermott (DRM) paradigm, which is also a word list learning task.
By using the DRM paradigm as a direct induction of false memory, Riba et al. (2015) demonstrated that the brain structures involved in adding episodic knowledge to a memory can display reduced activation due to chronic exposure to cannabis. This can lead to the deterioration of the recollection of autobiographical events, containing information about times, places, emotions and other contextual knowledge. The modification of our memory of autobiographical events due to cannabis use can lead to an increased susceptibility to produce false memories (Riba et al., 2015).
Since our knowledge on the effects of cannabis on memory deformation comes from a limited amount of studies using verbal stimuli (Broyd et al., 2016; Riba et al., 2015), generalizing these findings to criminal situations in limited (Yuille, Tollestrup, Marxsen, Porter, & Herve, 1998). The first empirical study into the effects of cannabis on memory deformation in a criminal situation was carried out by Yuille et al. (1998), examining the memory of a live mock crime scenario in eyewitnesses. The findings by Yuille et al. (1998) are in contrast with earlier research, indicating that cannabis only produces temporary negative effects on the eyewitnesses memory and only has small effects on recall accuracy or recognition.
Vredeveldt et al. (2018) also examined the effects of cannabis on eyewitness memory, by showing participants a video of a criminal event and showing them a target-present or target-absent line-up afterwards. It was shown that intoxicated participants memorized fewer accurate details of the crime. There were no effects found on inaccurate recall or recognition (Vredeveldt et al., 2018). A potential reason for the discrepancy between previous findings and the findings by Vredeveldt et al. (2018) and Yuille et al. (1998) is the application of a scenario as the to-be-remembered stimulus compared to a verbal stimulus (Yuille et al., 1998).
