As the movement to legalize marijuana marches across the United States, more people are wondering how this mind-altering drug affects memory and cognition. According to David Gallo, head of the Memory Research Laboratory in the Department of Psychology, the main psychoactive ingredient in cannabis – tetrahydrocannabinol (THC) – is known to cause forgetting, but it is not known if the drug also causes memory distortions. This is an important question because, unlike forgetting, memory distortion can fundamentally alter one’s sense of reality. Memory distortions may be especially problematic in individuals that are already prone to memory disturbances, such as those with post-traumatic stress disorder (PTSD) who often use cannabis to self-medicate.
To begin answering these kinds of questions, Gallo teamed up with Dr. Harriet de Wit, head of the Human Behavioral Pharmacology Lab in Psychiatry. Over the past few years they have conducted a series of experiments on the effects of different psychoactive drugs on human memory, supported by grants from the National Institute of Drug Abuse at the NIH. Their initial study was spearheaded by Michael Ballard, a graduate student at the time, and it was the first study to directly investigate the effects of THC on false memories. Healthy young adults were randomly assigned to ingest a pill containing THC or placebo, and once the drug had taken effect, they studied lists of related words (bed, rest, awake, tired…etc.). Unbeknownst to them, the lists were designed to generate false memories of nonstudied associates (sleep). Two days later the subjects took a memory test while sober.
How did THC impact this memory illusion? While it might be predicted that the drug would increase memory distortion, most individuals actually showed drug-induced reductions in false recognition of nonstudied words (like sleep), as well as a significant drop in the recognition of studied words (like bed). Moreover, these effects were strongly correlated, so that individuals who showed the greatest THC-induced forgetting of studied words also were poorer at recognizing nonstudied associates.
While it is tempting to conclude from these results that getting high can prevent false memories, Gallo argues that this would miss the bigger point (to say the least!). Instead, these findings suggest that THC impaired the brain’s ability to encode the associations that link all of the words together, thereby reducing the effect of these (false) associations on subsequent memory errors. These effects are analogous to those seen in amnesics with hippocampal damage, who also show reduced true and false memories in this task, and they are consistent with experiments showing that THC can disrupt processing in this part of the brain. By disrupting this system, THC interrupts the brain’s use of associations to help synthesize incoming information. In this way, reduced errors in this task is not necessarily a good thing.
In subsequent studies, Gallo and de Wit have discovered that THC and other psychoactive drugs can also distort memory in a different way: by disproportionately affecting emotional memories. Using a picture memory task, they have shown that drugs that impair memory (THC; alcohol) tend to impair the encoding of emotional pictures more than neutral ones, whereas stimulant drugs that improve memory (amphetamine, methamphetamine) tend to improve the encoding of emotional pictures over neutral ones. These results suggest that such drugs target the brain’s emotional memory mechanisms, including the amygdala as well as hippocampus, and Gallo and de Wit are planning to test these this hypothesis using fMRI brain scanning. The THC results also might explain why cannabis use is so prevalent in PTSD, as the drug might help to dampen the emotional memory “flashbacks” that cause stress and anxiety.
Gallo is excited to take this line of research to the next level. “While most of the work in my lab is focused on memory and aging, these drug studies have been every bit as rewarding and interesting” he writes. “Hopefully our work on these basic neurocognitive mechanisms will inspire more research into how these drugs affect cognition in different contexts, as well as a deeper understanding of the large individual differences that we often see in response to these drugs."
