The latest research on the effect of cannabis on memory has shows that it affects spatial memory but not associative memory. Smoking marijuana might not affect your memory of what you and your friends talked about while walking around town, but you might not remember exactly what route you took.
Human memory is generally divided into three types: sensory memory, short-term memory (which lasts less than a minute) and long-term memory (which can last a lifetime). Cannabis notoriously affects short-term or working memory in a way that… wait, what was I just talking about?
THC’s effect on long-term memory isn’t as drastic, partly because this type of memory is divided into two types: implicit memory (unconscious, muscle memory) and explicit memory. Explicit memory can be semantic (definitions, facts, concepts) or episodic (events, experiences, places). Out of all these kinds, THC deals a blow to episodic memory, or being able to recall things that have happened in your lifetime.
If cannabis affected all episodic memory in people everyone that consumed it, nobody would ever complain about old hippies talking their ears off with old stories, but that’s obviously not the case. The latest research concerning THC and memory used zebrafish, which have a remarkably similar brain and nervous systems to humans, and performed careful behavioral tests on them. The zebrafish helped scientists from the University of Bonn in Germany discover that THC impairs spatial memory but leaves associative memory largely intact.
This means marijuana’s effects on episodic memory aren’t uniform and certain details get left out. It may impair the spatial aspect of a long-term, episodic memory (as in where a past event occurred) but it seems to leave most of the rest intact. The effects of marijuana on memory can change from person to person and may be worsened when combined with alcohol, drugs and sleep-loss. Unlike “blacking-out” from heavy drinking cannabis will rarely give anyone legitimate amnesia, you’ll just forget some details. After waking up from a night of smoking with your buddies you’ll still have hazy recollections of everyone’s hilarious antics, but the exact location of the pizza place you ate at will always remain a mystery.