Everyone knows that THC can impair memory, but there are actually many different kinds of memory. Which specific ones are impacted by THC?
We all know that cannabis can have an effect on memory. If I think back to when I smoked the day before, everything just feels a bit…hazy. Of course, everybody responds differently, but I don’t know anyone who can get away without at least some memory impairment.
The story of how THC impacts your memory is more complicated than you may realize. Memory is more than just remembering what you had for lunch two days ago. This only represents one type of memory, and in reality, there are many different types.
This article will give you a brief overview of how THC impacts 5 different types of memory. I will focus on acute effects and not so much on chronic effects (which is much more difficult to study). In upcoming articles, I will review in depth the mechanisms behind how THC exerts these effects on memory.
Some Memory Basics
First, let’s get a couple of basic concepts of memory out of the way. Every type of memory fits into one of two categories:
- Explicit memory (also called declarative memory) is the conscious storage and recollection of information.
- Implicit memory (also called non-declarative memory) involves the unconscious storage and retrieval of information.
There are 3 stages to the formation and retrieval of any memory:
- Encoding involves the initial processing of the information to be stored
- Storage involves keeping the information in short-term or long-term memory
- Retrieval/recall involves calling back the information that has been stored at a later time
There are a couple other important concepts like extinction and reconsolidation, but I will cover those later. For now, let’s get on to the list!
#1. Episodic MemoryEpisodic memory is the collection of past personal experiences that are associated with specific places, times, and emotions. Remember that one time that you got really high? Neither do I. But if I did, it would be an episodic memory 🙂
THC impairs episodic memory during the period of intoxication. For a single dose of THC, these effects are transient and full memory function is quickly recovered after intoxication is over. For regular users, there is a period of continued minor impairment after intoxication is over.
Cannabis users do develop tolerance to the acute amnesic effects of THC. As highlighted by a 2008 study, THC produced clear memory impairment in non-users, but had little effect on memory in frequent users. Both groups felt equally high, so this was not due to general tolerance to all THC effects.
THC does not appear to impact recall of memories, but it does impact encoding. Encoding continues to occur for several hours after you have experienced something. The consequence of this: obviously you will not remember things as well when you are high, but memories from right before you got high can be impaired as well.
So if you are studying, probably better to wait a few hours to light up instead of doing it right after you finish.
The hippocampus is a crucial brain structure for this type of long-term memory. Although long-term memories aren’t actually stored inside the hippocampus, new memories cannot be formed without it.[More detail on this will be coming soon in the second article of this series.]
#2. Working MemoryWorking memory is a system your brain uses to temporarily hold information in order to manipulate it. Working memory is what most people think of as “short-term memory” (although in reality there are other types of short-term memory that are different from working memory).
This type of memory has very limited capacity in terms of the amount of information it can hold at one time. Working memory capacity is strongly correlated with performance in complex cognitive tasks, such as reading comprehension and problem solving.
As demonstrated by an extensive set of studies, working memory can be impaired by THC, with higher doses resulting in more significant impairment. However, the effects are more complicated in people who are chronically stressed. Stress itself significantly impairs working memory, so if THC decreases stress in some people, it is not as clear what the net effect of THC will be.
There are several areas of your brain which handle working memory. Although your hippocampus is still involved, many parts of the prefrontal cortex (PFC) are as well. Different parts of your PFC are specialized to handle different types of information (verbal, spatial, etc). So the part of the PFC that is activated depends on the specific type of working memory task at hand.
#3. Procedural MemoryProcedural learning is the process of acquiring new motor sequences by repeating an activity over and over again. Learning to drive, play a sport, roll a joint, or do a dab requires this procedural learning. Can you roll a joint without even thinking about it? You are accessing procedural memory
The effect of THC on procedural memory is one of the least studied. However, a few studies have indicated that the endocannabinoid system is involved in the acquisition and extinction of procedural memories. Therefore, THC is likely to impact procedural memory, although the full extent is not yet understood.
Although multiple brain areas are involved in procedural memory, the dorsal striatum is the main area where THC can act on it.
#4. Habit MemoryHabit memory (also called stimulus-response memory) is another type of implicit memory. This type of memory is when a specific behavioral response is paired with an external stimulus. This was made famous with Pavlov’s dogs, who would be fed after ringing a bell. After a while, the dogs start to drool from the bell alone.
You may use this type of memory more often than you realize. When you are driving or walking home from work, you probably take all of the correct turns automatically. Your brain has paired being at each intersection with making a specific turn.
THC slows the acquisition of new habit memories. On top of that, we sometimes need to change our habits. Using the example above, if you moved to a new home, you would need to form a new set of habit memories. For a while (if you are not paying attention), you may accidentally start driving to your old home. THC also slows our ability to update our habit memories.
Somewhat counter-intuitively, chronic cannabis users may actually depend more on habit memory due to impairments in other cognitive systems.
Habit memory is involved in innocuous day to day tasks, but can have serious consequences when the habits are maladaptive. Habit memory can contribute to obsessive-compulsive disorder (OCD), post-traumatic stress disorder (PTSD), and drug addiction and relapse.
Like procedural memory, habit memory is regulated by the dorsal striatum.
#5. Fear MemoryNot all types of memory are good. In fear memory, some stimulus (a sound, an object, a place, etc.) becomes associated with a threat. Fear memories are common after pain or trauma and are what drive phobias and PTSD.
THC can reduce the consolidation of fear memories. But more important for treatment of PTSD, it can also regulate the extinction and reconsolidation of fear memories.[More details on this will be coming soon in an article that focuses on research findings of cannabinoids and PTSD.]
Although fear memory also uses some of the brain systems listed above, it is unique in depending on the amygdala, a brain area which regulates emotional arousal and anxiety.
[Featured image: Flickr/ Taco Ekkel]
Last modified: August 12, 2017