Why We Forget and How to Remember Better, page 23
Procedural Memory
Does using cannabis impair procedural memory? As we discussed earlier, the most important procedural memory task you do is driving. Although several early studies suggested that cannabis users can functionally compensate for their intoxication and drive more cautiously,5 more recent studies have found that the proportion of cannabis-positive drivers involved in fatal crashes has dramatically increased with legalization. For example, in Washington state the percentage of drivers involved in fatal crashes who tested positive for cannabis rose from 9% to 18%.6 This statistic should be viewed with caution, however, because non-intoxicated cannabis users may test positive for several weeks. Nonetheless, acute cannabis intoxication is associated with an increased number of collisions, increased lateral movement such as lane weaving, increased brake latency, and slower reaction time.7 To summarize, cannabis impairs procedural memory.
Episodic Memory
Some of the prior research on cannabis and its users had been potentially confounded by the fact that there are usually a number of differences between individuals who chronically use cannabis and nonusers, in addition to their use of cannabis. It therefore makes it difficult to compare those populations directly. Researchers at Massachusetts General Hospital in Boston, however, conducted a clever experiment to get around this problem. They invited 88 cannabis users aged 16 to 25 into a study, tested their attention and episodic memory, and then randomly assigned two-thirds of them to stop using cannabis for a month, while the other one-third continued to use it. Use and cessation were monitored by urine tests. The results were clear: Cannabis cessation did not have an effect on attention, but it did have a significant beneficial effect on episodic memory—particularly during the encoding (learning) of material.8 Given that 16- to 25-year-olds are often in school and need to be learning new material, this is an important finding. A similar increase in episodic memory was observed in 30- to 55-year-old users when they ceased cannabis use for 28 days.9 Additionally, a meta-analysis that combined six studies found that compared to nonusers, cannabis users were impaired in prospective memory—the ability to remember to carry out intended actions in the future.10 So, it is clear that cannabis impairs episodic memory, but memory function appears to return to normal with cessation of cannabis use.
THC Versus CBD
Two of the many compounds in cannabis are Δ9-tetrahydrocannabinol and cannabidiol, known better as THC and CBD, respectively. It is primarily intoxication with THC that produces the subjective feeling of being “stoned” and impairs memory.11 Several studies (although not all) have found that CBD may be able to improve episodic memory in individuals with acute THC intoxication.1112 So, in theory, the CBD in cannabis might be able to reverse its THC-induced memory-impairing effects. Unfortunately, while that might have been true in the 1990s, when the ratio of the average concentrations of THC to CBD in cannabis were about 15 to 1, the ratio in today’s cannabis may exceed 80 to 1.13 This means there’s not enough CBD in most current strains of cannabis to counteract its memory-impairing effects.
To shed light on this issue, researcher Carrie Cuttler and her colleagues at Washington State University asked participants to use one of several cannabis strains with different percentages of THC, with or without CBD. They found that all strains of cannabis impaired some aspect of memory commonly used in everyday life, such as remembering a list off the top of your head or recalling where you learned a bit of information. Interestingly, the ability to distinguish true from false memories also was impaired by all strains.14
Can CBD improve memory in individuals who are not intoxicated by THC? Researchers at the University of Basel, Switzerland, set out to answer this question by asking 34 healthy young adults to participate in two conditions. In each condition they learned a list of 15 unrelated nouns; this was followed by vaping either 12.5 mg of CBD or a similarly flavored placebo. Participants remembered slightly more words in the CBD condition (7.7 words) compared to the placebo condition (7.0 words).15 Not a terribly huge effect or large study, but certainly intriguing and worthy of further research.
Given that CBD may be able to improve memory in healthy young adults, some people have wondered if CBD can help memory impairment in conditions such as Alzheimer’s disease. Although researchers are looking into this possibility, to date no studies have found that CBD produces improvement in memory or any other cognitive function in individuals with Alzheimer’s. And, for what it’s worth, when Andrew’s patients with Alzheimer’s disease or their families have asked him whether they should try CBD, he always encourages them to do so and let him know if it helps. Thus far, none have reported any beneficial effects in memory, mood, or anxiety. But none have reported any adverse effects of CBD on memory either, consistent with the published studies.
Some Thoughts on Cannabis Use
It is our job to interpret the data that make up the science of memory as we see it and present these findings to you. It is up to you, of course, to determine how such findings fit into your life. Cannabis has many documented medical uses. In fact, it was added to the U.S. Pharmacopeia in 1850 and remained there until 1942—when it was removed for political rather than scientific reasons. We view cannabis like any other medication, with beneficial effects and side effects. We believe the data is clear that the THC in cannabis interferes with procedural and episodic memory, just like alcohol and the more than 100 approved drugs we discussed in Chapter 14 and listed in the Appendix. Just like these other medications, cannabis may be the best option for your chronic pain or other medical problem, despite its effects on your memory. (In fact, one study showed that some aspects of thinking improved after initiation of medical cannabis, which the researchers speculated may have been attributable to participants’ decreased use of prescription medications known to impair memory and thinking—opioids, benzodiazepines, anticholinergic antidepressants, and mood stabilizers like valproic acid; see the Appendix.16) Whether used recreationally or for medical purposes, you now know that cannabis will likely impair your driving, your memory for new information, and your ability to remember to act in the future. So, if you do use cannabis, as with alcohol, we would simply recommend that you do so responsibly.
Drugs
Cocaine
Although much could be said about cocaine’s effect on memory, here we would like to highlight that cocaine predisposes you to strokes. It raises your blood pressure, causes your blood vessels to constrict, and disrupts normal heart function. There are many individuals with memory impairment—and full-blown vascular dementia—due to cocaine-induced strokes. Many of these individuals were in their 20s and 30s when they had their strokes. See Chapters 13 and 14 to learn more about strokes and vascular dementia.
Ecstasy
Ecstasy (MDMA or 3,4-methylenedioxymethamphetamine, also known as Molly) is a recreational drug that has been gaining in popularity. Ecstasy users show both working and episodic memory impairments compared with nonusers. Interestingly, their episodic memory for verbal information was particularly impaired, as was their prospective memory.1017 These impairments in episodic memory may be related to the diminished hippocampal activation observed in Ecstasy users.18
Methamphetamine
When used properly as a medication, stimulant drugs such as methylphenidate (brand name Ritalin as well as others) and amphetamine/dextroamphetamine (brand name Adderall as well as others) can improve both working and episodic memory in individuals with attention-deficit/hyperactivity disorder (ADHD). Methamphetamine (brand names Desoxyn and Methedrine) can also be used to treat ADHD as well as obesity, but it is more commonly used as a recreational drug. Despite its ability to boost memory when used properly, recreational methamphetamine users show impairments in their episodic memory, including their prospective memory, compared with nonusers.1019
Opioids
When fentanyl, heroin, Percocet, oxycodone, or other opioids are used—whether for prescribed pain management, for recreational purposes, or because of addiction—they also cause memory impairment and confusion. The good news is that not only will memory improve with abstinence, it will also improve with maintenance treatment with the less harmful opioid, methadone.20 So, if you or a loved one are suffering from memory problems due to opioid use, we recommend you seek treatment today. You’ll improve your memory—and your life.
Psychedelics
Classic psychedelic drugs include lysergic acid diethylamide (LSD), mescaline (the active chemical in peyote), psilocybin (the active ingredient in psychedelic mushrooms), and N,N-dimethyltryptamine (DMT, the primary hallucinogenic chemical in the Amazonian brew ayahuasca or yagé). These drugs have been used in the past by clinicians to facilitate individuals’ ability to recall childhood experiences from memory. It is therefore reasonable to consider whether such drugs may be beneficial to memory.
The scientific studies, however, clearly show dose-dependent impairment in working memory, semantic memory, and non-autobiographical episodic memory, such that low doses cause some impairment and higher doses cause greater impairment.21 The retrieval of autobiographical episodic memory may be facilitated, but it is generally only observed in individuals who have suppressed and/or unpleasant memories, such that the mechanism of this facilitation is thought to be disruption of inhibitory control processes. Lastly, it should be noted that individuals may be more likely to experience false memories under the influence of psychedelics, as memories of aliens, angels, and elves are not uncommon. So, even when retrieval of autobiographical episodic memories is facilitated with psychedelics, the accuracy of these memories may be unreliable.
Your Brain on Alcohol, Cannabis, and Drugs
Know the effects of substances on your memory:
• Even a single alcoholic beverage will interfere with procedural memory (for example, driving) and episodic memory (for example, remembering new facts and events of your life).
◦ Drinking alcohol will interfere with your retention of information you learned earlier in the day—and earlier in the week.
◦ As an adult, consuming one alcoholic beverage each day should not permanently damage your brain, but there also is no reason to drink in an effort to improve your brain health.
• Using cannabis will impair your procedural and episodic memory.
◦ CBD alone does not impair memory—and may even help it.
◦ Cannabis has not been shown to permanently damage memory.
• Cocaine, Ecstasy, methamphetamine, opioids, and psychedelics will all impair your memory.
◦ Cocaine can cause strokes—even in young people—and may lead to vascular dementia and life-long consequences for memory.
20
Sleep well
You’ve always been good at languages, and so, entering college, you decide you’re going to take two new ones, German and Arabic. You’re a planner, and you’ve mapped out your studying for the whole semester. You allocate equal studying time for each. For Arabic, in addition to class, you plan to study for 1 hour on Mondays, Tuesdays, Wednesdays, and Thursdays during the 10-week semester, totaling 40 hours. The German midterm and final, however, are each scheduled right when you return from vacation. You know the material will be fresher in your mind if you study right before the exams, so, in addition to class, you plan to study 10 hours a day on the 2 days prior to the midterm, and the same for the final, also totaling 40 hours. The semester starts and you follow your plan. To your delight, you receive straight A’s in both midterms and both finals. Returning to college in your sophomore year, sitting in the second-year classes for each language, you discover that you retained one of these languages quite well, while the other has almost completely vanished.
Which language was retained, and which vanished? And why did it happen like that? We will answer these and related questions in this chapter. The preview, as you no doubt have guessed, is that it is related to your sleep. Sleep is absolutely critical for normal memory function.
It’s Hard to Pay Attention When You’re Tired
The first reason that sleep is important for memory is the obvious one. As you learned in Parts 1 and 2, in order to learn new information so it can be remembered, you need to pay attention to the information! And we all know that it is hard to pay attention when you’re tired. This is one of many reasons why it is not beneficial to stay up all night studying—because your tired brain will do a poor job of encoding the information you’re trying to learn.
Two Drives to Sleep
Why do we get tired? Sleep is determined by two drives.
Sleep pressure (also called sleep debt) builds up while you are awake; the longer you are awake, the greater the pressure you feel to sleep. Sleep pressure is related to the accumulation of certain chemicals in your brain, whose levels return to normal when you sleep.
Circadian rhythm governs your normal, daily patterns of sleep and wakefulness; it is why you may wake up before your alarm goes off and why you suffer from jet lag. Your circadian rhythm is primarily set by the light you see during the day, which triggers the release of melatonin hours later. Melatonin is a hormone that tells your brain it is time to begin the process to go to sleep.
You become tired either when your sleep pressure builds up to sufficient levels or when your circadian rhythm has triggered the release of melatonin indicating it is time to sleep. When your sleep routine is consistent, these two drives are in sync, such that when it is time for you to go to sleep, your sleep pressure is at its maximum and your melatonin levels are rising rapidly. If, however, these drives are divergent, you will still feel tired from either one—and you’ll have difficulty learning new information.
For example, let’s say that you’re flying from Boston to London. You take a flight that leaves at 8:30 p.m. and, due to your hectic day preparing for your trip, you fall asleep before the flight even takes off. You wake up when the plane lands 6.5 hours later. Although it’s 8 a.m. London time and 6.5 hours is long enough to relieve your sleep pressure, you find you’re dead tired. Why? Because according to your circadian rhythm it’s 3 in the morning Boston time and you should be fast asleep.
Another time you’re at home and not changing time zones, but you’ve just finished pulling an all-nighter for work. It’s 8 a.m. and your circadian rhythm is ready for you to be awake for another day, but you’re exhausted because your sleep pressure has been building up for more than 24 hours.
Does Caffeine Help?
So, let’s say you’re tired because of either mounting sleep pressure from staying up too late, or because your circadian rhythm is out of whack from sleeping till noon every day of your week-long vacation and now you’re back at work 8 a.m. Monday morning. Will it help if you drink a cup of coffee, tea, or an energy drink with a shot of caffeine in it?
The answer, as you may know from your own experience, is yes and no. Caffeine temporarily blocks the effects of sleep pressure and can restore a level of alertness needed to adequately pay attention so you can remember information. However, caffeine can only do so much in the face of mounting sleep pressure. The bottom line is that caffeine can help you be more alert for a time, but it is no substitute for keeping a constant circadian rhythm and getting a good night’s sleep. Which leads us to the next reason as to why sleep is important for memory.
Sleep to Get Ready for New Learning
Remember the hippocampus, the part of your brain shaped like a seahorse that is involved in the storage, retention, and retrieval of memories discussed in Parts 1 and 2? It has a limited number of cells that must be used to store new memories. Exceed that limit and you may not be able to keep new information distinct from already-learned information—or you may overwrite one memory with another. Have you ever been studying the same type of material for hours on end and find that details of some facts start to get confused with that of other facts until much of what you’re trying to learn seems to be melting together? That may be a sign that you’ve reached your capacity for remembering that type of information—until you sleep.
As you learned in Chapter 4, when you sleep you are able to shift major responsibilities for retrieval of your recently acquired memories from your hippocampus to your cortex (the outer layers of the brain) where those memories can have more permanent, long-term storage. If the memory is about you, it will likely still contain a link to the hippocampus. If, however, the memory is for facts—such as who Rosa Parks was—the connection to the hippocampus can be severed without diminishing the accuracy of this semantic memory (see Chapter 5). In either case, you have succeeded in freeing up hippocampal capacity, so you wake with refreshed ability for new learning (Figure 20.1, top).
Figure 20.1. Sleep replenishes the binding capacity of the hippocampus (top). EEG waveforms of typical sleep cycles across a night’s sleep (bottom).
The Shrinking Brain
How do you free up brain capacity? There’s evidence that about 80% of the brain’s synapses—the connections between neurons—shrink while we sleep. Smaller connections indicate that the strength of memory traces is reduced or eliminated entirely. This shrinkage is selective, such that the most stable (presumably important) memory traces remain unaffected while most other connections shrink to get ready for new learning. This is thought to be one of the mechanisms of how, over time, we forget unimportant things that happened to us during the previous day yet still remember the important things.1 This model of memory selectivity is, however, not the only one. Reactivation of important memories during sleep is also critical, as we will discuss.
