Effects of Drugs on Sensation, Perception, and Memory

views updated

Effects of Drugs on Sensation, Perception, and Memory

Every behavior that a human or animal performs involves taking in information from the primary senses: vision, hearing, and touch. Many drugs of abuse change this sensory information. Mind-altering drugs can influence the way people perceive time, the way they think and behave, and their moods. Drugs also have an effect on memory. Just as people are always receiving sensory information, they are always learning things in the course of daily life. Imagine all of the sensory information—seeing, hearing, feeling—that a toddler must take in when taking a first, unassisted step. Very quickly children learn not to go near the street, that fire burns, that the fur of a kitten is soft, and that when pushed, the buttons on a remote control turn on a television. Within a few years, a young child has put thousands of pieces of information into his or her memory bank. Increasingly, the memories become more complex, involving numerous steps. For example, the very act of reading these words involves hundreds of memories, including information you stored in your brain many years ago about basic words and letters. Drugs can interfere with a person's ability to remember the things they learn.

Sensation and Perception

Naturally occurring drugs, such as mescaline from the peyote cactus, increase the user's awareness of visual and auditory sensations and also produce visual illusions and hallucinations . The psilocybin mushroom (Mexican or magic mushroom) produces similar effects. Because of these sensory changes, mescaline and psilocybin have been used for centuries in religious ceremonies by the peoples of Mexico and the American southwest.

Lysergic acid diethylamide (LSD), a drug first synthesized in a Swiss laboratory in the 1930s, has become well known for producing intense and colorful visual sensations. People also report changes in sensory perceptions with another class of synthetic drugs known as stimulant-hallucinogens, drugs that are chemically related to both methamphetamines and hallucinogens like mescaline or LSD. Ecstasy (MDMA) is the most popular drug of this type, but MBDB, MDE, MDA, MDEA, and 2CB are all chemical variations in circulation. The sensory effects of this class of drugs include hallucinations and "out-of-body" sensations. The active agent in marijuana, tetrahydro- cannabinol (THC), can also produce hallucinations, although less commonly than with LSD or mescaline. Cocaine and amphetamines sometimes produce hallucinations and other sensory distortions, but only when they are taken for long periods of time.

Various names are used to describe drugs that alter sensations and perceptions. One term is "psychedelic," which refers to mind- expansion or to experiencing events that go beyond normal boundaries. The term "hallucinogenic" refers to a substance that produces hallucinations. However, the term is slightly misleading, since not all drugs that alter sensations produce hallucinations.

Observations in Human Subjects. Most of the information about drugs and the ways in which they alter sensory behavior in people comes from individual reports (called anecdotal evidence) rather than from well-controlled laboratory studies. People have reported vivid images, changes in perception, and hallucinations after they have taken mescaline or LSD. Another alteration that sometimes occurs is synesthesia, which is a mixing of the senses. For example, people report hearing colors or seeing sounds.

Although these sensory disturbances stop within a few hours after taking LSD, some people experience confusion, sensory distortions, or poor concentration for longer periods of time. For some people, drug effects recur long after the drugs have left their systems. These brief episodes are called flashbacks.

Studies in the Laboratory. Since alterations in sensation and perception, such as hallucinations, cannot be observed directly, it is very difficult to examine these effects in laboratory animals. One way to investigate a drug's effect on sensation is to train animals to behave differently in the presence of different types of visual or auditory stimuli . If a drug changes the animal's behavior, it is possible that these changes in behavior are due to a change in how well the animal hears or sees the stimuli. Another type of procedure examines how intense (for example, how loud or how bright) a stimulus has to be for an animal to hear or see it. The point at which a stimulus can be seen or heard is called the threshold. In these procedures, the intensity required to hear or see a stimulus is determined before a drug is given, and then it is compared to the intensity required to hear or see the stimulus after the drug is given.

In general, drugs such as mescaline, LSD, THC, and ecstasy (MDMA) do not change an animal's ability to tell the difference between visual or auditory stimuli. In addition, they do not change visual or auditory thresholds. This lack of effect in animals suggests one of two explanations: (1) Drugs such as LSD may produce different effects in animals than they do in people. (2) The procedures used to study alterations in sensation and perception in animals given LSD and other drugs may be ineffective. The second explanation is more likely to be accurate. However, tests of the drug ecstasy in animals and humans have had similar results. Studies show that ecstasy damages memory in both animals and humans.

The changes in perception and sensation that occur when a person takes a drug can be very disturbing. Drug abusers often experience confusion, fear, anxiety, paranoia , and severe depression as a result of their drug experiences.


In addition to their effects on sensation and perception, alcohol and abused drugs have significant effects on memory. Much of our knowledge of the effects of commonly used substances on memory is based on experiments using laboratory animals.

In typical experiments, animals are first trained in a task that requires learning. For example, they may learn how to go through a maze to obtain a reward of food or water. Or they may learn to avoid or run away from an unpleasant stimulus. After a delay of one day or longer, the animals are given a test that measures how well they remember the learned task.

The researchers may give the animal a drug before the training, shortly after the training, or just before the memory test. When given within a few minutes after training, drugs of many classes can improve or impair memory. The effect on memory also depends on the size of the dose. Drugs that improve memory when given in low doses may impair memory when given in higher doses.

Alcohol. In experiments studying the effects of alcohol on rats and mice, animals given a large amount of alcohol before training usually have problems remembering the training. Overall, giving rats and mice high doses of alcohol for long periods impairs their memory.

Drinking large quantities of alcohol and long term–alcohol use also cause memory problems in humans. Large amounts of alcohol taken over a short period (hours or days) may cause a severe amnesia, or "blackout." The blackout wipes out the memory of events occurring during and/or shortly before the period of alcohol intoxication . State-dependent learning may play a role in alcoholic blackouts. When individuals become intoxicated on a later occasion, they may sometimes remember experiences that occurred during a previous blackout. Charlie Chaplin's 1931 film City Lights illustrates this effect on memory: the character of the hard-drinking millionaire remembers Charlie only when under the influence of alcohol.

Chronic, or long-term, drinking can produce three kinds of memory problems:

  1. The best-known memory problem is Wernicke-Korsakoff syndrome. This syndrome is due to Vitamin B 1 (thiamine) deficiency, resulting from the poor nutrition typical of heavy, long-term drinkers. Symptoms include mental confusion and difficulty with eye movements and walking. Treatment with thiamine can eliminate these symptoms. However, people who recover from Wernicke-Korsakoff syndrome may continue to have difficulties with learning and remembering new information.
  2. Alcoholic dementia is a condition in which a person suffers from severe memory problems. The person also suffers from major intellectual deterioration that resembles Alzheimer's disease. An individual with alcoholic dementia becomes confused and forgetful. He or she may have severe problems with memory, an inability to learn new things, and a hard time processing complex information. Eventually, severe alcoholic dementia may leave an individual unable to perform basic tasks of daily living, and unable to care for himself or herself. Patients who stop drinking often show improvement in these areas.
  3. The third type of memory problem linked to heavy alcohol use involves less obvious symptoms. The person may have trouble with abstract thinking, problem solving, and memory. Typically, patients recover when they stop drinking.

Benzodiazepines. Benzodiazepines, which are prescribed to treat anxiety disorders and insomnia, are among the most widely used (and abused) drugs. Benzodiazepines such as diazepam (Valium), triazolam (Halcion), and chlordiazepoxide (Librium) can cause amnesia in humans. Studies using laboratory animals show that benzodiazepines impair memory when given before training. They generally do not impair memory when given after training. This is probably because benzodiazepines are absorbed slowly and are therefore slow to reach peak concentrations in the brain. This slow absorption by the brain seems to dampen memory loss.

Marijuana. In studies of marijuana's effects on laboratory animals, animals were given THC, the active agent in marijuana. Some were given high doses of THC and some were given THC over long periods of time. Both sets of animals had difficulty in learning and remembering a very wide variety of tasks. There is also some evidence that high doses or chronic use of marijuana impairs memory in humans. These effects might be due to effects of THC on the brain processes underlying memory. It is also possible that these effects are simply due to the sedative quality of the drug. Stopping marijuana use typically results in quick recovery from the drug effects.

Opiates. In tests of the effects of opiate drugs on laboratory animals, morphine and heroin impaired memory when given to animals after training. No studies have been done on the effect of morphine, heroin, or other opiates on human memory. People who abuse opiates for long periods do have memory problems, but these problems may result from general poor health rather than from any specific effect of the drugs. Patients who are given opiates as anesthesia before surgery often fail to remember experiences immediately prior to surgery. This amnesia may be due, at least in part, to the effect of the opiates.

Amphetamines. Laboratory animals that are given high doses of amphetamines over long periods have problems performing many types of learning tasks. These effects occur when the drug is given before training. Other studies show that, depending on the dose, giving amphetamines after training can improve memory. Memory also improves when the drug is administered directly into several brain regions, including the amygdaloid complex, hippocampus, and caudate nucleus.

Amphetamine users often report that their ability to learn is enhanced by single doses of the drug. Since few studies have looked at the effects of amphetamine on memory in humans, it is impossible to say whether these reports show direct effects of the drug on memory. Improvements in learning might instead result from changes in the individual's perception and mood. Long-term amphetamine use usually causes memory to deteriorate, but this effect clears up when the person stops taking the drug.

Cocaine. In studies using rats and mice, high doses of cocaine given after training impaired memory. Low doses improved memory. Like amphetamine users, users of cocaine report that high doses improve memory and that long-term use impairs memory. However, the effect of cocaine on human memory has not been adequately studied.

Ectasy (MDMA). Both animal studies and human studies have found links between long term–MDMA use and memory impairment. In a recent study by the National Institute on Drug Abuse, researchers found that heavy MDMA users have memory problems that persist for at least two weeks after they have stopped using the drug. The extent of damage appears to be directly correlated with the amount of MDMA use. The changes in brain chemicals (neurotransmitters such as serotonin) thought to be responsible for this effect may be long-lasting or permanent. These changes were still present in monkeys seven years after their last exposure to MDMA.

State-Dependent Learning. Drugs affect the user's memory of things learned before the drug experience and also during the drug high. Material that a person learns while on a drug may be forgotten when the drug wears off. The person might not remember this material until he or she takes the drug again. This kind of learning is called state-dependent learning (SDL). It is also true that information a person learns while not on a drug may be forgotten when he or she takes a drug. In addition, material learned under one drug may be forgotten when another drug is used. SDL involves a "dissociation" of learning. This means that material learned while a person is on a drug is dissociated, or separated, from normal consciousness. Once it is dissociated, it cannot be retrieved—the person does not have access to the learned material.

SDL may offer important clues into multiple personality disorder. An area of current research concerns the dramatic increase in the number of reported cases of this disorder. According to one theory, the process underlying it is similar to the process that produces SDL while a person is under the influence of a drug.

Memory and Emotions. Many drugs alter moods, and users may experience emotions very intensely while under the influence of a drug. Researchers are currently exploring the possibility that emotions act as a cue for memory—in effect, calling it to a person's attention. They are trying to determine if memories learned in one emotional state are recalled best when that emotion occurs again and recalled less easily at other times. Overall, it is important to remember that the use of drugs and alcohol generally has a negative effect on learning and the ability to retain memories.

see also specific drug entries, such as Alcohol and Marijuana; Research.


Marijuana affects the parts of the brain that control emotions, memory, and judgment. Smoking pot habitually can weaken short- term memory but can also prevent the brain from creating new long-term memories.