Approximate reading time: 34 minutes

Biology of Memory

Just as information is stored on digital media, the information in long-term memory (LTM) must be stored in the brain. The ability to maintain information in LTM involves a gradual strengthening of the connections among the neurons in the brain. When pathways in these neural networks are frequently and repeatedly activated, the synapses become more efficient in communicating with each other, and these changes create memory. This process, known as long-term potentiation (LTP), refers to the strengthening of the synaptic connections between neurons as a result of frequent stimulation (Lynch, 2004). Drugs that block LTP reduce learning, whereas drugs that enhance LTP increase learning (Lynch, 2002). Because the new patterns of activation in the synapses take time to develop, LTP happens gradually. The period of time in which LTP occurs and in which memories are stored is known as the period of consolidation.

Long-term potentiation occurs as a result of changes in the synapses, which suggests that chemicals, particularly neurotransmitters and hormones, must be involved in memory. There is quite a bit of evidence that this is true. Glutamate is perhaps the most important neurotransmitter in memory (McEntee & Crook, 1993). When animals, including people, are under stress, more glutamate is secreted, and this glutamate can help them remember (McGaugh, 2003). Estrogen, a sex hormone, also seems critical for memory for both sexes (Cheung et al., 2013; Korol & Pisani, 2015; Maki & Resnick, 2000; Sherwin, 2012). Interestingly, women who are experiencing menopause, along with a reduction in estrogen, frequently report memory difficulties (Phillips & Sherwin, 1992). From an evolutionary perspective, emotion plays a highly adaptive role in memory. If you survive a frightening encounter with a bear, for example, you are likely to remember to avoid bears in the future. For this, we can thank the amygdala, as well as stress-related chemicals such as epinephrine (adrenalin) and cortisol.

Our knowledge of the role of biology in memory suggests that it might be possible to use drugs to improve our memories. Yet, controlled studies comparing memory enhancers, such as Ritalin, methylphenidate, ginkgo biloba, and amphetamines, with placebo drugs find very little evidence for their effectiveness (Gold et al., 2002; McDaniel et al., 2002). Memory supplements are usually no more effective than drinking a sugary soft drink, which also releases glucose and thus improves memory slightly. This is not to say that we cannot someday create drugs that will significantly improve our memory. It is likely that this will occur in the future, but the implications of these advances are as yet unknown (Farah et al., 2004).

Although the most obvious potential use of drugs is to attempt to improve memory, drugs might also be used to help us forget. This might be desirable in some cases, such as for those suffering from post-traumatic stress disorder (PTSD) who are unable to forget disturbing memories (Pitman et al., 2002). Although there are no existing therapies that involve using drugs to help people forget, it is possible that they will be available in the future. These possibilities will raise some important ethical issues: is it ethical to erase memories, and if it is, is it desirable to do so? Perhaps the experience of emotional pain is a part of being a human being, and perhaps the experience of emotional pain may help us cope with the trauma.

Where in the brain are memories made and stored? This is not an easy question to answer. Unlike physical storage devices, the brain cannot add another filing cabinet or hard drive. Instead, the many different neural systems that overlap and influence each other to create and retrieve memories make it complicated to study. As a result, scientists combine techniques that allow the study of areas in isolation with techniques that allow the study of memory systems.

One way to study an isolated region is to intentionally damage or remove it, a technique called ablation or lesioning. If the subject shows impaired performance on a memory task after lesioning, then the damaged area was likely involved in that task. Because it would be unethical to create lesions in healthy human subjects, lesion experiments are performed in non-human animals, often to create a model of human systems. This technique allows scientists to assess memory before damage occurs, ensuring that changes in performance are a result of the lesion. Experimental lesions are also specific in location, which is not often the case with naturally occurring brain injury. In contrast, studies of brain damage in humans rely on individuals who already have brain damage, or who are having brain regions removed for medical purposes. In these cases, we cannot be sure which effects result from the damage, or which specific regions are responsible.

Still, people with rare brain injuries can provide unique information about the nature of memory systems by examining the abilities that they lost. One famous case study of memory comes from a man named Henry Molaison, known as patient H.M., and his collaboration with researcher, Brenda Milner. H.M. suffered severe seizures from his temporal lobes, which were unresponsive to high-dose anticonvulsants. In 1955, he underwent experimental neurosurgery. Specific portions of his medial temporal lobes, including the anterior hippocampus and amygdala, were removed. Although the surgery controlled his epilepsy, it led to unexpected memory impairments (Milner, 2005). After the surgery, H.M. was unable to retain new experiences for more than 30 seconds. Every time H.M. met Milner, he introduced himself as if they had never met before. He was only able to remember small chunks of information through maintenance rehearsal (i.e., continuously repeating that information and keeping it alive in his working memory). However, H.M. could gradually form some new information through repeated exposure. For example, it took about five years to navigate the inside of the house he moved to after his operation. Likewise, he learned how to play tennis, although he could not remember being taught the skills and denied that he knew how when asked. He completed a motor task where he had to copy a drawing by only looking at a reflection of his hand through a mirror. After 30 trials over three days, he had no conscious awareness of doing the task before, but his performance improved significantly.

H.M.’s case helps to link the medial temporal lobe with explicit memory function and aids in distinguishing different types of memory. The hippocampus helps us encode and consolidate information into LTM, especially spatial and temporal relationships, the context in which events were experienced, and the associations among memories. Damage to the hippocampus results in the inability to transfer information from short-term into long-term memory, making it impossible to form new memories. This is known as anterograde amnesia.

In contrast, retrograde amnesia is a memory disorder that produces an inability to retrieve events that occurred before the trauma.

Imagine that you were in a car accident, suffered a head injury, and now have retrograde amnesia and do not remember anything about your life before waking up in the hospital. There are people surrounding your bed claiming to be your spouse, your children, and your parents but you do not recognise any of them. Hollywood has been fascinated with the amnesia plot for nearly a century, going all the way back to the film Garden of Lies from 1915 to more recent movies such as the Jason Bourne spy thrillers. However, for real-life sufferers of retrograde amnesia, like former NFL football player Scott Bolzan, the story is not a Hollywood movie. Bolzan fell, hit his head, and deleted 46 years of his life in an instant. He is now living with one of the most extreme cases of retrograde amnesia on record.

Watch this video: Scott Bolzan on Pat’s Portraits (7 minutes)

“Scott Bolzan on Pat’s Portraits” video by KTAR News is licensed under the Standard YouTube licence.

The hippocampus is important for laying down certain types of explicit memories, but its absence does not affect the ability to create implicit memories, such as procedural memory, motor learning, and classical conditioning. Three brain areas have been identified as critical for implicit procedural learning: cerebellum, basal ganglia, and frontal lobe (Vidoni & Boyd, 2007). Research shows that animals and humans with damage to the cerebellum have more difficulty in classical conditioning studies (Krupa et al., 1993; Woodruff-Pak et al., 2000). The basal ganglia and the frontal lobe are found to be involved in both explicit and implicit learning. For instance, people with basal ganglia stroke have difficulty in using explicit instructions to perform implicit motor tasks (Boyd & Winstein, 2004).

Why are some types of memory easier to remember than others? For example, why are emotionally arousing memories like your first kiss or your pet passing away easier to remember than your breakfast last week? Emotions provide a “boost” to our memory, but how? Neuroimaging studies show that the amygdala becomes highly active when viewing emotionally stimulating events. For example, one study had participants watch emotionally negative film clips (e.g., a violent crime), and emotionally neutral ones (e.g., court proceedings). Recall was better for emotional films and was positively correlated with amygdala activation, as measured by PET imaging (Cahill et al., 1996).

Forgetting

In ordinary life, we do not remember everything we experience. As we discussed earlier, we simply do not encode a lot of what we experience, but what about memories that we do encode but later forget? Being able to forget is adaptive; if our consciousness was continually bombarded with memories, it would be difficult to cope with reality. What happens to the information that was encoded, stored, and then forgotten? Is it lost for all time? Many students will have had the experience of learning something thoroughly for a course, but then later on, perhaps only months later, be unable to remember more than a few facts. Let us look at some of the processes that might be responsible for forgetting.

Encoding failure. Much of what we sense we never notice, and what we fail to encode, we will never remember. You probably have seen the Apple computer logo thousands of times. Can you draw it? As Adam Blake and colleagues’ (2015) study suggested, only 1 of the 85 undergraduates could do so accurately.

Storage decay. Probably the most intuitive way of understanding forgetting is “use it or lose it”: if we do not think about something for a long time, then we tend to forget it. Hermann Ebbinghaus’s (1885/1913) forgetting curve is evidence in the short term of decay theory — the notion that memories decay over time. He practiced memorising lists of nonsense syllables, such as DIF, LAJ, LEQ, and MUV, and tried to recall them over varying time intervals. He originally wanted to plot how many of the syllables he could remember against the time that had elapsed, but because his recall had failed completely after long delays, he had to invent another method: he measured the number of trials required to fully remember a list in the initial learning, and the number of trials required to relearn the lists. The difference is called “savings.” For example, if the initial learning required 10 trials and relearning required 7 trials, the savings was 30%. Then he plotted savings as a function of elapsed time and discovered an important principle of memory: memory decays rapidly at first, but the degree of decay levels off with time.

Retrieval failure. In some cases, forgetting is not memories faded, but memories unretrieved. We have all experienced retrieval failure in the form of the frustrating tip-of-the-tongue phenomenon, in which we are certain that we know something that we are trying to recall but cannot quite produce it. Forgotten words are particularly prone to the tip-of-the-tongue phenomenon. We might think of words that are similar in sound, have the same number of syllables, or mean the same thing — but we cannot bring the actual word to mind. The tip-of-the-tongue experience is a very good example of the inability to retrieve information that is actually stored in memory, although the word “tip-of-the-tongue” is a bit misleading – people with hearing impairments also experience a parallel “tip-of-the-finger” phenomenon in sign language (Thompson et al., 2005).

Retrieval failures may be caused by a lack of retrieval cues. For instance, we discussed mood-congruent memory earlier, which works for forgetting as well: when we are in a good mood, we tend to forget or ignore memories of things that are incongruent with happiness, and when we are unhappy, we tend to forget happy memories. This interplay of emotion and cognition is complex, but it may help to explain why some things are forgotten by some people but not others, or at certain times but not others. This congruence effect may broaden to other aspects of the situation; the less similar the recall context is to the context in which something was learned, the more we are likely to forget. Locations, times, people, facts, and pictures can all aid as memory cues to elicit memories. Most adults are not able to remember anything before ages three or four. This phenomenon is called infantile amnesia (or childhood amnesia). Infantile amnesia may be attributed to, among other explanations, a mismatch between the context upon encoding and the context at retrieval (Schachtel, 1947).

Retrieval failures may also stem from interference. As you collect more and more information, one retrieval cue may lead to multiple items in your memory. Retroactive interference occurs when learning something new impairs our ability to retrieve information that was learned earlier. For example, if you have learned to program in one computer language, and then you learn to program in another similar one, you may start to make mistakes programming the first language that you never would have made before you learned the new one. In this case, the new memories work backward (i.e., retroactively) to influence retrieval from memory that is already in place. The earlier memories were encoded and used, but then they were “forgotten” as they were interfered with by new information. In contrast, proactive interference works in a forward direction. Proactive interference occurs when earlier learning impairs our ability to encode information that we try to learn later. For example, if we have learned French as a second language, this knowledge may make it more difficult, at least in some respects, to learn a third language (e.g., Spanish), which involves similar but not identical vocabulary. In this case, we forget new information because the old memories have interfered.

Retroactive and proactive interference can both cause retrieval failure:

False Memory

Memory is constructive and reconstructive. Despite a widespread belief that memory is like the replaying of a videotape, the evidence suggests that our memories are active reconstructions. For example, you might think you remember something from childhood, but it might actually have been an event that was experienced by a sibling. The family stories have been unknowingly reconstructed with vividness. The likelihood of such reconstructions increases when a story is told and retold many times — perhaps over several years. The more colourful the story, the easier it is to imagine.

One potential error in memory involves source misattribution, which refers to mistakes in differentiating the sources of information. Unintentional plagiarism may occur due to source misattribution, wherein an author mistakenly attributes other people’s ideas as their own. For instance, George Harrison of The Beatles claimed that he was unaware that the melody of his song “My Sweet Lord” was almost identical to a song called “He’s so Fine” published seven years earlier by Ronnie Mack and popularized by The Chiffons. You can easily find versions of both tunes online and compare them for yourself. The judge in the copyright suit against Harrison ruled that Harrison did not intentionally commit the plagiarism. Harrison’s source misattribution is clearly expressed in his memoir:

“I wasn’t consciously aware of the similarity between ‘He’s So Fine’ and ‘My Sweet Lord’ when I wrote the song, as it was more improvised and not so fixed,” Harrison wrote in I Me Mine. “Although when my version of the song came out and started to get a lot of airplay, people started talking about it, and it was then I thought, ‘Why didn’t I realise?’ It would have been very easy to change a note here or there and not affect the feeling of the record.” (Mastropolo, 2016, para. 6)

Generally speaking, false memory refers to the phenomenon where someone recalls something that did not happen or recalls it differently from the way it actually happened. False memories can be elicited by presenting subjects with a list of words that are related to each other (e.g., bed, rest, tired, dream, snooze, yawn) and then testing them with a critical lure, which is, a word that is not from the original list but semantically related to the list words (e.g., sleep). This approach is referred to as the Deese-Roediger-McDermott (DRM) paradigm (Roediger & McDermott,1995).

A particular problem for eyewitnesses is that our memories are often influenced by the things that occur to us after we have learned the information (Loftus, 1979, 2005). This newer information can distort our original memories such that we are no longer sure what is the original and what was provided later. The misinformation effect refers to errors in memory that occur when new information influences existing memories.

In an experiment by Elizabeth Loftus and John Palmer (1974), participants viewed a film of a traffic accident and then randomly answered one of three questions:

Although all the participants saw the same accident, their estimates of the cars’ speed varied by condition. Participants in the “smashed” condition estimated the highest average speed at 40.8 miles per hour, and those in the “contacted” condition estimated the lowest average speed at 30.8 miles per hour.

In addition to distorting our memories for events that have actually occurred, misinformation may lead us to falsely remember information that never occurred. In Elizabeth Loftus and Jacqueline Pickrell’s (1995) famous “getting lost in the mall” study, the researchers first obtained three true events that happened to each participant (e.g., moving to a new house) from their close relative, and added a false event that did not happen (e.g., being lost in a shopping mall). The participants were then asked to provide details for each event. Out of the 24 participants, six “remembered” the false event. After being told that one of the four events was false and asked to identify it, 19 participants were able to identify the getting-lost memory as false, while the remaining five incorrectly thought that one of the true events was false. This study was replicated in 2023. The researchers found that 35% of the 123 participants generated a false memory for getting lost in a mall in childhood (Murphy et al., 2023).

The misinformation effect can have serious consequences in the criminal justice system if incorrect information is given to witnesses, causing them to change their memories, or to jurors, who may misremember evidence presented in court. In a study by William Crozier and colleagues (2020), mock jurors misremembered evidence from a police report containing misleading information. Exposure to misinformation made them believe more strongly in the suspect’s guilt. Even when warned about the presence of misinformation, the effect persisted, showing how robust this memory distortion can be.

Eyewitnesses to crimes are frequently overconfident in their memories, and there is only a small correlation between memory accuracy and the subjective confidence about the memory.

Depending on your age, you may have a clear memory of when you first heard about the 9/11 attacks in the United States in 2001, or the passing of Queen Elizabeth II in 2022. This type of memory, which we experience along with a great deal of emotion, is known as a flashbulb memory — a vivid and emotional memory of an unusual event that people believe they remember very well (Brown & Kulik, 1977). People are very certain of their memories of these important events, and they are frequently overconfident. Jennifer Talarico and David Rubin (2003) tested the accuracy of flashbulb memories by asking students to recall their memories of how they learned about either the September 11, 2001 terrorist attacks or an everyday event that occurred around the same time. The researchers recorded these memories on September 12, 2001, and then asked the participants to recall them again after one, six, or 32 weeks. As time passed, the accuracy of both emotional and everyday memories declined, but the participants’ confidence in their memory of learning about the attacks remained high and even became overconfident after 32 weeks. This study highlights how overconfidence can lead us to falsely believe that our memories are correct.

Do you think people can repress their traumatic memories? How reliable are the memories recovered during therapies using dream interpretation and hypnosis? This is referred to as the “Recovered Memory Debate” (Lindsay & Read, 2001). Studies indicate that individuals who have experienced childhood trauma may sometimes have difficulty recalling clear memories of those experiences (Dalenberg et al., 2020; Kate et al., 2021). The “repression” position states that such traumatic events are necessarily repressed and remain in the unconscious (Freyd, 1996; Solinski, 2020). Other researchers argue that painful memories, like sexual abuse, can naturally fade away over time due to decay or interference (rather than repression), much like other early childhood memories (McNally, 2007, 2012), and can be retrieved spontaneously by later encountering a reminder in the daily life (i.e., delayed recall, Elliott & Briere, 1995). In contrast, proponents of the “false memory” position argue that the procedures used by the therapists to recover the memories are more likely to actually implant false memories, leading the patients to erroneously recall events that did not actually occur (e.g., Loftus & Ketcham, 1994; Otgaar et al., 2022). This line of research challenges the accuracy of recovered memories and calls for a more careful investigation of the recovered memories (Geraerts et al., 2007). For example, cognitive interview techniques are shown to effectively help therapists reduce their clients’ memory errors upon retrieval (Memon et al., 2010).

Image Descriptions

Figure ME.15. Stress and Memory image description:

1. Stress (Go to step 2 or step 3)
2. Hypothalamus, Pituitary (gland), Adrenal cortex
3. Glutamate, Epiinephrine, Cortisol, Estrogen, etc.
4. Hippocampus, Amygdala
5. Processing
6. Long-term Memory [Return to Figure ME.15]

Image Attributions

Figure ME.15. Image created for this textbook, credit of bear Photo by mana5280 on Unsplash.

Figure ME.16. Figure 8.10 as found in Psychology 2e by OpenStax is licensed under a CC BY 4.0 License.

Figure ME.17. Figure 8.14 as found in Psychology 2e by OpenStax is licensed under a CC BY 4.0 License. 

Figure ME.18. Figure 8.19 as found in Introduction to Psychology (Libretexts) is shared under a CC BY-NC-SA 3.0 license.