In 1985, English musician Clive Wearing suffered a brain infection—herpes encephalitis—that left him with both retrograde and anterograde amnesia. He was unable to remember anything prior to waking up from the coma caused by the encephalitis, and he was unable to form new memories since the coma. “It was as if every waking moment was the first waking moment,” his wife Deborah wrote in the memoir Forever Today. “Clive was under the constant impression that he had just emerged from unconsciousness because he had no evidence in his own mind of ever being awake before.” Despite suffering from one of the most extreme cases of amnesia ever seen, Clive retained the ability to speak, think and write in the intervals between “waking up.” He couldn’t recall direct episodic memories, but he did retain certain semantic memories of events and facts from before his coma. And despite having no memory of ever learning it, Clive retained all of his skill and knowledge of playing and conducting music. Even more surprisingly, he retained all the same style and persona in his music and his conducting. Clive’s remarkably extreme case of amnesia is a perfect example of just how complex and multi-faceted memory and learning can be. Memory is inextricably tied up with our experiences of ourselves and the world. And yet, even without memory, so much of ourselves can remain. Scientists are nowhere close to completely untangling the complicated relationship between memory and consciousness, but extreme cases of amnesia like Clive’s have started to reveal some of the more unique intricacies of memory.
Clive’s unique case of amnesia perfectly illustrates the multifaceted nature of memory. Scientists typically categorize memories as either declarative or non-declarative. Non-declarative memories tend to be procedural memories—skills, habits, and reflexes. These memories can be explicitly built up over time—like learning how to play the piano—or they can be implicitly developed through associations or unconscious reflexes—like the negative association that prevents you touching a hot stove more than once. Since this form of memory involves subconscious and motor-based memory, it primarily takes place in the cerebellum.
On the other hand, declarative memories are primarily centered in temporal lobe structures like the hippocampus. Declarative memories include both basic semantic information and episodic recall of experienced events. While semantic memory can often be divorced from any sort of personal context, it can be much easier to recall when it has personal significance. You may struggle to remember the capital city of a foreign country but find it much easier to remember the capital of your home country or state. Episodic memories can be easy to remember in that we typically don’t need to explicitly learn them—you remember what you did yesterday because you experienced it. But episodic memories are the most vulnerable to corruption over time because there is no external way to accurately reinforce them (unless you happen to have video documentation).
The formation of memories starts with the perception of sensory information. Sensory input must be stored for a short period of time to build a continuous understanding of the world—a movie only makes sense if you can perceive how one image leads into the next. This first stage of memory, called sensory memory, only lasts a few seconds. A lot of the sensory information you are exposed to is irrelevant after those first few seconds—you need to remember the plot information revealed in a movie scene, not what every character was wearing. Any sensory information that is important enough to warrant more than a few seconds of memory gets moved into short-term memory or working memory, a type of short-term memory that allows you to hold onto old information while also processing new information. Working memory is necessary for continuous processing of more complex stimuli, like solving a math problem or reading a complex sentence. When you solve a mental math problem, you have to hold numbers in your working memory while performing calculations on those numbers.
Short-term memory only lasts around twenty seconds and has a limited capacity—most people can only hold around seven distinct items in their short-term memory simultaneously. And working memory adds the extra complication of processing while remembering. One way your brain tries to minimize the amount of information it’s juggling at once is through chunking, grouping items together into manageable bits of information. For example, if I were to ask you to memorize the phone number 5552315674, you might instinctually break it up into three pieces: 555-231-5674. Rather than learning ten distinct digits—too many for most people’s short-term memory—your brain only needs to focus on three small chunks of digits. But without reinforcement, your short-term memory will still lose hold of the information after twenty seconds or so. If you need to hold onto it for longer, your brain can keep information active in your short-term memory through rehearsal, repeating the information over and over again to keep it fresh in your mind. You’ve probably done this consciously countless times—whenever you need to remember a phone number or list of tasks until you can write them down. Rehearsal can be verbal or nonverbal, as long as it reinforces the information.
Meaningless repetitive rehearsal of the items in short-term memory can keep them active in your mind, but to actually stick those items in your mind more permanently you need to rehearse them in a way that imbues them with some significance. This elaborative rehearsal operates by connecting new information with existing schema in the long-term memory. This is why you tend to remember information better when you associate it with a mnemonic device, a song or rhyme, or a keyword. Do you still sing the alphabet song when you’re trying to alphabetize things? I do too. The way you encode information in your brain affects how you recall it later. The alphabet song acts as an anchor for your memory of those twenty-six letters. One of the best methods of encoding information into your long-term memory is through the self-reference effect. Our brains are innately self-obsessed, hardwired to pay more attention to information that directly involves us. Your self-identity, your personal memories, your interpersonal relationships are all major anchors in your long-term memory. If you can tie new information to one of those anchors, it’s much more likely to stick around permanently.
Once information is successfully encoded in the long-term memory, accessing it again requires active retrieval of the information, which can be its own complicated process. To some extent, your ability to retrieve a certain memory relies on the way it was encoded. Some memories can be tied to specific sensory cues that were recorded at the same time. For example, if you study something while listening to a particular song, you may find it easier to recall that information when the same song is playing. This phenomenon is called the encoding specificity principle, and it’s the reason why it can be helpful to study and learn material in the same environment you are going to take the test in later. But for cues like this to work, they have to be somewhat unique—too many memories tied to the same cue dilute the effect.
There are a few different ways your brain can retrieve information from your long-term memory. When you have to pull a piece of information out of your memory without contextual cues—like when you’re answering a fill-in-the-blank question on a test—you rely on recall to find and access that information. Alternatively, when you simply recognize a piece of information you’ve seen before—like when you’re answering a multiple choice question—you use recognition to identify the familiar information based on contextual cues. Recognition is often much easier than recall, as any test-taker can attest. Retrieval can also take the form of relearning, a process where information that is difficult to recall can be learned again more quickly. Relearning material is easier than learning it for the first time because the memory of that information still technically exists in the long-term memory, it just needs to be reinforced before it can be accessed.
Retrieving information from your memory can actually make it easier to retrieve again, which is why quizzing yourself before a test can be an effective study method. But retrieving a particular piece of information can also reduce your ability to retrieve other information in the same category—a phenomenon known as retrieval-induced forgetting. Since your memory is so focused on retrieving a specific item, it prevents similar but irrelevant information from interfering with your selective recall. But this mechanism can also lead to bias in terms of what information you perceive as important versus irrelevant, particularly when reconstructing an episodic memory. When we retrieve an episodic memory, we reconstruct the story from the sensory information we are able to access from our long-term memory. The details we retrieve and label as important are informed by our biased perception of the event. And the act of reconstruction solidifies those biases and allows errors to creep in. The more you access a memory, the more corrupted it can become. This is one of the reasons why eyewitness testimony can be so flawed—witnesses are susceptible to suggestion, bias, and flawed memory that can severely undermine the reliability of their testimony.
Even without suffering a severe case of amnesia like Clive’s, our memories can be full of flaws and bias. Our personal past is really just a story our brain tells us, cleaner and more narrative than reality ever could be. But our flawed memory informs the way we continue to perceive the world, reinforcing our biases and snowballing errors. Humans are chock full of cognitive biases that impact how we perceive our past, present, and future—sometimes with disastrous consequences.
But more on that next week! For now, check out last month’s series on the architecture of the nervous system and brain. Comment on this post or email me at contact@anyonecanscience.com to let me know what you think about this week’s blog post and tell me what sorts of topics you want me to cover in the future. And subscribe below for weekly science posts sent straight to your email!
