There Are Many Types of Memories

Human memory is too complex to be described by just one type — so experts use four categories. Long-term memory is likely the category that comes to mind when you think of “memories,” since this type holds important biographical information. This mental category can be further divided into semantic memory, the stuff that helps you recall bits of trivia and other learned skills, and episodic memory, which keeps track of the vital moments of your life.

Another type of memory is sensory memory, which is initially triggered by one of the senses but is incredibly fleeting (interestingly, experts believe all memories first form as sensory memories). There’s also short-term memory, the one that’s supposed to keep track of your keys. Finally, there’s working memory, which helps the brain hold onto small tidbits of information in order to perform cognitive tasks. Some categorize working memory as a type of short-term memory, but it typically lasts longer and is mostly confined to the manipulation of information.

The Brain Can Store 2.5 Petabytes of Information

With the average life span of a U.S. citizen in the upper 70s, that’s a lot of time (and a lot of memories) for the brain to process and store — but amazingly, our minds are more than up for the task. Scientists estimate that the human brain can store upwards of 2.5 petabytes of information. A petabyte equals a million gigabytes, so that is a lot of storage space. According to Scientific American, that amount of information is equivalent to recording your television for roughly 3 million hours (which would take roughly 342 years nonstop). It’ll be a while before the iPhone can match the amazing memory-storing power of the human mind — brought to you by millions of years of evolution.

Dreaming Is the Result of the Brain Organizing Memories

Theologians and philosophers since time immemorial have tried to make sense of dreams, and the ability to interpret dreams appears in religious texts and pagan myths alike. When a person sleeps, they fluctuate among light, deep, and rapid-eye movement (REM) sleep, and it’s during this last phase — which accounts for roughly one-fourth of our sleeping hours — that the brain consolidates memories. If you look at the activity of a person’s brain when in REM sleep, it fires as if it’s awake, but the chemicals in the brain — such as serotonin, norepinephrine, and histamine — are blocked so we don’t injure ourselves by physically acting out our dreams. Research shows that during REM sleep, dreams are a reflection of a biological process wherein the brain strengthens neural connections to important experienced events while ditching the inessential information — all while preserving the integrity of current memories and adding new information. You can kind of think of your brain as a very large filing cabinet, and every night, your brain has to add new files to its ever-growing record system while simultaneously not upsetting its organized methodology.

Babies Form Memories (We Just Don’t Remember Them)

Ask anyone to describe their very first memory, and one thing is common — no one remembers their first two years. Scientists sometimes call this biological phenomenon “childhood amnesia,” an inability for older children and adults to recall their earliest years on Earth. But this doesn’t mean babies don’t form memories at all — studies in the ’80s and ’90s found that within the first few months, infants could form memories that aided in learning and completing simple tasks. The problem is that babies don’t form episodic memories (see above), a form of long-term memory that helps us remember details from specific events. When we’re between the ages of 2 and 4, our brain’s hippocampus — the region responsible for tying fragments of information together — begins to make the necessary neural connections for episodic memory, which is why your very first memories lie somewhere within these toddler years.

Some People Can Recall Every Day of Their Lives

Those blessed with a good memory can still usually conjure only a fuzzy picture of the past, but for people with hyperthymesia (also known as “highly superior autobiographical memory”) — a condition that gives the brain the remarkable ability to remember every single day of a person’s life since adolescence — memory is both a blessing and curse. In a famous 60 Minutes interview in 2010, one person with hyperthymesia described her exacting recall as effortless, saying, “It’s almost as automatic as if you say, ‘What is your name and where do you live?’” Name any date, and they’ll almost immediately tell you what day of the week it was and what they did that day. But a superhuman memory means remembering things best left forgotten. The first person to ever be identified with this condition, Jill Price, has described the experience as haunting, telling the Seattle Post-Intelligencer in 2008, “I’ve been through hell in my life.” As of 2021, only 60 or so people have been diagnosed with the condition, but their superhuman memories give scientists an unparalleled opportunity to study the still-unknown marvels of the human mind.

We Don’t Know If Photographic Memory Exists

While hyperthymesia is real, there is no existing evidence that a “photographic memory” — that is, the ability to recall any previously glimpsed image and examine it in precise detail — actually exists. The belief in this kind of memory may have grown out of the biological fact that humans are generally better at recalling visual material than other forms of information. The only known example of a condition similar to photographic memory is something called “eidetic memory,” when someone can hold an incredibly detailed and accurate image in their mind but only for a short time. Strangely, this incredible ability is relatively common in young children and virtually absent in adults, leading scientists to believe that this snapshot memory plays some role in cognitive development in early childhood.

Smell Triggers Particularly Strong Memories

Unlike our other senses, our sense of smell is more directly hardwired into the parts of the brain that deal with memory. Instead of electrical signals being sent to the thalamus as in other senses, smell bypasses this region of the brain and instead travels to the olfactory bulb, which is directly connected to the hippocampus and amygdala — areas of the brain associated with processing memory and emotion. These strong connections are likely why smell can elicit such powerful memories throughout our lives. A study in 2017 found that one part of the olfactory bulb in particular — known as the piriform cortex — is responsible for depositing scents in our long-term memory. This cortex links up with other regions of the brain to determine whether a smell earns a spot in the filing cabinet of our mind. Often, these memories take us back to our childhood, as the recollections tend to be associated with the first time we smelled a certain scent, allowing us to relive those early years, if only briefly.