Memory is fundamental to our existence. Almost every aspect of our lives is touched by it in some way: Our sense of self is founded on knowledge of our personal past, our ability to learn relies on our capacity to recall stored information, our cat is only fed if we remember to do so.
The ubiquity of memory means we can sometimes take its function for granted. Indeed, memory failures are sufficiently alien and compelling that they form the basis of many of our favourite books, films, and television shows.
Memory itself has many different faces. Memory researchers might separately study a person’s capacity to temporarily maintain information (working memory), to recall specific and acontextual facts (semantic memory), to perform tasks without needing to consciously attend to them (procedural memory), or even the capacity to plan and carry out future actions (prospective memory). Each of these can be separated at the level of behaviour, cognition, and the brain.
But what of individual "memories"? What is the process that allows you to remember stubbing your toe on the sofa this morning before you were properly awake? Or eating the slice-of-cake-too-far on your latest birthday? Or how it felt to see a tiger at the zoo when you were five?
These are the remit of episodic memory.
Slevogt, M. (1900) Schreitender Tiger im Käfig. https://commons.wikimedia.org/wiki/file:slevogt_schreitender_tiger_im_käfig.jpg
The term "episodic memory" was coined by Estonian-born Canadian psychologist Endel Tulving in 1972. Tulving proposed that the episodic memory system is engaged when we consider events in our past. He suggested that episodic memory by necessity incorporates details that cover "what, where, and when" — that is, they refer to happenings that occurred in a specific place at a specific time. He noted that this differed from the memory tests in traditional laboratory experiments, which were concerned only with ‘what’ participants could remember.
Further, Tulving proposed that episodic memories are subjectively experienced. The experience of remembering can be easily distinguished from that of experiencing the present or imagining the fantastical. As such, episodic memory requires a sense of chronesthesia and autonoesis. Tulving described these “esoteric concepts” as differing subtly — autonoesis being an awareness of the self in relation to subjective time, and chronesthesia being an awareness of subjective time in relation to the self.
In this way, Tulving believed that episodic memory could be differentiated from all other forms of memory — the rememberer is required to mentally travel through time. Tulving eloquently referred to this as episodic memory’s unique ability to bend time’s arrow in a loop.
Although research in animals has demonstrated a memory for "what, where, and when", no evidence has been produced to suggest they are capable of subjectively re-experiencing events. As such, it is widely held (though hotly contested) that episodic memory is a uniquely human capacity that is connected with our higher state of consciousness. That is to say, your memory of the tiger and the tiger’s memory of you are likely to be fundamentally different.
One claim of Tulving’s stands out — that the subjective experiences of memory and fantasy are tangibly different in our consciousness. Indeed, it is difficult for us to think of picking up the Eiffel Tower or speaking with a Martian, and to believe that we are truly remembering these as genuinely experienced events.
For the most part.
Seurat, G. (1884/1885) Study of Figures for "La Grande Jatte". National Gallery of Art, Washington, D.C. https://www.nga.gov/collection/art-object-page.164967.html
Some of the clearest insight into the general and healthy function of memory is derived from scenarios where it fails. It is well-documented that patients with schizophrenia experience memory distortions. A common example is a source-monitoring error — in which the patient fails to remember the origin of information. The patient might hear about a robbery on the news, then later report erroneously (but genuinely) that they themself had been the victim.
Catastrophic memory failures are not limited to clinical populations. Neurotypical humans are also susceptible to the creation (and sometimes implantation) of false memories. In an elegant example, Kimberley Wade and colleagues presented participants with a doctored photograph, showing their child self in a hot air balloon. Even though it had not occurred, half of participants volunteered details about the event.
But memory failures do not need to be so dramatic. They can also be seen in the everyday misremembering of small details. How often do you hear a story about yourself, that the teller somehow manages to mangle?
The majority of these failures in memory can be explained by the cognitive process of ‘remembering’ itself.
It is perhaps natural to consider memories as extrasensory video clips, that we are able to access and replay on demand. However, a "perfect replay capacity" is hard to square with the less-than-perfect nature of our memories. Instead, memory researchers consider the process of memory to be constructive — in which snippets of sensory and contextual information are retrieved from a variety of sources and integrated to form a cohesive event. Notably, neuroimaging research has established that ‘imagining’ and ‘remembering’ share common functional pathways in the brain.
As such, the process of remembering a tiger at the zoo is anything but simple.
Episodic memory forms an impressionistic take on the past. Designed to be sufficiently accurate but not exactly precise. It relies on a distributed neuro-cognitive system that allows for relevant details to be accessed, integrated, and subjectively re-experienced.
Tulving considered human episodic memory to be a true marvel of the natural world. Given its apparently unique nature, it is hard to disagree.
Corballis, M. C. (2019). Language, memory, and mental time travel: an evolutionary perspective. Frontiers in Human Neuroscience, 13, 217.
Schacter, D. L. (1999). The seven sins of memory: insights from psychology and cognitive neuroscience. American Psychologist, 54(3), 182.
Schacter, D. L., & Addis, D. R. (2007). The cognitive neuroscience of constructive memory: remembering the past and imagining the future. Philosophical Transactions of the Royal Society B: Biological Sciences, 362(1481), 773-786.
Tulving, E. (2002). Episodic memory: From mind to brain. Annual Review of Psychology, 53(1), 1-25.
Vinogradov, S., Willis-Shore, J., Poole, J. H., Marten, E., Ober, B. A., & Shenaut, G. K. (1997). Clinical and neurocognitive aspects of source monitoring errors in schizophrenia. American Journal of Psychiatry, 154(11), 1530-1537.
Wade, K. A., Garry, M., Read, J. D., & Lindsay, D. S. (2002). A picture is worth a thousand lies: Using false photographs to create false childhood memories. Psychonomic Bulletin & Review, 9(3), 597-603.