Recalling an event can transport you into the past, bringing to mind the context of former similar events. For example, it’s easier to remember Jim’s great party this year if you can remember the horrible nosedive it took last year. Evidence of a neural basis for episodic memory — memory for the recall of events like Jim’s party — was found in a recent study by researchers from the University of Pennsylvania and Vanderbilt University. During the recall of retrieval cues, they found a neural signature (a pattern of brain electrical signals) of the temporal context in which the cues were encoded. In other words, when the brain formed a memory for an item, it did so by encoding it within a time interval that is later reinstated during recollection — thereby allowing for “mental time-travel.”
Published in the Proceedings of the National Academy of Sciences, the study recruited epilepsy patients who were part of a separate study so that researchers could use their already implanted electrodes to directly monitor brain activity.
Components of neural activity shown to play a role in memory creation and retrieval were measured. While brain activity was recorded, the participants took part in a free recall memory experiment. They first studied a list of common nouns, completed a brief distraction task, and were then asked to recall the words in no particular order. Upon recall, researchers found brain activity showed higher neural similarity in words that were positioned closer together in the studied list (hence, “temporal” context reinstatement). In recalling words, participants were also recalling the “contextual state” associated with the word. The authors of the study suggest that, “by showing that a component of the neural activity retrieved during memory search shows graded similarity to the brain states observed during the study of neighboring stimuli, we provide neural evidence for temporal context reinstatement in humans.”
This experiment provides the first neurobiological evidence of a phenomenon described by the temporal context model of episodic memory. The model, developed by Marc W. Howard and Michael J. Kahana in 2002, posits that when people recall an event, they also recall the context in which the event took place. This allows a memory to become episodic: linked to the time at which it occurred. The theory also suggests that recollection of the temporal context triggers recall of subsequent events — explaining why participants in the study tended to recall list items in succession.
Consequently, when we try to retrieve information about something that happened in the past we end up retrieving unrequested details about the experience as well. When Jim recalls his recent party, he may not only remember the sword swallowers but also that he was highly intoxicated. The authors believe this notion is in line with Tulving’s claim that episodic memory retrieval is like mental time travel. Kahana, a researcher in the study, describes temporal context reinstatement more intuitively: “When I remember my grandmother, for example, I pull back all sorts of associations of a different time and place in my life; I’m also remembering living in Detroit and her Hungarian cooking. It’s like mental time travel. I jump back in time to the past, but I’m still grounded in the present.”
Although this memory study doesn’t provide a complete description of how episodic memory works, it does mark an important step in understanding what happens in the brain during event recollection. More research has yet to be done; the authors suggest an important future goal would be to pinpoint the areas of the brain involved in the process of context reinstatement. In the meantime, others will discuss how these exciting new findings sprout changes in the study of episodic memory.
