|How does the mind file life’s episodes?|
Autobiographical or ‘episodic’ memory describes our ability to recall past experiences and is distinct from semantic memory, which is our factual knowledge about the world. So far so good, but according to Youssef Ezzyat and Lila Davachi, psychology until now has largely neglected to investigate exactly how the brain organises the continuity of lived experience into a filing system of discrete episodes.
Ezzyat and Davachi have made a start. They had 23 participants read six narratives containing dozens of sentences about a protagonist performing everyday activities. Each sentence was displayed one at a time on a screen. Crucially, a minority of sentences began: ‘A while later …’, thereby conveying a temporal boundary in the narrative; the end of one episode and start of another. For comparison, a small number of control sentences began: ‘A moment later …’, indicating that the ensuing sentence was part of the same episode, not a new one.
After a ten minute break, the participants were given a surprise memory test. Presented with one sentence from the earlier narratives, their task was to recall the sentence that had followed. The key finding here was that the participants were poorer at recalling a sentence that came after a temporal boundary. It’s as if information within an episode was somehow bound together, whereas a memory divide was placed between information spanning two episodes.
A second study was similar to the first except that nineteen participants had their brains scanned during the initial read-through of the sentences. Ezzyat and Davachi identified patterns of neural activity in distinct regions of the prefrontal cortex and the middle-temporal gyrus that either correlated with within-event processing or with forming boundaries between events. These neural activity patterns were more distinct in those participants who showed larger behavioural effects of episode boundaries in their memory performance.
‘Our experiments are an important step toward understanding how event perception and segmentation influence the structure of long-term memory,’ the researchers concluded. ‘The behavioural results support the hypothesis that event segmentation shapes the organisation of long-term memory; the fMRI [brain scanning] results link these memory effects to brain activity consistent with information maintenance and integration within events.’
Ezzyat, Y., and Davachi, L. (2010). What Constitutes an Episode in Episodic Memory? Psychological Science DOI: 10.1177/0956797610393742