In a nutshell: You remember what’s happening in the background better than you think, but scientists don’t yet know how this type of “incidental memory” works.

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You walk into a café, scanning the room for your friend. As your eyes flit from one person to the next, does your brain commit to memory all those faces?

A study led by CIBF associate investigator Naotsugu Tsuchiya of Monash University suggests that it does, at least briefly.

The team showed volunteers an image of a face, and then asked them to pick it out from an image of a crowd of 20 to 55 people. Their eye movements were tracked as they searched, and faces that were viewed and rejected along the way, recorded.

Immediately after, they were shown one of the rejected faces, and another new face, and asked to identify the one they had already seen in the crowd. They picked the correct one more than 70% of the time – far greater than chance.

The finding runs counter to the notion that things we only view briefly are quickly forgotten, says lead author Lisandro Kaunitz of the University of Tokyo.

The mechanism behind these incidental memories is a mystery. Working memory – the short-term, readily-accessed memory – lacks the capacity. It can hold only 3 to 4 images, whereas in this experiment the participants remembered faces they had viewed and rejected 7 faces before the target face just as well as a face viewed immediately before the target, suggesting greater capacity.

The Tsuchiya team has also ruled out unconscious memory – which marketers tap into to generate brand recognition. This type of memory is vast. However, when participants rated their confidence in their memories, the greater their confidence, the more accurate the memory. Their insight – knowing what they knew and what they didn’t – shows these memories are conscious, says Tsuchiya.

The Tsuchiya team is yet to test whether incidental memories last long enough to be relevant in a criminal investigation — they could be gone in a minute.

Next steps:
The team will look at how long our brains retain these incidental memories, and find out if other objects are remembered as accurately, or whether faces, because of their social importance, are a special case.

Kaunitz, L. N., Rowe, E. G., & Tsuchiya, N. (2016). Large capacity of conscious access for incidental memories in natural scenes. Psychological Science, 27(9), 1266-1277.

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