Can we deliberately forget specific parts of what we've read?

The majority of research on memory is focused, as you might expect, on the remembering side of things - how much, how accurate and so on. There is, however, a parallel, but less known, line of investigation into our ability to deliberately forget. This is no mere academic curiosity. The ability to forget selectively that to which we've been exposed would be, if we had it, an extremely useful ability - a kind of refuse collection service for the mind.

In one of the first studies of its kind, Peter Delaney and colleagues have now shown that people are indeed capable of reading a series of sentences and then selectively forgetting just some of those sentences, whilst remembering the rest.

Dozens of undergrad students were first told to memorise an initial list of 16 sentences about the imaginary men, Tom and Alex. Afterwards, half the students were unexpectedly told to forget the Tom sentences, so as to better remember the Alex sentences. The remaining students acted as controls and didn't receive this additional instruction. Finally, all the students attempted to memorise a second list of 14 random sentences about another man, Joe. A 90-second multiplication test acted as a filler task before the students were tested on their ability to recall as many sentences as possible from the two lists.

The key finding was that the students were able to follow the forget instruction so long as the sentences about Tom and Alex were of random meaning, with no discernible theme. In this case, students told to forget the Tom sentences subsequently recalled just 28 per cent of them, in a two-minute free recall test, whereas the control students recalled 39 per cent. Moreover, there was also a non-significant trend for the students who deliberately forgot the Tom sentences to remember more Alex sentences (37 per cent) relative to the controls (32 per cent) - showing that their deliberate forgetting really had been selective for the Tom sentences. Memories of the second test list (sentences about Joe) were unaffected by the forget instruction.

Another version of the experiment had the Tom and Alex sentences gradually forming discernible themes - Alex as a writer who liked snow sports and Tom as a lawyer and family man, or vice versa. Curiously, in this version, the students were not able to deliberately and selectively forget the Tom sentences. The researchers aren't sure why, but one possibility is that remembering just one sentence in a theme involuntarily cues all the other sentences, thus hampering attempts to forget.

So how do we deliberately forget a portion of previously-seen material? One possibility is that the forget instruction prompts people to cease mental rehearsal of to-be-forgotten items, in favour of extra rehearsal of the to-be-remembered items. Another possibility is that items are selectively inhibited at the retrieval stage. "Future studies should use recognition tests to determine whether forgotten items are available in memory but blocked from access, or if they are less well learned," the researchers said.
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Delaney, P., Nghiem, K., & Waldum, E. (2009). The selective directed forgetting effect: Can people forget only part of a text? The Quarterly Journal of Experimental Psychology, 62 (8), 1542-1550 DOI: 10.1080/17470210902770049

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