Memory Without Remembering... For 17 Years

Speaking of studies that make you go "wow," I recently read a very recent one that really surprised me, and thought I'd describe it here.

Memory researchers are famous for coming up with different types of memory, sparking years of research and debate just trying to determine whether two particular types of memory are really different. One such debate has been over the implicit-explicit distinction. Implicit memory is, to quote the classic paper from which I stole the title of this post, "the nonconscious influence of previous experiences on the performance of tasks that do not require explicit or conscious recollection"1. In other words, its the influence of previous experience on the performance of a task without consciously recalling the previous experience. In a typical implicit memory study, participants will be given a list of words or group of pictures and told to do something with them (rate how positive or attractive they are, for example). After a delay, participants will be given an ostensibly unrelated task in which they receive word (or picture) fragments, and are asked to name them. Word (or picture) fragments taken from the original list will be named faster and more accurately than fragments that weren't on the original list. Explicit memory is, well, the opposite. In explicit memory tasks, people generally learn some information (e.g., a list of words), and are later asked to either recall information when given a cue, or to indicate whether information (e.g., words on a new list) was in the original learned set.

One of the ways in which implicit and explicit memory have been dissociated has to do with forgetting. Some studies have shown that explicit memories are forgotten faster than implicit memories, while others have shown that the two have similar forgetting rates2. So the debate rages on. Until now, perhaps, because David Mitchell recently published a paper in Psychological Science that may show once and for all that implicit memory has a much, much longer retention rate3.

In 1982, Mitchell showed 48 participants drawings of common objects twice. After a short delay, they were again given the pictures, along with new ones, and asked to indicate whether they'd seen them before (an explicit memory task). The total time to which they were exposed to each picture varied from 1-3 seconds. In 1999, seventeen years after they'd participated in the original experiment, the original participants were mailed picture fragments, some of which were from the original set, some of which had been the "foils" (pictures not in the original set) in the 1982 explicit memory test, and some of which were new foils. They were instructed to name the object in each picture. As a control, new participants were also given the same picture fragments, and asked to name them.

Of the original 49 participants, 12 1982 participants sent back completed packets in 1999. Mitchell measured the priming rate (performance on the original pictures - performance on the two kinds of foils), and got priming rates of 24.7% for the 1982 participants, and 5.2% (a number not statistically significant from 0) for the 1999 participants. Furthermore, four of the 1982 participants indicated that they had no explicit memory of participating in the study 17 years prior. The mean priming score for these four participants was 19%. Clearly, their implicit memories of the original pictures (which, remember, they'd only seen for 1-3 seconds!) outlasted their explicit memories of the entire experiment experience! Interestingly, the priming rates for the 1982 participants were not that different from priming rates for much shorter intervals (e.g., 1 week). It really seems as though these people had some sort of memory trace (sorry Norman Malcolm) of pictures they'd briefly seen 17 years earlier. I don't know about you, but I think that's incredible.

1Schacter D L. (1987)> Implicit memory: history and current status. Journal of Experimental Psychology: Learning, Memory, and Cognition, 13, 501-518.
2McBridge, D.M., & Dosher, B.A. (1997). A comparison of forgetting in an implicit and explicit memory task. Journal of Experimental Psychology: General, 126(4), 371-392.
3Mitchell, D.B. (2006). Nonconscious priming after 17 years: Invulnerable implicit memory? Psychological Science, 17(11), 925-929.

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On the other hand . . .

Only 12 of the 49 returned the materials. Might there be some sort of bias in that only the ones who felt comfortable because of some recollection from 17 years ago returned the materials? In other words, only those with really good memories participated in the later study, while the ones with the poor memories just didn't bother?

Ahcuah - good question: from the article:

Various measures taken in 1982 (explicit memory, education, Wechsler Adult Intelligence Scale vocabulary, and picture-naming errors and priming) showed no reliable differences between the 12 subjects who responded in 1999 and the 17 who did not, ts(27) = 0.28 to 1.12, ps > .25.

Ahcuah, also, since 4 of them didn't remember the experiment at all, and the measure was of implicit memory (they didn't realize that memory was involved in the task they were doing in 1999), it's unlikely that there was a selection bias based on memory.

wow, that's incredible.