Interference in Memory

I wrote recently about interference processes that cause memory failure. As I wrote before, retroactive interference is when learning new information causes you to forget what you learned previously. In proactive interference, old information makes it hard to learn new information.

It turns out that there are (maybe) two types of proactive interference, and this may tell us a great deal about how memory works.

How Specific?

Half a century ago, Keppel & Underwood found that people quickly get worse at memory tasks. A basic task works like this: Remember the following letters: "etnmwo"

Now look away from the screen. After a few seconds, ask yourself what the letters were. How many could you remember?

Keppel & Underwood task was slightly different, but this gives you the basic idea. Again, what they found was that as people play this game, they actually do best on the first trial, worse on the 2nd, even worse on the 3rd, etc. (People bottom out fairly quickly, as we'll see in a future post.)

Keppel & Underwood suggested that this was due to proactive interference, which now seems pretty well established.

Later researchers discovered a curious thing. If the memory task is done with letters for a while, and then the experimenter switches to numbers, the participants suddenly get better. It doesn't have to just be letters and numbers. Switching from one type of item (say, names of car manufacturers) to another type (say, names of animal species) typically leads to an improvement in performance.

This has been called "release from proactive interference." But it is not the only kind.

More Specific

The type of proactive interference discussed above has been called "item-nonspecific" proactive interference. Learning information about one item made it harder to remember information about similar items.

This can be contrasted with "item-specific" proactive interference. As an example, go back to the sample memory test above. You were asked to remember "etnmwo." Suppose in the next trial, I asked you to remember "oaqzp" for a few seconds, after which I asked you:

"Is one of the letters are are supposed to remember an E?"

There is a decent chance you would incorrectly say "yes." This is because, although E was not one of the letters on this trial, it was one of the letters on the previous one. If I had instead asked about the letter C, which was not in either trial, you would be more likely to respond correctly and say "No."

This effect was discovered by Monsell using what is called the "Recent Probes Paradigm" -- which is basically what I just described.

Two Types or One?

One could legitimately wonder if these are really two different phenomena. That is, maybe item-specific proactive interference is simply a stronger version of item-nonspecific proactive interference.

It is hard to answer that question using behavioral experiments. Luckily, this is one of those places where neuroimaging can be helpful in understanding behavior. Recent neuroimaging results have found a strong overlap between the brain regions involved in the two types of proactive interference.

What Does this Say about Models of Memory?

Jonides and colleagues have been developing a model of memory that may both describe and predict the data on proactive interference.

In the model, to the extent that I understand it, you perform a short-term memory task like the ones described above by activating representations of the items. That is, to remember "aort," you would activate your long-term memory representations of A, O, R & T. But you do not actually hold those representations in consciousness; it is more that you make them easy to retrieve.

Now, suppose I ask you to repeat back those letters. You have to retrieve each of the four letters into consciousness so that you can give me your answer. You do this via something vaguely akin to a keyword search. That is, you search your memory for relevant features (e.g., a letter, recently encountered, seen on a blog, etc.). Since A, O, R & T all match those features and are all activated in memory, you retrieve them successfully.

Suppose on the next trial, though, you have to remember W, Z, P & E. So you activate those representations in memory. But A, O, R & T also remain somewhat active. And they also match most of the features (i.e., "keywords"). So you might accidentally retrieve one of them (item-specific proactive interference). In addition, since memories overlap, the still-active A, O, R & T representations make it harder to activate and maintain the representations of W, Z, P & E, since the compete for use of some of the same neurons. This might just make you fail to activate or retrieve anything at all.

Notice that if on the next trial, I ask you to remember 9, 3, 5, & 2, these items share fewer features with the letters on the previous trials, making the "keyword search" easier. Also, the representations of 9, 3, 5 & 2 in the brain are more distinct from the representations of the letters in trials one and two than either were from each other. Thus, you get release from item-nonspecific proactive interference.

Monsell, S. (1978). Recency, immediate recognition memory, and reaction time. Cognitive Psychology, 10(4), 465-501.

Keppel, G., Underwood, B.J. (1962). Proactive inhibition in short-term retention of single items. Journal of Verbal Learning & Verbal Behavior, 1, 153-161.

Jonides, J., Lewis, R.L., Nee, D.E., Lustig, C.A., Berman, M.G. (2008). The mind and brain of short-term memory. Annual Review of Psychology, 59, 193-224.