II Memory adaptations: Forgetting the past, remembering the future

The role of retroactive interference and consolidation in everyday forgetting


John T. Wixted

As the previous chapters in this book make abundantly clear, the subject of forgetting is as multifaceted as it is enigmatic. Why, exactly, do we forget? As noted by Levy, Kuhl, and Wagner (in Forgetting, chapter 7) we often use the term “forget” to refer to the inability to retrieve information that we failed to encode in the first place. Thus, for example, I might say that I forgot where I placed my keys, but the truth may be that I set them down without ever taking note of the fact that I put them on the kitchen counter. Although such absent-mindedness is an interesting issue in its own right, when experimental psychologists and cognitive neuroscientists study forgetting, they usually study the loss of information that was encoded, as shown by the fact that the information was once retrievable from long-term memory. What is it about the passage of time that renders once retrievable information ever more difficult to remember? That is the question I consider in this chapter.

The time course of forgetting was first experimentally addressed by Ebbinghaus (1885), who used himself as a subject and memorized lists of nonsense syllables until they could be perfectly recited. Later, after varying delays of up to 31 days, he relearned those same lists and measured how much less time was needed to learn them again relative to the time required to learn them in the first place. If ten minutes were needed to learn the lists initially, but only four minutes were needed to learn them again after a delay of six hours, then his memory was such that 60% savings had been achieved. As the retention interval increased, savings decreased, which is to say that forgetting occurred with the passage of time. When his famous savings function was plotted out over 31 days, what we now know as the prototypical forgetting function was revealed (Figure 5.1).

The Ebbinghaus (1885) savings data.The solid curve represents the least squares fit of the three-parameter Wickelgren power law, m

FIGURE 5.1 The Ebbinghaus (1885) savings data.The solid curve represents the least squares fit of the three-parameter Wickelgren power law, m = X(1 + pt)"4', where m is memory strength, and t is time (i.e., the retention interval).The equation has three parameters: is the state of long-term memory at t = 0 (i.e., the degree of learning), vp is the rate of forgetting, and (3 is a scaling parameter

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