Memory, Interference with

Retroactive Interference by Further Cognitive Activity

The notion that forgetting might occur due to memory interference was first put forward in 1900 by the German psychologist Georg Müller and medical student Alfons Pilzecker. They noted during their early experimental work on healthy participants that recently learned material often seemed to “pop into consciousness” in an unwilled manner, especially when their participants were not engaged in any mentally effortful activity following new learning. Müller and Pilzecker hypothesized that this “perseveration” of recently learned material reflected a transiently continued activity of learning-related physiological processes that served the “consolidation,” that is, the strengthening of new memory traces.

They posited that the consolidation of a recently acquired memory trace could be hindered by subsequent cognitive activity; they called this effect “retroactive interference.” Their work showed that, indeed, more nonsense syllables were forgotten by their participants when the learning of such nonsense syllables was followed by the learning of further nonsense syllables (filled delay) than when it was followed by a period of rest (unfilled delay). This retroactive interference effect was also apparent when the interpolated material was not similar to the earlier material, that is, when the learning of nonsense syllables was followed by a picture description task; this result indicated that this interference effect was nonspecific. Further evidence for such nonspecific effects comes from John Jenkins and Karl Dallenbach, whose much-cited study demonstrated that more nonsense syllables were forgotten when the learning phase was followed by routine daily activities as opposed to a period of sleep.

In an attempt to explain how (nonspecific) cognitive activity might impede the consolidation of an earlier memory trace, the psychologist John Wixted argues that any cognitive activity engages the consolidation system to allow for the retention of this activity and its associated material. Moreover, he hypothesizes that consolidation resources are not unlimited. Therefore, according to Wixted, post-learning cognitive activity impedes the consolidation of an earlier memory trace because it actively deprives the earlier memory trace of limited consolidation resources.

However, the magnitude of this interference effect is not constant over time. Indeed, the most important and influential finding to have emerged from Müller and Pilzecker's as well as Ernest Burton Skaggs's early memory research is that of a temporally graded effect of further activity on early memory traces: Retroactive interference is largest when further activity occurs immediately following the learning of early material, and it decreases with increasing delay in the onset of further activity. This temporal gradient of retroactive interference indicates that memory traces are initially highly fragile and vulnerable to interference but that they consolidate and become less susceptible to interference over time.

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