What does it take to demonstrate memory erasure? Theoretical comment on Norrholm et al. (2008).

An issue of increasing interest in Pavlovian conditioning is to identify ways to facilitate the development and persistence of extinction. Both behavioral and molecular lines of evidence demonstrate that learning during extinction can be enhanced. Similar evidence has been offered to support the idea that extinction causes the original association to be unlearned, or erased. Differentiating between extinction and erasure accounts is extremely difficult and requires many assumptions about the fundamental nature of how memory storage maps into memory expression. In this issue of Behavioral Neuroscience, Norrholm et al. (see record 2008-13280-002) describe a study of extinction with humans that has the potential to serve as a translational bridge between rodent work and clinical applications. They find less recovery of a conditioned fear response when extinction occurs 10 min compared with 72 hr after conditioning; however, the recovery of subjects' expectancies of the fearful stimulus is independent of when extinction occurred. These findings and others discussed in this article demonstrate some of the challenges in making inferences about memory erasure during extinction.