Implicit Memory Preserved in Amnesia

In addition to motor skills Opens in new window, there are a number of other types of learning behavior which amnesics seem to be able to learn, though without any conscious memory of the learning event.

Such learning can sometimes be demonstrated by tests of implicit memory Opens in new window, in which patients’ behavior is shown to have been influenced by some previous experiences despite their inability to consciously recall it.

Claparede noted that the patient who had fallen foul of this trick refused to shake hands with him the following day, but could not explain the reason for this unwillingness. The cautious behavior of the patient thus revealed evidence of learning without any conscious awareness of the learning episode.

Similar effects were later demonstrated by Warrington and Weiskrantz (1968) using the technique of repetition priming. They showed Korsakoff patients a series of degraded pictures of common objects or words (see Figure X-1 "A" for an example), starting with the most incomplete version and then showing increasingly complete versions until the word or object was correctly identified.

Figure X-1 | Focus on A. An example of an incomplete or fragmented word stimulus

When the same procedure was repeated at a later time, the Korsakoff patients showed a marked reduction in the number of trials required to identify the object, thus providing clear evidence of learning.

A similar study was carried out on the patient HM Opens in new window, who also showed an improvement in the identification of degraded pictures following repetition priming Opens in new window (Milner et al., 1968).

Graf et al. (1984) used the priming of verbal material to demonstrate intact implicit memory in Korsakoff patients.

Following a priming task, subjects were presented with word fragments and asked to complete them with the first word that came into their heads.

In most cases the Korsakoff patients were found to respond with previously primed words, even though they revealed no memory of those words in a test of explicit recall or recognition. In fact the Korsakoff patients achieved similar wordstem completion scores to the control subjects.

Graf et al. concluded that implicit memory was unimpaired in Korsakoff patients, whereas explicit memory was severely impaired. Several other studies have confirmed that implicit memory, but not explicit memory, appears to be preserved in amnesics (Graf and Schacter, 1985).