Recognition memory and recollective experience in Alzheimer's disease

The phenomenal experience that accompanies the recognition of a previously presented stimulus seems to take at least two distinct forms. Recognition can occur when the stimulus evokes some specific experience in which the stimulus was previously involved, or, alternatively, when the stimulus gives rise only to feeling of familiarity without any recollective experience. These two kinds of conscious awareness can be measured in laboratory conditions by "remember" and "know" responses. A "remember" response indicates that recognising the stimulus brings back to mind some conscious recollection of its prior occurrence, whereas a "know" response indicates that recognising the stimulus is not accompanied by any conscious recollection of its prior occurrence. In the experiments reported here the relationship between recognition memory and conscious experience in Alzheimer's disease (AD) patients was investigated. The purpose of the experiments was to compare "remember" measures of conscious awareness in free recognition and forced-choice recognition memory for words and unfamiliar faces. The point of the experiments was to see whether AD patients' performance might be associated with a decrease in the relative incidence of "remember" responses as compared to normal controls (NC), and whether there was an effect of experimental material (words versus faces) on recognition performance and on recollective experience. In both experiments AD patients produced significantly fewer correct responses and fewer "remember" responses for correctly recognised items than NC. By contrast AD patients produced the same proportion of "know" responses to target items as compared to NC in all recognition conditions, with the exception of forced-choice recognition of faces where they gave more "know" responses to target faces than NC. These results are consistent with the assumption that recognition memory may entail two processes, only one of which gives rise to conscious recollection, and they suggest that an impairment of conscious recollection is responsible for the poor performance of AD patients in recognition memory. Implications of these findings for current theories of retrieval are discussed.