Knowing “What” and Knowing “When”

Abstract

The ability to situate autobiographical memories accurately in the “time-line” of one's own life is a particular aspect of retrograde memory that has received little attention in well-controlled, systematic studies. Here, we addressed this issue by testing the hypothesis that patients with basal forebrain damage would be impaired in their ability to place various autobiographical memories accurately on a ‘time-line’ of their life. Seven such subjects were contrasted with 11 medial temporal lobe subjects, 8 brain-damaged comparison subjects, and 18 normal comparison subjects, using a procedure in which subjects placed autobiographical events on a time-line of their life. The basal forebrain group was very impaired in this task, relative to the other groups, and on average, misplaced events by more than five years. Although the basal forebrain group was also impaired in retrieving the contents of autobiographical memory, they did not differ statistically from the medial temporal lobe group in this regard (and the medial temporal lobe group did not have impaired time placement of memories). The results indicate an intriguing dissociation between “knowing what” and “knowing when,” and suggest that the basal forebrain contains structures that are especially important for “knowing when.” Our findings are compatible with the view that the basal forebrain participates critically in retrieval strategies important for memory chronology, which contrasts with the medial temporal lobe's critical role in relational memory per se.

Acknowledgments

We thank Robin Mitchell for help in collecting the experimental data. Kathy Jones and Ruth Henson helped schedule the subjects.

Supported by Program Project Grant NINDS P01 NS19632.

Notes

^aHandedness was measured with the modified Oldfield-Geschwind questionnaire, which provides an index ranging from full right-handedness (+100) to full left-handedness (−100).

^bChronicity refers to lesion vintage, i.e., the amount of time between lesion onset and execution of the current experiments.

*ACoA = Anterior communicating artery; ACE = Anterior cerebral artery.

¹Another domain that is frequently assessed in retrograde memory is “public” events, for example., famous faces, knowledge of TV programs, political events, and so on (e.g., Albert, Butters & Levin, 1979; Cohen & Squire, 1981; Zola-Morgan, Cohen & Squire, 1983). We did not explore the “public” domain in the current study, although this is a valid domain of inquiry. On the face of it, we do not have specific reasons to predict that there would be key differences between the “personal” and “public” knowledge domains in terms of the key findings of our study, although this is an empirical question that is open to further investigation.

²This 30-item threshold was selected based on pilot work, which had indicated that even the most severely impaired of our subjects was able to recall accurately about 30 pieces of information on the IAMQ. The exact number of items for the Time-Line Test varied slightly from subject to subject, but these variations were minimal (i.e., 30–32 items) and not systematically different across the 4 experimental groups.