HOW THE BRAIN ASSIGNS an event to a specific time and then puts that event in a chronological sequence—or in the case of my patient, fails to do so—is still a mystery. We know only that both the memory of facts and the memory of spatial and temporal relations between those facts are involved. Accordingly, when I was at the University of Iowa, my colleagues Daniel Tranel and Robert Jones and I decided to investigate how an autobiographical timeline is established. By looking at people with different kinds of memory impairment, we hoped to identify what region or regions of the brain are required to place memories in the correct epoch.
We selected four groups of participants, 20 people in total. The first group consisted of patients with amnesia caused by damage in the temporal lobe. Patients with amnesia caused by damage in the basal forebrain, another area relevant for memory, made up the second set. The third group was composed of patients without amnesia who had damage in places other than the temporal lobe or basal forebrain. We chose as control subjects individuals without neurological disease who had normal memories and who were matched to the patients in terms of age and level of education.
Every participant completed a detailed questionnaire about key events in their life. We asked them about parents, siblings and various relatives, schools, friendships and professional activities. We verified the answers with relatives and records. We also established what the participants remembered of key public events, such as the election of officials, wars and natural disasters, and prominent cultural developments. We then had each participant place a customized card that described a specific personal or public event on a board that laid out a year-by-year and decade-by-decade timeline for the 1900s. For the participants, the situation was an experience similar to playing the board game Life. For the investigators, the setup permitted a measurement of the accuracy of time placement.
Predictably, the amnesiac patients differed from the controls. Normal individuals were relatively accurate in their time placements: on average, they were wrong by 1.9 years. Amnesiac patients made far more errors, especially those with basal forebrain damage. Although they recalled the event exactly, they were off the mark by an average of 5.2 years. But their recall of events was superior to that of temporal lobe amnesiacs, who were nonetheless more accurate with regard to time stamping—they were off by an average of only 2.9 years.
The results suggest that time stamping and event recall are processes that can be separated. More intriguingly, the outcome indicates that the basal forebrain may be critical in helping to establish the context that allows us to place memories in the right epoch. This notion is in keeping with the clinical observation of basal forebrain patients. Unlike certain of their counterparts with temporal lobe damage, these patients do learn new facts. But they often recall the facts they have just learned in the incorrect order, reconstructing sequences of events in a fictional narrative that can change from occasion to occasion.