Time and Memory
I WAS FIRST DRAWN TO the problems of time processing through my work with neurological patients. People who sustain damage to regions of the brain involved in learning and recalling new facts develop major disturbances in their ability to place past events in the correct epoch and sequence. Moreover, these amnesiacs lose the ability to estimate the passage of time accurately at the scale of hours, months, years and decades. Their biological clock, on the other hand, often remains intact, and so can their abilities to sense brief durations lasting a minute or less and to order them properly. At the very least, the experiences of these patients suggest that the processing of time and certain types of memory must share some common neurological pathways.
The association between amnesia and time can be seen most dramatically in cases of permanent brain damage to the hippocampus, a region of the brain important to memory, and to the nearby temporal lobe, the region through which the hippocampus holds a two-way communication with the rest of the cerebral cortex. Damage to the hippocampus prevents the creation of new factual memories. The ability to form memories is an indispensable part of the construction of a sense of our own chronology. We build our timeline event by event, and we connect personal happenings to those that occur around us. When the hippocampus is impaired, patients become unable to hold factual memories for longer than about one minute. Patients so afflicted are said to have anterograde amnesia.
Intriguingly, the memories that the hippocampus helps to create are not stored in it. They are distributed in neural networks located in parts of the cerebral cortex (including the temporal lobe) related to the material being recorded: areas dedicated to visual impressions, sounds, tactile information, and so forth. These networks must be activated to both lay down and recall a memory; when they are destroyed, patients cannot recover long-term memories, a condition known as retrograde amnesia. The memories most markedly lost in retrograde amnesia are precisely those that bear a time stamp: recollections of unique events that happened in a particular context on a particular occasion. For instance, the memory of one's wedding bears a time stamp. A different but related kind of recollection—say, that of the concept of marriage—carries no such date with it. The temporal lobe cortex that surrounds the hippocampus is critical for making and recalling such memories.
In patients who sustain damage to the temporal lobe cortex, years and even decades of autobiographical memory can be expunged irrevocably. Viral encephalitis, stroke and Alzheimer's disease are among the neurological insults responsible for the most profound impairments.
For one such patient, whom my colleagues and I studied for 25 years, the time gap went almost all the way to the cradle. When this patient was 46, he sustained damage both to the hippocampus and to parts of the temporal lobe. Accordingly, he had both anterograde and retrograde amnesia: he could not form new factual memories, and he could not recall old ones. The patient inhabited a permanent present, unable to remember what happened a minute earlier or 20 years before.
Indeed, he had no sense of time at all. He could not tell us the date, and when asked to guess, his responses were wild—as disparate as 1942 and 2013. He could guess time more accurately if he had access to a window and could approximate it based on light and shadows. But if he was deprived of a watch or a window, morning was no different from afternoon, and night was no different from day; the clock of body time was of no help. This patient could not state his age, either. He could guess, but the guess tended to be wrong.
Two of the few specific things he knew for certain were that he was married and that he was the father of two children. But when did he get married? He could not say. When were the children born? He did not know. He could not place himself in the timeline of his family life. He was in fact married, but his wife divorced him more than two decades before. His children had long been married and had children of their own.