We wake up to time, courtesy of an alarm clock, and go through a day run by time--the meeting, the visitors, the conference call, the luncheon are all set to begin at a particular hour. We can coordinate our own activities with those of others because we all implicitly agree to follow a single system for measuring time, one based on the inexorable rise and fall of daylight. In the course of evolution, humans have developed a biological clock set to this alternating rhythm of light and dark. This clock, located in the brain's hypothalamus, governs what I call body time [see Times of Our Lives, by Karen Wright, on page 26].

But there is another kind of time altogether. Mind time has to do with how we experience the passage of time and how we organize chronology. Despite the steady tick of the clock, duration can seem fast or slow, short or long. And this variability can happen on different scales, from decades, seasons, weeks and hours, down to the tiniest intervals of music--the span of a note or the moment of silence between two notes. We also place events in time, deciding when they occurred, in which order and on what scale, whether that of a lifetime or of a few seconds.

How mind time relates to the biological clock of body time is unknown. It is also not clear whether mind time depends on a single timekeeping device or if our experiences of duration and temporal order rely primarily, or even exclusively, on information processing. If the latter alternative proves to be true, mind time must be determined by the attention we give to events and the emotions we feel when they occur. It must also be influenced by the manner in which we record those events and the inferences we make as we perceive and recall them.

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 amnesics 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 ability 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 memories. The ability to form memories is an indispensable part of the construction of a sense of our own chronology. We build our time line 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 the hippocampus. 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 that surrounds the hippocampus is critical in the making and recalling of 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 have studied for 25 years, the time gap goes almost all the way to the cradle. When my patient was 46, he sustained damage both to the hippocampus and to parts of the temporal lobe. Accordingly, he has both anterograde and retrograde amnesia: he cannot form new factual memories, and he cannot recall old ones. The patient inhabits a permanent present, unable to remember what happened a minute ago or 20 years ago.

Indeed, he has no sense of time at all. He cannot tell us the date, and when asked to guess, his responses are wild--as disparate as 1942 and 2013. He can guess time more accurately if he has access to a window and can approximate it based on light and shadows. But if he is deprived of a watch or a window, morning is no different from afternoon, and night is no different from day; the clock of body time is of no help. This patient cannot state his age, either. He can guess, but the guess tends to be wrong.

Two of the few specific things he knows for certain are that he was married and that he is the father of two children. But when did he get married? He cannot say. When were the children born? He does not know. He cannot place himself in the time line of his family life. He was indeed married, but his wife divorced him more than two decades ago. His children have long been married and have children of their own.

Time Stamps

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 relationships 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 time line 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, and then 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 time line 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 amnesic patients differed from the controls. Normal individuals were relatively accurate in their time placements: on average they were wrong by 1.9 years. Amnesic 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 patients, 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.

Being Late for Consciousness

MOST OF US do not have to grapple with the large gaps of memory or the chronological confusion that many of my patients do. Yet we all share a strange mental time lag, a phenomenon first brought to light in the 1970s by neurophysiologist Benjamin Libet of the University of California, San Francisco. In one experiment, Libet documented a gap between the time an individual was conscious of the decision to flex his finger (and recorded the exact moment of that consciousness) and the time his brain waves indicated that a flex was imminent. The brain activity occurred a third of a second before the person consciously decided to move his finger. In another experiment, Libet tested whether a stimulus applied directly to the brain caused any sensation in some of his surgery patients, who were awake, as most patients are in such operations. He found that a mild electrical charge to the cortex produced a tingling in the patient's hand--a full half a second after the stimulus was applied.

Although the interpretation of those experiments, and others in the field of consciousness studies, is entangled in controversy, one general fact emerged from Libet's work. It is apparent that a lag exists between the beginning of the neural events leading to consciousness and the moment one actually experiences the consequence of those events.

This finding may be shocking at first glance, and yet the reasons for the delay are fairly obvious. It takes time for the physical changes that constitute an event to impinge on the body and to modify the sensory detectors of an organ such as the retina. It takes time for the resulting electrochemical modifications to be transmitted as signals to the central nervous system. It takes time to generate a neural pattern in the brain's sensory maps. Finally, it takes time to relate the neural map of the event and the mental image arising from it to the neural map and image of the self--that is, the notion of who we are--the last and critical step without which the event will never become conscious.

We are describing nothing more than mere milliseconds, but there is a delay nonetheless. This situation is so strange that the reader may well wonder why we are not aware of this delay. One attractive explanation is that because we have similar brains and they work similarly, we are all hopelessly late for consciousness and no one notices it. But perhaps other reasons apply. The brain can institute its own connections on the central processing of events such that, at the microtemporal level, it manages to antedate some events so that delayed processes can appear less delayed and differently delayed processes can appear to have similar delays.

This possibility, which Libet contemplated, may explain why we maintain the illusion of continuity of time and space when our eyes move quickly from one target to another. We notice neither the blur that attends the eye movement nor the time it takes to get the eyes from one place to the other. Patrick Haggard of University College London and John C. Rothwell of the Institute of Cognitive Neuroscience in London suggest that the brain predates the perception of the target by as much as 120 milliseconds, thereby giving us all the perception of seamless viewing.

The brain's ability to edit our visual experiences and to impart a sense of volition after neurons have already acted is an indication of its exquisite sensitivity to time. Although our understanding of mind time is incomplete, we are gradually coming to know more about why we experience time so variably and about what the brain needs to create a time line.

THE AUTHOR

ANTONIO R. DAMASIO is professor and director of the Institute for the Neurological Study of Emotion, Decision- Making and Creativity at the University of Southern California and adjunct professor at the Salk Institute for Biological Studies in La Jolla, Calif. He is recognized for his studies of neurological disorders of mind and behavior. Damasio is also author of three books: Descartes' Error, The Feeling of What Happens and Looking for Spinoza.