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See Inside A Matter of Time

Remembering When [Preview]

Several brain structures contribute to mind time, organizing our experiences into chronologies of remembered events

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 the late 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 awareness) 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 patients undergoing brain surgery, 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 of 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 awareness 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 talking about 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. At the microtemporal level, the brain 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 and John C. Rothwell of University College London suggest that the brain predates the perception of the target by as much as 120 milliseconds, thereby giving us the perception of seamless viewing.

The brain's ability to edit 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 timeline.

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