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Biology 202
2001 Second Web Report
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The Time Percept

Or, If a Clock Ticks in a Forest and No One is Around to See it, Does Time Still Pass?

Daniel J. Burdick

Time is the most elusive physical element. Despite familiarity with the concept, time is difficult to describe. Time is always the underlying assumption in our descriptions of the universe. In physics, it remains the largest barrier to the unification of relativity and quantum theory; some physicists believe time will have to be dismissed altogether if that unification is to occur (1). In more common experience, time appears to be an immutable and often lamented truth; who hasn't wished to "have more time," or to be able to "go back and do it over?"

But does time exist, or is it the creation of a brain eager to render input comprehensible? Examples abound of the brain's ability to invent perception. Color is created entirely by the brain; no single physical phenomenon is responsible for a specific color (2). Similarly, the lonely tree falling in a forest does not make a noise because no one hears the sound. If the brain can invent color and noise, can it also invent time? Or in other words, is time a function of reality or a function of our brains?

The situation is somewhat worse for time than for color or noise, in fact. Whereas light and sound can be objectively observed, time can only be inferred from events; even the motion of a clock is merely a series of events. Einstein's Relativity Theory (3), (4) may be extended to propose that "time" is created by the brain to explain motion through a series of stationary and discontinuous universes. In this way, the "percept of time" would be similar to the percept of motion created by watching a series of still pictures in a movie theater (1). It has also been suggested that time is a distinction between "conscious" and "unconscious" (5). In this conception, the sense of time is created by the sequential logic necessary for thought, when, in fact, reality exists independently of thought and thus of time, a fact that is understood by the "unconscious."

The question of time's reality is probably as unanswerable as any other metaphysical question, but in either case, the question of how the brain creates a percept of time can be approached. To do this, it is first necessary to define what is meant by a "time percept." This should have three meanings: first, an ability to estimate accurately the passage of time; second, a knowledge of past, present, and future, and a knowledge of the logical and predictable progression through time; and third, an ability to distinguish simultaneous events from non-simultaneous events, and to order non-simultaneous events chronologically.

Neurobiological research in these areas is limited, directly approaching only the first aspect of this definition. Investigators have found evidence that specific brain regions are responsible for the perception of time duration. The basal ganglia and cerebellum have been thought to be most responsible for assessing duration (11). However, cortical activity is also suspected, as researchers at the University of New Mexico and the University of California, Davis, report that patients with right-hemisphere cortical damage - specifically, to the prefrontal and inferior parietal cortices - were less able to judge the duration of controlled events than patients with left cortical damage or normal control subjects (10). The activity of neurotransmitters also helps regulate an internal clock; dopamine has been linked to duration determination and acetylcholine to temporal memory (12). Specific mechanisms for any of these activities remain speculative.

This research is significant in pointing to a specific role of the brain in creating an element of the time percept; it does, however, only apply to a specific aspect of the time percept, and does not address our perception of time's linearity or continuity. Indeed, it appears that the available academic literature primarily investigates how organisms estimate the passage or duration of time. All such studies assume the fundamental reality of time. This seems equivalent to assuming that color is real and then setting out to discover how humans recreate color in the brain, when it seems more accurate to say that the brain is the originator of color. Nonetheless, in the absence of hard scientific evidence, it can only be supposed that, since our concept of time involves a number of abstract components, this percept, like vision, is the result of coordinated activity in many parallel neural networks.

To approach the question of time's reality, our survey needs to include other observations. Consider the argument with color. The key to the conclusion that color is "made up" by the brain is that the same light may be perceived differently in different situations or by different people (a "red" rose is not red in low light or to someone who is color blind (2)) and that different light may be perceived to be the same (a single wavelength or a combination of wavelengths may both produce "yellow" (2)). We may thus attempt to judge time's reality by asking the same questions: Can time be perceived differently? What would our experience be if we could not perceive time, or perceived it differently? Can our sense of time get confused?

To start, one may ask, as with color, whether an individual may perceive time differently in different situations. The answer is yes. It is commonly reported by survivors of traumatic experiences that events seemed to occur "in slow motion," or that "time slowed down" (7) - if you've ever been in a car accident, you probably know the sensation. This seems to imply that the sense of time, at least as it's reported by the "I-function," is determined by sensory input. Moreover, it implies that the sense of time is created by the I-function. An ability to be aware of and to estimate the passage of time relies on the interpretation of events reported to the I-function. During trauma, fear stimulation enhances awareness of sensory input (8): more events reach the I-function, and this is interpreted as time slowing down.

It has also been reported that time may be perceived differently in different light. A 1999 study by German researchers concluded that rod-mediated vision reports motion to be slower than it is in actuality (9). Again, time is a matter of perception, and this perception varies by situation.

To continue, consider these questions with regard to the second aspect of the time percept, the logical progression of events. While there are no common "pathologies of time" analogous to visual pathologies, there are certain pathologies that have been termed "thought disorders" - the most prevalent being schizophrenia - that I submit may, in part, be considered as "time disorders." One of the most salient features of schizophrenia is an inability to "connect thoughts into logical sequences, with thoughts becoming disorganized and fragmented" (6). Thoughts come and go without the usual, predictable causal relationships. Hallucinations further distort a schizophrenic's reality. To a schizophrenic, pictures of reality are not connected in a way that appears continuous; at times, it's as if the movie that the rest of us are seeing has been cut up and spliced together in a bizarre order. The schizophrenic periodically loses that knowledge of an ordered and predictable past-present-future that is characteristic of the time percept. It should be emphasized that this hypothesis is not widely accepted or even heard of (to my knowledge, it has never been proposed before), and whether this conception will be useful in helping us understand the time percept or schizophrenia remains to be tested, but it seems quite possible that the two are related. Regardless, because schizophrenics lack an element of the time percept, it must be agreed that, like color, time may be perceived differently by different people.

Clearly, the time percept is as complex as time itself. Much of what has been discussed here raises more questions than it answers. Not yet addressed (and which will, for reasons of space, be saved for future queries) is the issue of simultaneity. Much of our concept of time and of our behavior relies on our ability to discriminate events that are not simultaneous and to connect those that are. This ability has been studied in owls, which use differences in time to locate the source of sounds (13). From a cognitive science point of view, time and simultaneity rely on parallel and serial processing. Those events that are processed in parallel are simultaneous; those in serial are not, and it is this serial processing that gives rise to a concept of time. Some have argued that consciousness, because it depends on serial thought, necessitates a concept of time, whether or not time actually exists. At least one cognitive scientist, Patricia Smith Churchland, dismisses this notion, though, noting that simulated serial processing is not necessary for ordered, logic-based behavior, and that competing parallel networks are equally effective in this respect (14).

Regardless of such arguments, it is inescapable that the time percept is created by the brain and is not a product of reality. Just as with other perceptions, time can be perceived differently by different people or by the same person during different events. Does this mean that time itself is not real? That time does not pass if no one is there to perceive it? This is a more difficult question, in large part because, while we can distinguish light from color or sound from noise, we have no concept of the difference between time and the percept of time. In some respect, this makes the argument all the more conclusive: time cannot exist outside our perception of it. If a distinction can be made, though, it is one that we need to make. This will require some fundamental changes in our thinking. Whether we are successful in making those changes, well, that will only come with time.

WWW Sources

1) From here to eternity, by Tim Folger, in December 2000 issue of Discover, 21(12):54-61.

2) Serendip, notes of 29 March 2001, by Paul Grobstein.

3) Relativity: The Special and General Theory, by Albert Einstein, 1920. Translated by Robert W. Larson.

4) Theory: special relativity, by Helen Quinn, 1999.

5) Why time moves forward, by Mark Germine, 1995. In Dynamical Psychology.

6) Schizophrenia, from the National Institute of Mental Health, 1999.

7) PTSD, from the United Kingdom National Workplace Bullying Advice website, Bully Online.

8) The neurobiology of fear: emotional memory and post-traumatic stress disorder, by Daniel Burdick, 2001.

9) Slow motion sets in when the light dims, by L. Helmuth, 1999, Science News Online.

10) Cortical networks underlying mechanisms of time perception, by Harrington, D.L., K.Y. Haaland, and R.T. Knight, 1998, in the Journal of Neuroscience, 18(3):1085-1095.

11) Toward a neurobiology of temporal cognition: advances and challenges, by Gibbon, John, Chara Malapani, Corby L. Dale, and CR Gallistel, 1997, in Current Opinion in Neurobiology, 7:170-184, on BioMedNet.

12) Neuropharmacology of timing and time perception, by WH Meck, 1996, in Cognitive Brain Research, 3(3-4):227-42, on PubMed.

13) Locating a mouse by its sound: the value of having two ears, from the Howard Hughes Medical Institute, 1997.

14) Can neurobiology teach us anything about consciousness?, by Patricia Smith Churchland, 1995.




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