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Originally published November 21st, 2012.
By Michelle Lim
A woman walks around her office, excavating messy piles of books and papers until she finally sees it: a daily planner bound in red leather. It is thick with appointments and reminders – work things – but also: birthdays, dinner promises (of which about 70% are kept), unexpected moments of joy and grief immortalized next to their dates, a chronicle of the last eight months and a blueprint for the next four, a fossil record continuously in the making.
She brings it over to her desk by the frosty window. In a few hours the sun will sit high in the noon sky, flooding the room with its bright rays. In subsequent hours it will drop slowly into the horizon, painting the sky a mélange of hues in its wake. Over the day’s course, the woman perceives a variety of changes in her physical environment through the classical senses—sight, smell, hearing, taste, and touch. She also inevitably perceives the passage of time.
The existence of the past in human perception depends wholly on memory, or what the brain chooses to remember, while the future lies just beyond reach at all times, getting converted into the present at a rate beyond our control.
The classical senses inform her understanding of space, but what about time? The woman cannot see or touch time, yet she structures her daily activities according to it and the watch on her wrist keeps track of it in seconds, minutes, and hours. Her perception of time, governed by complexly woven cerebral networks, often disagrees with these measurements—days drag on while months seem to fly by.
For such a simple word, “time” packs a formidable can of worms. As a physical concept, it describes a fourth dimension that exists in addition to the three dimensions of space. Like length, width, and depth, time can be measured and measured against; a clock ticks off seconds, and seconds can then be utilized to quantify motion.
Human time perception, however, resists both physical theory and the institution of clock time. The brain’s highly subjective treatment of time gives rise to what psychologist Luke Jones describes as “a persistent feeling of events receding into a past of non-existence, of the future as a nebulous void of possible existences to come, and of the ‘now,’ to which we grant a higher level of existence” (Eaves, 2008). The existence of the past in human perception depends wholly on memory, or what the brain chooses to remember, while the future lies just beyond reach at all times, getting converted into the present at a rate beyond our control.
This model of perception, specific to the brain, does not reflect any true reality of time but only what is necessary for survival. It is necessary, for example, to correctly time how long it would take to cross the street without being hit by a car. Down at the synaptic level, the timely integration of sensory inputs can mean the difference between proper function and schizophrenic delusion.
Time can be understood according to three different facets: physical time, internal timekeeping, and the experience of time distortion.
Time and motion are inextricably linked. The measure of time relies on periodic phenomena, or processes that constantly repeat at a regular rate. Clocks function according to some sort of repeating process—a swinging pendulum, an oscillating balance wheel, the regular vibrations of an electrically stimulated quartz crystal, or the resonance frequency of atoms—balanced against the universal unit of time, seconds.
Though the measure of time is absolute and constant, Einstein’s special theory of relativity shows that time itself is not. The special theory of relativity postulates that the speed of light is a constant for all observers, and that the laws of physics are the same for all observers moving at uniform speed. The latter implies that passengers on a ship moving at constant velocity cannot tell that the ship is in motion. They can, however, see movement through the changing scenery outside their windows. Scientifically speaking, there is no way to prove that the ship moves while everything else remains still, just like the woman in her office cannot say that the sun moves across the sky while her window remains stationary (Ostdiek et al., 2010).
An extrapolation of this theory—time dilation— describes the way in which time appears to slow down as it approaches the speed of light. To a stationary observer, a clock moving uniformly at the speed of light appears to tick more slowly than an identical stationary clock. To an observer traveling with the clock, however, the stationary clock appears to be ticking at a slower rate. From the latter point of view, the stationary clock is not so stationary after all; it appears to be moving away from him or her with the same uniform velocity in the opposite direction.
Observers perceive the rate of flow of time differently as a result of their relative motion and each observer’s perception has as much validity as another’s because no experiment can determine which observer is really in motion (Ostdiek et al., 2010).
Perception of time by the human brain displays similar features of relativity, albeit according to a whole different set of laws. Since special relativity applies only at velocities approaching the speed of light, or 299,792,458 meters per second, this discussion of everyday human perception assumes a relatively fixed progression of time with which the brain interacts subjectively.
Moving in scale from the physical universe to the biological, emphasis shifts from the nature of time itself to the perception and utilization of time by living organisms through experienced events. While physics relies on a stable, unchanging measure of time, the body operates according to a mutable clock of varying intervals.
Not only do the intervals vary, but the clock changes as well, with different mechanisms for different periods of time (Wittmann et al., 2009). Unlike the visual cortex or olfactory bulb, the parts of the brain involved in time sense are elusive and sundry: “a complex network of circuits, some interlocking, some whirring away independently” (Healy, 2009).
A 2006 study by Rufin VanRullen, neuroscientist at the University of Toulouse in France, provides some insight into the possible nature of internal clock intervals. He based this study on the wagon wheel illusion, whereby a movie depicting the forward rotation of a wheel gives the illusion of a slowed down or backward-spinning wheel. This occurs because each clockwise rotation captured by the successive movie frames could also represent a counterclockwise rotation, given the symmetry of the wheel. The illusion also exists in real life, as VanRullen confirmed by spinning a wheel in front of subjects who all reported seeing it turn the “wrong way” at certain speeds. This implies that perception, like a film roll, is not continuous but rather a series of very quick snapshots. From this experiment, VanRullen determined an average visual frame rate of thirteen frames per second, which corresponds to a certain rhythm of brainwaves observed in the right inferior parietal lobe.
While physics relies on a stable, unchanging measure of time, the body operates according to a mutable clock of varying intervals.
Using EEG electrodes on the scalp to measure subjects’ brainwaves, VanRullen found that waves in this region oscillate at a frequency of thirteen cycles per second. Interfering with these waves through transcranial magnetic stimulation reduced the probability of subjects seeing the illusion by thirty percent, showing a correlation between brainwave rhythm and visual frame rate (VanRullen et al., 2008). What is more, each neural process seems to have its own stream of discrete frames, which feeds into blocks of information in a higher processing stream—what neuroscientist Ernst Pöppel calls “the building blocks of consciousness” (Fox, 2009). Pöppel’s research at the Ludwig Maximilian University in Munich looks at the way in which temporally and spatially distributed neuronal activities are brought together into one time window, a cohesive perceptual frame. He found that this type of integration requires thirty to fifty milliseconds, making continuous perception impossible (Pöppel, 2009).
In 2001, neurologists Stephen M. Rao and Deborah L. Harrington identified some areas in the brain critical for brief interval time-keeping, the right parietal lobe being one of these. They used functional magnetic resonance imaging (fMRI) to track second-by-second changes in the brain activity of seventeen volunteers. Subjects listened to two consecutive tones followed by another pair of tones and were asked to judge whether the time between the second pair of tones was longer or shorter than the duration between the first two tones. Control tasks involved simply listening to the tones as well as estimating their pitch. Subjects conclusively showed activity in the basal ganglia and right parietal cortex when specifically attending to considerations of time. Cells in the basal ganglia contain large amounts of the neurotransmitter dopamine, which has long been associated with time perception (Marphetia, 2001).
Warren Meck, an experimental psychologist at Duke University, describes the pharmacological isolation of an internal clock based on systematic changes in time perception following the administration of certain drugs. Antipsychotic drugs such as haloperidol, which are strongly antidopaminergic, decrease internal clock-speed while stimulants such as methamphetamines, which facilitate dopamine activity, speed it up. This is indirect evidence that dopamine-releasing cells, such as those found in the basal ganglia, play an important role in temporal integration for time estimation (Meck, 1996). It also points to neurobiology’s very own form of time dilation.
Drugs aside, perceived time can slow down or speed up depending on emotional or situational factors. Experiences of heightened fear or excitement, for example, appear to go by in slow motion. According to neuroscientist David Eagleman, an intense experience demands heightened attention, which translates into more neurons firing across the brain for an increased input of sensory details. Frame by frame, the experience is converted into memory—as the present invariably turns into the past—and because each frame is so dense with detail, it seems much longer in memory. “You assume you would have needed more time to record so many details,” Eagleman explains (Fox, 2009).
The human brain can only perceive the past through memory and new memories are always being created.
This might also explain why days feel longer than months, as well as the perceived elevation of the present described earlier by Jones. The human brain can only perceive the past through memory and new memories are always being created. Fresher memories tend to contain more detail, while older memories thin out to create space for new ones; once again, the richer memories will seem to last longer. The human brain, after all, is not designed to perceive objective time—especially time in which “the dinosaurs, your birth, Christmas morning 2012 and your death bed [would] all have the same level of existence [in your perception] as this very moment” (Jones).
Luke Jones studied ways in which time dilation could occur as a result of the brain actually speeding up. Expanding on experiments by John Weardon, he exposed subjects to ten seconds of fast clicks (about five per second) and then measured how quickly they were able to do basic arithmetic, memorize words, or hit a specific key on a computer keyboard. Weardon had found in an earlier study that subjects exposed to the clicks would overestimate the duration of a subsequent light or sound by about ten percent, suggesting that their internal clocks had accelerated.
This could alternatively be attributed to memory distortion, but Jones’ subjects showed accelerated performance in all three tasks by ten to twenty percent, establishing a direct correlation between the clicks and increased rate of mental processing. According to neuroscientist Edward Large, rhythmic sounds can entrain gamma brainwaves so that they burst into especially strong wave peaks at the beginning of each sound. If this is also true of waves that correspond to temporal windows, the click train was actually able to reset the brain’s frame capture rate to a higher speed (Fox, 2009).
Human perception of time, however inaccurately it reflects actual physical time, forms the basis of consciousness—how we make sense of space, motion, events, relationships, change, and ultimately, meaning.
Timing is Everything
The process of temporal integration is crucial to survival but also the maintenance of perceptual identity. It underlies all mental machinery, essentially defining subjective experience and the state of consciousness (Pöppel, 2009). Subtle errors in the perception of time can give rise to highly altered states of being. Schizophrenics, for example, have a hard time discerning tenth of a second time intervals. Since normal neural processing relies on temporal intervals much smaller than that, their thoughts and perceptions become mixed up in the brain and expressed out of sync. They might act, say, or think something before they are conscious of their own decision to do so, producing the feeling of being controlled by another entity (Fox, 2009). This makes it impossible to manage paranoia, insecurities and impulses, things we all experience and learn to moderate through self-awareness and interaction with others.
Human perception of time, however inaccurately it reflects actual physical time, forms the basis of consciousness—how we make sense of space, motion, events, relationships, change, and ultimately, meaning. This processing delay could mean the difference between pitch black loneliness and a chance at a meaningful life. Neuroscience offers the possibility of a solution as well as a new mode of tolerance: the understanding that “reality” is a subjective experience mediated by waves and synapses, that every brain has unique chemical variations and that for some, these are more extreme. What seems so alien to the rest of us may be just a matter of milliseconds.
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