A woman in profile is shown contemplating time and thought, symbolized by a watch and brains inside light bulbs.

Keeping Time

What then, is time? If no one asks me, I know. If I
wish to explain it to someone who asks, I know it not.
–St. Augustine, Confessions, Book II, Sec. 14, from http://www.nytimes.com/books/first/l/levine-time.html

“How long will it take?” and “When will it happen?” have always been important questions, for childbirth, travel, and rituals of every sort. It’s only recently that we have been able to answer the questions for events that are over in a fraction of a second, but it’s surprising that humans have succeeded so well for so long with longer-lasting or more widely spaced events. Physicists still argue over what time is, but psychologists are pretty sure we can’t sense time directly. We have no sensory receptors for time, so time is perceived rather than sensed.

We’re good at making comparisons, though: before, after, quicker, slower, shorter, longer. We lack a universal body clock, but the body has a number of rhythmic processes, excluding the heartbeat.In addition, to measure time we need to be able to pay attention and remember.

It’s worth noting that neither attention nor memory has a fixed time limit on latency, duration, capacity or repeated use. The so-called attention span varies so much with the situation in which it is measured that we should probably not refer to it as a trait. As for memory, the “engram” also  comes in a variety of sizes and durations. But we can switch attention in two shakes of a lamb’s tail (try it here), and the same holds for retrieving a memory: a fraction of a second, if you don’t suffer the agony of interference.

Originally, humans had only the rhythmic oscillations of nature to go by: the rising and setting of the sun and moon, the planets, and the stars. The Egyptians may have been the first to use dripping water as a timer. Almost exactly 3000 years later, if you believe the speculations, Christiaan Huygens invented a pendulum clock that was more compact and more accurate as long as you stayed on land. At sea, a clock’s pendulum not only told the time, but also the pitch, yaw, and roll of the ship, and made the determination of longitude a chancy guess.

Today our GPS compasses use clocks with nanosecond (billionths of a second) accuracy, which brings to mind the original meaning of “good enough for government work”.

To listen to this astrophysicist tell it, time perception rests on pulses, sensory focus, and memories. It’s an attractive mashup of (mostly) psychology, neurology, physiology, and psychiatry. The concepts are not consistent or predictable from each other, but a little further investigation will give them more meaning.

Picture of an hourglass

BIO: The model of time perception that we encounter most widely in biology is the Scalar Expectancy Theory, a three-stage model that posits a pacemaker in the body that feeds pulses to an accumulator in the brain, so that time intervals may be judged (see Figure 2).

Such a model works over seconds and minutes. We have other timekeepers that work across sub-second ranges, and others that monitor days and years, called circadian and circannian rhythms.

For the sub-second range, we have neurons in the supramarginal gyrus (research here) that appear to compare intervals of one-fourth to three-fourths of a second. By adapting to a test interval, these neurons reveal retention of the length of time intervals. It’s possible that this effect is responsible for the reduction in time awareness caused by close attention to an object, discussed in the next section, PSYCHO.

Still other neurons program events in the brain across day-long or year-long intervals. Daily, or circadian, rhythms are phylogenetically ancient. In one sense there is not a single biological clock in the brain, because each body cell contains the machinery for rhythmic processing. One part of the brain, the suprachiasmatic nucleus (SCN), has the neuronal connections to couple rhythmic molecular processes with entrainment to day-night cycles of sunlight that are signaled by ganglion cells in the retina that sense light but do not contribute to image formation. These photosensitive retinal ganglion cells absorb light directly with a different pigment, melanopsin, and signal the SCN to regulate day-night rhythms. send signals about day-night cycles to the hypothalamus along the retinohypothalamic tract, a phylogenetically ancient pathway*.

Sensing day-night, or diurnal, rhythms, permits many body processes to be synchronized without our conscious awareness.

However, there are a number of ways that people find themselves at odds with their circadian rhythms and unable to sleep.  Rotating shifts are one kind of offender, and lack of sunlight is another. Everyone needs to get outside every day or two to keep their circadian rhythms on schedule, or entrained, properly. Old people may need to make a special effort just to sit outside for 30 minutes a day to overcome the aging lens’s tendency to absorb short wavelengths of light before they can do anything more than contribute to cataracts**.

For most of us, our biological clocks even dictate when we should take our pills. There’s some evidence that humans can change their sleep-waking rhythms, but the adaptation does not seem to be complete. On the other hand, it’s possible that getting eight hours’ sleep is not our historical pattern anyway.

One virtue of such cycles is that they synchronize a lot of interrelated events in the body, so that we digest our food before we need to sleep without a trip to the bathroom. Paleomom likens this to a conductor bringing all the instruments in a symphony into harmony.

Another virtue of cycles is that they don’t just synchronize; they anticipate. For example, the respiratory cycle is set to a rate that anticipates how fast we use up oxygen and build up carbon dioxide. Of course it’s not a circadian rhythm. (That is, our capacity to breathe is circadian, but we don’t breathe on a 24-hour rhythm, thankfully.)

However much we might need to synch processes that depend on sunlight, culture further entrains our rhythms needlessly. We might not need to work the usual shifts. Standardization and uniformity are cultural values (of 19th-century socialism), not evolutionary imperatives. In fact species depend on genetic variation to avoid extinction!

(You may object that people who work on personal schedules will still be working cyclically, and that’s OK. If a rhythm is meaningful, use it. The question I’d ask is whether rhythms, like soldiers marching in synchrony across a bridge, might prove inefficient in the end. But maybe we just can’t help it.

 In any case, the synchrony we experience depends on a shared sense of “now”, which is hard to establish. We are all 80 milliseconds behind the clock to begin with.)

* How do blind people maintain normal circadian rhythms? They have normal light-dark cycles because of a visual sense that we are not aware of consciously.

There are retinal ganglion cells that act as photoreceptors for our daily rhythms. They use a special pigment called melanopsin to absorb light. Instead of forming an image, they pass on information about the timing of light and dark to the suprachiasmatic nucleus in the hypothalamus along a phylogenetically ancient pathway. In other words, this stimulation probably doesn’t reach the lateral geniculate nucleus by the usual visual pathway (optic nerve, optic tract), yet in some people the information may eventually reach the visual cortex with rather baleful results.

The retina is mostly clear–transparent–except for blood vessels and the photopigments in the rods and cones, which light reaches only after it has already passed through most of the retina. So light can reach these ganglion cells almost undiminished. The ganglion cells also receive some input from rods and cones, but this influence is still not understood well.

**In fact we all benefit from exposure to morning light (sorry, owls). Short-wavelength light in the morning suppresses melatonin secretion in the brain (tho’ less in brown-eyed people), banishing sleep and keeping us on schedule. (Melatonin? Melanopsin? What about melanin?)

The color of light–especially blue light–affects more than sleep.

PSYCHO: “Like sands through the hourglass, so are the days of our lives.” Remember that intro? We are aware of time passing chiefly by watching the changes around us. It has been demonstrated over and over again that every daily (or episodic) memory is time-stamped when it is formed, owing largely to processing in the hippocampus. When events are over, they exist only in memory, but the order of those memories evokes the sense of time passing by a kind of “comet’s tail” of personal experiences trailing behind each of us.

In the hippocampus there are “time cells” that support such this time-stamping, assisted by other members of a generously defined (if poorly pronounced***) hippocampal formation including the entorhinal cortex and subiculum. (You can get some idea of the interrelationships of the hippocampus, subiculum and entorhinal cortex here and there.)

Experience modifies time perception. Attention switches back and forth four times per second. Focusing our attention on a specific behavioral act reduces our sensitivity to time, making time seem to fly by. A welter of surprising challenges can make events seem to be speeding. On the other hand, increasing the predictability of well-practiced events has the opposite effect, making time seem to slow down. Slowing down body movements can have the same effect. The duration of facial movements alone is positively correlated with judgments of time intervals. See how time speeds up or slows down in this demo.

Healthy, normal people tend to overestimate the duration of a 90-second interval, reporting it as longer than it is. But losing track of time altogether points to dyschronometria. Losing track of time can also be symptomatic of attention deficit hyperactivity disorder (ADHD). Either ADHD or external pressures such as a lockdown may lead to a confusion of time perception that is labeled “time blindness”, and appears similar to a disturbance of time perception accompanying stroke, alcoholic coma or head injury called time agnosia.

For responding to longer intervals, it’s plain that our circadian rhythms are established before birth, programmed by genes, which account for about half of the variability in our rhythms. They are strongly entrained by day-night cycles and further matched to the mother’s rhythms. Why should there be individual variability when nearly every cell in the body has a circadian rhythm?

Alas, it turns out that some clocks run faster than others. People with a faster clock grow up as “larks” who are most active in the morning, while those with a slower clock become “owls“.

If the lark-owl distinction were hard and fast we might become separate species, since people who were active at the same time would more likely choose each other as mates. The special genes lodged in those of the other time preference would be unavailable. It happens (described by ungrammatical researchers–see “arose” that should have been “arisen” in the abstract). But maybe there are more than two chronotypes, or maybe most of us fall in the middle.

I usually have reservations about typologies such as the Myers-Briggs or Maslow’s-needs or even masculinity-femininity varieties, and I doubt that the lark-owl difference is absolute. I haven’t seen the relevant experiments, but chances are that its effects on behavior are on a level with birth order and handedness–that is, rather small.

***Stress the first syllable in “perforant” and the second in “pyramidal”. Anatomists I’ve talked with also emphasize the second syllable in “Ammonis”, but Lord Byron’s legacy pops up everywhere.

SOCIAL: The experience of time varies not only with personal experience but with cultural influences. Languages differ, and they lead us to speak of time in different figures of thought. This variation suggests that, although time has a physical meaning like an arrow in many people’s minds, our experience of it is a social construct. It is not an arrow.

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