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August 1 - October 17, 2021
As we begin, however, we must first acknowledge that our ability to answer questions pertaining to time is constrained by the nature of the organ asking them.
Time is a road without any bifurcations, intersections, exits, or turnarounds. Perhaps for this reason, there was relatively little evolutionary pressure for animals to map, represent, and understand time with the same fluency as space.
The brains of all animals, humans included, come better equipped to navigate, sense, represent, and understand space than time. Indeed one of the theories of how humans came to understand the concept of time is that the brain co-opted the circuits already in place to represent and understand space (chapter 10). As we will see, this may be one reason all cultures seem to use spatial metaphors to talk about time (it was a long day, I’m looking forward to the eclipse, in hindsight I should not have said that out loud).
Geometry was one of the first true scientific fields for a reason: science is much simpler if one can get away with ignoring time.
The work of many scientists, including the French mathematician Henri Poincaré and the American meteorologist Edward Lorenz, revealed that tiny differences in the state of a system can lead to vastly different future outcomes (the most famous example being the butterfly effect in weather prediction). This is called chaos, and
First, the problem of time in neuroscience and psychology is not a single problem, but a set of interconnected problems relating to how the brain tells time, generates complex temporal patterns, consciously perceives the passage of time, recollects the past, and thinks about the future. Second, significant progress was made in many subfields relating to the psychology and neuroscience of time.
the fact that “information about the past is useful only to the extent that it allows us to anticipate what may happen in the future.”12 Memory did not evolve to allow us to reminisce about the past. The sole evolutionary function of memory is to allow animals to predict what will happen, when it will happen, and how to best respond when it does happen.
introduce the two most important philosophical theories on the nature of time: presentism and eternalism. Presentism, as the name hints, states that only the present is real. Under presentism, the past is a configuration of the universe that once existed, and the future refers to some yet-to-be-determined configuration. Eternalism, in sharp contrast, states that the past and future are as equally real as the present.
Although eternalism is agnostic as to whether time travel is achievable, it validates the discussion because there would be “places” (moments) in time to travel to.
But in practice, neuroscientists are implicitly presentists.
But despite its intuitive appeal, presentism is the underdog theory in physics and philosophy.
Now for the clash between neuroscience and physics: if all moments in time are equally real, and all events in our past and future are eternally embedded within the block universe, then our perception of the flow of time must be an illusion (chapter 9). In other words, if all of time is already “out there,” then time is not flowing or passing in the normal sense of those words.
“The objective world simply is, it does not happen. Only to the gaze of my consciousness, crawling upward along the world line of my body, does a section of the world come to life as a fleeting image in space which continuously changes in time.”15
Clock time is a local measure of change, neither absolute nor universal.
the brain is a time machine: a machine that not only tells time and predicts the future, but one that allows us to mentally project ourselves forward in time.
the concept of time travel is conspicuously absent from most of human history. The Bible, along with other religious texts and orally transmitted folktales, are full of stories of talking animals, gods, and other supernatural beings. They tell of animals transmuting into humans and vice versa, epic voyages over vast spatial distances, Methuselah-like humans whose lives have spanned centuries, magic, and resurrections. But, oddly enough, little or no time travel. Even Shakespeare, who seems to have anticipated the plots and twists of almost every modern movie, never touched upon the subject of
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Telling time is a critical component of predicting the future. As any meteorologist knows, it is not sufficient to announce that it will rain; one must also predict when it will rain.
Classical conditioning is the primordial algorithm animals use to predict what is about to happen next.
synaptic algorithms—so called synaptic learning rules—programmed into our genes.
But STDP is simply one of many learning rules in the brain’s arsenal. Indeed, STDP operates near the finest temporal resolution of the nervous system—a difference of a few milliseconds in the timing of a postsynaptic spike can determine whether a synapse becomes weaker or stronger. STDP cannot capture the relationship between events separated by seconds and beyond.
The parts of your brain responsible for processing sound must measure these delays to calculate the location of sound sources. Evolution has exploited the fact that because the speed of sound is fairly constant, space and time are complementary—thus telling time allows us to “tell” space.
are able to grasp the whole from the temporal relationship between the parts of speech or music. But we can only detect such temporal patterns on the very narrow time scale of around a second.
The brain is a product of natural selection, and was thus “designed” to survive in a harsh and continuously changing world. As it turns out, one of the best ways to prosper in such a world is to be able to predict what will happen in the future, and when it will happen. So the brain is both an anticipation machine and a machine that tells time. It quantifies the passage of time across a range of over twelve orders of magnitude—from the tiny difference in the time it takes a sound to arrive at the right ear versus the left ear, to the ability of some animals to anticipate the seasons.
But it turns out that we are not only surrounded by clocks, we are also filled with them.
multiple clock principle—stands in contrast to man-made clocks. Even the simplest of digital wristwatches can accurately measure hundredths of a second, seconds, minutes, hours, days, and months. In the brain, however, the neural circuits responsible for timing Beethoven’s Fifth have no hour hand, and the circuits that govern our sleep-wake cycle have no second hand.
For millennia it was thought that the daily fluctuations in sleep and activity of humans and other animals was governed by external cues, most importantly sunrise and sunset. But experiments similar to those shown in Figure 3.1 established that even in the absence of external cues, animals continue to exhibit daily oscillations in their sleep, activity, eating, and body temperature. These cycles prove that there must be some internal clock—a circadian clock (circa meaning approximately, and dian meaning day)—governing the daily rhythms of our lives.
In many subjects the temperature rhythm remained close to 24 hours, even when their sleep-wake cycle was as short as 20 hours or as long as 40. Providing an important clue that we have more than one circadian clock within us and that these clocks may not always agree with each other.
How does a single bacterial cell keep track of the time of day? Before answering this question, it is worth pointing out, that an equally valid question might be: why would bacteria care what time it is?
Thus, one of the driving forces for the evolution of circadian clocks was the highly adaptive coordination of cellular functions with cycles of light and dark produced by the Earth’s rotation.
A cell that divides under UV light risks damaging its DNA, a danger that recedes at night. Some chronobiologists thus endorse the so-called escape from light hypothesis, which posits that the original driving force for the evolution of circadian clocks was to help cells divide at night.11
Like the clockmakers of the eighteenth century who struggled with the effects of temperature on pendulum and mechanical clocks, evolution had to overcome the problem that the speed of biochemical reactions changes with temperature. We still do not fully understand how ectothermic organisms, such as cyanobacteria, plants, and flies, maintain a period of approximately 24 hours over daily and seasonal temperature fluctuations.
Which raises the question: how does the suprachiasmatic know the correct external time—that is, if it is day or night? In chronobiological lingo, the suprachiasmatic nucleus has to be entrained by external cues, the zeitgebers (time givers). Sunlight is, of course, the most important zeitgeber. The location of the suprachiasmatic nucleus at the intersection of the left and right optic nerves is not coincidental; this location makes it ideally situated to receive raw data about whether it is light or dark outside the skull.
In sharp contrast, the pendulum of the circadian clock requires a full day for a single swing, so resetting it requires the much more delicate task of mucking with the actual oscillator, akin to advancing a pendulum in mid-swing. The “swing,” in this case, refers to the rise and fall of the circadian proteins within our cells. It is simply not possible to instantly reset the concentration of the circadian proteins within a cell, any more than it is possible to instantly reset an hourglass that is half full.
Eastward travel is significantly harder to adjust to than westward travel. Eastward travel requires a phase advance of our circadian clock—when traveling from Los Angeles to New York we must set our wristwatch forward three hours—whereas westward travel requires a phase delay.
circadian clocks are harder to phase advance than delay, although the mechanistic reasons for this are not fully understood.
raises the question: might it be better to have no clock at all than a clock that is perpetually out of synch? Surprisingly, the answer may be yes.
The absence of any relationship between the circadian period and temporal judgments on the scale of seconds to a few minutes is consistent with a large body of work that shows we have distinct circuits devoted to telling time across different scales.22
In other words, the circadian clock does not have a minute hand, much less a second hand.
Additionally, the fact that the human menstrual cycle is very close to the lunar month hints at a role for the moon in human reproduction. This appears to be a mere coincidence, as the menstrual cycle of other primates can be significantly shorter or longer.
Indeed, our subjective sense of time is actually quite inaccurate. A watched pot never boils and time flies when you’re having fun, precisely because there are countless circumstances that warp our subjective sense of time.
chronostasis—the sensation that time is standing still.
Before we address these questions, we must first distinguish between two distinct types of timing. PROSPECTIVE AND RETROSPECTIVE TIMING
Starting a stopwatch is an example of prospective timing: determining the passage of time starting from the present into the future. In contrast, if you walk into a room just in time to see the last grains of sand trickle through the neck of an hourglass, you can deduce something about how much time has elapsed since a past event: an hour ago someone flipped the hourglass over. But unless you flip it over again, the hourglass provides no information about how much time has elapsed since you entered the room. This an example of retrospective timing: estimating the passage of time from some
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As far as the brain is concerned, these two timing tasks are fundamentally different from each other. In the first case you know in advance that you will be performing a timing task, you can start a hypothetical stopwatch at t=0, and track the passage of time until approximately five minutes have elapsed. But in the second case—where Bert asked how long ago Amy left—your stopwatch is useless because you were never told when to start it. Prospective timing is a true temporal task in that it relies on the brain’s timing circuits. In contrast, retrospective timing is in a sense not a timing task
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William James wrote in 1890: “In general, a time filled with varied and interesting experiences seems short in passing, but long as we look back. On the other hand, a tract of time empty of experiences seems long in passing, but in retrospect short.
TIME COMPRESSION AND DILATION On the scale of seconds, the differences between prospective and retrospective timing can be easily studied, and manipulated in the laboratory. One of the most common ways to surreptitiously alter people’s perception of time is by changing the cognitive load of the task they are performing. Cognitive load is just a fancy term to describe how easy or hard a task is.
Dozens of subsequent studies have established that prospective timing is strongly modulated by cognitive load: the more complex or challenging a task, the shorter the estimates of how much time was spent performing the task (53 versus 31 seconds). The opposite can happen with retrospective timing: the higher the cognitive load, the longer the task can seem (28 versus 33 seconds). Retrospective timing, however, is not as strongly modulated by cognitive load as prospective timing.8
Studies demonstrating large differences in prospective and retrospective time estimates, and the susceptibility of these estimates to cognitive load, also reveal just how inaccurate and unreliable our judgments of elapsed time are.
Additionally, people perceive novel or unexpected stimuli to last longer than familiar or expected ones.11
stopped clock illusion.
