The Brain’s Clock

time and memory intertwine

In a fascinating talk for the LongNow Foundation, David Eagleman describes the interrelation of time and memory. David suggests the less we remember about a given time period the shorter the experience feels. He uses this idea to explain why time seemingly speeds up as we get older. When we are young we are “learning the rules” of society. More of our experiences are novel to us and therefore more memorable. Ergo, that impossibly long summer break.

As we age, our daily experiences are generally less novel and so we remember less of those summer vacations and most everything else. To combat the retreat of memory, Eagleman suggests reintroducing novelty into our lives with the aim of slowing down our perception of time and getting out of “autopilot.” He suggests small changes like taking a different route to work each day or wearing your watch on the other wrist (I do this sometimes but mostly to avoid a tan line). It follows that new hobbies and more significant life changes would have a larger effect. Indeed, Eagleman’s own near-death experience falling off a roof inspired his choice of career and area of research.

David noticed that when he fell off of a roof he had the perception of time slowing down. This is a common report amongst people in this type of situation and I’ve experienced it myself on more than one occasion. Eagleman formed two hypotheses: (1) our conscious perception actually speeds up during these incidents or (2) we reconstruct the event from memory after-the-fact.

He came up with a clever experiment to test these hypotheses but first let me jump to the beginning of his talk. He starts out explaining his experiments that measure a delay of 50–80 ms in our visual cortex. Unsurprisingly, the delay is longer in dim light than bright light. Our auditory cortex is a bit faster than our visual cortex perhaps because “hearing” is more primitive from an evolutionary standpoint. However, as far as all the various sensory inputs coming together in what we call consciousness, Eagleman ascribes a total delay of around half a second (500 ms).

Part of what is going on in consciousness is a synchronization of these various sensory signals so we can make sense of them. When our nose and toes are touched at the same time we perceive them happening simultaneously even though the signal from our toes takes longer to reach the brain — particularly for tall people! As Eagleman tells it, the ability to establish motive causation is extremely important to an animal’s survival: we need to know whether we caused a event or if it was an external force.

David mentions that early TV broadcasters learned that as long as audio and video were within 80 ms of each other our brain would perceive them as simultaneous. This example reminds me of the persistence of vision that occurs when images are displayed at 24 frames-per-second or greater. So there’s a time threshold within which the brain successfully maintains synchronization. What happens if we move outside the threshold?

One scenario is transitioning from bright light to dim light. David points out that when a volleyball match transitions from daytime to evening we temporarily make more mistakes. What’s remarkable is our brains dynamically recalibrate the synchronization of sensory inputs. The calibration is based on observations of our own interactions with the world. For example a hand clap could be a means to synchronize our visual and auditory cortices.

In the lab, David had subjects press a button that flashed a light with an 100 ms delay. Once they had “calibrated” on this delay, he removed it. The subjects perceived the light as occurring before they pressed the button. David found that schizophrenic patients were not able to calibrate in this way. This difficulty with synchronization may lead to the misattribution of their own thoughts and actions to others (i.e. the voices in my head). Eagleman is now researching whether training on a video game with a slowly increasing delay can help patients recalibrate. An audience member pointed out that VR, by its immersive nature, may be even more effective.

So back to his experiment to test what happens during those near-death moments of fear. How to determine if time slows down because of heightened perception or if it is an illusion of after-the-fact memory consolidation? Well, Eagleman devised a terrifying yet purportedly safe experiment in which a test subject is pushed backwards off a ledge into a safety net.

Falling backwards goes against our every instinct and reliably provokes instantaneous terror. To his credit, Eagleman tried the experiment himself first. While falling, the test subject glances at a special LED watch that rapidly changes between numbers just faster than normal perception can make out. So could the terrified subjects make out the numbers? Eagleman found the answer to be “no.”

So what might be going on that “slows down” time and makes memories so vivid? Eagleman suggests our brain’s memory-making faculties go into overdrive and store as much as possible, perhaps to allow us to figure out what went wrong in retrospect. Since our perception of time is largely a function of memory this oversized “download” conflates the length of the event.

I’d read elsewhere that through experimentation with LSD, Aldous Huxley, amongst others, came to understand that consciousness acted more like a funnel on vast unbuffered sensory intake rather than a light selectively illuminating the dark. Eagleman is making a similar hypothesis about how memory is stored. During a fear inducing event, adrenaline and cortisol rise and a hyperactive hippocampus commits as much to memory as possible.

Why not do this all of the time? Well for one thing, it takes additional resources and the brain is already consuming an outsized 20% of our energy intake. Our body is frugal, evolutionarily conditioned in an environment of ancestral scarcity. Another reason might have to do with the long-term toll activating stress hormones such as cortisol takes on our body.

Yet in an age of ADD it’s hard not to fantasize about superhuman memory. One of the most well-read writers of all time, Jorge Luis Borges, explores the ramifications in his short story Funes the Memorious. After a horseback riding accident young Ireneo Funes is endowed with a prodigious memory (and also rendered immobile). He is able to quickly learn multiple languages and can completely reconstruct a day in the past from start to finish.

However, the solipsistic reverie of memory consumes his consciousness, detaching him from the world and disrupting his sleep. In the end, Borges speculates Funes was little capable of thought, for “to think is to forget differences, generalize, make abstractions.” At some level memory is antagonistic to new thoughts and experiences.

When the Funes connection occur to me, I couldn’t remember the details of the story so I had to read it again. In this delicious irony a consolatory thought: forgetfulness loosens the snares of the past innoculating against boredom such that we glean novelty from new and familiar experiences alike.

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