A&O READING – TIME and HOW THE BRAIN CREATES NOW (Laura Spinney reporting 2015)



WE can consider MEMORY and muse upon … 


The time illusion: How your brain creates now  By Laura Spinney.  7 January 2015  https://www.newscientist.com/article/mg22530030-500-the-time-illusion-how-your-brain-creates-now/


WHAT is “now”? It is an idea that physics treats as a mere illusion, yet it is something we are all familiar with. We tend to think of it as this current instant, a moment with no duration. But if now were timeless, we wouldn’t experience a succession of nows as time passing. Neither would we be able to perceive things like motion. We couldn’t operate in the world if the present had no duration. So how long is it?

That sounds like a metaphysical question, but neuroscientists and psychologists have an answer. In recent years, they have amassed evidence indicating that now lasts on average between 2 and 3 seconds. This is the now you are aware of – the window within which your brain fuses what you are experiencing into a “psychological present”. It is surprisingly long. But that’s just the beginning of the weirdness. There is also evidence that the now you experience is made up of a jumble of mini subconscious nows and that your brain is choosy about what events it admits into your nows. Different parts of the brain measure now in different ways. What’s more, the window of perceived now can expand in some circumstances and contract in others.

Now is clearly a slippery concept. Nevertheless, it would be good to pin it down because it could tell us something about the bigger picture of how the brain tracks time. Not just that, the perception of the present is also crucial to how we experience the world. If events appear simultaneous when they aren’t, that has implications for our understanding of what causes what. “Your sense of nowness underpins your entire conscious experience,” says Marc Wittmann at the Institute for Frontier Areas of Psychology and Mental Health in Freiburg, Germany. Understanding now even helps us address the question of whether we have free will.

We have long known that the brain contains structures that use cycles of light and dark to set its daily clock. How it tracks the passing of seconds and minutes is much less well understood. At this level, there are two broad types of timing mechanism, an implicit and an explicit one. The explicit one relates to how we judge duration – something we’re surprisingly good at. The implicit mechanism is the timing of “now” – it is how the brain defines a psychological moment and so structures our conscious experience.

Our implicit sense of time is itself made up of two seemingly incompatible aspects: the fact that we exist permanently in the present yet experience time flowing from the past towards the future. So how do successive nows get sewn together into the smooth-flowing river of time? Wittmann has addressed this question by drawing on the mass of psychophysical and neuroscientific data gathered in recent decades (Frontiers in Integrative Neuroscience, vol 5, article 66). He believes this points to a hierarchy of nows, each of which forms the building blocks of the next, until the property of flow emerges (see diagram).

If Wittmann is correct, to understand the now that we experience, we first need to understand its subconscious component, the “functional moment”, which operates on the timescale at which a person can distinguish one event from another. This varies for different senses. The auditory system, for example, can distinguish two sounds just 2 milliseconds apart, whereas the visual system requires tens of milliseconds. Detecting the order of stimuli takes even longer. Two events must be at least 50 milliseconds apart before you can tell which came first.

“We exist permanently in the present yet experience time flowing from the past towards the future”

The brain must somehow reconcile these different detection thresholds to make sense of the world. Its task is made more difficult by the fact that light and sound travel through air at different speeds and can reach our sensory apparatus at different times, even if they were emitted by the same object at the same time. How does the brain bind all the dislocated stimuli into a single psychological event, a functional moment?

There’s good evidence that even at the subconscious, millisecond level, the brain makes predictions. This is what happens when you watch a badly dubbed movie. Your brain predicts that the audio and visual streams should occur simultaneously and – as long as the lag between them doesn’t exceed about 200 milliseconds – after a while you stop noticing that the lip movements and voices of the actors are out of sync. Exploiting this effect, Virginie van Wassenhove at the French medical research agency’s Cognitive Neuroimaging Unit in Gif-sur-Yvette and her colleagues have been investigating how the brain might bind incoming information into a unified functional moment. What they found is intriguing.

Mind meld

They exposed people to sequences of beeps and flashes, both occurring once per second, but 200 milliseconds out of sync. Brain imaging then revealed the electrical activity produced by these stimuli. This consisted of two distinct brain waves, one in the auditory cortex and another in the visual cortex, both oscillating at a frequency of 1 hertz – once per second. At first the two oscillations were out of phase, and the volunteers experienced the light and sound as out of sync. But as people reported that they started to perceive the beeps and flashes as being simultaneous, the auditory oscillation became aligned with the visual one (NeuroImage, vol 92, p 274). “The changes predict participants’ conscious timing,” says van Wassenhove, “so we have to hypothesise that this reflects an active mechanism of the brain to deal with time in the world.” In other words, your brain seems to physically adjust signals to synchronise events if it thinks they should belong together.

This is the first time that a biological basis has been found for implicit timing. It also suggests that, even at the subconscious level, the brain is choosing what it allows into a moment. However, this functional moment is not the now of which we are conscious. That comes at the next level of Wittmann’s hierarchy, with the “experienced moment”. So what do we know about this?

“The now we are conscious of seems to last between 2 and 3 seconds”

It is this now that seems to last between 2 and 3 seconds. A neat demonstration of that was provided last year by David Melcher at the University of Trento, Italy, and his colleagues. They presented volunteers with short movie clips in which segments lasting from milliseconds to several seconds had been subdivided into small chunks that were then shuffled randomly. If the shuffling occurred within a segment of up to 2.5 seconds, people could still follow the story as if they hadn’t noticed the switches. But the volunteers became confused if the shuffled window was longer than this (PLoS ONE, vol 9, p e102248). In other words, our brains seem able to integrate jumbled stimuli into a cohesive, comprehensible whole within a time frame of up to 2.5 seconds. The researchers suggest that this window is the “subjective present”, and exists to allow us to consciously perceive complex sequences of events.

Melcher likens the effect to the way we are able to guess a written word even if some of its letters are missing or out of place. Because we have a cohesive concept of the word, we can fill in the gaps, but comprehension breaks down if the words either side of it don’t provide context, or the first and last letters have been tampered with. Melcher thinks the 2 to 3 second window provides a sort of bridging mechanism to compensate for the fact that our brains are always working on outdated information. Right now, your brain is processing stimuli that impinged on your senses hundreds of milliseconds ago, but if you were to react with that lag you wouldn’t function effectively in the real world.

“Our sense of now can be viewed as a psychological illusion based on the past and a prediction of the near future,” says Melcher. “And this illusion is calibrated so that it allows us to do amazing things like run, jump, play sports or drive a car.” Consciously or not, Hollywood movie editors take account of our experienced moment. In the cutting room, they rarely create shots that last less than 2 or 3 seconds, unless the director is aiming to create a sense of chaotic or confusing movement. “Three seconds is long enough to understand what’s going on, but not so long that you have to rely too heavily on memory to maintain access to all the relevant information,” says Melcher. “It’s the sweet spot.”

Wittmann acknowledges that it is not clear how a group of subconscious functional moments are combined to create the conscious experienced moment. The biological signature of the experienced moment has yet to be found, although neuroscientist and philosopher Georg Northoff at the University of Ottawa in Canada has proposed one possibility. In his 2013 book Unlocking the Brain, he speculated that implicit timing could be related to slow cortical potentials, a kind of background electrical activity measurable across the brain’s cortex. It’s telling, says Wittmann, that these waves of electrical activity can last several seconds. He also points out that consciousness is itself a kind of filter because it focuses our attention on some things to the exclusion of others. Influenced by factors such as emotion or memory, it might tag or label a subset of functional moments as belonging together, to create an experienced moment.

Creating flow

However the present moments we experience arise, they are combined to give us a sense of continuity or “mental presence”, the final now in Wittmann’s hierarchy. This operates over a timespan of about 30 seconds and gives you a sense of continuity. According to his model, the glue that holds the experienced moments together to create an impression of time flowing is working memory – the ability to retain and use a limited amount of information for a short time. Mental presence is what underpins the sense that it is you who is experiencing events. “It is the now of ‘I’, of your narrative self,” Wittmann says.

The implications of this new view of nowness are potentially mind-boggling. Take, for example, the debate over free will. In the 1980s, US physiologist Benjamin Libet found that people reported deciding to flick their wrist about 500 milliseconds after he had detected activity in their brains that preceded each wrist-flick. His now controversial conclusion was that we have less conscious control over our actions than we think. But, given what we know about implicit timing, it is possible that what he actually detected was an artefact of the brain’s insensitivity to order at very small time scales. At 500 milliseconds, says Wittmann, “we are definitely within margins of temporal resolution where you cannot distinguish which event came first”.


“With a bit of effort we are all capable of manipulating our perception of now”

Then there’s the issue of the stretchiness of now. There is plenty of anecdotal evidence that time can seem to expand or contract depending on what’s happening around us – for example, that events seem to unfold in slow motion during car accidents. Such expansion has been reproduced in the lab, when people are presented with a succession of stimuli of equal length yet report that an oddball event in the series seems to have a longer duration. What’s more, Melcher has preliminary findings showing that, when people perceive an event to have lasted longer than it actually did, they also take in more detail about it, describing it more accurately. In his opinion, this shows that temporal stretchiness reflects real changes in sensory processing, which in turn may have conferred an evolutionary advantage. By ratcheting up the brain’s processing rate at critical moments and easing back when the environment becomes predictable and calm again, we conserve precious cognitive resources.

The now Illusion

Such changes in sensory processing would be subconscious, but might we be able to take control of our perception of now? Regular meditators often claim that they live more fully or intensely in the present than most people. To test the claim, Wittmann asked 38 people who meditate and 38 who do not to look at an ambiguous line drawing of a cube, known as a Necker cube, and press a button each time their perspective of it reversed. The reversal time in this kind of task is considered a good estimate of the length of the psychological present. By this measure, people in both groups perceived now to last about 4 seconds, seeming to confound the claims of some meditators. However, when Wittmann asked participants to try to hold a given perspective for as long as possible, the meditators managed 8 seconds on average, compared with 6 seconds for the others.

Meditators tend to score highly in tests of attention and working memory capacity, says Wittmann. “If you are more aware of what is happening around you, you not only experience more in the present moment, you also have more memory content.” And that in turn affects your sense of the passing of time. “Meditators perceive time to pass more slowly than non-meditators, both in the present and retrospectively,” he says.

This suggests that with a bit of effort we are all capable of manipulating our perception of now. If meditation extends your now, then as well as expanding your mind it could also expand your life. So, grab hold of your consciousness and revel in the moment for longer. There’s no time like the present.

This article appeared in print under the headline “Once upon a time”

Like this? Read: “The human universe: Does consciousness create reality?