La ilusion del tiempo

* 07 January 2015 by Laura Spinney

Laura Spinney is based in Lausanne, Switzerland

Time is not out there – “now” is a strange trick of the mind. The
good news is that with training you can live in the moment for

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

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.

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

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?

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

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”.

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

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.

La ilusion del tiempo