Although we do not have any specific sensory system that is associated with the perception of time, we do have different systems that help us to do so: the cerebral cortex, the cerebellum and the basal ganglia . What’s more, in the central part of the hypothalamus is the suprachiasmatic nucleus, which controls circadian cycles and if it is destroyed, all our regular rhythms disappear.
Our implicit sense of time has within it two seemingly incompatible aspects : the fact that we exist permanently in the present, and the experience that time flows from past to future continuously, seamlessly. How do we make a series of ‘nows’ become that gently flowing river of time? Marc Wittmann, from the Institute for Frontier Areas of Psychology and Mental Health in Freiburg (Germany), thinks that the brain takes 3 seconds to create the conscious ‘now’ . But not everything ends here; in the end, the brain must build the image that time flows continuously; for this he invests the not insignificant amount of 30 seconds.
Wittmann asked 38 people who meditated regularly and another 38 who did not to look at what is called a Necker cube : it is a cube with an ambiguous image, which can be seen one way as soon as another. Wittmann asked them to press a button every time they saw him change perspective. Why? Because in this way we can estimate how long the psychological present lasts. Well, he found that both groups perceived it in the same way: the ‘now’ lasted 4 seconds. Apparently, meditating doesn’t make you more stuck to the present.
How does the brain process information?
The neuroscientist at Baylor College in Houston, Texas, David Eagleman, points out a factor that we must take into account to understand the problem of ‘now’: our brain processes information at different speeds depending on the sense with which we perceive it . Our auditory system, for example, can distinguish between two sounds separated by a millisecond, while the visual system can hardly perceive two images separated by less than ten milliseconds as different. This implies that the brain must learn to reconcile these disparities to create a unified picture of the outside world . As if that were not enough, the work of the brain is complicated if we take into account that light and sound travel at different speeds, so they reach our senses at different times even if they have been generated at the same time, as happens when lightning strikes . How does he end up handling all of this?
Neuroscientists believe that for this our brain makes predictions about what is going to happen . This is what happens when we watch a movie that has been badly dubbed: our brain thinks that the voice and the movement of the lips are in sync until it realizes that they do not match. And this happens if the difference between the two is 200 milliseconds. This was discovered by engineers in the early days of television, as they had to worry about synchronizing the image with the audio. It was then that they realized that they had a hundred milliseconds of margin: if the signal arrived within this window, the viewers’ brain synchronized the video signal with the audio signal on its own.
The unconscious brain decides the ‘now’
Virginie van Wassenhove from the Gif-sur-Ivette Cognitive Neuroimaging Unit (France) has dedicated herself to investigating why this happens. To do this, he exposed a group of volunteers to a series of flashes and beeps once a second, but separated by 200 milliseconds. At the same time, he did a scanner to record what their brain activity was at that time. He found two brain waves, one in the visual cortex and one in the auditory, with a frequency of 1 Hz (that is, once per second). At first the two waves were out of phase, which made the volunteers perceive the flashes and sounds as out of sync. But they began to say that they perceived them as simultaneous just as the auditory and visual brainwaves were synchronizing, implying that there is an active mechanism in the brain that tells us how to deal with time in the outside world , although we don’t know what it is. . But the most interesting thing about this work is that it has another deeper implication: it is the unconscious brain that decides what the ‘now’ is and its duration is 2 to 3 seconds.
This was demonstrated in 2014 by David Melcher from the University of Trento (Italy). What he did was prepare a very particular film; at certain times some scenes of it had been cut into smaller pieces : in some cases the scene was cut into pieces that lasted a few milliseconds, in others they lasted up to several seconds. Melcher found that they followed the plot of the film as if nothing had happened if the duration of those small cuts was not more than 2.5 seconds ; if they exceeded that duration, the volunteers noticed that something strange was happening. For Melcher, this is similar to when we read a letter where we have deleted or changed some letters for each word: despite this, we are able to read it because our brain is capable of filling in the absences. Of course, it fails when, for example, we systematically modify the first and last letter of each word. For Melcher, this window of 2 to 3 seconds that we have to perceive the now is the consequence of some kind of unknown mechanism that solves the problem that our brain always handles outdated information. Because, whether we like it or not, our perception of the world is placed in the past .
Similarly, as we age our perception of time speeds up . Confronting a group of people aged between 19 and 24 with another group between 60 and 80, it was found that if they were asked to estimate the duration of 3 minutes, the young people missed, on average, about 3 seconds while the older ones went to the 40 seconds. The why is something that eludes us.
Fairhall S.L., Albi, A. y Melcher, D. (2014). Temporal Integration Windows for Naturalistic Visual Sequences. PLOS One. 9(7):e102248.
Wittmann, M. (2016) Felt Time: The Psychology of How We Perceive Time, MIT Press