When we observe an hourglass, when we fix our gaze on the falling grains of sand, we have the feeling that time is running continuously. We believe this has been the case since the birth of the world, and that nothing can contradict this universal truth. However, our sensory perceptions and the neurons which are at the origin of them have a completely different way of chanting time. A subjective and sensual way, in the proper sense of the term.
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The example of vision
To tell you more about this “time of the senses”, I will take the example of vision. It works immediately and effortlessly, with fast, efficient, automatic learning. No need for instructions to learn to see! But in reality, the visual system must overcome a number of difficulties to achieve this efficiency. Perceptible difficulties when working on an artificial vision system, for example, to make a smartphone more intelligent or autonomous cars of the future.
Take for example the so-called delayed flash illusion. Look at this scene: the red point scrolls on the screen and you will then see a green flash appear when the red point is aligned with the vertical of the green point.
A majority of you will perceive that the position of the red point is shifted to the right relative to the flash, in the direction of its trajectory. Look again: Equivalently, the flash is perceived to be late relative to the moving point. However, if we watch the video in slow motion, we see that the physical reality is different. This simple device, therefore, shows that instead of being synchronized, visual objects can be perceived at different subjective moments and “travel” in the time of our senses.
The most astonishing thing is that this illusion is universal, it is in a way anchored in our senses. And it changed my way of thinking about time. Engineer and physicist by training, I conceived of time as an external and immutable variable: one that allows us to measure any physical phenomenon, whether it be the movement of planets, a brain activity, etc. So where does this “time of the senses” come from? Does it have the same linear and continuous form that we have generally attributed to time? And how does its definition shed light on the mysteries of the brain?
The brain, black interior
Some people imagine vision as producing an “internal light screen”. But in reality, apart from the light that reaches the retina, the outgrowth of our brain that lines the back of the eye, there is no light in the brain. Full black. Solidly encased in the hermetic space of the skull, the brain is protected from direct contact with the external world. Inside, its approximately 10 billion neurons form large networks, organized on multiple scales – from the simple population of neurons to the network of brain areas.
We know that all the information circulates there through electrochemical messages distributed on all the membranes of nerve cells. Constantly shared messages from neuron to neuron, within each network, thanks to numerous synapses. And it is these messages, and only these, that create in you at this precise moment the simultaneous access to your senses, thoughts and actions. It remains to be seen how this network can be organized over time, and how information flows are coordinated and synchronized.
On the temporal scale of perception, there is not in the brain a central clock giving a synchronous beat to the different parts, like a conductor. More proof that the brain is not comparable to a computer. We must, therefore, face the facts: as in a jazz group improvising on the same theme, this capacity is internal to the brain and emerges from distributed and self-organized interactions. But what are the processes at work?
Inevitable transmission delays
Let’s go back to the anatomy of the visual system. The physiology of nerve cells means that the speed of information transmission in our brain varies on the different transmission channels to reach a maximum of 100 km / h for the fastest. Due to the volume of the skull, this inevitably results in transmission delays: thus an image that illuminates the retina excites the primary visual cortex only after approximately 50 milliseconds. There, visual information is transformed and distributed to other areas of the brain, which requires approximately 50 additional milliseconds. Finally, the information transmitted can generate muscular activity and, for example, induce an eye jerk movement after a total time of approximately 150 milliseconds.
Let’s try to visualize these propagation delays with a simple spot. You hold a ball in the right hand, and you watch it fall in the left hand on 10 cm: its fall takes approximately 150 milliseconds. Knowing that the image is delayed by 50 to 100 milliseconds in your visual cortex, this means that when the left hand received the ball, the image of this ball that the cortex receives is always in the middle of its trajectory!
In other words, like stars whose light does not reach us until after a journey of several years, it is a past image of the ball which reaches our visual cortex. For the brain, this is a real problem. Because knowing the time from decision to action and to be able to close your hand at the right time on the ball, the decision must be made upstream. Future action, as it is formed in the present, must, therefore, be built from the past … Complicated, right?
A timed puzzle
Here we are in front of a real-time puzzle. On the one hand, absolute and external time is inaccessible to the neurons involved in capturing the ball, except for the sensory neurons. On the other, subjective and internal time is subject to the proper functioning of the brain and the synchronization of past, present and future information. This scientific problem seems too complex to be solved …
Let’s take a step back: as a general rule, physical systems are transformed by exchanging energy and matter with their environment. Now in any system, according to the second principle of thermodynamics, the disorder measured by entropy must increase. This is why there is an asymmetry in the flow of time, that is to say, an arrow of time. The result, if we film a game of billiards, we will find this sequence incongruous if we project it in the opposite direction of time. The fact remains that among all physical systems, there is a class that has acquired the property of going up this arrow of time: the living.
Compared to billiard balls, and more broadly to passive systems, the living corresponds to systems whose organization allows their structure to be preserved for as long as possible. Such a system is therefore capable of obstructing the constant flow of hazards imposed by the arrow of time, by interacting with its environment through various processes. These latter are opposed to chance, they are said to be predictive. And in theory, they can overlap and interact on different scales of space and time: natural selection for a species, learning for an individual or simple prediction, as what interests us here.
Let’s go back to our visual illusion. We explained it by the delays in transmitting information, of the order of 50 to 100 milliseconds, at work in the visual system. The perceptual system would, therefore, do its best to compensate for this systematic shift, and predict the trajectory of the elements seen. And the image of a moving point would be projected forward relative to its physical position.
The necessary manipulation of information
Ultimately, our visual system would only interpret the image transmitted by the retina, to make it closer to that which it considers perceiving at the present moment: knowing the time taken to transmit vision and the speed of the point. , it “manipulates” its position on its trajectory, and therefore causes the red point to “advance” to its present position. However, there remains a problem: how to explain that the green point, at the instant of the flash, is not, in the same way, shifted in time? In other words, where does the sensory processing differential between the moving point and the light flash come from?
As we have just mentioned, our visual system is equipped with several predictive systems based on the information acquired by experience. Our brain can indeed learn that an object is likely to follow a coherent trajectory (as for the ball or the point), or that the nose is in the middle of the figure, that natural light generally comes from above, etc.
The idea of such a predictive brain with “a priori” knowledge of the structure of the world seems daring and pleasant. But can we formalize it, make it a mathematical theory and thus provide a unified conceptual framework of the functioning of the brain? The answer is yes if one believes in the researches of the British Karl Friston.
For this neuroscientist, a theory of a predictive brain is part of the broader theoretical framework of “minimizing free energy”. According to its author, it is a “mathematical formulation of the way in which biological agents resist the natural tendency to disorder” and “maintain their state in a changing environment”. To do this, they must minimize entropy, and therefore, “the long-term average of surprise …”, which amounts to minimizing free energy.
Information sorted to better predict
In summary, we are talking about a quantity of information measuring the degree of surprise of a system, a quantity that is simply measured in bits, just like the size of a computer file. And in this new theoretical framework, we can describe any behavior (action, perception, learning …) in the form of minimizing the amount of surprise to deduce the best direction to go up the arrow of hazards. More importantly, the principle of minimizing free energy makes it possible to describe and predict phenomena hitherto difficult to explain, both for the behavior of animals or humans and with regard to the functioning of the brain.
There was, however, no explicit model of the time of the senses. This is what we have attempted to sketch here. Such a sense of time explains the illusion of the delayed flash by differentiating the predictable moving point from the flash whose appearance time is unpredictable. But it is also useful for the brain to predict more complex trajectories: for example, that of a ball which we know will reappear after having passed behind a wall and become temporarily invisible. With this model, which has a hierarchical treatment similar to that existing between the different cerebral areas, we open up to other facets of the representation of time in the brain, even to forms different from normal, for example in schizophrenics.
Starting from the simple examination of a visual illusion, we were able to touch on the temporal paradoxes with which our senses are confronted. And to better understand them, we had to replace the linear and continuous time of the physicist with subjective time representing – at the present moment – past and future. For the moment, there is no physiological evidence to confirm the existence of this sense of time. But the new generations of scientists will certainly be able to take up this challenge.