The fourth dimension is a favorite subject of science fiction writers and remains a fascinating enigma for mathematical and physical geniuses. Einstein, for example, conceived of time as a river that flowed, fast or slow, in front of massive bodies, such as stars and galaxies, that is to say that it was not fixed and universal, but a relative dimension, conditioned by gravity and speed and, therefore, “deformable”, despite the idea of constancy and linearity that clocks and calendars would have us believe.
We live in 3D – we navigate through space through length, width and depth – but we know little about our internal head clock. In classical antiquity, the Greeks already spoke of the dual nature of time: Chronos would be the linear dimension and Kairós the subjective aspect, measured according to the intervals between remarkable experiences, contributing to unusual perceptual distortions. And the examples abound.
As we get older, reaction time slows down and time seems to “fly”; if we feel painful or uncomfortable emotions, we feel like it never goes away; what we record in memory along our “timeline” creates an effect of surprise that leads us to make comments like “yesterday he was a baby and he’s already a man”; and there is even the feeling that time ceases to exist, which is also possible in certain states of mind – the ecstasy induced in various ways (during a concert, during sexual activity, under the influence of drugs or through meditation – giving the feeling that time is transcended.
During the first decade of this century, American social psychologist Philip Zimbardo said that “each of us has a skewed view of time, with obvious implications for the conduct of our lives”. How then does our “internal clock” work, in addition to what we already know about circadian rhythms, which influence sleep and wake cycles and our metabolism?
To refine the neural mechanisms involved in the decisions we make on a scale that can range from seconds to minutes – when we wait at a red light or during a tennis match, for example – a team of five researchers from the Learning Laboratory of Champalimaud Research has put forward the hypothesis of the “population clock”: coherent patterns of brain activity capable of evolving, during decision-making over a time scale, into neuronal clusters in an area of the brain, the striatum (deep region involved in motor control and which connects to the cortex).
Starting from previous studies with birdsong, which was more or less rapid depending on the manipulation of the temperature of brain regions, the team coordinated by Joe Paton developed a thermoelectric device capable of heating or cooling the striatum of guinea pigs, simultaneously recording their neuronal activity.
The results of the study, published on July 13 in the journal Natural neurosciencehas demonstrated that there is a causal relationship between neural activity and the processing of time involved in behaviors, as the scientists explain, in a video that you can watch here.
In the near future, this pioneering discovery could make a difference in the understanding and treatment of dementia, problems in terms of motor control and even mental illnesses such as obsessive-compulsive disorder, since the striatum is directly affected in these pathologies.
How the brain perceives time
The team created a game with three gates and a reward, and trained subjects to insert their snout into the middle gate whenever they heard two sounds, one and a half seconds apart.
For two months, the rats learned to make decisions based on time intervals (between 600 milliseconds and 2.4 seconds): if it was less than a second and a half, the reward was on the left door; above this range it would be to the right.
Once the ability to discriminate time was acquired, the animals’ movement management was monitored and, dozens of neural pattern recordings later, the temperature variable was introduced. Jo
The study’s lead author explains, “Each time the animal has to make a decision, its brain’s activity state follows a path, like the hands of a clock, continuously for 12 hours.” If the route is faster, the mouse tends to classify the time interval as long and vice versa.
The investigator continues: “Striatal cell velocity was manipulated with temperature variations, leaving the pattern of brain activity more or less intact.”
The results were telling: “Cooling the corpus striatum made the time interval appear shorter for the guinea pigs, while heating accelerated the neuronal population, driving behaviors that indicated the time was longer than it actually was.”
This means that the hands of the “internal clock” have a variability that manifests itself at different speeds: “The slower the activity pattern of the striatum, the faster the perception of time and vice versa (from the responses that the rats exhibited in the experimental model).”
The innovative nature of the work justifies the use of the expression “a stone in the pond”, a metaphor used by the neuroscientist for the hypothesis of the “population clock” which exists in the head: “The rhythm of the neuronal patterns resembles the concentric circles which form and evolve in a lake into which a stone has been thrown”.
The preliminary results of Champalimaud’s team showed another curious fact: “The manipulation of temperature in the cerebellum (located at the base of the brain and responsible for motor functions) had an impact on behavior, which was manifested by slower movements.”
These data are consistent with what is described in the scientific literature on movement disorders, such as Parkinson’s disease, “in which patients’ vigor is affected and they have difficulty initiating motor responses.”
Confirming the existence of “waves” of neural activity that mark time, or the perception of it, and the extent to which they guide our actions, is a breakthrough in basic science.
For now, it is too early to say about future applications in medicine or even in the fields of learning and artificial intelligence, but the potential is there. Matter to say: time will tell.