The Predictive Brain. The Secrets of Time and Rhythm

Have you ever wondered how we manage to keep up with a song, clap our hands following the beat, or even predict when the semaphore light will turn green? These apparently simple abilities hide a sophisticated brain mechanism that allows us not only to react to the world but to anticipate it. The ability to measure time and predict when events will occur is critical to our daily lives. For a long time, it was thought that the brain had a kind of internal clock dedicated exclusively to measuring time. However, recent research from our lab at UNAM’s Institute of Neurobiology suggests a different and fascinating perspective. The ability to manage time and rhythm may not depend on a specialized clock, but on a much more general and powerful brain capacity: that of actively predicting the world around us. Our brains are constantly building internal models of how the world works, running simulations to anticipate what will come next. This idea shifts our view of the brain from an organ that reacts to stimuli to one that actively predicts its environment to generate anticipatory behaviors.

PREDICTION, KEY FOR KEEPING THE BEAT
Imagine you hear a series of regular knocks, like the clock’s ticking. Your brain not only registers each sound but predicts when the next one will occur. If the rhythm is slightly altered, you notice the irregularity almost immediately. How does our brain detect this? Our studies suggest an ingenious strategy: the brain seems to continually compare its predictions about when the next stimulus should arrive with when it actually arrives. The difference between expected and actual is what we call a “prediction error.” 

In experiments with humans and monkeys we observed that accumulating these prediction errors is key to identifying whether a rhythm is regular or irregular. For example, when the intervals between sounds were variable, participants quickly recognized the rhythm as irregular. However, when the variations were minimal, they took longer to decide and made mistakes because the small discrepancies between the prediction and reality were difficult to detect. This suggests that our brain does not wait for an obvious error to occur but adds up small differences over time until they reach a threshold that makes us react.

A METRONOME SHARED BETWEEN SPECIES
Is the ability to keep a rhythm only human? To find out, we designed a task in which both humans and rhesus monkeys observed a visual metronome alternating between the left and right sides of a screen. After three cycles, the metronome disappeared and participants had to estimate its current position based only on the elapsed time (figure 1). The results showed that both groups managed to maintain the rhythm internally. However, thanks to intensive training, monkeys showed more accuracy than humans in this task. This indicates that the ability to generate “internal clocks”—without physical movements—is an evolutionarily conserved ability. When analyzing errors in behavior, we observed that both monkeys and humans tend to be ahead at slow rates and behind at fast paces, suggesting that both species use a predictive model based on the accumulation of prediction errors.

Figure 1. Visual metronome

A circle alternates to the left and right of a point where participants fix their gaze. The circle alternates three times (visible stage) and then disappears (non-visible stage). Participants must be able to follow the position of the circle (right or left) even when it is no longer visible.

EL ÁREA MOTORA SUPLEMENTARIA: MÁS QUE SÓLO MOVIMIENTO
THE SUPPLEMENTAL MOTOR AREA: MORE THAN JUST MOVEMENT
What happens in the brain when we keep an internal rhythm? As a starting point to answer this question, we recorded neuronal activity in an area of the cortex known as the supplementary motor area, a region traditionally associated with the control and planning of movements (figure 2). Results show that even when the monkeys were stationary, mentally following the invisible metronome, this brain region generated patterns of activity as if they were performing alternating movements (figure 3). We observed that the activity sped up for fast paces and slowed down for slow paces, reflecting the tempo that the monkey maintained mentally. By analyzing the errors, we were able to see how neural activity predicted whether the monkey’s internal metronome was ahead or behind the actual pace.

Figure 2. Supplemental motor area

The supplemental motor area in the monkey brain helps plan possible future events and movements of the body.

Figure 3. 

The brain’s electrical activity oscillates at the same rate as the internal metronome. The spectrogram above indicates the rhythmic increase in the power of the gamma oscillations (~40 Hz). The panel below shows individual voltage traces, with marked increases in gamma frequency associated with the rate at which the non-visible stimulus changes position.

The fact that a movement-related area is so active during a purely mental rhythm-keeping task supports the idea that the brain could use simulations of movements (imagine moving your hand back and forth) as a way to measure time. In addition, we observed that the intensity of this rhythmic activity increased as more time passed since the start of the test, suggesting that the supplementary motor area not only marks the intervals, but could also keep track of the total time elapsed (figure 3). Although the monkeys did not move their hands, their neurons mentally reproduced the alternating movements of the metronome as if they were rehearsing the rhythm in their minds. This finding directly links motor planning with temporal perception, reinforcing the idea that the motor system not only executes actions, but also serves as a tool for predicting and modeling the world.

THE MOTOR SYSTEM NOT ONLY EXECUTES ACTIONS, BUT ALSO SERVES AS A TOOL FOR PREDICTING AND MODELING THE WORLD

SIMULATING THE FUTURE, THE BRAIN’S MASTER STRATEGY
These findings lead us to a unifying hypothesis: perhaps the brain doesn’t need a dedicated clock. Instead, it could be using its uncanny ability to internally simulate future events. In our most recent experiments we went a step further and also recorded activity in visual areas of the brain (specifically, the V4 area) during the metronome task. Surprisingly, we found that even this visual area, responsible for processing what we see, showed rhythmic activity synchronized with the internal metronome, even when there were no visible stimuli on the screen. This suggests that, in order to keep up, the brain could be internally recreating or simulating both sensory stimuli (imagining the visual point appearing left and right) and associated motor actions (imagining moving the eyes or hand). In essence, the brain generates a complete predictive model of the task.

THE ESSENCE OF INTELLIGENCE IS NOT IN REACTING, BUT IN ANTICIPATING

CONCLUSION
This research from our lab suggests that our abilities to measure time and perceive rhythm are manifestations of the brain’s fundamental ability to shape and predict the world. Instead of simply reacting, our brain is constantly anticipating, using internal models to generate behaviors that are not only responsive, but predictive. Understanding this not only redefines how we think about time in the brain, but it also sheds light on how we proactively and dynamically plan, anticipate, and engage with our environment. The next time you follow the beat of a song, remember that you’re not just listening, but your brain is actively predicting and simulating the future. As these studies suggest, the essence of intelligence is not in reacting, but in anticipating.

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