Insight into how brain tells time brings Einstein's theory of relativity down to earth

The stronger the gravitational field, the more the dilation slows time. And so, time passes more slowly near massive objects such as planets, stars, or black holes compared to regions with weaker gravitational fields.

But just as gravity affects time, our own neural circuits can dilate or contract our own experience of time, according to a recent study published in the journal Nature Neuroscience.

Looking closer to home than his studies of the universe, Einstein also referred to different human perceptions of time, saying: "When you sit on a hot stove for a minute you think it's two hours. When you sit with a nice girl for two hours you think it's only a minute."

Much human and animal behaviour is time-based. People and animals alike must extract chronological signals and structures from the environment to learn to anticipate events, understand the relationships between actions and outcomes, and estimate time – implicitly or explicitly – to plan, sequence, and coordinate action.

When a lion attacks a deer, timing becomes crucial. When humans plant crops, timing is also crucial; even when we walk or run, the timing of the movement of our feet is essential.

According to the study, timing mechanisms appear to be distributed across the nervous system, reflecting the importance of information about time in many brain functions.

Evidence that the brain directs an internal clock

In the new research – as revealed in Nature Neuroscience – scientists distorted the way mice perceive time by artificially slowing down patterns of neural activity, then accelerating them. The change in the animals' judgement of time provided the most convincing causal evidence yet that the behaviour of the brain directs an internal clock.

The idea of this kind of moment-by-moment biological timekeeping – involving internal mechanisms or processes within organisms –  is relatively recent.

Such mechanisms are associated with circadian rhythms and internal biological clocks which can affect the behaviour of an organism by influencing time perception by organising physiological and cognitive processes accordingly.

These processes help organisms adapt to their environment, anticipate recurring events, and improve their behaviour in line with their longer-run internal timing systems.

But little is known about how the body measures time on a scale from seconds to minutes. The recent study focused precisely on this time range – seconds to minutes – in which much of our behaviour unfolds.

Unlike the precise beats of mechanical and digital clocks, our brains maintain a decentralised and flexible sense of time, thought to be shaped by the dynamics of neural networks spread across the brain.

Consistent patterns of activity

The internal clock hypothesis found that brains track time by relying on consistent patterns of activity that develop in clusters of neurons during behaviour.

The study's lead author, Joe Patton, compares this to ripples: "Every time a stone is thrown into a lake, ripples form outward on the surface in a repeatable pattern. By examining the patterns and locations of these ripples, one can infer when and where the stone was dropped into the water."

Just as the speed at which the ripples move can vary, so can the pace at which these activity patterns progress in neural clusters.

Patton adds: "Our lab was one of the first to show a close correlation between how quickly or slowly these neural 'ripples' and time-dependent decisions develop."

In the new study, the researchers trained mice to distinguish between different time periods. They found that activity in the striatum, a deep brain region, followed predictable patterns that changed at different speeds.

This means that neural activity within the striatum is associated with time perception. Observing patterns suggest that the rate at which activity develops in that brain region is related to mice's self-experience over time.

Brain manipulation

But scientists are well aware of the famous rule in science that correlation doesn't mean causation.

So, the study manipulated the brains of the mice.

The researchers say that manipulating neural activity to change time perception is crucial for several reasons: by artificially slowing or accelerating patterns of neural activity, researchers can gain insights into how the brain's internal timing mechanisms work.

This research provides causal evidence that the brain is involved in time perception and processing.

To determine causality, the team turned to the old-school approach known to all neuroscientists: temperature.

In previous studies, scientists used temperature to manipulate the temporal dynamics of behaviours. Cooling a particular area of the brain slows down behaviours while warming it up accelerates them without changing their structure.

The researchers say this is akin to changing the rhythm of a piece of music without affecting the notes themselves: "We thought the temperature could be ideal because it would probably allow us to change the speed of neural dynamics without disrupting their pattern," they said.

To test this tool on the mice, they developed a thermoelectric device designed to heat or cool the striatum deep in the brain, while simultaneously recording neural activity.

In these experiments, the mice were anaesthetised, so the researchers used optogenetics — a technique that uses light to stimulate specific cells — to create waves of activity in the striatum.

The researchers found that cooling the striatum expanded the activity pattern, while heating it reduced activity, without disturbing the brain wave pattern itself.

This not only means that there's a neural basis for time perception. It also means that time perception can be manipulated and controlled externally.

Relativity in the brain as well as the universe

Neural activity manipulation allowed researchers to establish causal relationships between brain activity, behaviour, and time perception; by proving that altering neural activity can distort the mice's judgment of time duration, researchers can now establish a direct link between specific neural processes and time perception.

This study also empirically and systematically confirms the validity of the individual experience of the passage of time that scientists and philosophers have long talked about.

When we engage in a fun activity, time can seem to pass quickly. In contrast, when we are bored or engaged in a monotonous task, it may seem that time drags on; our emotional state can affect our perception of time.

In extreme or emotionally charged situations, time may seem to slow down or accelerate. For example, during moments of fear or excitement, time may seem to pass more slowly. The study says that self-perception of time has a neurological basis.

Of mice and men

Mice are often used as model organisms in scientific research because of some similarities in brain structure and function to those of human beings. Studying the effects of manipulation of neural activity in mice could provide valuable insights into human time perception.

Understanding the neural mechanisms behind time perception in mice could help inform our understanding of human time perception and potentially contribute to the search for time-related disorders or conditions in humans.

New treatments as well as new understanding

By providing new insights into the causal relationship between neural activity and time self-perception, the study's findings may lead to the development of new treatment for debilitating diseases such as Parkinson's and Huntington's, which includes time-related symptoms.

In Parkinson's disease, symptoms can include difficulties with time estimation and duration perception. Some studies suggest that individuals with Parkinson's disease may have a poor perception of accurate time periods. In addition, they may experience fluctuations in response times, which can affect tasks that require precise timing.

Timing-related symptoms in Huntington's disease can also appear in the form of disturbances in time perception, impairment in time processing, and difficulties in timing and coordinating movements.

Individuals with Huntington's disease may show irregular timing, poor synchronisation, and difficulty accurately estimating durations, and may also experience difficulties with tasks involving timing and rhythm, such as walking or maintaining a steady pace.

Distortions in time perception have also been observed in some neurological and psychiatric disorders, such as schizophrenia and attention-deficit/hyperactivity disorder; understanding the neural basis of time perception through studies in animal subjects can lead to insight into these disorders and the development of therapeutic interventions.

Time perception plays an important role in various aspects of behaviour and cognition.

The researchers say that by investigating how altered neural activity affects time perception in mice, the broader effects of decision-making, memory, and other cognitive processes that depend on accurate time perception can be highlighted.