How the brain perceives time: Welcome back to "The Science Of Health", ABP Live's weekly health column. Last week, we discussed the different kinds of diseases caused by insects and arachnids. This week, we discuss how the brain perceives and understands time, and how the organ's inner clockwork drives one's behaviour. According to Einstein's theory of relativity, time can stretch and contract, and this phenomenon is known as time dilation.
Similar to the universe warping time, the brain's neural circuits can stretch and compress the subjective experience of time.
Scientists from the Champalimaud Foundation, a private Portugal-based biomedical research foundation, recently conducted a study in which they artificially slowed down or sped up patterns of neural activity in rats in order to warp their judgement of time duration.
The research, published in the journal Nature Neuroscience, offers new insights into how the brain's inner clockwork, or the internal clock, drives how one behaves.
Check all stories published in "The Science Of Health" series here.
Brain maintains a decentralised internal clock
A lot is known about how circadian clicks govern the 24-hour biological rhythms of the body and shape one's daily life, from sleep-wake cycles to metabolism. However, not much is known about how the body measures time on the scale of seconds to minutes, which is why researchers from the Champalimaud Foundation analysed the seconds-to-minutes timescale which determines one's behaviour.
A computer maintains a centralised clock, which has a precise ticking pattern. Meanwhile, the brain maintains a decentralised and flexible sense of time. The dynamics of neuronal networks dispersed across the brain shape the organ's sense of time.
What is the population clock hypothesis?
The brain maintains a sense of time by understanding consistent patterns of activity evolving in groups of neurons during a certain kind of behaviour. This is known as the population clock hypothesis.
In a statement released by the Champalimaud Foundation, Joe Paton, the study's senior author, compared the population hypothesis to dropping a stone into a pond, and said each time a stone is dropped, it creates ripples that radiate outward on the surface in a repeatable pattern, and examining the patterns and positions of these ripples can help one deduce when and where the stone was dropped into the water.
The speed at which these ripples move can vary. Similarly, the pace at which activity patterns progress in neural populations can also shift.
Paton explained that the Champalimaud Foundation's laboratory was one of the first to demonstrate a tight correlation between how fast or slow these neural 'ripples' evolve and time-dependent decisions.
Striatal activities determine how the brain perceives time
As part of the study, the researchers trained rats to distinguish between different intervals of time. The striatum is a deep brain region in which activities follow predictable patterns. In the instances in which rats reported a given time interval as longer, the activity in the striatum evolved faster. Meanwhile, when rats reported a given time interval as shorter, the activity in the striatum evolved slower.
However, the fact that there is a correlation does not mean that there is causation. The aim of the study was to find if the variability in the speed of striatal population dynamics merely correlates with or directly regulates timing behaviour, Paton said. He explained that in order to find out if the speed of striatal activities directly regulates timing behaviour, the researchers needed a way to experimentally manipulate the striatal population dynamics as animals reported timing judgements. This means that the team had to perform certain experiments on rat brains to regulate the dynamics of the striatal population, and see how the animals perceived time.
In order to perform this experiment, the researchers used an old-school technique: temperature. They manipulated the temperature of the striatum to obtain a deeper understanding of time perception, and how brains process and coordinate actions.
Tiago Monteiro, one of the lead authors on the paper, said temperature has been used in previous studies to manipulate the temporal dynamics of behaviours, such as bird song.
Monteiro explained that cooling a specific brain region slows down the song, while warming speeds it up, without altering the structure of the song. Therefore, the researchers used temperature to change the speed of neural dynamics without disrupting the pattern.
The team developed a custom thermoelectric device that could warm or cool the striatum of rats focally, and also record neural activity at the same time. The researchers anaesthetised the rats, and used a technique called optogenetics to stimulate specific cells using light. Optogenetics helped the team create waves of activity in the striatum, which is an otherwise dormant region.
Margarida Pexirra, a co-lead author on the paper, said they were careful not to cool the area too much because it would shut down the activity, or warm it too much, risking irreversible damage.
The researchers observed that cooling the region dilated the pattern of activity, or decreased its speed, while warming contracted the patterns, or increased the speed. The pattern was not disrupted.
Filipe Rodrigues, another lead author on the paper, said the researchers trained animals to report whether the interval between two tones was shorter or longer than 1.5 seconds. When a rat's striatum was cooled, the activity became slower, and the animal reported the time interval as being shorter than it actually was.
Meanwhile, when the striatum was heated, the activity became faster, and the animal judged the time interval as being longer than it actually was.
The study found that slowing down or speeding up the patterns of activity in the striatum did not slow down or speed up the movements of the animals in the task. Paton explained that organisms face two fundamental challenges while controlling movement. The first challenge is to choose from among different potential actions - for instance, whether to move forward or backward, and the second challenge is to be able to adjust and control the activity continuously to ensure that it is carried out effectively.
The striatum helps resolve the first challenge, which is to determine 'what' to do and 'when', while the second challenge of 'how' to control the ongoing movement is determined by other brain structures.
The researchers are now conducting a separate study to determine the correlation between the cerebellum and how it helps execute actions.
Paton said that their preliminary data shows that applying temperature manipulations to the cerebellum, unlike the striatum, does affect continuous movement control.
He explained that one can see this division of labour between the two brain systems in movement disorders like Parkinson's and cerebellar ataxia. Parkinson's affects the striatum, and hampers patients' ability to self-initiate motor plans such as walking. However, providing sensory cues, like lines of tape on the ground, can facilitate walking, because the cerebellum is still intact.
Meanwhile, patients with cerebellar damage struggle with executing smooth and coordinated movements but not necessarily with the initiation or transition between movements because the striatum is still intact.
The new study is important because it provides new insights into the causal relationship between neural activity and timing function, and these findings can help advance the development of novel therapeutic agents for treating diseases such as Parkinson's and Huntington's which involve time-related symptoms and a damaged striatum.