We may have finally unlocked the secrets of how our brains keep track of time
You may not even realise you’re doing it, but every time you sing a song, play an instrument, swing a tennis racquet, or even have a conversation, your brain is keeping track of time.
It knows exactly when to hit the right note, or swing the bat to hit the ball, and knows from the rhythm of a person’s voice when it’s time to speak.
For decades it was believed we had an internal brain clock; one that’s different from our body clock, capable of recording an interval of time without the need for a watch or timer, but new research has found to the contrary.
By studying brain activity in monkeys, researchers from MIT have discovered that instead of a centralised clock, or pacemaker, that is always tracking passages of time for the entire brain, neurons in the parts of our brains needed for certain activities change their behaviour depending on what time interval is needed.
Depending on the time interval required, these neurons compress or stretch out the steps they take to generate a behaviour at a specific time. For example, when playing baseball, the brain uses various stimuli to establish the location of the ball and the neurons needed to make you swing the bat will change their speed just so, in order for the bat to connect with the ball at the correct time.
“What we found is that it’s a very active process. The brain is not passively waiting for a clock to reach a particular point,” said senior author Mehrdad Jazayeri, a member of MIT’s McGovern Institute for Brain Research.
One of the earliest models of timing control, known as the “clock accumulator model”, suggested that we each have an internal pacemaker that keeps time for the rest of the brain. A later version of this model moved this theory on, suggesting that instead of using a central pacemaker, the brain measures time by tracking the “synchronisation between different brain wave frequencies.”
These theories “don’t match well with what the brain does,” explained Jazayeri. Namely, because no-one has found evidence of this central clock. This led Jazayeri and his team to ponder whether parts of the brain that control the behaviours which need precise timing could perform the timing function themselves.
“People now question why would the brain want to spend the time and energy to generate a clock when it’s not always needed. For certain behaviours you need to do timing, so perhaps the parts of the brain that subserve these functions can also do timing,” he continued.
In an attempt to address their theory, the researchers recorded neuron activity from three brain regions (the dorsomedial frontal cortex, the caudate, and the thalamus) in monkeys as they performed a task at two different time intervals – 850 milliseconds or 1,500 milliseconds.
The researchers found a complicated pattern of neural activity during these intervals. Some neurons fired faster, some fired slower, and some that had been oscillating began to oscillate faster or slower. However, the key discovery was that no matter how the neurons responded, the rate at which they adjusted their activity depended on the time interval required.
When the interval required was longer, the neurons took more time to reach the so-called “final state” – or the point at which the action is required. When the interval was shorter, the neurons were faster; kicking in to get ready sooner.
While a distinctive neural pattern was seen in the dorsomedial frontal cortex, an area of the brain involved in cognitive processes, and the caudate, responsible for motor control, inhibition, and some types of learning, a different pattern was spotted in the region responsible for motor and sensory skills, the thalmus. Instead of altering the speed of their trajectory, many of the neurons simply increased or decreased their firing rate, depending on the interval required.
Jazayeri said this is likely because the thalamus is telling the cortex how to adjust its activity to track the correct time.
The work follows on Jazayeri’s 2015 research in which he tested humans’ ability to measure and reproduce time using a task called “ready, set, go.” In this experiment, the volunteer measured the time between two flashes (“ready” and “set”) and then pressed a button (“go”) at the right time.
This study revealed that we don’t simply measure an interval and reproduce it. Rather, after measuring an interval we combine that measurement, which is imprecise, with prior knowledge of what the interval could have been. This prior knowledge, which builds up as we repeat a task, allows us to reproduce the interval more accurately. This is why people who practice tennis or piano gradually improve their timing and skill.
“When people reproduce time, they don’t seem to use a timer,” Jazayeri said at the time. “It’s an active act of probabilistic inference that goes on.”
The team now hopes to study how the brain generates the neural patterns seen during varying time intervals, and also how our expectations influence our ability to produce different intervals. For example, if we’re excited about something does that make time feel like it’s going slower in anticipation.
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