What the research is about
How long has it been since you ate breakfast? When should you clap to match the next beat of a song? Our brains do more than keep track of time that has already passed-they also help us anticipate when to act. Previous research has shown that the passage of time, particularly on the scale of seconds, is represented not in a single part of the brain but across multiple regions.
Two regions known to play important roles in timing are the secondary motor cortex (M2), located toward the front of the brain, and the posterior parietal cortex (PPC), located farther back. Neurons in these two regions are connected to one another. Yet it has remained unclear whether the regions always keep time in the same way or can sometimes track time independently.
So how do these two regions actually keep track of time?
A research team led by graduate student Hiroto Imamura, Associate Professor Riichiro Hira, and Professor Yoshikazu Isomura at Institute of Science Tokyo (Science Tokyo) studied mice performing a task in which they learned to anticipate alternating intervals of 6 and 12 seconds. The researchers simultaneously recorded the activity of thousands of neurons in M2 and PPC. They then built a computational model incorporating key features of the neural activity to explore how the two regions coordinate and process time.
Why this matters
The researchers first analyzed neural activity recorded from the mice and found that M2 and PPC did not always keep time in lockstep. When they estimated elapsed time from neural activity, the timing represented by the two regions sometimes shifted together, but in other cases only one region showed a shift. Cases in which only one region shifted occurred about twice as often as cases in which both shifted together. In other words, the two regions could keep time together while still retaining a degree of independence.
A closer look at the experimental data also revealed neural activity in M2 and PPC that slowly strengthened and weakened in coordination with each other. The researchers incorporated this feature into a computational model to investigate how the two regions could cooperate while still operating independently.
In the model, sparse connection between two regions helped keep their timing representations aligned. By contrast, when the both networks were affected by the same slow fluctuations, each network could first settle back into its own stable activity state before the two synchronized again. This behavior may allow the networks to track time independently while remaining part of a coordinated system.
What's next
Unlike spatial coordinates, time has no single absolute reference point. The brain therefore needs to keep track of how much time has passed since many different events. The fact that all brain regions do not always follow exactly the same internal clock may not be a limitation. Instead, it may provide the flexibility needed to track multiple time intervals at once.
The findings offer new clues to how different brain regions can work together while still processing information independently. By extending this research to additional brain areas, the team hopes to better understand how the brain as a whole shifts between more coordinated and more independent modes of processing depending on the situation.
Comment from the researcher
Most of the time, the brain can focus on only one thing at once. For example, it is difficult to listen to and enjoy two different pieces of music simultaneously, and few people would try to watch a different movie with each eye at the same time. The brain is not very good at dealing with things that do not form a coherent whole. When things come together as a whole, however, we can focus on them and enjoy them. We are interested in how large numbers of neurons create this kind of coherence. Perhaps this kind of coherence is related to what the "mind" that the brain creates.
(Riichiro Hira, Associate Professor, Department of Physiology and Cell Biology, Graduate School of Medical and Dental Sciences, Institute of Science Tokyo)
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