Time

Time is an inevitable part of everything that happens in the brain, and it is represented in many different ways across different brain areas, with no evidence of a single central “clock”. Perhaps the closest to such a thing is the theta rhythm which is a prominent timing signal at ~5 Hz (200 ms period) coordinated across wide areas of the spatial processing networks including the hippocampus. However, there are many other timing systems in the brain. The massive array of granule cells in the cerebellum are widely thought to provide a high-resolution timing mechanism for coordinating motor actions, and dynamic activity patterns in the prefrontal cortex monitor progress toward accomplishing goals.

There is considerable evidence that people use representations of space in the parietal lobe to reason about time, and in general, the slippery nature of time is often rendered more concrete by turning it into a distributed representation encoded across a population of neurons, effectively spatializing time. The large language models for example take this approach to the extreme by encoding large streams of text across increasingly large context input layers, used to predict the next word in that language stream.

Theta rhythm

The theta rhythm (~5 Hz, 200 ms period) is generated in the brainstem ventral tegmental nucleus (VTN) (of Gudden) and the medial mammillary bodies (MMB) to which it connects. From there, this timing signal spreads widely to the medial temporal lobe and the hippocampus.