Multiple timescales

The command “Take a left turn at the next junction”, while seemingly easy to interpret, entails integration of information across at least two different timescales. On the one hand the sensory information of the word “Take” has to be retained for about a second until the sentence is over. On the other, the meaning of the sentence has to be retained for about a minute until the junction is actually reached. This example presents two timescales out of a large range relevant for daily functioning.

When considering possible solutions utilized by the brain for these tasks, it is worthwhile to note that the bio-chemical processes in the brain have inherent time scales spanning many orders of magnitude such as milliseconds (action potentials), seconds (Calcium dynamics) and hours (protein synthesis).

On the functional level, we know that the nervous system can adapt to the statistics of the environment over a wide range of timescales, but the mechanisms for doing so are yet unknown.

As an example of two different ways to solve a task, consider the problem of holding the identity of a stimulus in memory for a delay of a couple of seconds. The prevailing dogma is that the brain does it by keeping the neurons which are related to the stimulus firing action potentials throughout the delay. Although every action potential is a process of a few milliseconds, if the neurons are recurrently connected to each other then positive feedback can maintain this activity over the two seconds. This is reminiscent of the way RAM works,   or a car engine produces electricity.

Another option, which I explored during my PhD [Barak et al. 2007, Mongillo et al. 2008], is that the brain utilizes the slower dynamics of Calcium in the neurons in order to store the information there. According to this view the relevant neurons only fire transiently during the stimulus, but their associated synaptic Calcium levels remain elevated throughout the delay.

I am now exploring a more general formulation of this specific problem: given several mechanisms with different timescales, that could all solve the same task, can the brain effectively choose a mechanism such that its timescale matches that of the task. Potential advantages of such a solution are metabolic – slower processes are usually less energetically demanding than maintaining fast processes across a long time, and robustness – a slow process is less likely to be disrupted by fast noise. On the other hand, an obvious disadvantage is the loss of flexibility – a system tuned to a specific timescale (or a specific range of timescales) will perform poorly if forced to operate in a different regime.