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Nonlinear encoding of temporal information in rat prefrontal cortex

Kim, Jieun; Ghim, Jeong-Wook; Lee, Ji Hyun; Jung, Min Whan
Lab. of Neuroscience, Institute for Medical Sciences, Ajou University School of Medicine
Involvement of the prefrontal cortex (PFC) in interval timing is well supported by several lines of converging evidence. Brain imaging studies in humans have found that blood oxygenation level dependent signals are enhanced in the PFC during various timing tasks, and local lesion/inactivation studies have shown that the PFC is crucially involved in time interval discrimination in humans, monkeys, cats and rats. We also have found that inactivation of the medial PFC (mPFC) profoundly impairs rat’s ability to discriminate time intervals in the range of a few seconds. However, neural mechanisms underlying the role of the PFC in interval timing are currently unclear. In particular, it has been controversial whether time is represented on a linear or logarithmic scale in the nervous system. We investigated this issue by recording single unit activity in rats discriminating six randomly presented interval durations (approximately 3~5 s) as either short or long ones. The recorded neuronal ensemble conveyed significant amount of information on the elapse of time. A principal component analysis revealed that the largest variance of neuronal activity was accounted for by a monotonically increasing, concave-shaped activity profile over time. Also, there was a significant correlation between PC1 loading value and the amount of temporal information conveyed by each neuron. These results indicate that the mPFC keeps track of time based on linearly changing neuronal activity on a logarithmic scale. Therefore, monotonically changing neuronal activity on a logarithmic scale appears to be at least one way of representing the elapse of time in the brain, which might be the reason why the precision of time interval discrimination is lowered in proportion to its duration according to Weber’s law.
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