A Heterogeneous Population Code for Elapsed Time in Rat Medial Agranular Cortex

The neural mechanisms underlying the temporal control of behavior are largely unknown. Here we recorded from medial agranular cortex neurons in rats while they freely behaved in a temporal production task, the peak-interval procedure. Due to variability in estimating the time of food availability, robust responding typically bracketed the expected duration, starting some time before and ending some time after the signaled delay. These response periods provided analytic " steady state" windows during which subjects actively indicated their temporal expectation of food availability. Remarkably, during these response periods, a variety of firing patterns were seen that could be broadly described as ramps, peaks, and dips, with different slopes, directions, and times at which maxima or minima occur. Regularized linear discriminant analysis indicated that these patterns provided sufficiently reliable information to discriminate the elapsed duration of responding within these response periods. Modeling this across neuron variability showed that the utilization of ramps, dips, and peaks, with different slopes and minimal/maximal rates at different times, led to a substantial improvement in temporal prediction errors, suggesting that heterogeneity in the neural representation of elapsed time may facilitate temporally controlled behavior.