Climbing activity as an event-based representation of time

Jan Reutimann, Stefano Fusi, Walter Senn

The brain has the ability of predicting the time of occurrence of a behaviorally relevant event. Recordings from inferotemporal (IT) cortex of monkeys performing a delayed match-to-sample task suggest the existence of neurons representing the passage of time between two events. These IT neurons exhibit climbing activity which is triggered by the sample stimulus and which peaks at the expected time of the match stimulus. When the typical interval between the two stimuli is changed, the slope of the climbing activity adapts to the new timing. We present a model in which the climbing activity is an indirect result of a slow firing rate adaptation. Simple Hebbian synaptic modifications allow to learn the proper slope. The model reproduces the experimental data, and is consistent with Weber's law on the scalable nature of interval timing.