SUMMARY
This manuscript is intended as a theoretical companion to Hamiloset al., 20201, in which we examined the role of dopaminergic neurons (DANs) in self-timed movements. In that study, we recorded DAN signals in mice trained to initiate a licking movement after a self-timed delay following a start-timing cue. DAN signals both before the start-timing cue and during the timing interval predicted the timing of movement onset, up to seconds before the movement itself. In particular, dopaminergic signals “ramped up” from the time of the cue to the time of the movement. On a given trial, the slope of the ramping was predictive of when the movement would occur, with steep slope associated with early movement and shallow slope with late movement, reminiscent of a ramp-to-threshold process.
Ramping dopaminergic signals were recently proposed in a theoretical framework that examined temporal-difference learning under resolved state uncertainty (Mikhael et al., 20192; Mikhael & Gershman, 20193; Gershman, 20144). Here, we show that an adapted version of Mikhael et al.’s model recapitulates the ramping dopaminergic signaling observed in our self-timed movement task. We also applied the model to results reported in a recent temporal bisection study, in which mice categorized time intervals as relatively short or long compared to a criterion interval (Soares et al., 20165). The model successfully predicted the relative amplitude of dynamic DAN signals observed in the bisection task. These combined results suggest a common neural mechanism that broadly underlies timing behavior: trial-by-trial variation in the rate of the internal “pacemaker,” manifested in DAN signals that reflect stretching or compression of the derivative of the subjective value function relative to veridical time. In this view, faster pacemaking is associated with relatively high amplitude dopaminergic signaling, whereas slower pacemaking is associated with relatively low levels of dopaminergic signaling.
Competing Interest Statement
The authors have declared no competing interest.