Pallikaras, V. | Search bioRxiv

bioRxiv2022

Does phasic dopamine release cause policy updates?

Phasic dopamine activity is believed to both encode reward-prediction errors (RPEs) and to cause the adaptations that these errors engender. If so, a rat working for optogenetic stimulation of dopamine neurons will repeatedly update its policy and/or action values, thus iteratively increasing its work rate. Here, we challenge this view by demonstrating stable, non-maximal work rates in the face of repeated optogenetic stimulation of midbrain dopamine neurons. Furthermore, we show that rats learn to discriminate between world states distinguished only by their history of dopamine activation. Comparison of these results to reinforcement learning simulations suggests that the induced dopamine transients acted more as rewards than RPEs. However, pursuit of dopaminergic stimulation drifted upwards over a time scale of days and weeks, despite its stability within trials. To reconcile the results with prior findings, we consider multiple roles for dopamine signaling.

neuroscience↗

bioRxiv2021

The trade-off between pulse duration and power in optical excitation of midbrain dopamine neurons approximates Bloch's law

BackgroundOptogenetic experiments reveal functional roles of specific neurons. However, such inferences have been restricted by widespread adoption of a fixed set of stimulation parameters. Broader exploration of the parameter space can deepen insight into the mapping between selective neural activity and behavior. In this way, characteristics of the activated neurons, such as temporal integration, can be inferred. ObjectiveTo determine whether an equal-energy principle accounts for the interaction of pulse duration and optical power in optogenetic excitation. MethodsSix male TH::Cre rats worked for optogenetic (ChannelRhodopsin-2) stimulation of Ventral Tegmental Area dopamine neurons. We used a within-subject design to describe the trade-off between pulse duration and optical power in determining reward seeking. Parameters were customized for each subject on the basis of behavioral effectiveness. ResultsWithin a useful range of powers (~12.6-31.6 mW) the product of optical power and pulse duration required to produce a given level of reward seeking was roughly constant. Such reciprocity is consistent with Blochs law, which posits an equal-energy principle of temporal summation over short durations in human vision. The trade-off between pulse duration and power broke down at higher powers. ConclusionsOptical power can be substituted for pulse duration to scale the region of neuronal excitation in behavioral optogenetic experiments. Power and duration can be adjusted reciprocally for brief durations and lower powers. The findings demonstrate the utility of within-subject and trade-off designs in optogenetics and of parameter adjustment based on functional endpoints instead of physical properties of the stimulation. HighlightsO_LIWe provide behaviorally derived intensity-duration curves for ChannelRhodopsin-2. C_LIO_LIDuration trades off almost perfectly with power within useful ranges. C_LIO_LIThis trade-off breaks down at high optical powers. C_LIO_LIPulse duration and optical power scale the area of neuronal excitation equivalently. C_LIO_LIBehaviorally derived trade-offs can reveal optogenetic excitation mechanisms. C_LI

neuroscience↗