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

Abstract

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