Abstract
The hippocampus, a region critical for memory and spatial navigation, has been implicated in delay discounting, the decline in subjective reward value when a delay is imposed. However, how delay information is encoded in the hippocampus is poorly understood. Here, we recorded from CA1 of mice performing a delay-discounting decision-making task, where delay lengths, delay positions, and reward amounts were changed across sessions, and identified subpopulations of CA1 neurons that increased or decreased their firing rate during long delays. The activity of both delay-active and -suppressed cells reflected delay length, delay position, and reward amount; but manipulating reward amount differentially impacted the two populations, suggesting distinct roles in the valuation process. Further, genetic deletion of the N-methyl-D-aspartate (NMDA) receptor in hippocampal pyramidal cells impaired delay-discount behavior and diminished delay-dependent activity in CA1. Our results suggest that distinct subclasses of hippocampal neurons concertedly support delay-discounting decisions in a manner that is dependent on NMDA receptor function.
Highlights
Animals faced with multiple options optimize their decisions through a complex cost-benefit valuation
Much of the rodent hippocampal physiology literature has focused on the spatial code present in hippocampal place cell activity (Jung and McNaughton, 1993; O’Keefe and Dostrovsky, 1971; Wilson and McNaughton, 1993), subsequent work has demonstrated that the circuit is capable of encoding a variety of spatiotemporal features beyond the animal’s current position, including
With the inclusion of a delay, preference for the large reward arm decreased as a function of delay length (Figure 1C)
Summary
Animals faced with multiple options optimize their decisions through a complex cost-benefit valuation. When trying to understand the links between behavior and physiological data, several factors must be considered, including the variable(s) correlated with the activity, the regions or cell assemblies engaged and the mechanisms of representation on both the single-cell and population levels To this end, studies combining imaging and recording with optogenetic manipulation and identification suggest that subsets of CA1 neurons can encode distinct features of a task (Cembrowski et al, 2016; Danielson et al, 2016). We first examine the two schemes for encoding delay length: population coding and rate coding, and found that both schemes were employed by a significant fraction of CA1 neurons, including two populations that demonstrated increased or decreased delay-period activity The activity of these distinct populations reflected delay length, delay positions, and/or reward size, manipulation of reward size resulted in these populations’ having opposite responses. These results suggest that distinct subpopulation of neurons in the hippocampus can have unique contributions to the valuation processes that are required for delay-based decisions
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