Suppression of Ventral Hippocampal Output Impairs Integrated Orbitofrontal Encoding of Task Structure

Abstract

The hippocampus and orbitofrontal cortex (OFC) both make important contributions to decision making and other cognitive processes. However, despite anatomical links between the two, few studies have tested the importance of hippocampal-OFC interactions. Here, we recorded OFC neurons in rats performing a decision making task while suppressing activity in a key hippocampal output region, the ventral subiculum. OFC neurons encoded information about expected outcomes and rats' responses. With hippocampal output suppressed, rats were slower to adapt to changes in reward contingency, and OFC encoding of response information was strongly attenuated. In addition, ventral subiculum inactivation prevented OFC neurons from integrating information about features of outcomes to form holistic representations of the outcomes available in specific trial blocks. These data suggest that the hippocampus contributes to OFC encoding of both concrete, low-level features of expected outcomes, and abstract, inferred properties of the structure of the world, such as task state.

Keywords: decision making; decoding; electrophysiology; hippocampus; orbitofrontal.

Figure 1. Task and histology

a) Rats performed an odor-guided decision making task. Odor cues delivered to the central port instructed rats on which action (go left, go right, go either direction) would be rewarded on that trial. Rats responded to either the left or right fluid well, where reward was delivered if they chose correctly on forced-choice trials, or for a response to either well on free-choice trials. The outcomes delivered at each fluid well differed in size and flavor, and changed across blocks of trials as illustrated in the example block sequence. b) The ventral subiculum was injected bilaterally with an AAV virus carrying the eNpHR3.0-eYFP construct under the control of the CamKII promoter. Immunohistochemistry was used to identify eNpHR expressing neurons. Green staining indicates eNpHR expression, while the blue DAPI counterstain labels nuclei. Ventral subiculum (vSUB) and dentate gyrus (DG) cell layers are labelled. Scale bar indicates 1 mm. c) A magnified view of the area marked by the white box in panel b, showing individual neurons expressing eNpHR. Scale bar indicates 100 microns. d) Expression was confirmed in the ventral subiculum for all rats. Green shading indicates the maximal (light) and minimal (dark) extent of expression. Dots indicate optical fiber placements. e) Approximate neural recording locations in the OFC are indicated with red boxes.

Figure 2. Ventral subiculum inactivation impaired free-choice but not forced-choice behavior

a) The fraction of large choices rats made on free-choice trials was computed and aligned to block switches (when the location of large and small reward outcomes reversed positions; dashed vertical line). With eNpHR stimulation, rats adjusted their behavior in response to block switches more slowly than in control stimulation sessions. Overall, the large outcome choice rate was lower in eNpHR sessions, compared to control sessions (panel a inset). b) Rats performed correctly on a high fraction of forced-choice trials, when the odor cue instructed them which fluid well to select. There was no difference in accuracy between eNpHR and control sessions, although in both session types rats were significantly more likely to select correctly when cued to select the large outcome. Similarly, during both control and eNpHR sessions, reaction times were faster on forced-choice trials directing rats to the large outcome. For all bar graph plots, bars indicate mean values computed across all rats and sessions, and connected dots show mean values computed separately for each rat.

Figure 3. OFC firing rate dynamics were altered by ventral subiculum inactivation

a) Trials were divided into seven 0.5 s epochs bounded by important task events. b) Average, peak-normalized firing rates were computed for cells within task epochs. Each row represents the firing rate of a single unit (brighter colors = stronger activation), with time on the x-axis (each panel = 0.5 s; bin size was 45 ms). c) In control sessions, most OFC neurons reached their maximal firing rate during the outcome anticipation epoch—after rats indicated their decision, but before outcomes were delivered. With subiculum inactivated, significantly fewer OFC neurons reached their peak firing rate during reward anticipation. d) Raw, un-normalized population firing rates showed a similar pattern—in eNpHR sessions the OFC population firing rate was lower than in control sessions. This difference began during the movement epoch (as animals left the odor port and moved towards one of the fluid wells), and was sustained through the reward anticipation period. Bin size was 45 ms; error bars indicated the SEM. See also Figure S1.

Figure 4. Single unit encoding of action and outcome information

The average firing rates for three example units (recorded during control sessions) are plotted, with activity aligned to outcome delivery. The plots show the unit’s average firing rate for each of eight possible outcomes animals selected on forced-choice trials. The shaded region indicates the reward anticipation task epoch. The unit on the left discriminated the size of outcomes; dashed lines group all trials types in which the animal selected the large reward outcome, and solid lines indicate trial types when the rat selected the small outcome. Note that neurons can encode outcome size information with either increases or decreases in firing rate. The unit in the middle discriminated outcome flavor, firing more for trials in which chocolate was selected (solid lines) than trials in which vanilla (dashed lines) was chosen. The unit on the right distinguished response direction, firing faster when the animal chose the right fluid well (solid lines) than when it selected the left fluid well (dashed lines). The bin size used to compute firing rate was 45 ms. Outcome abbreviations: LLV = left, large, vanilla; LLC = left, large, chocolate; LSV = left, small, vanilla; LSC = left, small, chocolate; RLV = right, large, vanilla; RLC = right, large, chocolate; RSV = right, small, vanilla; RSC = right, small, chocolate.

Figure 5. Ventral subiculum inactivation reduced OFC selectivity for response direction

a) We fit regression models to neural data to examine how well the firing rates of individual OFC neurons were explained by the variables outcome size, outcome flavor, and response direction throughout the course of task trials. The fraction of neurons whose responses were significantly modulated by each of these variables increased during odor sampling, and was sustained throughout the trials. While similar proportions of neurons were selective for size and flavor during control and eNpHR sessions, response direction selectivity was strongly attenuated in eNpHR sessions. The bin size for the neural activity used to fit regression models was 45 ms. b) Venn diagrams show the fraction of neurons that were selective for size, flavor, and/or direction during odor sampling, movement, or reward anticipation. c) Ventral subiculum inactivation selectively reduced the proportion of neurons modulated by response direction. See also Figure S2.

Figure 6. Ventral subiculum inactivation altered OFC outcome representations

a) Decoding chosen outcome was more accurate for control neurons than for eNpHR neurons for most pseudoensemble sizes (paired t-tests, corrected for multiple comparisons). Error bars indicate the SEM in classification accuracy across 250 random pseudoensembles; dashed horizontal line indicates chance level for classification. b) Confusion matrices show the proportion of test samples classified correctly (along the main diagonal, from the upper-left to the lower-right corner) and incorrectly (off the main diagonal) for each trial type using pseudoensembles of 50 neurons. Control session errors in classification were largely confined to the upper left and lower right quadrants, indicating that direction information was generally encoded correctly even when flavor or outcome information were not. In contrast, classification errors in eNpHR sessions were more dispersed. c) Decoding variables in pairs (rather than the full action–outcome triplet) revealed pronounced deficits in eNpHR pseudoensembles when pairs included response direction, along with a more subtle deficit in decoding flavor and size. Error bars indicate the SEM in classification accuracy across 250 random pseudoensembles; dashed horizontal line indicates chance level for classification. Asterisks indicate significant differences between control and eNpHR pseudoensemble classification performance (paired t-tests, corrected for multiple comparisons).

Figure 7. OFC represents task structure information beyond outcome features

a) With selectivity for action and outcome features removed, pseudoensembles drawn from control session neurons achieved significantly better classification performance than eNpHR pseudoensembles. Error bars indicate the SEM in classification accuracy across 250 random pseudoensembles; dashed horizontal line indicates chance level for classification. Asterisks indicate significant differences between control and eNpHR pseudoensemble classification performance (paired t-tests, corrected for multiple comparisons). b) For control data, errors in classification were concentrated along the minor diagonal from the lower-left to upper-right corner, corresponding the unchosen action and outcome combination that accompanied the option rats chose. This pattern was substantially weaker in eNpHR sessions.

Figure 8. Ventral subiculum inactivation abolishes OFC task block representations

a) With selectivity for action and outcome features removed, control session pseudoensembles were significantly better at trial block decoding. Error bars indicate the SEM in classification accuracy across 250 random pseudoensembles; dashed horizontal line indicates chance level for classification. Asterisks indicate significant differences between control and eNpHR pseudoensemble classification performance (paired t-tests, corrected for multiple comparisons). b) However, when trial block was included as a predictor in the regression models, no differences in performance were observed between eNpHR and control data.