A Role for Dorsal and Ventral Hippocampus in Inter-Temporal Choice Cost-Benefit Decision Making

model: Group₄ × (Triple-block₃ × S₃₉)]. The ANOVA found no main effect of group,F(3, 35) = 2.14; p = .11, but there was a main effect of triple-block, F(2, 70) = 7.79; p < .001 and a group × triple-block interaction, F(6, 70) = 2.25; p < .05. Analysis of simple main effects and subsequent pairwise comparisons (Fisher's LSD) revealed an effect of block within the cHPC group, F(2, 34) = 6.33; p = .01, in that they chose the HR significantly less often in the third triple-block, compared to the first (p < .001) and second triple-blocks (p < .05). In contrast, there was no change in HR arm choices across this stage of testing in the sham animals (no effect of triple-block; p > .2). Pairwise comparisons (Fisher's LSD) of the effect of lesion group within triple-block also revealed that the sham group chose the HR significantly more often than the vHPC group during the first and second triple-blocks (both p < .05). The shams also chose the HR significantly more often than the dHPC group during the second triple-block (p < .05). There was a trend toward the shams choosing the HR more than the cHPC group during the third triple-block (p = .07).

In summary, combining the results of stages 1 and 3, rats with dorsal, ventral, or complete hippocampal lesions exhibited a reduced preference for a higher reward that was subject to a 10s delay when the alternative was a low reward that was available immediately. In contrast, when the delay to reinforcement was equivalent (stage 2), all groups showed a consistent preference for the higher reward.

Experiment 2: Effortful reward

With no barrier in place, all groups chose the HR arm on more than 90% of trials, further demonstrating that the HPC is not required for simple reward discrimination (Figure 2a). When the 15 cm barrier was first introduced (Figure 2b), the sham and vHPC groups dramatically reduced their preference for the HR arm in the first block but increased their HR choices in subsequent blocks. In contrast, the dHPC and cHPC groups did not show this dramatic reduction in HR choices following the introduction of the barrier. After the first three blocks with the 15 cm barrier in place, 20 forced trials were given to ensure that all groups had adequate exposure to the reward contingencies. Upon retesting, there were no clear differences between the groups (Figure 2, c-h). Preference for the HR arm remained over 80% for all groups over the first four barrier heights (15–30 cm) but declined over the last two barrier heights (35 cm and 40 cm).

To analyze the effect of introducing the barrier, the three blocks with no barrier in place were compared to the first three blocks with the 15 cm barrier in the HR arm