The hippocampus dissociates present from past and future goals

Abstract

Our brain adeptly navigates goals across time frames, distinguishing between urgent needs and those of the past or future. The hippocampus is a region known for supporting mental time travel and organizing information along its longitudinal axis, transitioning from detailed posterior representations to generalized anterior ones. This study investigates the role of the hippocampus in distinguishing goals over time: whether the hippocampus encodes time regardless of detail or abstraction, and whether the hippocampus preferentially activates its anterior region for temporally distant goals (past and future) and its posterior region for immediate goals. We use a space-themed experiment with 7T functional MRI on 31 participants to examine how the hippocampus encodes the temporal distance of goals. During a simulated Mars mission, we find that the hippocampus tracks goals solely by temporal proximity. We show that past and future goals activate the left anterior hippocampus, while current goals engage the left posterior hippocampus. This suggests that the hippocampus maps goals using timestamps, extending its long axis system to include temporal goal organization.

Fig. 1. Experimental design.

a The experiment took place over two days. Participants learned when they would need to complete each goal during a training session on the first day of the experiment. Participants were familiarized with the paradigm in the mock scanner and then embarked on their mission inside the 7T scanner on the second day. b Example screen shown during year 1. While lying in the scanner, participants were shown a goal and asked when they need to accomplish this goal, with buttons at the bottom of the screen to indicate whether the goal was applicable for the current Mars year (CY), or relevant in the distant future (DF), near future (NF), near past (NP), distant past (DP), always (AN, always needed), or never (NN, never needed). The QR code grants access to the video shown to the participant at the beginning of the experiment to actively involve participants in the task by immersing them in an engaging experience, depicting a rocket’s journey from Earth, through space, and landing on Mars. c Example trajectory of the space helmet goal across the mission, which is relevant in the 3^rd Mars year for participants completing version 1. d Example set of goals. Participants learned 6 sets of 5 goals, each set associated with either year 1, year 2, year 3, year 4, always, or never.

Fig. 2. Participants took longer to process temporally removed goals than current goals.

a Participants (n = 31) shifted their thinking of the goals as the goals’ temporal relevance changed. This plot visualizes the trajectories of the goals, separated by when they had to be accomplished in the game. Error bars = 95% confidence interval (CI). b Post hoc comparisons of the estimated marginal means revealed that current goals were processed faster than all temporally removed goals. ***p < 0.001. (Supplementary Table 3 provides a comprehensive overview of all contrasts). Error bars = 95% CI. c Individual participants’ average log reaction time for each of the temporal conditions.

Fig. 3. Temporally removed goals activated the left anterior hippocampus and current goals activated the left posterior hippocampus.

a Activation maps for the contrasts comparing the remote (distant future + near future + distant past + near past) > current are overlaid in yellow. b Activation maps for the contrasts comparing the current > remote are overlaid in green. All z-statistic images were thresholded parametrically using GRF-theory-based maximum height thresholding with a (FWE-corrected) significance threshold of p = 0.025. c Activation for the temporally removed goals (yellow) and the current goals (green) shown concurrently on the brain. d The same goal, for instance fixing the space helmet, was anatomically dissociated along the longitudinal axis based on whether it was currently relevant, or relevant at a point removed in time. Contrast maps were overlaid and rendered onto a 3-dimensional MNI 152 brain using MRIcroGL. The left hippocampal region of interest (ROI) is displayed in white. Color bars reflect the thresholded z-statistic scores.