Human hippocampus represents space and time during retrieval of real-world memories

Abstract

Memory stretches over a lifetime. In controlled laboratory settings, the hippocampus and other medial temporal lobe brain structures have been shown to represent space and time on the scale of meters and seconds. It remains unclear whether the hippocampus also represents space and time over the longer scales necessary for human episodic memory. We recorded neural activity while participants relived their own experiences, cued by photographs taken with a custom lifelogging device. We found that the left anterior hippocampus represents space and time for a month of remembered events occurring over distances of up to 30 km. Although previous studies have identified similar drifts in representational similarity across space or time over the relatively brief time scales (seconds to minutes) that characterize individual episodic memories, our results provide compelling evidence that a similar pattern of spatiotemporal organization also exists for organizing distinct memories that are distant in space and time. These results further support the emerging view that the anterior, as opposed to posterior, hippocampus integrates distinct experiences, thereby providing a scaffold for encoding and retrieval of autobiographical memories on the scale of our lives.hippocampus | representational similarity analysis | lifelogging | episodic memory T he hippocampus plays a critical role in remembering the events of our lives (1). Direct evidence from single-neuron recordings in rats indicates that cells in the hippocampus fire in specific spatial locations (2-6) or at specific times during a temporal delay (7,8). Single-neuron and functional MRI (fMRI) studies in individuals navigating virtual environments have confirmed that cells coding for spatial location are also present in the human hippocampus (9-11). Similarly, place-responsive cell activity recorded in the hippocampus of patients with epilepsy during navigation of a virtual town was shown to reinstate during episodic memory retrieval of the previous virtual navigation (12). Together, these studies provide evidence that the same neurons in the medial temporal lobe (MTL) that are active during an experience also help represent the memory for that experience. These results, however, are limited to simple events in laboratory settings that occur on the scale of minutes and meters, thereby leaving unanswered whether we harness similar mechanisms in more natural settings and over larger temporal and spatial scales.Recent studies have used more naturalistic designs with incidentally acquired memories recorded via lifelogging devices that automatically capture photographs from the participants' lives (13,14). The typical finding is increased hippocampal activation when participants view images from their cameras as opposed to images from other participants' cameras (15-17), and this activation decays over the course of months (14). Still, there is no evidence to date that the hippocampus or other MTL structures actually represent space or time of autobiographical experiences. We a...

Episodic specificity induction impacts activity in a core brain network during construction of imagined future experiences

“…This finding converges with evidence suggesting that the anterior hippocampus supports the relational processing of elements of an encoded memory at retrieval (29,30), as well as the flexible recombination of previously learned elements into a novel representation (31). Evidence has also indicated that the anterior hippocampus tracks the content (vs. the temporal ordering) of imagined events (32) and the specificity (vs. abstractness) of imagined events (18,19) and autobiographical plans (33).…”

Memory integration constructs maps of space, time, and concepts

“…Consistent with this hypothesis, anterior hippocampus is more likely to demonstrate integrated codes for overlapping events that are separated by long time intervals than posterior hippocampus (Fig. 2C,D) [22, 23, 63]. The different representational capacities of anterior and posterior hippocampus may thus simultaneously support behaviors that rely on different levels of memory detail [83], with posterior hippocampus supporting fine judgments relating individual memory elements and anterior hippocampus supporting abstractions across broader experiential scales.…”

“…2C) [63], is also observed within hippocampus. Memory integration across longer temporal intervals may result from a different mechanism than that supported by time cells.…”

“…Adapted from Ref. [63]. (D) Memory elements that share a common association with an overlapping item are represented differently within anterior and posterior hippocampus.…”

Temporal and spatial context in the mind and brain

“…2B). These findings have been extended to real world memory [68], where hippocampal ensembles capture not only the spatial distance between memories and the temporal distance between memories, but also the interaction of spatial and temporal distance. Rodent work demonstrating that entorhinal grid cells have time-cell-like properties in time cell experiments [52], suggesting that spatial and temporal context representations are supported in some cases by the same neurons.…”