Neural Activity in the Medial Temporal Lobe Reveals the Fidelity of Mental Time Travel

Abstract

Neural circuitry in the medial temporal lobe (MTL) is critically involved in mental time travel, which involves the vivid retrieval of the details of past experience. Neuroscientific theories propose that the MTL supports memory of the past by retrieving previously encoded episodic information, as well as by reactivating a temporal code specifying the position of a particular event within an episode. However, the neural computations supporting these abilities are underspecified. To test hypotheses regarding the computational mechanisms supported by different MTL subregions during mental time travel, we developed a computational model that linked a blood oxygenation level-dependent signal to cognitive operations, allowing us to predict human performance in a memory search task. Activity in the posterior MTL, including parahippocampal cortex, reflected how strongly one reactivates the temporal context of a retrieved memory, allowing the model to predict whether the next memory will correspond to a nearby moment in the study episode. A signal in the anterior MTL, including perirhinal cortex, indicated the successful retrieval of list items, without providing information regarding temporal organization. A hippocampal signal reflected both processes, consistent with theories that this region binds item and context information together to form episodic memories. These findings provide evidence for modern theories that describe complementary roles of the hippocampus and surrounding parahippocampal and perirhinal cortices during the retrieval of episodic memories, shaping how humans revisit the past.

Temporal and spatial context in the mind and brain

“…It is also clear that this recovery can cause associations that bridge across different experiences with a stimulus, as predicted by retrieved context models of memory. It is also known that behavioral markers of contextual retrieval correlate with hippocampal activity [35]. However, it has not yet been definitively established that gradually-changing brain states are in fact recovered during episodic memory.…”

Integrating theoretical models with functional neuroimaging

“…Other studies first fitted a model to the behavioral data and then used the resulting fitted model to make inferences about the neural signals (Ide et al, 2013). The opposite approach has also proven effective, in which fMRI signals from a voxel or brain region of interest are incorporated within a model in order to make better predictions of behavioral performance (Kragel et al, 2015). Finally, Mack et al (2013) fitted multiple models to the behavioral data, and determined which model produced the highest correspondence with patterns of brain activity.…”

“…Recently however, Kragel et al (2015) successfully established links between a well-studied model of free recall and fMRI measures of brain activity, providing insights into neural mechanisms underlying memory processing that would not have been possible otherwise. A challenge in understanding human memory is the organization and structure that people naturally impose upon any set of learned items.…”

“…Kragel et al (2015) posited that changes in brain activity during the recall period may reflect these dynamics, driven by two different processes: 1) More or less brain activity in an area might correspond to how well a word is being recalled, termed retrieval success , and 2) Activity may be related not to the item strength, but rather to the degree with which the context surrounding the item is being activated, termed temporal reinstatement .…”

“…Here, a multivoxel pattern classification approach was used to transform multivariate fMRI data, obtained from multiple levels of the visual hierarchy, into an interpretable measure of information representation, and the theoretical model was fitted to the result. Whereas in both of the earlier studies, a formal model was fitted to information decoded from the mutlivariate fMRI signal (Figure 1B), we next consider a study in which the time course of the fMRI signal on individual trials was incorporated within a theoretical model of behavioral memory performance (Figure 1C), to determine whether this neural signal can lead to more accurate predictions of free recall performance (Kragel, Morton, & Polyn, 2015). An fMRI study of categorization highlights yet another approach (Figure 1D), by assessing the degree to which competing models of behavioral categorization performance can account for observed patterns of neural data (Mack, Preston, & Love, 2013).…”