Spatiotemporal Trajectories in Memory
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Spatiotemporal Trajectories in Memory
Author: Myelin
Reviewed by: 食野
Episodic memory is a form of long-term memory, defined as memory for events that occurred at a particular time and place. It is inseparable from personal experience, and its defining feature is its narrative, event-based character—for example, remembering an activity you attended or a place you once visited. [1]
In daily life, we usually imagine space and time as separate dimensions of experience, yet we often combine them when describing episodic memories. If I ask what you did this morning, you may mentally retrace the events of the entire morning as they unfolded across times and places. This reflects a widely held view: episodic memory embeds our records of events in a unified representation of their spatial and temporal context. [2]
In applied physics and cosmology, moreover, “time” is treated directly as the fourth dimension. Most famously, a key component of Einstein's special theory of relativity is that time dilates with velocity in different frames of reference. This observation underlies our modern concept of “spacetime”, the unification of spatial and temporal dimensions. [3]
This article, however, is mainly about how the brain processes space and time. Do the neural pathways and mechanisms involved reflect distinct spatial and temporal codes? Or do they instead reveal a unified representation of spacetime in which memories themselves are located?
“Space”
The hippocampus has long been recognized as crucial both to memory and to the brain's representations of space and time. One possible link among these functions is that the hippocampus organizes memories in space. Pioneering research distinguishing the “what” and “where” streams of visual processing identified a specialized pathway for spatial cognition and action, separate from pathways that support particular visual properties such as color and brightness.
Research over recent decades has mapped the details of the “where” pathway. Spatial memory is encoded across a succession of cortical regions extending from cortex to the parahippocampal cortex—called the postrhinal cortex in rodents—and the medial entorhinal cortex. [4]
Many functional-imaging studies have observed selective activation in the parahippocampal cortex and medial entorhinal cortex when participants recall the setting in which they studied a particular object. This has allowed researchers to identify these regions as representing the spatial elements of memory. [5]
In 2005, May-Britt and Edvard Moser discovered grid cells, which map our surroundings at different scales by dividing space into hexagonal units. In 2014, the Mosers shared the Nobel Prize in Physiology or Medicine with their University College London colleague and mentor John O'Keefe for discovering cells that constitute a positioning system in the brain.
Inspired by the Mosers' discovery of spatially encoding grid cells, Albert Tsao—then a doctoral student at the Kavli Institute—began in 2007 to decipher what was happening in the enigmatic lateral entorhinal cortex (LEC). This region lies immediately beside the medial entorhinal cortex (MEC), where his supervisors, the Mosers, had discovered grid cells.
Today, we have a fairly good understanding of how the brain processes space, but our understanding of time is less coherent. Time is a non-equilibrium process: it is always unique and constantly changing. If this network truly encodes time, its signal must change with time so that experiences can be recorded as distinct memories. [6]
“Time”
Researchers at Norway's Kavli Institute for Systems Neuroscience have discovered a network of brain cells that represents our sense of time within experience and memory. This neural clock works by organizing the flow of experience into an ordered sequence of events. Its activity creates the brain's clock for subjective time. Experience—and the sequence of events within it—is therefore the material from which the brain produces and measures subjective time.
For example, we coordinate our activities by clock time. Your brain, however, does not perceive duration in the standardized minutes and hours shown on your watch. Time as it appears in experience and memory belongs to an entirely different kind of temporality. [7]
Over the course of evolution, organisms—including humans—have developed several biological clocks to help keep track of time. The brain's timers differ not only in the timescales they measure, but also in the phenomena to which they are tuned. Some are set by external processes. The circadian clock, for instance, is tuned to the rising and fading of daylight and helps organisms adapt to the daily cycle. [8]
Other forms of timing are set by more internal phenomena. Time cells in the hippocampus, for example, form a domino-like chain of signals that tracks intervals of up to ten seconds with precision. Yet few known mechanisms operate on the timescales the brain uses to record experiences and memories, which may last from seconds to minutes or hours. [7]
In 2016, doctoral student Jørgen Sugar joined Norway's Kavli project and conducted a new series of experiments to test the hypothesis that the LEC network encodes episodic time. In one experiment, a rat encountered a broad sequence of experiences and choices. As it visited a series of open environments, it could run freely, explore, and chase pieces of chocolate.
The unique temporal signal in this experiment showed that over its two-hour duration, the rat kept a very good record of time and of the temporal order of events—for instance, when and where it found chocolate, and whether it remembered when to go where for its next treat. The researchers could use signals from the time-coding network to track accurately when the various events in the experiment occurred.
In the second experiment, the task was more structured and offered a narrower range of experiences and choices. The rat was trained to chase pieces of chocolate by turning left or right in a maze [Figs. 8(b) and 8(c)]. During this activity, the time-coding signal changed from a unique temporal sequence into a repeating, partly overlapping pattern.
In a repetitive task, on the other hand, the temporal signal became more precise and predictable. The data showed that the rat had a finely resolved sense of time within each lap, but a poor sense of time from one lap to the next and from the beginning to the end of the experiment as a whole.
Professor Moser believes this research shows that by changing the activity and the content of an experience, a person can change the progression of the temporal signal in the LEC—and thereby change how time is perceived. [6, 8]
The structure of time has long been a subject of debate among philosophers and physicists. What can the brain's newly discovered mechanism for episodic time tell us about our perception of time? Is our perception linear, like a flowing river, or cyclical, like a wheel or a spiral? Data from O'Keefe's research suggest that both are true, and that signals in the time-coding network can take many forms depending on the experience.
Space and time are initially processed by overlapping brain networks and encoded at different scales. Their signals are then integrated in the hippocampus to form a spatiotemporal framework for organizing memory. Although movement through space and time is intrinsically coupled, under some behavioral demands spatial and temporal codes can be observed separately in the hippocampus; alternatively, neurons encoding the two dimensions may combine to form an integrated population representation.
The hippocampus can be a little willful, though—just kidding. Rather than using a unified spatiotemporal coordinate system, the hippocampal network mixes separately encoded space and time and selectively reuses their representations. In other words, spatial and temporal memories can occasionally become confused.
The hippocampal network also maps a wide range of other continuous dimensions of experience, organizing knowledge through the mixed selectivity of neuronal elements within the same population. Temporal-context models applied to hippocampal networks with broad mixed selectivity for different inputs point to a simple mechanism for associating adjacent events that together integrate space, time, and other dimensions.
And those dimensions are the features of an abstract relational space.
Citations
Literature
[1] Buzsáki, G. Time, space and memory. Nature 497, 568–569 (2013). https://doi.org/10.1038/497568a
[2] Sarah DuBrow, Lila Davachi, Temporal binding within and across events, Neurobiology of Learning and Memory, Volume 134, Part A , Pages 107-114(2016), https://doi.org/10.1016/j.nlm.2016.07.011.
[3] Brunetti, Romeo, Franceschini, Lorenzo & Moretti, Valter, 2009. Topological features of massive bosons on two-dimensional einstein space-time. Annales Henri Poincaré, 10(6), pp.1027–1073.
[4] Robert E. Clark, Larry R. Squire, Animal model of human memory impairment, Proceedings of the National Academy of Sciences Jun 2013, 110 (Supplement 2) 10365-10370
[5] ALEXANDRA O. CONSTANTINESCU, JILL X. O’REILLY, TIMOTHY E. J. BEHRENS, Organizing conceptual knowledge in humans with a gridlike code, SCIENCE17 JUN 2016 : 1464-1468
[6] Edvard I. Moser, Emilio Kropff and May-Britt Moser ., Place Cells, Grid Cells, and the Brain’s Spatial Representation System
Annual Review of Neuroscience.(2008) , Vol. 31:69-89
[7]O’Keefe, J., and Nadel, L. (1978). The Hippocampus as a Cognitive Map(Oxford University Press).
[8]Roberts, B.M., Hsieh, L.-T., and Ranganath, C. (2013). Oscillatory activityduring maintenance of spatial and temporal information in working memory.Neuropsychologia51, 349–357.
Images
Fig. 1: Spacetime, http://www.thephysicsmill.com/2012/12/02/ftl-part-3-general-relativity-shortcuts/
Fig. 2, Fig.3, Fig.4: The hippocampus, cortical regions, and spatial memory https://www.researchgate.net/profile/Neil-Burgess-2/publication/5580634/figure/fig1/AS:267308312559622@1440742676688/The-hippocampus-and-its-connectionsa-The-hippocampus-lies-in-the-medial-temporal_Q320.jpg
Fig. 6: https://i1.wp.com/www.lec.guru/wp-content/uploads/2017/12/Decoding-the-brain.jpg?resize=300%2C227&ssl=1
Fig. 7: https://norwegianscitechnews.com/wp-content/uploads/2018/08/illustration-1_a-neural-clock-for-timesmall.jpg
Fig.8 (a) (b) © : K.Jeffery. University College London, Topics in Neurobiology
Fig. 9: https://www.geon.us/Memory/images/DentateGyrus.jpg
Author: Myelin
Copyright Notice: All articles in this blog are licensed under CC BY-NC-SA 4.0 unless stating additionally.
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