本文作者：小红猪小分队

Source:http://blogs.scientificamerican.com/mind-guest-blog/2013/06/03/decoding-space-and-time-in-the-brain/

“…henceforth, space by itself, and time by itself, are doomed to fade away into mere shadows, and only a kind of union between the two will preserve an independent reality.”

This now iconic quote spoken by Hermann Minkowski in 1906 captured the spirit of Albert Einstein’s recently published special theory of relativity. Einstein, in a stroke of mathematical genius, had shown that both space and time as independent mathematical constructs were mere illusions in the equations of relativity, conceding instead to a 4-dimensional construct which Minkowski adroitly termed space-time. While most people are familiar with the ensuing influence Einstein’s ideas had on both the academic and public conception of the physical universe, few people are aware a similar revolution against space and time is underway in the fields of experimental psychology and neuroscience.

Space in the Brain
Spatial cognition is the study of how the mind’s cognitive architecture perceives, organizes and interacts with physical space. It has long been of interest to philosophers and scientists, with perhaps the biggest historical step towards our modern ideas occurring within Immanuel Kant’s Critique of Pure Reason (1781/1787). Kant argued that space as we know it is a preconscious organizing feature of the human mind, a scaffold upon which we’re able to understand the physical world of objects, extension and motion. In a sense, space to Kant was a window into the world, rather than a thing to be perceived in it.

While philosophers following Kant have debated his theory on space perception, it served to lay the groundwork for the twentieth century empirical investigation into how the mind constructs the space that we experience. A key piece to how this happens was provided in 1948 by American psychologist Edward Tolman.

Tolman’s main interest was studying the behavior of rats in mazes – specifically, he was interested in whether a rat came to understand the layout of an environment through purely behavioral mechanisms, or if there was a cognitive process underlying their navigation ability. In his studies, Tolman found that rats were able to efficiently navigate to locations in a maze that had never been behaviorally reinforced, suggesting that rats spontaneously formed a mental representation of the maze which allowed them to mentally identify locations and plan routes to reach a specific destination. This mental representation was termed a ‘cognitive map’, which Tolman hypothesized as the primary means through which mammals – rats and humans alike – learned about and navigated through spatial environments.

Although the idea of a cognitive map became widespread in the 1960s with the growth of cognitive psychology, Tolman himself did little to elaborate on the processes involved in forming and using a cognitive map. Particularly, it remained unclear how cognitive maps differed from other potential strategies of navigation and spatial learning, and whether scientists could identify its neural basis.

These issues were addressed by John O’Keefe and his colleagues in the 1970s through a series of studies that cumulated in an elegant theory proposed in the aptly titled book The Hippocampus as a Cognitive Map (1976). In this publication, O’Keefe and Lynn Nadel proposed that a specific population of neurons in the hippocampus – a brain region implicated in various memory processes – were responsible for encoding the location of a mammal within space. This group of neurons were dubbed place cells, and by using direct recordings in the rat hippocampus were shown to have increased firing frequency as a rat entered a particular location within an environment.

Strikingly, the locations in which place cells fire appears fixed over repeated exposure to an environment, anchoring themselves to environmental landmarks. O’Keefe and Nadel believed that these place cells form the neurological basis of a cognitive map – a map defined by the interrelations of the different elements that compose an environment. Research in the early 2000s on epileptic patients undergoing seizure monitoring confirmed the existence of place cells in the human hippocampus, which were shown to function in similar manner to what had previously been documented in studies on other mammals.

Place cells themselves appear sufficient to represent locations within an environment. However, due to the malleability of their firing locations in response to certain experimental manipulations such as rearranging the location of environmental landmarks, it is unclear whether they are capable of providing the spatial framework through which we construct our experience of the world.

A second class of cells first identified by the husband and wife team of Edvard and May-Britt Moser and their students in 2005 may provide the answer. Termed grid cells, these neurons exhibit firing patterns that closely resemble a hexagonal grid. Unlike place cells, the regularity observed in the firing patterns of grid cells does not appear to be derived from environmental features, or any type of sensory information. Rather, they appear to code a spatial structure that is generated internally within the brain and use it to scaffold the external environment, much in the same manner that Kant had anticipated. Interestingly, grid cells have been identified primarily within an area of the brain called the entorhinal cortex, one of the primary neural inputs to the hippocampus, suggesting that grid cells provide a source of the spatial framework upon which cognitive maps of environments are formed.

Time in the Brain

Time has proven to be a much more elusive concept for both psychology and neuroscience. Despite numerous decades of research, the majority of what we know about time representation in the brain comes from two lines of research: how overlapping events are parsed into discrete episodes and the sequential ordering of those events into a temporal framework.

It had been hypothesized since the 1970s that the hippocampus is critical for separating patterns of experience into the independent episodes that occupy the content of our episodic memory system. However, this hypothesis rested largely on findings from neuropsychology, where brain lesions to the hippocampus impaired both pattern separation and pattern completion ability, and from computational modeling studies deconstructing how episodic memory systems operate.

In the early 2000s, direct evidence to support the role of the hippocampus in parsing and sequencing episodic events began to emerge from both animal and human studies. Using an array of experimental methodologies, researchers found that the hippocampus is crucial for encoding the order of visual stimuli – whether pictures on a computer screen or landmarks in an environment – and that it expresses unique patterns of activity during overlapping segments of routes through an environment.

The latter finding is particularly important, as it counters a purely place cell model of hippocampal function during navigation. In such a model it would be expected that hippocampal activity is consistent during overlapping route segments, as a person’s physical location is the same through these portions of an environment. This suggests that the hippocampus is involved in representing more than simply the spatial layout of an environment.

A key breakthrough in identifying which types additional information the hippocampus processes was provided by Howard Eichenbaum and his colleagues at Boston University. In a 2011 paper, the authors proposed a new type of neuron population within the hippocampus which they labeled as ‘time cells’. Through a series of studies with rats, it was found that time cell activity could uniquely code successive events and were able to disambiguate overlapping sequences in temporally organized episodes.

If translated to humans, these findings have important implications for understanding how the hippocampus subserves various cognitive functions that rely on it, such as episodic memory, navigation and imagination. Specifically, it suggests that the hippocampus is able to tune its activity to both spatial and temporal aspects of an experience, depending on what type of information needs to be encoded or recalled.

Binding Space and Time in the Brain

If our experience of time and space share similar neural correlates, it begets a fundamental question: are space and time truly distinct in the mind, or are they the product of a generalized neurocognitive system that allows us to understand the world? While Kant had much more to say about space than time, contemporary cognitive neuroscientists have begun composing theories to address this question. One proposal by Demis Hassabis and Eleanor Maguire suggests that the primary function of the hippocampus isn’t to think about past and future, or to move about through space per se. Rather, through cooperation in a larger network spread throughout the brain, the hippocampus allows us to construct a representation of the world in a spatiotemporal context that affords the ability to simulate past experiences in order to make predictions about the future, and to ultimately use this information to direct action in the present.

However, while this primary role of spatiotemporal context generation has caught the interest of notable scientists throughout the past decade, may discrepancies between space and time perception remain. For instance, our perception of space appears to remain stable, while time regularly dilates at the whim of attention – moments stretch and contract as we pay differing degrees of focus to our actions in the world.

Which ever side the truth is found to reside in, the coming years will undoubtedly provide a wealth of insight into how the mind represents space and time, and whether the two are destined to yield to a unified account of how we experience the world.

Image: Hartwig HKD on Flickr.

About the Author: Aiden Arnold is a PhD candidate in NeuroLab at the University of Calgary. The focus of his research is on understanding how neural networks in the brain shape our ability to think about and interact with space. While not doing science or writing, he is busy traveling with his fiancée, photographing life and seeking out the perfect espresso. You can find out more about Aiden and his work at www.aidenarnold.com. Follow on Twitter@aidenarnold.