Decoding Space and Time in the Brain
2013 06 04

By Aiden Arnold | ScientificAmerican


“…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.

[...]

Read the full article at: scientificamerican.com




Related Articles
"Laws of Physics for a Holographic Universe" --New Theories of Space-Time
The Brain’s Stopwatch – Emotions and Time Perception
Why Time is a Social Construct
Low Doses of THC Can Halt Brain Damage
"The Google Brain" --Are Humans Entering a New Epoch of Evolution?
You’ll Probably Never Upload Your Mind Into A Computer
Mind Science Kept Hidden


Latest News from our Front Page

Easter - Christian or Pagan?
2014 04 18
From: truthbeknown.com Contrary to popular belief, Easter does not represent the "historical" crucifixion and resurrection of Jesus Christ. In reality, the gospel tale reflects the annual "crossification" of the sun through the vernal equinox (Spring), at which time the sun is "resurrected," as the day begins to become longer than the night. Rather than being a "Christian" holiday, Easter celebrations date back ...
Man-Made Blood Might Be Used in Transfusions by 2016
2014 04 18
Researchers in the U.K. have created the first man-made red blood cells of high enough quality to be introduced into the human body The premise of the HBO show and book series True Blood revolves around a technological breakthrough: scientists figure out how to synthesize artificial human blood, which, as an ample new source of non-human food, allows vampires to "come ...
The Trials of the Cherokee Were Reflected In Their Skulls
2014 04 18
Researchers from North Carolina State University and the University of Tennessee have found that environmental stressors – from the Trail of Tears to the Civil War – led to significant changes in the shape of skulls in the eastern and western bands of the Cherokee people. The findings highlight the role of environmental factors in shaping our physical characteristics. ...
Our Fears May Be Shaped by Ancestral Trauma
2014 04 18
Last December, an unsettling Nature Neuroscience study found that mice who were taught to associate the smell of cherry blossoms with pain produced offspring who feared the smell of cherry blossoms, even if they had never been exposed to it before. We knew that the process was epigenetic—that it was not hard-wired in the permanent genetic structure of the mouse—but ...
Did vitamin B3 come from space?
2014 04 17
Ancient Earth might have had an extraterrestrial supply of vitamin B3 delivered by carbon-rich meteorites, according to a new analysis. The result supports a theory that the origin of life may have been assisted by a supply of key molecules created in space and brought to Earth by comet and meteor impacts. "It is always difficult to put a value on ...
More News »