Human Hippocampus: The Cognitive Map

Oct 10, 2014 Ruchira Dhoke Did You Know?, Lifestyle & Health 0

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Memory is the treasury and guardian of all things,” the Roman philosopher and statesman, Cicero once wrote; but have you ever given a thought as to how we remember the route to our office, the way back to home; the remembrance of places visited is the past?

The answer to this question may be found in hippocampus a major component of human brain belonging to the limbic system. The hippocampus is widely accepted to play a pivotal role in memory and is thought to function as a ‘cognitive map’, which stores non-spatial information such as items and events in a spatial framework.

According to the news article published in Scientific American; Decoding Space and time in the Brain(JUNE 2013, by Aiden Arnold)

‘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.”

Einstein, a mathematical genius, had shown that both space and time were mere illusions in the equations of relativity, conceding into a 4D construct which was later termed as space-time by the German mathematician Hermann Minkowski. The now iconic quote “…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” by Hermann Minkowski in 1906 captured the spirit of Albert Einstein’s recently published special theory of relativity.

Einstein’s ideas had influenced both the academic and public conception of the physical universe which led to the revolution against space and time in the fields of experimental psychology and neuroscience.

The study of cognition of mind’s perception, organization and its interaction with physical space known as spatial cognition has long been of interest to philosophers and scientists

Immanuel Kant a philosopher and author of “Critique of Pure” (1781/1787) 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.

While philosophers following Kant have debated his theory on space perception, it served to lay the groundwork for the twentieth century scientists to further dig 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.

Cognitive maps differed from other potential strategies of navigation and spatial learning, and its neural basis was addressed by John O’Keefe and his colleagues in the 1970s through a series of studies that were 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. These groups of neurons were dubbed as “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 2000 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.

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 and 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.

The human hippocampus acts primarily to create and store representations of the physical environment is suggested by the context of evidence from both neuropsychological analyses of amnesic patients and neuroimaging studies on healthy subjects, indicates  that the human hippocampus plays a role not only in spatial memory but in many other aspects of memory .

Thus, although many studies suggest that the human hippocampus has a role in spatial navigation and episodic memory (Scoville and Milner, 1957; Tulving, 2002; Squire et al., 2004) ,additional work is needed to explore how this relates to the more pervasive role of the hippocampus in many aspects of memory.

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• Brain, cognitive map, Human Hippocampus

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