Introduction
Spatial navigation is the ability to track our changes in position and orientation by integrating self-motion cues from linear and angular motion. It is a complex process involving a number of brain regions, most centrally the retrosplenial cortex, involved in translating spatial representations based on egocentric and allocentric reference frames. Spatial navigation is a function needed in everyday life, letting us travel around familiar environments such as homes or workplaces, and helping in learning new routes and adaptation to changing environments. Spatial navigation is also closely related to the formation of declarative memories, requiring conscious and voluntary processing. For instance, the location of a favourite restaurant or how one gets to a friend’s nearby house depends on the hippocampus’ processing ability to encode spatial information. As we will later discuss in this blog, the hippocampus plays a vital role in our spatial navigation skills, and it confers enormous advantages to appreciate how it handles and establishes stored representations of spatial information.
A Hub for Spatial Information Processing: The Hippocampus
The hippocampus has habitually been referred to as the “hub” of spatial information processing within the temporal lobe. Its involvement in the formation of memories is irrevocably established; through its activity, new experiences and events are able to be encoded into long-term conscious memory. Less known is its pivotal role in processing spatial information about our sense of direction and how we navigate. It contains a variety of cells, including place cells, head direction cells, and grid cells; all these help in mental mapping of the environment. Place cells are neurons that are activated by being at a particular place; therefore, we are able to associate specific locations with memories or experiences. The other constituents include the head direction cells, which provide information about our orientation in space, and grid cells, offering spatial layout information about our surroundings. It is by means of such complex interactions that the hippocampus develops a mosaic of integrated spatial information with the rest of the brain, offering us the ability to move around in our environment with ease.
The Different Types of Cells within the Hippocampus
Different kinds of cells exist within the hippocampus, whose interactions underlie our navigational competencies in space.
Place cells are best defined as neurons that are fired by the presence of an animal at a certain location in space, letting them form a cognitive map of their environment.
Another kind of cells called the head direction cells detects changes in direction and orientation and gives vital information about spatial position and movement of a living being.
Grid cells take it one better and mentally create a grid of the environment where each cell fires when an individual is at a particular place in that grid. This allows reconstruction of distance and direction from a starting point—a process known as path integration.
Other cells involved in spatial navigation include the boundary cells, which fire when one reaches the edge of one’s environment, and border cells, putatively involved in the detection and processing of information about boundaries between different spaces. The work of such specialised cell types empowers our ability to navigate complex environments easily.
Role of Hippocampus in Spatial Navigation
The hippocampus is important for building cognitive maps of our environment, and we move with ease through space. As we wander through our environments, it goes on to redevelop and refines these mental maps through the integration of new information that reaches us from our senses onto existing knowledge. Place cells, head direction cells, and grid cells are all part of the range of cell activation taking place within the hippocampus in this process. While place cells can be said to handle the mapping of space around, head direction cells are part of the orientation in that given space. On the other hand, grid cells underlie navigation in which we move through familiar environments by providing a mental grid system aware of knowing one’s place and movement. The hippocampus has a large role in spatial navigation; it underlies our learning of new routes and remembering where we have gone, even remembering events that happened in a particular place. In this respect, it acts as a kind of central hub for spatial information processing, which helps us create mental maps of our surroundings so we can travel through space with confidence.
Case Study: Henry Molaison (H.M.)—The Curious Case of Patient H.M.
Henry Molaison, better known as H.M., suffered from such severe epilepsy that in the year 1953, he had to go through a bilateral medial temporal lobectomy. This surgery damaged his hippocampus to a great extent—an area of the brain important for forming new memories and performing spatial navigation. Research into the case of H.M. showed that he held deep impairments in episodic memory, such as recalling recent events, learning new skills, and complex visual scenes. Surprisingly, though, his ability to move around familiar spaces remained quite intact. For example, despite dense amnesia, H.M. Decades after surgery he could still draw faithful maps of his apartment, showing that spatial navigation had been taken over by other parts of his brain. These findings could have broad implications for how we think about spatial memory and navigation: while the hippocampus does indeed participate crucially in the formation of new memories, other regions can compensate and allow us to navigate through familiar environments.