By Esha Kulkarni
Like the GPS System in our smartphones, the human brain is suggested to also consist of an inner control system, or an ‘inner GPS system’. This GPS system is thought to be responsible for spatial orientation, recognizing its surroundings, creating a cognitive map, finding directions, and retaining spatial memory (Chaparro, 2021). Mammalian brains are thought to consist of a special network of neurons located within the hippocampus— a region within the brain, responsible for memory retention, that ultimately allow us to navigate in space. The recent cumulative findings of three scientists, has created a paradigm shift in this field of neuroscience, laying down the foundation for other researchers to tackle specific diseases such as Alzheimer’s, where one of the early onset symptoms can be spatial disorientation (ScienceDaily, 2021).
The first discovery of the navigation system of the brain was discovered by John O’Keefe in 1971. He investigated the hippocampus, a region of the brain, previously thought to be solely critical for memory retention and processing. John O’Keefe challenged this theory, by conducting an experiment where he examined the neural activity of the hippocampus region when a rat was allowed to move around freely in a designed enclosure. He concluded that there were specialised units in the hippocampal region called “place units”, responsible for creating a cognitive map of the environment (Chiu, 2021). To examine the exact role of the place units, a series of experiments were conducted to also look at the effects of any confounding variables. For example, he monitored the firing activity of the place units in response to several visual sensory stimuli such as turning off the room lights, etc (O’Keefe, 1976). It was concluded that the firing mostly occurred irrespective of the changes in visual sensory stimuli hence, the firing was not influenced by any physical manipulations in the enclosure. Additional experiments also concluded that the firing of the place cells, occurred at certain points or places of the enclosure (Chiu, 2021). Ultimately, a cognitive map theory was formulated, which states that the hippocampal place cells form an overall map of the environment, and a single place cell, is representative of a part of an environment. In simpler words, each place cell was thought to receive two types of inputs that are independent from each other: from environmental stimuli and a spatial navigation system, which decides where the rat is located at a specific moment in time (O’Keefe, 1976). So, when the rat passes through a particular space, a particular environmental stimulus specific to that location is able to excite each place cell. This is almost like the brain is receiving special hidden “cues” in the surroundings that associates that cue with that that particular space hence creating this intricate map of our surroundings. John O’Keefe states that the navigation system is “shielding” the environmental stimuli, where only an environmental stimuli located in a specific location is able to excite the place cell (O’Keefe, 1976). Hence, John O’Keefe’s research has shaped the overall understanding of hippocampus and has helped one understanding its crucial role in spatial mapping and orientation.
During the 1970s (when O’Keefe was conducting his experiments), the cognitive map theory was widely accepted hence multiple hypothesis were also derived from it. For example, researchers hypothesized that an absence of hippocampus would result in low spatial exploration and possibly a loss in spatial disorientation. However, decades later, a Norwegian- husband and wife research duo, May-Britt Moser and Edvard Moser, discovered that the disruption of the hippocampus neural circuit, still shows place cell function (Chiu, 2021). This means that there is an additional neural circuit that is also partly responsible for spatial orientation. The Mosers, investigated individual cells from another region of the brain, that directly communicates with the hippocampus called the entorhinal cortex. They examined these cells were not firing constantly and had periods of “silence” like place cells. The pair increased the size of the enclosure and observed that there was a hexagonal, regularly spaced grid pattern of the firing fields of these special cells, meaning the period at which they went silent, was at equal intervals (Moser, Rowland and Moser, 2015). They named these cells “grid cells”, suggesting that these cells, are responsible for creating an intricate “chart” of the entire environment, like creating latitudes and longitudes of a place. The automated firing of these grid cells at specific locations helps the brain memorize reference points in the environment, like coordinates, to help navigate the environment more efficiently. Additional studies have also discovered head-direction cells, cells that fire in response to a specific direction, and border cells that fire specifically along the border of the environment (Moser, Rowland and Moser, 2015). Hence, place cells, grid cells, head-direction cells and border cells all form a massive neural circuit in the hippocampus and entorhinal cortex which overall helps an individual to navigate through the environment, and possibly even judge distance and re-map or re-orient themselves.
The Moser couple, and John O’Keefe received the 2014 Nobel Prize in Physiology and Medicine for their findings. This area of neuroscience is still an area of investigation. For example, studies have tried to raise new-born rats in a spherical enclosure to examine the response of the rats due to the possible lack of neuronal input from border cells. It has also been discovered that grid cells develop later than place cells. Since grid cells are closely connected to border cells, it is interesting to see whether this has an impact on the rats’ navigation system later life. Most importantly, the recent discoveries of the navigation system of brain have crucial clinical significance, such as in Alzheimer’s disease. Studies have shown that there is mild deterioration of place cells and severe impairment on grid cells in Alzheimer patients which attribute to the symptoms of Alzheimer’s, i.e, loss of spatial memory orientation (Jun et al., 2020). The reasons as to why these symptoms occur and the role of place and grid cells are still under question.
References:
Chaparro, L., 2021. This is how your Brain’s GPS Orients Itself – OpenMind. [online] OpenMind. Available at: <https://www.bbvaopenmind.com/en/science/research/this-is-how-your-brains-gps-orients-itself/> [Accessed 5 October 2021].
Chiu, L., 2021. Exploring the Brain’s GPS. [online] Brainfacts.org. Available at: <https://www.brainfacts.org/thinking-sensing-and-behaving/learning-and-memory/2017/exploring-the-brains-gps-012417> [Accessed 5 October 2021].
Jun, H., Bramian, A., Soma, S., Saito, T., Saido, T. and Igarashi, K., 2020. Disrupted Place Cell Remapping and Impaired Grid Cells in a Knockin Model of Alzheimer’s Disease. Neuron, 107(6), pp.1095-1112.e6.
Moser, M., Rowland, D. and Moser, E., 2015. Place Cells, Grid Cells, and Memory. Cold Spring Harbor Perspectives in Biology, 7(2), p.a021808.
O’Keefe, J., 1976. Place units in the hippocampus of the freely moving rat. Experimental Neurology, 51(1), pp.78-109.
ScienceDaily. 2021. Oscillations provide insights into the brain’s navigation system. [online] Available at: <https://www.sciencedaily.com/releases/2018/10/181012110201.htm> [Accessed 5 October 2021].
Imperial Bioscience Review 