We often think about where we are physically, which direction we are facing, and the environment that’s surrounding us. Without even looking, we can discern how far away we are from the door which we just walked through or know where we left our charger plugged in just a few feet away. We can conjure a map in our minds and determine the distance of objects as well. How can we do that? The answer lies in the hippocampus and spatial cognition. The hippocampus is a crucial brain structure with many functions, notably in long-term memory and spatial navigation. Although important, however, the hippocampus is not the only player. A variety of cells, cortical regions, and even a couple structures play an important role. And damaging any of these can prove to have drastic consequences to spatial navigation, such as in Alzheimer’s disease.
Spatial cognition is divided into two frameworks, one which is activated when you are reaching your hand out to pick up a pencil or throwing a softball to a person, and the other is activated when you are driving a jeep through an unknown savannah or running a marathon across a new town. The latter framework is focused on in the article by researchers Hartley, et al., titled, “Space in the brain: how the hippocampal formation supports spatial cognition.” It explores the allocentric spatial framework, which utilizes a distinct system in the hippocampus to form a long-lasting map or spatial representation of an individual’s current location in their environment, including other objects, structures, living beings, even things like hazards outside of and surrounding the person themself. Although the majority of the research referenced in the article was conducted on rodents such as mice and rats, evidence has also shown that this extends to humans as well, among other mammals. This fascinating system is called the hippocampal formation, and it consists of the hippocampus proper and the surrounding cortical regions that it connects to. One of these connections is to the medial septum and diagonal band of Broca, resulting in the theta oscillation, a significant component contributing to spatial representation among other functions. The researchers also outlined the four major types of spatial cells residing in the hippocampal formation and their most important properties. The four types of spatial cells are place cells, head direction cells, grid cells, and boundary cells with individually distinct firing patterns. Place cells typically fire at a low rate, which increases in its place field, portraying the animal’s physical location. Head direction cells depict direction irrespective of location in a world-centered point of view, with increased firing as the person or animal faces the preferred direction of the cell. Grid cells are like place cells, except they have many fields that they fire in. They consist of three characteristics: orientation, scale, and spatial phase. And lastly, boundary cells fire at a preferred distance of an environmental boundary with direction. Along with giving background information on the anatomy of the involved structures and cells, Hartley, et al. also explored some areas of current research, including in Alzheimer’s disease, and provided implications for future studies.
Like a part of researchers Hartley, et al.’s article, the article by researchers Silva and Martinez titled, “Spatial memory deficits in Alzheimer’s disease and their connection to cognitive maps’ formation by place cells and grid cells,” gives an overview of cognitive maps and the key cells, primarily place cells and grid cells, and structures involved in cognitive map formation. Hartley, et al., briefly mentioned atrophy in the hippocampus, specifically in the hippocampal formation, being a distinct symptom in patients with progressing Alzheimer’s, and how the disease includes deficits in spatial navigation. But Silva and Martinez go a step further, expanding further on Alzheimer’s. According to them, Alzheimer’s disease is characterized as a multi-faceted disorder primarily affecting the hippocampus-entorhinal cortex (HP-EC). Patients affected by Alzheimer’s experience disruptions in spatial memory and navigation and have trouble forming their cognitive maps and remapping, among other symptoms. Like Hartley, et al.’s article, Silva and Martinez discuss the important role of place cells and grid cells in cognitive map formation. They affirm how the different types of cells in the hippocampus and the HP-EC circuit allow the cognitive map to form for navigation through the environment. Place cells and grid cells engage in remapping, which is a process through which representations specific to various places are created. Different environments cause place and grid cells to reorganize their activity, which is crucial to creating long-term memory of different spatial representations. In people with Alzheimer’s, this ability is impaired.
Additionally, both articles highlight the importance of grid coding, and how it plays an important role in organizing information for spatial memory. Hartley et al. mention a theme of research focusing on the relationships between grid, head direction, and boundary cells in providing input to hippocampal neurons. Silva and Hartley expand on this to connect specifically to humans, talking about research of place cells, path cells, and grid-like activity in various brain structures. Both articles discuss the versatility of grid coding in spatial navigation, among other functions, that showcase how the HP-EC is a multifaceted structure complex. They also highlight implications for further research on the hippocampus, specifically in humans including patients living with Alzheimer's disease. Furthermore, they name a variety of techniques that can be used to conduct this research as well, such as optogenetics, human neuroimaging, and even virtual navigation in those with Alzheimer's specifically.
Spatial cognition is a fascinating area of study that gives us insight and answers into how we are able to form cognitive maps and conjure an allocentric image of our surroundings and future path without much effort. Both articles by researchers Hartley et al. and Silva and Martinez offer immensely valuable information on the structures, cortical regions, and neurons involved in making spatial frameworks and cognitive maps a reality. They also highlight the role it plays in some important symptoms of Alzheimer's disease, and give suggestions on techniques that can be used for further research on this and relating topics.
References
Hartley, Tom, et al. “Space in the Brain: How the Hippocampal Formation Supports Spatial Cognition.” Philosophical Transactions of the Royal Society B: Biological Sciences, vol. 369, no. 1635, 5 Feb. 2014, p. 20120510, www.ncbi.nlm.nih.gov/pmc/articles/PMC3866435/, https://doi.org/10.1098/rstb.2012.0510.
Silva, Azul, and María Cecilia Martínez. “Spatial Memory Deficits in Alzheimer’s Disease and Their Connection to Cognitive Maps’ Formation by Place Cells and Grid Cells.” Frontiers in Behavioral Neuroscience, vol. 16, 12 Jan. 2023, www.ncbi.nlm.nih.gov/pmc/articles/PMC9878455/, https://doi.org/10.3389/fnbeh.2022.1082158.
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