• Zemina

Could Climate Change contribute to Spatial Memory Instability?

The Intro:


There are different types of spatial memories, but of course, the common thread being that an aspect of space or a spatial location is the foundation unto which that memory or information is built upon. Examples of spatial memory could be remembering a path or route that you take and the strategies you use to navigate in your environment. A more holistic type of spatial memory are episodic memories.


Episodic memories are ones in which you remember the rich details surrounding an autobiographical event. It gives you information on the where and when. Due to the nature of episodic information, these memories are quite precious to us humans. They allow us to recall significant life events such as graduations, heartbreaks, and getting married. They are rich because they are associative. Episodic information is connected to different components involving the space within which the events occurred and the salience of those experiences.


The function and role of space nested within episodic information and its ability to link across our own experiences to space, may help explain why a certain spatial location can have significant meaning to us. Walking through a familiar park can offset a slew of memories past, especially if the memories have an associated emotional valence to them. Feelings of happiness, sadness, resolution; can all be felt, simply by re-contextualizing a memory (walking through the same park as the event you're recalling or rather, recalling an event by simply walking through said park).


The importance of space appears to be shared among both settler and Indigenous populations:


...he demonstrates how many traditional Apache stories are connected to particular places, allowing the land to continuously remind people of social and moral code even when storyteller is absent or deceased. Basso also explores the notion that what matters most to Apaches is where events occurred, not when (my emphasis).

-- Kim Anderson, Life Stages and Native Women


suggesting that place itself can be broadened out of a narrow, physical concept/understanding of space or place. We could envision space as being a core, fundamental component of humanness. The significance of space and understanding the space around us is further reinforced by the specific spatial functions our brain has to understand it as I've eluded to above.


Space in the Brain: How do our Brains do it?


Edward Tolman once theorized that experiences are organized and linked via episodes across multiple domains of information into a cognitive map (Tolman, 1948). Hence, space is fundamental to the theory and construction of the cognitive map. Afterall, how can you develop a map sans space?


The significance of this mental cognitive map is many-fold. Tolman believed that the map would help facilitate the planning of future trajectories, determine the most efficient routes, and manage expectations based on goals and outcomes (Moser, Moser, & McNaughton, 2017; Tolman, 1948) suggesting that space is linked not just with navigation but decision making and goal-fulfillment--important human cognition.


The discovery of place cells in the Hippocampus (HPC; a medial temporal lobe structure in the brain) provided first-level validation of Tolman’s theory (O'Keefe, Burgess, Donnett, Jeffery, & Maguire, 1998; O'Keefe & Dostrovsky, 1971; O'Keefe & Nadel, 1978). Cells located in CA1 regions of the dorsal HPC fired in relation to a specific spatial location.


Place Cell Characteristics:


A thing to note about these place cells is that they have shown to be quite dynamic and adaptive as they appear to re-map themselves based on changes in the environment. Meaning that if place cell A fires in a specific location of a room, if any configuration of said room changes, place cell A may fire but not in the same location or maybe even not at all (see Figure 1).



Figure 1. Colour-coded firing rate maps (yellow = 0 Hz, purple = maximum rate) for hippocampal place cells recorded in a cylinder containing either a white or a black intra-maze cue card. (a) An example place cell recorded across four sessions in a cylinder with either the white or black cue card (as indicated above). Note the difference in place field locations for white and black sessions. (b) Three additional cells recorded in the cylinder for white and black cue card sessions. In these examples, place fields changed location or disappeared in response to the cue card substitution. Modified, with permission, from Bostock et al. Source: , Colgin et al, 2008, Understanding memory through hippocampal remapping

There are different types of re-mapping. A place cell can rate re-map (meaning the way in which the place cell, itself, fires; its rate code) to global re-mapping (whole ensembles of place cells change their configurations based on changes in the external environment, including not firing at all).


What is of most relevance to this post, however, is the notion that re-mapping is thought to help differentiate between two similar (but slightly different) experiences (see Figure 2), suggesting that re-mapping might be connected to memory and therefore may have actual behavioural function.



Figure 2: Schematic illustrating how remapping might help prevent interference between similar memories. Separate memory representations for similar experiences can be formed when patterns of neural activity from the entorhinal cortex (EC) are remapped onto largely nonoverlapping cell ensembles in the hippocampus (subfield CA3 in this example). Source: Colgin et al, 2008, Understanding memory through hippocampal remapping

Memory + Behavioural Consequences of Re-mapping on short-time scales:


Studies have demonstrated alteration in memory performance following displacement of environmental cues in an already previously coded hippocampal cognitive map (Colgin et al., 2008). Specifically, Lenck-Santini et al (2001) developed an experimental protocol that required the alternation of goal locations of a previously established paradigm (see Figure 3):



Source: Lenck-Stantini, 2001, Evidence for a Relationship Between Place-Cell Spatial Firing and Spatial Memory Performance

This alternation task is known to test memory, specifically, short-term memory. Basically, the rat has to alternate arms (between arm A and arm B) to get the goal (G; food reward). So for instance, a correct sequence would be ...G-A-G-B, etc. Researchers found that when re-mapping occurred in place cells due to changes in the room or the task itself, so to did differences in performance arise, and in this case, performance as a proxy to memory. Performance was measured in terms of correct and incorrect choices (i.e., the correct sequence of goal to arm alternations). Specifically, in this study, they found that when place cell firing did not adapt to the changes in goal location (i.e., did not re-map); memory performance suffered (though it is important to mention that this study and the subsequent behavioural result was correlative and not causal).


It is also important to point out here the temporal components to this study. Although the task itself tests short-term memory, the rats in the above study had several days, in addition to pre-training trials, to solidify the cognitive map for the standard configuration of this task (aka session 1). Although the alternation component of this task tests shorter-term memory, comparison of performance between alternation and the standard configuration (sessions 2-4 for the former and sessions 1 and 5 for the latter, respectively) also makes this an examination of longer-term memory.


The results of the study shows that changes in goal location on a short term scale (sessions 2-4) exhibit deteriorated performance and subsequently, poor memory while performance shot up at session 5 when the configuration went back to the standard configuration as it was in session 1 (possibly because it had more time to solidify).


Place Cells (+ re-mapping) are about more than just place:


The discovery of place cells in the dorsal CA1 region of the HPC provided a basis into how precisely these cells may be contributing to the formation and maintenance of episodic-like information with a spatial emphasis (O'Keefe, 1976; O'Keefe et al., 1998; O'Keefe & Dostrovsky, 1971). Place cells are thought to be a critical building block in the formation of the cognitive map Edward Tolman initially envisioned. This cognitive map is thought to facilitate the ongoing, mnemonic (associative) navigation and planning of trajectories in the rat and perhaps the human model (O'Keefe & Nadel, 1978; Tolman, 1948). Thus, place cells are a lot more dynamic and contribute much more to information than just spatial navigation. And naturally then, space is tied to more fundamental components of human life.


For our example study, the place cells and subsequent re-mapping (or lack of re-mapping) appear to have correlative consequences to memory as demonstrated by associated behaviour which happen to be the deterioration of memory leading to incorrect decision-making. An interesting property of place cells is that they show stability for the same environment for long periods of time after they form. This was demonstrated in the example study (sessions 1 and 5). The consequences of changing our environments too quickly (sessions 2-4 in the example study) results in decreased performance.


Essentially, if environments change too quickly, we may lose our ability to understand the space around us and lose out on developing the required associations of our space; putting us at risk of losing our ability to form autobiographical information.


When change happens too quickly:


The discussions above demonstrate the important link between time, space, experiences and our (brains') ability to consolidate important spatial (and episodic information).


That said, we also have to consider the possibility that since our brains are limited, we may not be able to process changes in our environment that happen too quickly as demonstrated in these studies. In response, you may tell me that the above discussions are only limited to the laboratory. This is a perfectly reasonable response, but I would like to offer a real-life example.


The Current Rate of Climate Change:


Such quick changes and the consequences of these quick changes can be observed outside the laboratory. Studies suggest that climate change is happening too fast for animals to be able to adapt to their new climate niche and that we are making our way into a mass extinction.


A study of more than 250 species found their ability to change their "climactic niche", the conditions under which they can survive, will be vastly outpaced by future changes in rainfall and temperature.

-- Source: Hayes et al., 2018


The ability to adapt to changes requires information retained from previous experiences that assisted in survival which can also include episodes/events that happened in a particular place. If we’re not being given sufficient time to consolidate said information because our environments are changing too quickly then we’re also losing our ability to survive.


The situation is even more dire for species that depend on land due to fundamental changes in habitat (see Polar Bears).



Wrapping it Up:


I will admit that this post is purely speculative as there are currently no scientific studies linking the notion that drastic climate change may affect our memories. I would however point you to something related to the topic.


There is substantial literature demonstrating that climate change negatively affects mental health. This is due to the deterioration of resources to sustain livelihoods and the destruction of precious ecosystems.


The expanding research on climate change and mental health includes increasing evidence that extreme weather events—which are more frequent, intense, and complex under a changing climate—can trigger post-traumatic stress disorder (PTSD), major depressive disorder (MDD), anxiety, depression, complicated grief, survivor guilt, vicarious trauma, recovery fatigue, substance abuse, and suicidal ideation.

--Source: https://ijmhs.biomedcentral.com/articles/10.1186/s13033-018-0210-6.


It also just so happens that the listed illnesses above have an affect on memory.


Add that to the drastic changes that are occurring due to habitat destruction, changes in temperature, and the other associated problems that come with accelerated climate change


Plus you're new found (introductory) knowledge on how experiences of ourselves are so closely tied to our environment + space and that our brains not only track changes in our environment but that this information requires sufficient time to solidify


All suggests that perhaps the notion that we’re messing up our spatial memory systems because of human-made climate change doesn’t seem like too far of a leap anymore.


References:


Anderson, K. (2011). Life stages and Native women: Memory, teachings, and story medicine (Vol. 15). Univ. of Manitoba Press.


Colgin, L. L., Moser, E. I., & Moser, M. B. (2008). Understanding memory through hippocampal remapping. Trends in neurosciences, 31(9), 469-477.


England, C. (2016). Climate change happening "too fast" for people to adapt". Independent.


Hayes, K., Blashki, G., Wiseman, J., Burke, S., & Reifels, L. (2018). Climate change and mental health: Risks, impacts and priority actions. International journal of mental health systems, 12(1), 28.


Lenck‐Santini, P. P., Save, E., & Poucet, B. (2001). Evidence for a relationship between place‐cell spatial firing and spatial memory performance. Hippocampus, 11(4), 377-390.


Moser, E. I., Moser, M. B., & McNaughton, B. L. (2017). Spatial representation in the hippocampal formation: a history. Nat Neurosci, 20(11), 1448-1464. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/29073644. doi:10.1038/nn.4653


O'Keefe. (1976). Place units in the hippocampus of the freely moving rat. Exp Neurol, 51(1), 78-109.


O'Keefe, Burgess, Donnett, Jeffery, & Maguire. (1998). Place cells, navigational accuracy, and the human hippocampus. Philos Trans R Soc Lond B Biol Sci, 353(1373), 1333-1340. doi:10.1098/rstb.1998.0287


O'Keefe, & Dostrovsky. (1971). The hippocampus as a spatial map. Preliminary evidence from unit activity in the freely-moving rat. Brain Res, 34(1), 171-175.


O'Keefe, & Nadel. (1978). The Hippocampus as a Cognitive Map: Oxford: Clarendon Press.


Rosenzweig, E. S., & Barnes, C. A. (2003). Impact of aging on hippocampal function: plasticity, network dynamics, and cognition. Prog Neurobiol, 69(3), 143-179.


Tolman, E. C. (1948). Cognitive maps in rats and men. Psychol Rev, 55(4), 189-208. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/18870876.






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