The Hippocampus and the Caudate Nucleus

This post is based on text from Wayfinding, a book by M.R. O’Connor. When Bohbot did her doctoral research with Lynn Nades, coauthor of The Hippocampus as a Cognitive Map, the hippocampus was a fascinating brain structure to study, and it was the only structure known to be involved in spatial memory. But it was conjectured that there were other brain structures involved in different ways to navigate in the environment. At McGill in the mid 1990s fellow researchers Normal White and Mark Packard discovered one other brain circuit. That circuit was the caudate nucleus.

Bohbat wondered if it was possible that people use very different brain structures for different strategies in navigation. So she began to conduct experiments in humans designed to distinguish which strategies were dependent on the hippocampus and which involved the caudate nucleus. She found that there were different circuits with corresponding differences in strategy.

Bohbat explained that the hippocampus is involved in learning to navigate using the relationships between landmarks. Once one has learned the relationship between landmarks, she can derive a novel route to any destination from any starting position in the environment. Spatial memory is allocentric in that it’s independent of one’s starting position. We use spatial memory when we can picture the environment in our mind’s eye. That’s when we are using our internal map to find our way. The caudate nucleus is not involved in creating maps; it’s a structure of directional cues such as “turn right at the corner with the grocery, and turn left at the tall white building”. This creates what are called stimulus-response memories. Bohbot notes if every day we use the same route, at some point it becomes automatic. We don’t think about it anymore. We don’t ask, where do I have to turn? Autopilot takes over. We see the white building, it acts as a stimulus that triggers a response to turn left at the bakery.

Bohbot states that there are three types of stimulus-response strategies. The egocentric strategy involves a series of right and left turns that begin with the starting point. When one leaves home (the stimulus), one turns right (the response). There is also a beacon strategy where one can reach a target location from many different starting positions: the beacon, say a tall white building, is the stimulus and one heads toward it, turning at every corner in its direction (the response). The most common form of stimulus-response: a series of turns in response to various landmarks in the environment.

Even though the caudate nucleus uses repetition to navigate successfully, it’s actually not a spatial strategy. The key difference being that the response strategy does not involve learning the relationships between landmarks, so it becomes impossible to generate a novel trajectory in the environment. All the caudate does is signal—left or right—in response to a cue without engaging one’s active attention.

There is an evolutionary explanation for why nature invented this other circuit; it means we don’t have to retrieve a memory of a route or make spatial inferences every time we need to go home. It gives us the advantage of not needing to make calculations or decisions—or pay very much attention—to where we are going and how we are getting there. Autopilot is fast and efficient, and we don’t have to think.

Bohbot discovered a negative correlation between the two strategies: the human brain is using either the hippocampus or the caudate nucleus to get somewhere, but it never engages the two brain areas at the same time. So, the more we use one, the less we use the other, and the weaker it becomes.

Bohbot conducted a study of 599 children and adults and compared the hippocampal spatial strategies to increased automation. Each of our life histories traces this trajectory: we go from using hippocampal spatial strategies to increased automatization. Bohbot and her researchers compared the spatial strategies preferred to solve tasks. They found that children rely on hippocampal spatial strategies some 85% of the time, but adults over the age of sixty used this strategy just under 40% of the time. The critical question is whether the preferences of one strategy over the other led to physiological differences in gray matter density and volume in the hippocampus.

Bohbot and several researchers published two studies focused on measuring activity and gray matter in both the hippocampus and the caudate nucleus. They mimicked the classical spatial test for routes and applied it to humans by creating a radial maze in a virtual setting and asking participants to navigate it while they tracked their brain activity with fMRI. As expected, individuals who used spatial memory strategies shows increased activity in the hippocampus. Those who used the stimulus response strategy had increased activity in the caudate nucleus.

Then they measured the morphological differences in the two brain regions in each individual. They found a high probability that people who used a spatial strategy had more gray matter density in the hippocampus, and the inverse was also true: those who used a response strategy had more gray matter in the caudate nucleus.

So the big questions are what if we persistently prioritize the caudate nucleus over a hippocampal strategy? And what if this prioritization is happening at an endemic scale?

HM navigates with a response strategy. It is efficient as advertised. There are shortcomings with this strategy that are not made explicit in the text. What happens if there is an accident or heavy traffic that makes one want to alter the route? That can’t be done. HM not only uses a response strategy, he uses a lazy version of the response strategy. He just uses the visual references and pays no attention to street names. This make HM appear to be an idiot (and this might be more than an appearance) because he cannot very readily explain the directions to others.

But these dangers are relatively minor given the weaknesses that develop in the hippocampus. The hippocampus is one of the most important, if not the most important, organ relevant to a healthy memory. One both wants and needs a very healthy or extremely healthy hippocampus.

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