Humans and Rats Process Space Similarly!

Ooh Ooh Ooh! They Finally Did It!

I am sure by now everyone has figured out I am sort of what I like to call a mouse psychologist. One of the things I have always loved is when research in rodents has been replicated in humans using a different technique.

Since my background is in studying spatial and temporal memory and the hippocampus in rats, I was ecstatic when I came upon Dr. Jason Mattingley’s work a few years back. Particularly, they replicated a few studies of mine in college students using fMRI, suggesting what we were reporting in rats may actually be relevant to human spatial cognition. Since they have been replicating my work as well as others studying rat spatial memory and navigation, it goes without saying that I am excited about these studies.

Today I am going to talk about a couple of papers from Mattingley’s lab. Namely: “Distinct neural networks underlie encoding of categorical versus coordinate spatial relations during active navigation” by Oliver Baumann, Edgar Chan, and Jason Mattingley in (NeuroImage), and “Dissociable representations of environmental size and complexity in the human hippocampus” by Oliver Baumann and Jason Mattingley in (The Journal of Neuroscience).


What is Being Studied and Why?

The work from Baumann and colelagues has been to evaluate how humans perceive spatial relationships among objects and the environment using fMRI methods. What makes their work unique is the fact that they have leaned rather heavily on rodent work as a starting point to develop their experimental paradigms.

Intriguingly, these authors also started to replicate and extend research into rodents evaluating spatial memory processes. Specifically, they have been studying categorical and coordinate relationships among items in space. But rather than using the standard paradigms in humans, they used an experiment in rats as a starting point (Link). As a quick primer, categorical relationships among objects can be expressed by prepositions (above, below, next to, etc.), and coordinate relationships are based on angles and distances among objects.

They also have replicated the work from Dr. Neil Burgess’s lab among others that have demonstrated the importance of the overall size and complexity of an environment on spatial processing in rats. They also studied whether different brain regions would be recruited in larger and more complex environments as seen in rats-specifically is the anterior-posterior axis of the hippocampus respond similarly in humans in fMRI as rats neural firing.


An Innovative Approach

A reliance on rodent paradigms to develop tasks is not a minor point since most of the work in humans and animal models do not share common paradigms and often do not even share similar fundamental mechanisms (i.e., different brain functions underlie performance across the species). Unfortunately, as I have mentioned in an earlier post, it is often extremely difficult for rodent researchers to modify a task developed for humans for use in an animal model. Research in human participants, however, can use exactly the same or only slightly modified protocols from rodent research to search for behavioral homologies.

Using retrieval-based tasks in fMRI, Mattingley’s group have been able to replicate previous rodent studies with uncanny specificity. Specifically, they replicated a task in rats showing the hippocampus processes coordinate space and parietal cortex mediates categorical processes (example of this task shown below). Previous work in humans had not looked at anatomical specificity of these processes per se, but rather limited study to hemispheric specialization or strictly behavioral studies.

Figure 1

This is the virtual environment used for these categorical and coordinate processing tasks. The relationship between the yellow pyramid and the other objects is changed (Source).

Figure 2

You can see the caudate and hippocampus activation in the coordinate task condition in plate c of this figure (Source).


They also demonstrated that, like rats, virtual maze size and complexity are processed by the hippocampus. More so, that size and complexity can be dissociated. Rat studies have demonstrated that the dorsal hippocampus (or posterior in humans) has very fine resolution for spatial processing, whereas the ventral hippocampus (or anterior in humans) has much less specific firing, but some short term memory functions not present in the dorsal hippocampus. Based on these hypotheses, the authors developed tasks that, when participants performed the tasks in an fMRI, the resulting fMRI activation data were reminiscent to the firing patterns observed in rodents in similar paradigms. Specifically, the anterior hippocampus appears to be recruited more for complex environments than larger ones. The posterior hippocampus, however, appears to be more important for larger environemnts than complx ones. These results intuitively map onto the results from rat studies that wherein the dorsal hippocampus processes fine-scale space for navigation and the ventral hippocampus is more involved when short-term memory is required to navigate.

Figure 3

This is an example of an environment used to study the effects of size and complexity on hippocampus firing. Other mazes were smaller and more simple than this one. The red dots are locations of navigational landmarks on the walls (Source).


Take Home Message

By replicating these findings in rodents, the authors demonstrated that despite the differences among species, the way the brain fundamentally processes spatial relationships may be homologous between rodents and humans. This is important as this work suggests that it is possible for the chasm between rodent and human research to be bridged, so long as a conscientious effort is made to use similar tasks.


Conclusion

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p>These studies from Baumann and colleagues from Jason Mattingley’s lab provide clear evidence that rats and humans use very similar processes to compute spatial relationships and build mental maps of space. By developing extremely simple tasks based on rodent research, they have been able to uncover fundamental processes underlying human navigation that to date have only been clearly demonstrated in rodents.

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