Monkeys With Damaged Hippocampi Show No Cognitive Impairment

The hippocampus is generally believed to be the brain's memory center. Image: ktsdesign/Shutterstock

The mystery of consciousness has just become a whole lot more complicated, thanks to a new study that casts fresh doubt on our understanding of how the brain works. Appearing in the journal Science Advances, the new research indicates that monkeys with damaged hippocampi can perform just as well as intact monkeys on a range of tasks that require visual memory, pattern recognition, and reasoning.

While much remains unknown about how cognition is coordinated within the brain, it is widely accepted that the hippocampus is fundamental to memory. This belief dates back to 1953, when a man named Henry G. Molaison – known in the scientific literature as patient H.M – had part of his hippocampus removed as an experimental treatment for epilepsy. The surgery did help to reduce the frequency and severity of his seizures but also left him with amnesia, and our understanding of the hippocampus and its function is still largely based on this one case study.

However, the authors of this latest study point out that there is some uncertainty over how much of H.M’s hippocampus was left intact and how much the procedure damaged his other brain regions. As such, they suggest that some of the assumptions drawn from this example may not be as clear-cut as they are often thought to be.

To test some of the established theories about the hippocampus, the researchers surgically damaged the hippocampi of five rhesus monkeys before subjecting them to an array of cognitive tests that were designed to measure numerous aspects of neurological function. For example, in one task the monkeys had to recreate a shape that they had previously seen by touching different areas of a screen, while in another they were required to touch a set of images in a particular order that they had learned.

Another test challenged the monkeys to place items in a relational order, indicating which were greater and which were smaller, while a further task involved recognizing images.

Amazingly, these monkeys scored just as well on every test as a group of control monkeys that did not have damaged hippocampi. Based on this finding, the study authors suggest that current understandings of how certain brain regions work may be wide of the mark, and that “we should reassess the relative contributions of the hippocampus proper compared to other regions in visual memory and relational cognition.”

Interestingly, this is not the first time that our understanding of how the brain works has been contradicted. In 2007, a middle-aged French man was found to be walking around with a large portion his brain missing, yet was still able to live a normal life, working as a civil servant while raising a family. His condition was caused by an overproduction of cerebrospinal fluid, which filled the ventricles of his brain and caused them to inflate so much that they replaced pretty much all of his other brain regions, leaving just a thin layer of cortical neurons.

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