Mind the (small) gap between the mammal and non-mammal navigation system

Remembering where we left our keys, the distance between the platform and the train, or where the TV remote control was placed last night, are examples of daily events that require spatial memory, i.e., the ability to recall spatial locations. This ability is made possible by neurons called place cells, which are mainly located within a brain region known as the hippocampus. While we navigate spaces, place cells encode information from environmental cues, creating a sense of space. Place cells were discovered by Dr. John O’Keefe, who, together with Drs. May-Britt Moser and Edvard I. Moser, was awarded with the Nobel Prize in Medicine and Physiology in 2014. The so-called ‘inner GPS’ of the brain is shared across species and has been identified in mammals including rodents, primates, and bats, which are particularly known for their spatial abilities.

In a recent article in Science, Columbia postdoc Dr. Payne and colleagues investigated whether this navigation system also exists in non-mammals with pronounced spatial memory skills. The authors chose to study tufted titmice, birds that accumulate seeds in multiple locations to prevent a shortage in case of decreased food supply. In the lab, the authors recorded hippocampal activity of tufted titmice while they were navigating in an arena for sunflower fragments. They identified neuronal activity similar to rodent place cells. Also, they observed a mammalian-like distribution of those cells along the hippocampus. These observations suggest that the mechanisms of hippocampal coding in birds exhibit similarities to those identified in mammals.

The authors compared results from the titmouse with the activity and hippocampal organization of zebra finch, a non-food-caching bird. They found that the subregion of the hippocampus with the highest density of place cells was larger and more populated in tufted titmice than in zebra finches. These findings suggest that place cells are more spatially informative and stable in tufted titmice compared to the place cells in zebra finches. Such differences in spatial coding between bird species might be due to differences in demands of food caching.

In this work, Dr. Payne and colleagues identified for the first time neuronal activity in a non-mammalian hippocampus similar to mammalian-like place cells. Furthermore, they reveal evidence that spatial encoding may differ between species mainly due to its functional demand. Despite a divergent evolution from millions of years, mammals and non-mammals possess a similar version of their GPS cells.

 Dr. Hannah Payne is a postdoctoral fellow at Dr. Dmitriy Aronov‘s lab in the Zuckerman Mind Brain Behavior Institute at Columbia University.

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