Mice, as we are learning more and more, are surprisingly smart. One way scientists know this is through the categorization capacity mice have. Categorization is a fundamental element of our thinking. It is the brain’s tool to organize nearly everything we encounter in our daily lives. Grouping information into categories simplifies our complex world and helps us to react quickly and effectively to new experiences. Now, scientists at the Max Planck Institute of Neurobiology have shown that also mice categorize surprisingly well. In fact, the researchers identified neurons encoding learned categories in mice and thereby demonstrated how abstract information is represented at the neuronal level.
Researchers showed mice different pictures of stripe patterns and gave them a sorting rule. One animal group had to sort the pictures into two categories based on the thickness of the stripes, the other group based on their orientation. The mice were able to learn the respective rule and reliably sorted the patterns into the correct category. After this initial training phase, they even assigned patterns of stripes they had not seen before into the correct categories.
And not only that: when the researchers switched the sorting rules, the mice ignored what they had learned before and re-sorted the pictures according to the new rule — something we humans do all the time while learning new things. Therefore, the study demonstrates for the first time to what extent and with which precision mice categorize and thereby approach our capacity for abstraction.
Neurons gradually develop a category representation
With this insight, the researchers were now able to investigate the basis of categorization in the mouse brain. They focused on the prefrontal cortex, a brain region which in humans is involved in complex thought processes. The investigations revealed that certain neurons in this area become active when the animals sort the striped patterns into categories. Interestingly, different groups of neurons reacted selectively to individual categories.
Category-selective neurons are part of long-term memory
The scientists argue that the category-selective neurons in prefrontal cortex only play a role once the acquired knowledge has been shifted from short-term to long-term memory. There, the cells store the categories as part of semantic memory — the collection of all factual knowledge.
By investigating category learning in the mouse, the study adds important details to the neuronal basis of abstract thinking and reminds us that complex thoughts are not only reserved for us humans.
Journal reference: Sandra Reinert, Mark Hübener, Tobias Bonhoeffer, Pieter M. Goltstein. Mouse prefrontal cortex represents learned rules for categorization. Nature, 2021; DOI: 10.1038/s41586-021-03452-z