Answer:
The activities of the prefrontal cortex.
Explanation:
Use it or lose it refers to the idea of "if something is not used or practiced, it will decay or be lost over time.". Neuronal activities that take part in the prefrontal cortex of the brain can relate to this idea. Activities such as decision making, learning languages, communicating, rational thinking etc which are activities that can decrease over time if they are not used.
I hope this answer helps.
