Carrick Insight: Could vitamin D deficiency lead to decreases in brain plasticity? Researchers discovered that vitamin D levels had an effect on the perineuronal nets located in the hippocampus, suggesting a link between vitamin D deficiency and cognition. 
Perineuronal nets are like a strong mesh that surrounds the neurons and stabilizes them, allowing them to form new connections with other neurons. 
This study compared perineuronal nets in healthy adult mice who had vitamin D removed from their diet. They found that the mice had a pronounced reduction in the number of these supportive nets in the hippocampus, the area of the brain responsible for memory formation. This loss of support structure in the hippocampus caused a large reduction in the number and strength of connections between the neurons in this region and ultimately led to a loss in cognition.
This study compared perineuronal nets in healthy adult mice who had vitamin D removed from their diet. They found that the mice had a pronounced reduction in the number of these supportive nets in the hippocampus, the area of the brain responsible for memory formation. This loss of support structure in the hippocampus caused a large reduction in the number and strength of connections between the neurons in this region and ultimately led to a loss in cognition. Vitamin D in Synaptic Plasticity, Cognitive Function, and Neuropsychiatric Illness
Mayne, Phoebe E. et al.
Trends in Neurosciences , Volume 0 , Issue 0
Image from: https://www.pnas.org/content/115/3/433