In collaboration with his supervisors, Arturo Rosenblueth and Hallowell Davis, he analyzed the cycle of excitability of cortex neurons (nerve cells) after the convulsive phenomena, and was the first to identify an important phenomenon, that of a decrease of the excitability which spread in increasing circles around the initial focus, which he named spreading depression.
In both cases, these changes in excitability are typically due to mutation of the sodium channel New Forest pony
DNA sequencing revealed that the affected foal was homozygous for a missense mutation in the gene encoding CLCN1, a protein which regulates the excitability of the skeletal muscle.
The Los Angeles Times called Hagiwara "a pioneer in understanding the mechanisms of excitability in nerve and muscle cells".