What is the fine-grained topic of the following text: In 1971, Dr. John Olney, neuroscientist and one of the world's foremost experts on excitotoxins, informed G.D. Searle that his research had revealed that aspartic acid caused holes in the brains of mice. Only return the topic.
Dr. John Olney's research on the effects of aspartic acid on mouse brains.