Answer:
case study
Explanation:
A case study is defined as a process or a method of research where a detailed information or a detailed study is done on a person or a population over a period of time. It is an in-depth thorough study done on the subject of interest to come to conclusion within the real world context.
In the context, Dr. Okawa first collects the data or information form a small number of individuals suffering from brain damage and then creates a in-depth study on the information provided by the interviews conducted or the from the medical records.
Thus, Dr. Okawa uses case study as a research method for this study.