I've been interested in this question for some time. I've had the unfortunate experience to watch my mother and mother-in-law decline with Alzheimer's. Deep into the disease, they had no memory of what happened 5 minutes ago or yesterday, although they had memories from childhood. So I wonder whether it would be interesting to visit nursing homes that offer memory care, and ask them if any of their clients stuttered. i.e. if you can't remember what happened 5 minutes ago, maybe you also can't remember that you stuttered 5 minutes ago? The unit my Mom was in had maybe two or three dozen residents. So you would have to visit maybe 10 nursing homes to get a reasonable sample size.