Not a study, but...in Ron Chernow's bio of Alexander Hamilton there are a couple of places where he mentions that AH was notable for talking to himself while walking from place to place. [1] Your comment made me think of it because his tremendous impact and voluminous output made that behavior seem odd. But, perhapse they where related.

[1] https://books.google.com/books?id=4z5eL5SGjEoC&pg=PT438&lpg=...