I just came across this: "You're Not As Smart As You Could Be", about Dr. Samuel Renshaw and the tachistoscope. This is a device used for exposing an image to the human eye for the briefest fraction of a second. In WWII he used it to train navy and artillery personnel to instantly recognise enemy aircraft, apparently with great success. He also used it for speed reading training; this application appears to be somewhat controversial.

I remember the references to Renshaw in some of Heinlein's stories, and I knew he was a real person, but this is the first time I've seen a substantial account of his work.

A few more references:

Wikipedia is rather brief.

Open access review article about work with the tachistoscope, in the Journal of Behavioral Optometry, 2003. This is the closest thing I've found to a modern reference.

An academic paper by Renshaw himself from 1945. Despite its antiquity, it is paywalled. I have not been able to access the full text.

This information is mostly rather old and musty, and there appears to be little modern interest. With current computers, it should be very easy to duplicate the technology, although low-level graphics expertise is likely needed to get very short, precise exposure times.