I'd be very interested in seeing that, given that much of the research that is readily available discusses lowering glucose levels below 1mmol/l in order to cause neuronal damage in rats.Every time you go below 3.9, you are risking a small amount of brain damage, even if you feel fine. 9I do have a paper on it somewhere, but can't find it to upload.....sorry).
All of the research that I can find linked to hypoglycaemia states that it is severe hypoglycaemia that causes the issues, and that mild hypoglycaemia is does not cause affect cognition. This slidedeck is quite interesting in that respect, especially the final slides talking about following 1144 T1s over 18 years and seeing no reduction in cognitive function where 0 severe hypos have taken place.
With reference to non-diabetic blood glucose levels, they do get down below 3.9mmol/l as part of normal life. As this article in diabetes care journal shows, across all populations, up to 1.7% of the day can be spent below 3.9mmol/l, with this occurring most frequently in the sample set in those aged between 29 and 45.