Figure 4. Overview of study procedures.

(A) Overview of study timeline for each experimental visit (insulin and saline clamp conditions) conducted within the Massachusetts General Hospital, Clinical Research Center (CRC). Each infusion condition was implemented following an overnight 12-hour fast. Study tasks (consisting of electroencephalography [EEG], cognitive testing, and functional MRI [fMRI]) commenced as soon as steady state was achieved. (B) Schematic of clamp setup. A continuous infusion of either insulin or saline was administered alongside a variable dextrose infusion to maintain normoglycemia, as indicated by 5-minute-interval blood glucose level checks. (C) Overview of N-back working memory task that was given during each fMRI session concurrently with infusion procedure. For the 2-back trials, participants were required to respond by button press if the letter presented was exactly the same as 2 stimuli before. For the 0-back control trials, participants were instructed to press a response button immediately upon seeing the letter X.