Unconditioned Stimulus
The US was 20 units (0.2 ml) of fast-acting insulin (Insulin NovoRapid; Novo Nordisk), administered with the MAD Nasal Intranasal Mucosal Atomization Device (Teleflex) by a trained member of the research team. Six administrations of insulin were done on day 1 in the conditioned group with a break of 15 minutes between the administrations. The spray was administered alternating between the left and then right nostrils. The same dosage of insulin has been successfully used in previous research on insulin conditioning in healthy volunteers (Placebo nasal spray was used in the control group on day 1 and day 2 and in the conditioned group on day 2. The spray was prepared by the Department of Clinical Pharmacy of the Leiden University Medical Center. Because of unavailability of meta-cresol, the preservative that gives a particular smell to the insulin nasal spray, another preservative, chlorobutanol, was used to add a smell to the placebo.