Elevated insulin levels engage the salience network during multisensory perception

Description: This study provides a placebo-controlled double-blind within crossover design. On separate mornings, twenty-six healthy normal-weight males aged between 19 and 31 years received either 40 IU intranasal insulin or placebo vehicle. Subsequently, they underwent 65 min of functional magnetic resonance imaging whilst performing an odor identification task. Odor stimuli were applied birhinally for 5.2s with continuous air flow (3.0 L/min) using a computer controlled olfactometer. The application of the odors was conducted using the respiration-triggered method RETROS (Hoffmann-Hensel et. al. 2016). As baseline, aqua conservata was used to control for sniffing artifacts.Two food odors, ‘apple’ and ‘strawberry’ and two nonfood odors, ‘wood’ and ‘grass’ were presented along with four odor-matching pictures. During the stimulus presentation the participants pressed response buttons with the right hand to identify the odor category (food, nonfood, control). Any metabolic diseases, chronic or acute respiratory diseases, neurological or psychiatric disorders, skull injury or surgery, pharmaceutical medication, or drug abuse in medical history led to exclusion. Functional MRI images were acquired on a 3 Tesla scanner using a T2*weighted blood oxygenated level-dependent sensitive, gradient echo-planar imaging sequence. Data preprocessing and analysis was based on SPM12. The main aim of the study was to examine whether intranasal insulin would modulate the activity of areas in charge of olfactory-visual integration. Functional brain activations of olfactory, visual and multisensory integration as well as insulin vs. placebo were assessed.