Altered frontal and temporal brain function during olfactory stimulation in adult attention-deficit/hyperactivity disorder.

Objective: Olfactory processing depends on dopamine metabolism and orbitofrontal cortex functioning, both known to be disturbed in attention-deficit/hyperactivity disorder (ADHD). Some investigations suggested alterations in olfactory processing (identification and sensitivity) in childhood and adult ADHD.

Methods: In the present study we investigated olfactory function (Sniffin' Sticks) of 29 adult patients with ADHD (17 combined, 11 inattentive, and 1 hyperactive/impulsive subtype) and 29 controls matched for sex, handedness, age, intelligence, and education. Additionally, we measured frontal, temporal, and somatosensory cortical activity during olfactory perception. This was performed with functional near-infrared spectroscopy (fNIRS) during presentation of 2-phenylethanol (olfactory stimulant) and linalool (mixed olfactory/trigeminal stimulant) in two concentrations each.

Results: Adult patients with ADHD and controls did not differ in sensitivity for and discrimination and identification of olfactory stimuli. Functional brain imaging measures with fNIRS generally revealed diminished activation in olfaction-associated brain regions in patients with ADHD. Only for a high concentration of linalool, oxygenated haemoglobin (O₂Hb) concentrations in patients were similar to controls (significant increase in the temporal, somatosensory, and inferior-frontal cortex). O₂Hb concentrations in active brain regions positively correlated with ADHD symptoms during childhood and trait impulsivity. These effects were carried for the subgroup with combined subtype.

Conclusions: Although we could not replicate altered clinical performance in ADHD, our fNIRS findings suggest an association of cortical olfactory processing with hyperactivity and impulsivity in adult ADHD.