Brain olfactory activation measured by near-infrared spectroscopy in humans

In recent years, near-infrared spectroscopy (NIRS) has been used to study functional activation of various areas of the brain. This is based on the assumption that an increase in the recorded oxygenated haemoglobin (HbO2) concentration represents an increase in blood flow, which in turn reflects neuronal activation. The aim of this preliminary study was to use NIRS to monitor the activity of the olfactory cortex, as mirrored by the haemodynamic response, when subjects were exposed to olfactory stimuli. A NIRO 300 (Hamamatsu Photonics, Hamamatsu, Japan) device was used. The optodes were placed on the right forehead and right temporal, parietal and occipital regions. Changes in the concentration of HbO2 and deoxygenated haemoglobin during olfactory stimulation were monitored. Olfactory stimulation was performed with vanilla essence, strawberry essence and scatol. During olfactory stimulation, cerebral HbO2 concentration increased over the frontal region. However, in the temporal, parietal and occipital regions, little or no HbO2 changes were recorded. This study shows that human brain cortical activation following olfactory stimulation can be recorded by NIRS. This NIRS analysis may therefore provide the basis for future development of an objective olfactory test in humans.