Cortical hemodynamic responses to intravenous thiamine propyldisulphide administration detected by multichannel near infrared spectroscopy (NIRS) system

Intravenous injection of thiamine propyldisulphide (TPD), which induces sensation of a garlic-like odor, has been used as a representative subjective olfactory test in Japan. However, cortical loci activated by TPD still remain unclear. We recorded cerebral hemodynamic responses (changes in Oxy-Hb concentrations) induced by TPD administration using whole-head multi-channel near infrared spectroscopy (NIRS) system based on 3D-MRIs. TPD as an odorant and saline as a control were injected from the cephalic vein in the left forearm in ten male normosmic (five young and five elderly) subjects and five dysosmic elderly patients. The all normosmic, but not dysosmic, subjects felt the garlic-like odor in the all TPD trials. There was no significant difference in hemodynamic responses between the young and elderly normosmic subjects. However, TPD injection induced significantly larger hemodynamic responses in the bilateral operculums, bilateral dorsolateral prefrontal cortices (PFC) and anteromedial PFC in the normosmic subjects, compared with saline injection. Onset latencies of these hemodynamic responses were significantly correlated with onset latencies of subjective odor sensation in the normosmic subjects. Comparison of hemodynamic responses between the normosmic and dysosmic subjects indicated a significant difference in the bilateral operculums. The results demonstrated that Oxy-Hb increases in the bilateral operculums reflected olfactory sensation induced by TPD injection. Consideration of a route for intravenous TPD to reach the olfactory mucosa suggests that these hemodynamic responses might be attributed to food-related retronasal olfactory responses to TPD.