Auditory Responses Measured by Topographic Near-infrared Spectroscopy

Abstract

By near-infrared spectroscopy (NIRS), the changes in the local concentrations of oxygenated (oxy-Hb), deoxygenated (deoxy-Hb) and total hemoglobin (total-Hb) are measured. NIRS was used as a non-invasive functional brain mapping method of near-threshold auditory responses. Bilateral auditory areas of 14 healthy subjects were studied with a 24-channel NIRS optical topography system (ETG-100, Hitachi Medical Co.) during attentive hearing of toremolo bandnoise (0.5, 1 and 4kHz, 0 and 30dBSL) and of 1kHz tremolo tone (0 and 30dBSL), and non-attentive hearing of 1kHz bandnoise at 0dBSL. A combination of significant changes of oxy-Hb and total-Hb, or of deoxy-Hb and total-Hb, was observed in the attentive hearing of bandnoise even at 0dBSL in all the subjects. A significant response was also observed to 1kHz tone at 30dB above threshold, although only two out of five subjects showed a significant change of Hb concentrations in response to 1kHz tone at 0dB. In the non-attentive condition, a significant response to 1kHz bandnoise was observed in 80% of the subjects.