Our Research Lab
Spectro-Temporal Processing – FNIRS
Functional near-infrared spectroscopy (fNIRS) is a non-invasive neuroimaging technique used to measure hemodynamic activity in cortical tissue. When areas of the brain are stimulated (e.g., by external sound), the neural activity triggers a hemodynamic response (HR) from the tissue due to neurovascular coupling. That is, neurons depend on the delivery of oxygen via blood to support their metabolic needs when they become activated, thus local changes in oxygenated (HbO) and deoxygenated (HbR) hemoglobin concentrations can be used to index cortical activation. The fNIRS technique capitalizes on this hemodynamic process by directly measuring these intrinsic optical properties cortical tissue. Near infrared (NIR, 660-830 nm) light of different wavelengths is presented to the surface of the scalp via optical fibers. NIR light scatters and diffuses though the tissue, samples cortical tissues up to 1 cm below the scalp, and re-emerges a couple of centimeters away from the source. The differential absorption of the detected NIR light is then used to quantitatively compute changes in in local concentrations of oxygenated (HbO) and deoxygenated (HbR) hemoglobin that can be used to index neural activation.
We have been using fNIRS to understand hemispheric differences in auditory cortical function. Using different stimulus types (pure tones, noise bursts, temporally modulated noises, spectrally modulated noises), we use two fNIRS approaches to measure hemodynamic changes associated with functional stimulation of the human auditory cortex. One approach is based on single-distance continuous wave NIRS (CW-NIRS) and uses relatively simple instrumentation and the Modied-Beer Lambert (MBL) law to estimate activation induced changes in tissue oxygenation (ΔHbO and ΔHbR). A second approach, frequency domain NIRS (FD-NIRS), is more complex and employs a photon diffusion model of light propagation through tissue to measure both baseline (CHbO and CHbR), and stimulus induced changes in oxygenated and deoxygenated hemoglobin. FD-NIRS is more quantitative but requires measurements at multiple light source-detector separations and thus its use in measuring focal changes in cerebral hemodynamics have been limited.
We have shown a comparable increase in oxygenated hemoglobin in the right hemisphere for both pure tone and broad band noise stimuli when analyzed by the MBL method at the 'best' source-detector separation. In contrast, the frequency domain analysis method estimated a greater increase in oxygenated hemoglobin for pure tone than broad band noise stimulation. These results suggest that although more quantitative, multi-distance FD-NIRS may underestimate focal changes in cerebral hemodynamics that occur due to functional activation. Additional studies are underway to evaluate cortical functioning across hemispheres with more complex acoustic stimulus in different listener populations.