Absolute quantification of oxygenated hemoglobin within the visual cortex with functional near infrared spectroscopy (fNIRS).

PURPOSE To examine absolute changes in oxygenated (HbO) and deoxygenated (Hb) hemoglobin concentrations over the visual cortex in response to visual stimulation. Before this study, only relative changes have been reported at the visual cortex. METHODS Near infrared spectroscopy (NIRS) can be used to assess changes in hemoglobin concentration in tissue. A frequency domain oximeter with a specially designed probe was placed over the visual cortex while the participant viewed a checkerboard stimulus. The stimulus was alternated at 7.5 Hz for 30 seconds before being replaced by a control gray screen. RESULTS The mean HbO concentration when the stimulus was presented was 26.8+/-3.9 microM, as opposed to 25.9+/-3.9 microM for the control condition, and the mean Hb concentration was 18.8+/-2.3 microM during stimulation and 19.1+/-2.3 microM during the control condition. The greatest change in HbO concentration occurred within the first 10 seconds. It did not increase significantly (P>0.01) after that time. CONCLUSIONS This study is the first to demonstrate absolute quantification of HbO and Hb concentrations in the visual cortex with functional changes in hemoglobin concentrations in response to a visual stimulus. NIRS has the potential to be a valuable clinical tool in assessing the hemodynamics of the visual system in a quantitative and localized manner.