Comparison Of Hemodynamic Response Non-linearity Using Simultaneous Functional Near Infrared Spectrsoscopy And Magnetic Resonance Imaging Modalities

Blood oxygenation level dependent (BOLD) response, which is measured by functional magnetic resonance imaging (fMRI), is known to be a combination of various vascular parameters, among which deoxy-hemoglobin is argued to be a major contributor. Functional near infrared spectroscopy (fNIRS) though limited in its spatial resolution provides a promising tool to study cortical activations, due to its specificity of independent measurement of blood parameters (Oxy, De-oxy and Total Hemoglobin), high temporal resolution and ease of use. To integrate the best aspects of two imaging modalities, it becomes important to study the close relationship between the two imaging modalities. A finger tapping task with different stimulus durations (2, 4, 8 & 16 sec) with variable inter-stimulation intervals was chosen to compare spatio-temporal properties and non-linearity of BOLD signal with HbO, HbR and HBT signal. This study helped determine what parameter (HbO, HbR and HbT) does BOLD correlate to most and how factors like neural adaptation that cause non-linearity can affect the hemodynamic behavior. It investigates the non-linearity in oxy, deoxy and total hemoglobin concentrations as compared to BOLD signal obtained using simultaneous fNIRS and fMRI measurement. Separating non-linearity from hemodynamic response could lead us to a better understanding of neuronal function by modeling neural adaptation. The paper also discusses a method to model the neural adaptation and hemodynamic response. This information if available could be valuable, especially for the cases where MRI is not so convenient to use (for example, be children with cerebral palsy).