A THEORETICAL-STUDY OF THE SIGNAL CONTRIBUTION OF REGIONS OF THE ADULT HEAD TO NEAR-INFRARED SPECTROSCOPY STUDIES OF VISUAL-EVOKED RESPONSES

Near-infrared (NIR) spectroscopy has been used in studies of the cereb ral hemodynamic response to visual processing. In this paper, we prese nt theoretical results from finite element and Monte Carlo modeling in order to help understand the contribution to the NIR signal from diff erent parts of the head. The results from the models show that at the typical optode spacings used in these studies, an infrared spectroscop y measurement of intensity is sensitive to the outer 1-2 mm of the cor tical gray matter and the partial optical path length in the gray matt er is approximately 10 mm, compared with a total optical path length o f 400 mm. When the NIR measurement is of change in mean photon arrival time (or phase shift), the signal comes from the upper 2-4 mm of the cortical surface and there is an increased lateral spread of the contr ibuting tissue. We predict that for a 4-cm separation of input and det ection optodes at 800 nm, a 1 mu M change in hemoglobin concentration in the cortex corresponds to an attenuation change of approximately 0. 001 OD (optical density) or 1 ps mean time change. Movement of the bra in caused by this increase in volume will cause an absorption change o f approximately half this magnitude, but does not affect the photon ar rival time at 4-cm spacing. A discrepancy between the predicted and th e experimentally measured intensities may support the supposition that the NIR signal is actually very sensitive to changes occurring in the pial cerebral vessels lying on the brain surface. (C) 1998 Academic P ress.