Implications of NIRSbrain signals

Abstract

Near Infrared Spectroscopy（NIRS）is commonly used for functional brain studies. With this method, brain signals can be easily obtained, but the interpretation of these signals still remains unclear. The continuous wave method, which is usually used in the optical topographic instrument, cannot measure an absolute value of the tissue hemoglobin. There are many hemoglobin-containing tissues other than brain such as head skin, muscle and skull bone between source and detector by using reflectance configuration of optodes. This paper recommend to use a block design or event related design to reduce the effect of hemoglobin other than brain tissue and to use of a ratio to eliminate an effect of personal difference of path-length. This paper also provides a simple model to interpret the NIRS signal, which is based on the following assumptions : 1. The NIRS signal may reflect Hb levels only in the capillaries and not in large vessels. The following features of the brain can explain this assumption. First, the brain has a lighter color than the other tissues, indicating that the Hb concentration in brain tissue is very low and intensity level of the NIRS signal is very high. Second, a photon that hits a large vessel is too weak to be detected in the surrounding high signal environment. 2. Cerebral blood flow（CBF）can be separated into cross-sections（S : the number of capillary beds） that are multiplied by the velocity（v）. This model can explain the typical signal pattern observed during task performance, where oxy-Hb levels increase and deoxy-Hb levels slightly decrease. This model also leads several features of NIRS signals. 1. An increase in total-Hb indicates an increase of S. 2. An increase in oxy-Hb indicates both increase of S and v. 3. A decrease in deoxy-Hb indicates an increase of v. These features of NIRS signals may be useful to interpret obtained experimental data. Finally, This paper presented an effect of a facial skin blood flow change during task, such as verbal fluency task, which is very important factor should be cleared in near future.