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In recent years investigation and understanding of the interaction between brain activity and human hemodynamics is receiving much attention. Such investigations are generally confined to few select premier research institutions where expensive and sophisticated facilities like PET, NMRI, etc. are available. Actual advancement in optoelectronics and computer technology have accelerated the development of new measuring systems and methodologies for use in this medical field. Specifically, transcutaneous assessment of skin perfusion changes (blood volumetric measurement) through optical sensor based Photoplethysmography (PPG) has rapidly gained an important role. This basically noninvasive measuring procedure is devoid of harmful radiation and ionizing phenomena, simple in construction and connection to the measuring setup and is easy to use in all areas of human body. The relatively low cost of these sensors have resulted in their use in varied medical fields for extensive clinical applications. In the past, the application of PPG had been limited by technical difficulties involving calibration of the data. The development of quantitative PPG technique based on computer-aided data processing has removed this obstacle. In co-operation between Indian Institute of Technology at Chennai and Aachen University of Technology an Indo-German Project was initiated in 1996 for advanced studies in this research area. Measuring system design, experimental details and some preliminary results obtained so far within the framework of this project are presented in this paper. From the investigations carried out so far using the PPG sensors in conjunction with breathing sensors, it has been possible to monitor the 0.125 to 0.15 Hz rhythms in the arterial volumetric changes and to study the influence of breathing on them. These rhythms, which according to medical experts have relevance to psychosomatic conditions e.g. stress or relaxation, can also be addressed to by ancient Indian practices like yoga and meditation.
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