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Journal of Scientific Research and  Studies

Journal of Scientific Research and Studies Vol. 7(3), pp. 41-53, September, 2020

ISSN 2375-8791

Copyright © 2020

Author(s) retain the copyright of this article


Full Length Research Paper

Inferring causal relationship in the human cardio-respiratory system from the time series of skin color changes associated with heartbeat and breathing activity


Jung Y. Huang

The T.K.B. Research Center for Photonics, Chiao Tung University, 1001 Ta-Hsueh Road, Hsinchu 300, Taiwan. E-mail:


Accepted 17 August, 2020




A sensitive handheld color-sensing module has been developed to examine the physiological status of the human cardio-respiratory system. This portable device can provide color coordinates of the skin during both breathing and heartbeat rhythms. Color changes occur in the skin due to more blood cells and hemoglobin filling the arterial skin tissue during the heartbeat-induced blood volume pulsing. To mitigate small distortions arising from projection of high-dimensional data onto one-dimensional time series, we deduced a state-space portrait from the measured skin color changes by implementing Takensís time-embedding approach. Based on the state-space picture, we can apply a cross-recurrence plot to disclose nonlinear interrelations and similarity in the time series data. Finally, clear evidence for the directionality of cardio-respiratory coupling was yielded by integrating heartbeat- and breathing-induced skin color data with convergent cross-mapping, which reveals that heart-to-respiration coupling is stronger than respiration-to-heart coupling in healthy test subjects. Based on these findings, we conclude that the cardiovascular process causally drives respiratory activity in the cardio-respiratory system.

Key words: Skin color, cardio-respiratory, heartbeat rate, blood volume pulsing, causality, Takensís theorem, time-embedding, convergent cross-mapping.

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Vol. 7 Issue 3

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