1. A method for analyzing a physiological signal obtained from a subject, the method comprising: transforming a first physiological signal based on light transmission at a first wavelength to generate a first transformed signal; transforming a second physiological signal based on light transmission at a second wavelength to generate a second transformed signal; deriving a ratio surface from the first transformed signal and the second transformed signal; identifying a first region of interest on the ratio surface indicative of venous perturbation; calculating a representative value for the first region of interest on the ratio surface; and making a determination regarding the physiological signals based on the calculated representative value.