Abstract: The physiological or mental functioning of a patient is assessed by subjecting a patient to a protocol expected to activate or suppress the neurhormonal systems that regulate blood flow. A physiological signal indicative of the cardiac cycle, such as the electrocardiograph signal, is then monitored in synchrony with performance of the protocol. The physiological signal is processed to quantify the low frequency oscillation during different conditions during the protocol, and the results are interpreted in terms of cardiovascular changes, such as changes in blood flow to active tissues in the body. The results are then compared to normative data to assess the physiological or mental functioning of the patent.