The study of physiological time-series can reveal underlying mechanisms of the physiological system, which usually contains both deterministic and random components. In a time-series, memory is a statistical feature that lasts for a period of time and distinguishes the time-series from a random, or memory-less, process. In the present study, memory was ‘quantified’ by determining the time scale over which rare fluctuations do not appear randomly within a physiological time-series. We provided empiric evidence to show that rare fluctuations in both respiratory and cardiac cycles are ‘forgotten’ quickly in healthy subjects while their memory is kept for longer in pathological conditions such as asthma and liver cirrhosis.
