One or more physiological conditions of a patient can be observed by obtaining a photoplethysmograph (“PPG”) signal from the patient and by only lightly filtering that signal. The light filtering of the PPG may be such as to only remove (for example) high frequency noise from that signal, while leaving in the signal most or all frequency components that are due to physiological events in the patient. In this way, such physiological events can be observed via a visual display of the lightly filtered PPG signal and/or via other signal processing of the lightly filtered PPG signal to automatically extract certain physiological parameters or characteristics from that signal.