Fig. 2. Photoplethysmogram time series (PPG;  A ) and arterial blood pressure time series (ABP;  B ), measured in an ipsilateral hand, using a laboratory prototype photoplethysmography sensor and the Finapres (Finapres Medical Systems, Amsterdam, The Netherlands), respectively. In this representation, several minutes of waveforms are compressed into a single image (specific waveform morphologies are illustrated in the call-outs). Before 1 = baseline. 1: Over 20 s, the subject lowers his hand, raising the local blood pressure. 2: The photoplethysmogram indicates that the hand has filled with more blood (higher waveform baseline) and the blood vessels have grown less compliant as a result of the increase in blood pressure (decreased amplitude of the photoplethysmogram, while the blood pressure's amplitude is unchanged). 3: The hand's vascularity returns toward baseline even though arterial blood pressure remains elevated, due to autoregulation. The data above were collected in our laboratory with institutional human use committee approval and subject consent.