Editing Biofeedback (section)

===Photoplethysmograph===
[[File:EmWave2, powering up.jpg|thumb|180px|upright|An ''emWave2'' photoplethysmograph for monitoring heart rate variability]]

[[File:Photoplethysmograph - Biofeedback HRV with ear sensor.jpg|thumb|180px|upright| ''Stone'' computer-based photoplethysmograph with ear sensor]]

A [[photoplethysmograph]] (PPG) measures the relative blood flow through a digit using a photoplethysmographic (PPG) sensor attached by a Velcro band to the fingers or to the temple to monitor the [[Superficial temporal artery|temporal artery]]. An infrared light source is transmitted through or reflected off the tissue, detected by a [[phototransistor]], and quantified in arbitrary units. Less light is absorbed when blood flow is greater, increasing the intensity of light reaching the sensor.<ref name="Combatalade, D. 2009">Combatalade, D. (2009). ''Basics of heart rate variability applied to psychophysiology''. Montreal, Canada: Thought Technology Ltd.</ref>

A photoplethysmograph can measure blood volume pulse (BVP), which is the phasic change in blood volume with each heartbeat, heart rate, and [[heart rate variability]] (HRV), which consists of beat-to-beat differences in intervals between successive heartbeats.<ref name="Lehrer, P. M. 2007 P. M">{{cite book | vauthors = Lehrer PM | date = 2007 | chapter = Biofeedback training to increase heart rate variability. | veditors = Lehrer PM, Woolfolk RM, Sime WE | title = Principles and practice of stress management | edition = 3rd | location = New York | publisher = The Guilford Press }}</ref><ref>{{cite journal | vauthors = Peper E, Harvey R, Lin IM, Tylova H, Moss D | year = 2007 | title = Is there more to blood volume pulse than heart rate variability, respiratory sinus arrhythmia, and cardio-respiratory synchrony? | journal = Biofeedback | volume = 35 | issue = 2| pages = 54–61 }}</ref>

A photoplethysmograph can provide useful feedback when temperature feedback shows minimal change. This is because the PPG sensor is more sensitive than a thermistor to minute blood flow changes.<ref name="Shaffer" />  Biofeedback therapists can use a photoplethysmograph to supplement temperature biofeedback when treating chronic pain, edema, headache (migraine and tension-type headache), essential hypertension, Raynaud's disease, anxiety, and stress.<ref name = Yucha2008/>