54 Summer 2007 Ô Biofeedback Keywords: bloodvolumepulse, heartrate variability, respiratory sinusarrhythmia, cardiorespiratory synchrony or CRShave beenusedtoaddressillnessaffectedbythe Whereas clinicalapplications andfeedbacktrainingofRSA and offers hopeformany chronicillnesses(Gevirtz, 2003). in strengtheningsympathetic-parasympathetic balance protocol. Itappears that learningtoincreaseHRV results pulmonary diseaseusingHRV training andanexercise reported abenefi their HRV. Inaddition, Giardino, Chan, andBorson (2004) Guarneri, 2004)hasshownthatcardiacpatientscanimprove by DelPozo andcolleagues(DelPozo, DelPozo Scher, & asthma reducestheirseverity. Similarly, research 2004) hasdemonstratedthatHRV training inpatientswith by Lehrerandcolleagues(Lehreretal., 2006; Lehreretal., expansion oftheabbrevationsintext). For example, research sympathetic-parasympathetic balance(see Table forthe clinical applicationsandtrainingproceduresforimproving areas ofcurrentresearch, expandingourknowledge ofnew and cardiorespiratorysynchrony(CRS)arepromising rate variability(HRV), respiratorysinusarrhythmia(RSA), migraine andthemonitoringofhumansexualarousal. addition, BVPtrainingofferspromiseforthetreatmentof refl protocols. For example, theshapeofBVPwaveform and thepotentialbenefi methodology ofBVPrecording, theunderlyingphysiology, as aconvenientmeasureofHRV. This articleexploresthe the BVPsignalisvaluableinitsownright, notmerely the electrocardiogramusedinmedicalsettings. However, photoplethysmograph, becauseitismoreuserfriendlythan systems relyonthebloodvolumepulse(BVP)sensor, or many ofthesophisticatedbiofeedbackinstrumentation in recentyears. Manyofthehometrainingdevicesand use ofHRV trainingprotocolshasincreaseddramatically of trainingheartratevariability(HRV), andtheclinical A growingbodyofresearchreportsthehealthbenefi 1 Erik Peper, PhD, Cardiorespiratory Synchrony? Variability, RespiratorySinus Arrhythmia, and Is There MoretoBlood Volume Than HeartRate SPECIAL TOPICS Volume 35, Issue2, pp. 54-61 Biofeedback San Francisco StateUniversity, SanFrancisco, CA; ects arterialchangescorrelatedwithhypertension. In Studies ofmethodsfortrainingimprovementinheart t forpatientswithchronicobstructive 1 RickHarvey, PhD, ts fromBVPtreatmentandtraining 2 National ChungChengUniversity, Taiwan; 1 I-MeiLin, MA, ts 2 to-beat variabilityinthesinusrhythmoveragivenperiod HRV andhealth(Kleiger, Miller, Bigger, &Moss, 1987). mortality. We willelaboratelateronthisrelationshipbetween index hashealthimplications, predictingmorbidityand to-normal heartbeats(SDNNorSDANN). This statistical calculating thestandarddeviationofaverage ofnormal- rhythm overagivenperiodoftime, isusuallydoneby protocols couldbeemployedaswell. sympathetic-parasympathetic imbalance, newHRV training Hana Tylova, DH Standard deviationofaverage heartrate SDAHR Standard deviationofaverage normal-to-normal Respiratory sinusarrhythmia SDANN Pulsetransit time RSA Photoplethysmography PTT Normal-to-normalbeat(interbeatinterval) PPG Heartrate variability Heartrate NN HRV Fast Fourier transform HR FFT Electrocardiography ECG Cardiorespiratory synchrony EKG Bloodvolumepulse CRS Bloodvolumeamplitude BVP BVA Table. Alphabet soupofabbreviations There aremanytechnologiesthatcanestimatethebeat- Measuring HRV, thebeat-to-beatvariabilityinsinus 3 Saybrook Graduate School, SanFrancisco, CA of thenormal-to-normalbeat) beat (alsoknownasSDNN: standard deviation 1 ©Associationfor Applied Psychophysiology&Biofeedback andDonaldMoss, PhD 3 www.aapb.org Peper, Harvey, Lin, Tylova, Moss

Figure 2. volume pulse, blood volume amplitude, rate, and Figure 1. Blood volume pulse, blood volume amplitude, , and respiration during training designed to increase cardiorespiratory synchrony. respiration before training. of time using measures of the volume of blood that passes breaths per minute with a corresponding heart rate of 73 over a photoplethysmographic (PPG) sensor with each beats per minute (SD = 10.1 beats). pulse, also called blood volume pulse (BVP). Recently, HRV Whereas designing training protocols for improving training using BVP measurements has escaped out of the HRV or RSA/CRS is possible using BVP technologies, the laboratory and into the marketplace. BVP training devices BVP signal holds even greater promise as a tool for use in are commercially available as portable units such as the improving other clinical applications and training protocols. StressEraser and the emWave or as units built for use with The remainder of this article explores various components computers such as the FreezeFramer or the Journey to Wild of the BVP technology. Divine. By using these BVP training devices, individuals can become aware of factors that increase or decrease their Background HRV. For example, excessive sensory arousal; shallow, rapid The BVP signal is derived with a PPG sensor that measures breathing; and excessive emotions of fear, worry, anger, changes in blood volume in arteries and by shining or panic will decrease HRV. On the other hand, reducing an infrared light (a light-emitting diode) through the tissues. sensory arousal, breathing slowly at approximately fi ve to This infrared light is selectively transmitted, backscattered, seven breaths per minute, and experiencing emotions of refl ected, and absorbed. The amount of light that returns appreciation and love will increase HRV. In clinical settings, to a PPG sensor’s photodetector is proportional to the practitioners use sophisticated multichannel biofeedback volume of blood in the tissue. The PPG signal represents systems to train clients to increase HRV and CRS. In most an average of all blood volume in the arteries, capillaries, clinical settings, heart rate information is derived from a and any other tissue through which the light passed. The BVP signal rather than from an electrocardiography (ECG) PPG signal depends on the thickness and composition of the signal. Whereas the ECG signal is more precise (e.g., with tissue beneath the sensor and the position of the source and fewer movement artifacts), applying an ECG sensor is also receiver of the infrared light. more cumbersome and obtrusive compared to applying a Most PPG sensors can be placed anywhere on the body, BVP sensor (e.g., without gel or tape or placement on a chest). from the earlobe to the vaginal wall, with the fi nger as the Using PPG feedback devices alone and/or in conjunction most common location for recording a BVP signal. The PPG

with respiratory feedback devices such as respiratory strain sensor measures relative changes in the of the Biofeedback gauges, clients can learn to increase HRV/RSA, as shown in blood through the tissue underneath the sensor. For example, Figures 1 and 2. changes in the BVP signal can indicate relative changes in The participant data from Figure 1 represent an average the vascular bed due to or

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respiration rate of 14 breaths per minute, with a corresponding (increase or decrease in blood perfusion) as well as changes Summer 2007 heart rate of 68 beats per minute (SD = 5.0 beats). The data in the elasticity of the vascular walls, refl ecting changes in from Figure 2 represent an average respiration rate of 7 . Because the blood volume in the arteries and

55 Blood Volume Pulse

Figure 3. Heart rate is derived from measures of blood volume pulse by Figure 4. Recording of the heart rate derived from the raw blood volume pulse measuring the interbeat interval and then transforming this information into during the baseline period. The average heart rate included a segment of very beats per minute. For example, the interbeat interval of 0.80 seconds is equal rapid heart beat refl ecting movement artifact rather than a true increase in to a heart rate of 75 beats per minute, whereas the interbeat interval of 0.93 average heart rate. seconds is equal to a heart rate of 64.5.

bed increases with each arterial pulsation, heart rate can be estimated from the BVP signal. Heart rate, or the number of heartbeats per minute, is calculated by estimating the time interval between the heartbeats, called the interbeat interval. The time in seconds of the interbeat interval is divided into 60 seconds to calculate the beat-by-beat heart rate, as shown in Figure 3. The time between each beat can vary; therefore, the estimated beat-by-beat heart rate can also vary. There are two estimates of the variability of the heart rate: variability estimated by the SDANN or variability estimated by the standard deviation of the average beat-by-beat heart rate (SDAHR). A methodological issue includes the fact that the raw BVP signal must be artifact free before meaning is Figure 5. When movement artifact is excluded from the signal shown in Figure assigned to the SDANN and SDAHR measures of variability 4, the actual heart rate is estimated at 61.05 beats per minute. in the heart rate data. The next section address artifacts in the BVP signal. This dialogue illustrates issues related to artifacts in the Inspecting the Raw BVP Signal for Artifacts BVP data. The data included movement artifacts, as shown in When analyzing average heart rates, it is important to Figure 4. After eliminating the movement artifact as shown eliminate artifacts before calculating estimates of HRV. Heart in Figure 5, the heart rate information was nearly fi ve beats rate averages calculated from the data may not be reliable if lower. Note also that in this participant, there was a slight artifacts exist, as illustrated in the following vignette: increase in heart rate during the stressful imagery rehearsal once the segments of data affected by artifact were removed. What happened? He reported increased arousal during Once movement artifacts are removed from BVP the stressful imagery, yet his heart rate was higher during estimates of heart rate, the BVP measure of HRV becomes the initial baseline condition. It was only when I included an easy-to-administer technology for use in clinical settings. Biofeedback

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the raw blood volume pulse (BVP) signal from which the HRV is usually a sign of cardiac health, as it suggests that the heart rate was calculated that I realized the average signal heart has fl exibility in response to the demands of the body. was incorrect. HRV refl ects cardiovascular health as a sign of sympathetic Summer 2007 56 Peper, Harvey, Lin, Tylova, Moss

caused by the heart contracting, as shown in Figure 6. The elasticity in the arterial vasculature is partially refl ected in the magnitude of the dicrotic notch signal, shown as marker 3 in Figure 6. The dicrotic notch signal depends on the interaction of the initial pressure wave when the heart contracts, arterial stiffness that decreases the pulse transit time, and the refl ected pressure wave from the peripheral arterial bed. The loss of arterial wall elasticity is usually an indicator of aging and suggests an increased risk of cardiovascular disease, especially (Izzo & Shykoff, 2001). Increasing arterial stiffness decreases the pulse transit time (PTT). PTT can be time referenced to the R-wave of the ECG signal corresponding time between P1 and V of the BVP signal graph in Figure 6. The stiffer the arterial walls, the faster the Figure 6. Example of blood volume pulse signal with amplitude and timing PTT, with the effect most pronounced in the periphery of markers. Time t1 (between markers 1 and 5) indicates the interbeat interval and the toes compared to the fi ngers and ears (Allen & Murray, is used to calculate the heart rate. Pulse measure P1 (marker 1) is a measure of pulse amplitude. Volume at V (marker 3) is the indicator of the blood volume 2002). Decreases in PTT have been signifi cantly correlated infl uenced by the dicrotic notch. Reprinted with permission from Hlimonenko, with an increase in blood pressure and age. The decrease in Meigas, and Vahisalu (2003). PTT affects the BVP waveform by appearing as a diminution of the dicrotic notch as the initial and refl ected pressure and parasympathetic nervous system balance. When HRV waves come closer together. The relationship between the is absent or reduced, it may signal pathology (Del Pozo et PTT and BVP waveforms is illustrated by the BVP and the al., 2004; Kleiger et al., 1987). Namely, when HRV is low, as blood pressure recordings of parents and children of varying indicated by an SDANN <50 milliseconds and an SDAHR ages, as illustrated in Figure 7. <2.5 beats, there is a fourfold increase in relative risk of death Even though HRV training offers great clinical potential, after myocardial infarction compared to those who have it is important to remember that many factors affect blood a high HRV (SDANN >100 milliseconds and SDAHR >5 circulation and thereby the BVP signal. These include beats; Kleiger et al., 1987). Whereas artifact-free BVP signals sympathetic arousal, which induces vasoconstriction; can be used for training of HRV, BVP measures may also be decreased sympathetic arousal and increased parasym- used for estimating cardiovascular health in terms of blood pathetic arousal, which induce vasodilation especially as the pressure because the BVP signal also refl ects changes in the person relaxes; low external temperature, which induces elasticity of the vascular walls (Asada, Shaltis, Reisner, Rhee, vasoconstriction; recreational and prescription drugs (e.g., & Hutchinson, 2003; Babchenko et al., 2001; Speckenbach & alcohol increases vasodilation, whereas nicotine increases Gerber, 1999; Weng, Matz, Gehring, & Konecny, 2002). vasoconstriction); and illnesses such as Raynaud’s disease, which is characterized by arteriole vasoconstriction and Changes in BVP Waveform Shape Refl ect reduced blood fl ow in the fi ngers. Changes in the BVP signal Other Types of Cardiovascular Health can be very rapid and thereby may refl ect sudden shifts in The shape of the BVP waveform may be an indicator of arousal or cognitions. Because arousal and cognitions change cardiovascular variables such as blood pressure because rapidly, the BVP signal may also be used as an indicator of stiffer arterial walls are associated with higher blood transient processes. pressure (Speckenbach & Gerber, 1999). The raw BVP signal is the product of many factors that infl uence blood fl ow Changes in BVP Amplitude May Refl ect Mo-

through the vascular bed. The raw BVP wave pattern is the ment-by-Moment Sympathetic/Parasympa- Biofeedback result of the recording location, the heart’s left ventricular thetic and Cognitive/Emotional Activity ejection, and the elasticity/stiffness of the aorta and arteries. The BVP amplitude displays moment-by-moment HRV The shape of BVP signal can be used for deriving interbeat and may offer signifi cant insight into individual emotional

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interval (t1), which in turn can be transformed into an responses, as illustrated in Figure 8. Figure 8 shows Summer 2007 estimate of heart rate as well as the pulse amplitude (P1), psychophysiological responses during a standardized stress both of which represent the relative increase in blood volume protocol. The participant’s responsiveness to internal and

57 Blood Volume Pulse

Figure 7. Comparison of fi nger blood volume pulse recording of parents (62- Figure 8. Example blood volume pulse amplitude analysis by measuring the year-old father and 52-year-old mother) and child (17-year-old daughter). The percentage change for each condition [(maximum – minimum)/maximum × mother has borderline hypertension. The absence of the dicrotic notch in 100]. the borderline hypertensive (top) tracing suggests a stiffening of the arteries, indicating increased blood pressure. This exploration of emotions and BVP training could include some of the following: external physical and emotional stressors is vividly depicted in the variations of BVP amplitude presented Figure 8. The • Identifi cation of the thoughts, feelings, and emotions pattern portrays decreases in amplitude in the BVP signal in associated with the responses response to prompts such as sighs and claps that triggered • Exploration of past and present patterns that let the sympathetic activation. In this participant, eye closure during person interpret the world as unsafe the protocol evoked an unanticipated and large decrease • Exploration of how to reframe the experience so that the in the BVP amplitude compared to any of the physical or vigilance response is not evoked imagined stress conditions. This unanticipated decrease in • Practice of physiological desensitization so that the BVP may be interpreted as a kind of anticipatory anxiety. response is extinguished Initially, the eye closure was only an instruction before asking the person to think about a stressful experience. The The BVP signal may also be interpreted in terms of idiosyncratic BVP amplitude changes are a window into her peripheral temperature changes because the signal refl ects psychological processes. The large response (vasoconstriction) changes in vasoconstriction and vasodilation. to the instruction “close your eyes” indicates the participant’s general ongoing arousal and vigilance. Instead of relaxing The Use of BVP Amplitude as an Indicator to the “close your eyes” instruction, she reported later that of Changes in Peripheral Temperature she started to think, “What will happen now?” She was Changes in BVP represent changes in the blood volume. If continuously checking if her experience of the world was BVP amplitude increases, it refl ects an increase in vasodilation, safe. This cognitive reaction is often found in people who which leads to an increase in peripheral temperature; if the are anxious, fearful, or want to please others. The BVP BVP amplitude decreases, it refl ects a decrease in peripheral amplitude can also indicate the extent to which the person circulation and a decrease in peripheral temperature. BVP has been captured by his or her emotions; the more captured, amplitude changes are very rapid and precede changes in the longer it takes for the BVP signal to return to some temperature, as temperature is a slow-averaging signal, as baseline level. In the example above of anticipatory anxiety, shown in Figure 9. the BVP signal returned rapidly to baseline, refl ecting a When BVP is monitored simultaneously with temp- state of vigilance rather than the activation of a memory erature, the rapid BVP amplitude changes can facilitate of an emotional trauma. By observing her response pattern peripheral warming. For example, when the BVP amplitude Biofeedback

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during the training session, the participant could explore started to increase, even though the peripheral temperature strategies that would enable her to feel safe and to prevent was still decreasing, it may help the clinician and participant a decrease in her BVP amplitude when she closes her eyes. to identify emotional and cognitive images and sensations Summer 2007 58 Peper, Harvey, Lin, Tylova, Moss

Figure 10. The vaginal blood volume and pulse amplitude of a woman who is shown a neutral stimulus and an erotic stimulus. The genital response occurs Figure 9. Simultaneous recording of right and left blood volume pulse within seconds of the presentation of an erotic stimulus in sexually healthy (BVP), temperature, and electromyography signals. Temperature changes lag women, regardless of age or menopausal status. Reprinted with permission behind the BVP amplitude changes. Even as BVP amplitude is increasing, the from Brotto and Gorzalka (2006). temperature is still decreasing. From Zoe Talbot and Sarah Langensiepen, personal communication. and temporal BVP reduction biofeedback in the treatment of migraine. Results showed that temporal constriction that facilitate peripheral warming. Among these strategies and fi nger temperature biofeedback were equally effective are slower and diaphragmatic breathing, autogenic phrases, in controlling migraine headaches and produced greater relaxation instructions, and imagery to increase peripheral benefi ts than the waiting list condition. Nestoriuc and warmth. In addition to BVP signals being used as indicators of Martin (2007) performed a meta-analysis examining the peripheral temperature, the BVP signal may also be used for effi cacy of biofeedback training in treating migraine. Results treating headaches as well as for monitoring sexual arousal. showed that biofeedback training was more effective than control conditions. The strongest improvements were in the Overlooked BVP Applications: Treatment frequency of migraine attacks and perceived self-effi cacy. of Migraine and Monitoring Sexual BVP feedback yielded higher effect sizes than peripheral Arousal skin temperature feedback and electromyography feedback. Using BVP feedback to encourage vasomotor control offers Moderator analyses revealed biofeedback training in numerous clinical applications such as the treatment for combination with home training to be more effective than migraine by recording the BVP from the temporal artery therapies without home training. (Feuerstein & Adams, 1977). Allen and Mills (1982) used BVP has also been used as an indicator of sexual arousal. photoelectric plethysmograph feedback to train to self- For example, PPG sensors measure tissue engorgement by regulate BVP amplitude in eight female migraine sufferers. blood from any location ranging from the nasal septum to Participants learned to increase and decrease BVP amplitude measure cerebral blood fl ow to the vaginal wall to measure on the scalp at superfi cial temporal artery (STA) and fi nger sexual arousal. As early as 1967, Palti and Bercovici used locations. The results of the research showed a signifi cant a PPG sensor to measure vasoengorgement of the vaginal relationship between voluntary pulse amplitude changes in wall as an indicator of sexual arousal to erotic stimuli, as the BVP amplitude measured at the STA and corresponding shown in Figure 10 (Brotto, Basson, & Gorzalka, 2004; Palti pain reports during a migraine. Hoelscher and Lichstein & Berovici, 1967). Other applications of BVP measurement (1983) used a temporal BVP biofeedback protocol in treating and feedback have yet to be fully explored

chronic cluster headache patients. The result shows a 70% Biofeedback reduction in daily headache frequency and a 45% decrease Exploring Other Uses of the BVP Signal in headache severity. Improvement was maintained at 1, 3, The biofeedback applications of BVP are expanding rapidly 6, 12, and 21 months of follow-up. Large decreases in the with the availability of the new economic HRV/CRS home

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consumption of migraine abortives, narcotic analgesics, and trainers. These portable and relatively inexpensive devices Summer 2007 antiemetics were also observed. Gauthier, Lacroix, Coté, allow people to become aware of factors that affect the BVP Doyon, and Drolet (1985) reported using fi nger warming signal. Through HRV training, participants can develop

59 Blood Volume Pulse

sympathetic/parasympathetic balance and master strategies Allen, R. A., & Mills, G. K. (1982). The effects of unilateral to prevent and reverse illness. In addition, easily available plethysmographic feedback of temporal artery activity computerized monitoring providing the corresponding during migraine head pain. Journal of Psychosomatic Research, 26, 133–140. digital analysis such as power spectrum analysis (fast Fourier Asada, H. H., Shaltis, P., Reisner, A., Rhee, S., & Hutchinson, transform) and feedback may allow new areas of biofeedback R. C. (2003). Mobile monitoring with wearable BVP applications to emerge. Among those are the following: photoplethysmographic biosensors. IEEE English Medical Biology Magazine, 22(3), 28–40. • Analysis and training of specifi c components of the BVP Babchenko, A., Davidson, E., Ginosar, Y., Kurz, V., Faib, signal, such as increasing the dicrotic notch to improve I., Adler, D., et al. (2001). Photoplethysmographic cardiovascular fl exibility or reducing high blood pressure. measurement of changes in total and pulsatile tissue blood • Correlating BVP waveforms recorded from multiple volume, following sympathetic blockade. Physiological locations on the body with Chinese pulse used as a Measurement, 22, 389–396. diagnostic tool in traditional Chinese medicine (TCM). Brotto, L. A., Basson, R., & Gorzalka, B. B. (2004). In TCM, for more than 2000 years, a stiff pulse has been Psychophysiological assessment in premenopausal an indicator of aging and/or pathology. sexual arousal disorder. Journal of Sexual Medicine, 1, 266–277. • Exploring the changes in blood fl ow as indicators of Brotto, L. A., & Gorzalka, B. B. (2006). The vaginal illness or health by recording the BVP response patterns photoplethysmograph. Retrieved November 10, simultaneously from multiple locations such as from 2006, from http://www.psych.ubc.ca/~bglab/equipment. different sympathetically enervated dermatomes. html • Ongoing analysis and treatment of male and female Del Pozo, J. M., Gevirtz, R. N., Scher, B., & Guarneri, E. (2004). sexual dysfunctions as well as development of strategies Biofeedback treatment increases heart rate variability in to enhance orgasmic quality. This could possibly include patients with known coronary artery disease. American correlations of penile and vaginal wall engorgement with Heart Journal, 147(3), G1–G6. peripheral blood fl ow changes in other areas of the body Feuerstein, M., & Adams, H. E. (1977). Cephalic vasomotor that may contribute to sexual arousal. feedback in the modifi cation of migraine headache. • Combining BVP monitoring with psychotherapy, as the Applied Psychophysiology and Biofeedback, 2, 214– rapid changes in BVP amplitude may indicate cognitive 254. Gauthier, J., Lacroix, R., Coté, A., Doyon, J., & Drolet, M. and emotional responses. (1985). Biofeedback control of migraine headaches: A • Teaching increasing BVP amplitude to improve circulation comparison of two approaches. Applied Psychophysiology and thus regeneration in cases of diabetic ulcers and and Biofeedback, 10, 139–159. frostbites (Rice & Schindler, 1992; Graul, Stanculescu, Gevirtz, R. N. (2003). The promise of HRV biofeedback: Peper, Johansen, & Doyle, 2004). Some preliminary results and speculations. Biofeedback, 31(3), 18–19. The goal of this article was to raise awareness of the Giardino, N. D., Chan, L., & Borson, S. (2004). Combined use of BVP technologies beyond the calculation of HRV or heart rate variability and pulse oximetry biofeedback RSA/CRS indicators. Whereas other uses of the PPG/BVP for chronic obstructive pulmonary disease. Applied technology will emerge in the future, those described here Psychophysiology and Biofeedback, 29, 121–133. offer a good start for stimulating new areas of research. Graul, M., Stanculescu, A., Peper, E., Johansen, K. L., & Doyle, J. W. (2004). Possible treatment of diabetic ulcer Acknowledgments for a patient: A case report [Abstract]. Applied Psychophysiology and Biofeedback, 29, 299. This article was adapted from E. Peper, H. Tylova, K. H. Hlimonenko, I., Meigas, K., & Vahisalu, R. (2003). Waveform Gibney, and R. Harvey, Mastery Through Experience: A analysis of peripheral pulse wave detected in the fi nger- Self-Teaching Laboratory Manual for Biofeedback and Self- tip with photoplethysmograph. Measurement Science Regulation skills (unpublished manuscript). We thank Fred Review, 3(2), 49–52. Shaffer for his insightful feedback. Hoelscher, T. J., & Lichstein, K. L . (1983). Blood volume pulse biofeedback treatment of chronic cluster headache. References Biofeedback and Self-Regulation, 8, 533–541. Biofeedback

Ô Allen, J., & Murray. A. (2002). Age-related changes in Izzo, J. L., & Shykoff, B. E. (2001). Arterial stiffness:

peripheral pulse timing characteristics at the ears, fi ngers Clinical relevance, measurement, and treatment. Review and toes. Journal of Human Hypertension, 16, 711–717. Cardiovascular Medicine, 2(1), 29–34, 37–40. Summer 2007 60