complex). the interbeat interval, divided into 60 seconds, determines the heart rate at that moment. the statistical variability of that interbeat interval is what we call heart rate variability. the ecg method is more accurate than the ppg method in measuring heart rate variability. biofeedback therapists use heart rate variability (hrv) biofeedback when treating asthma, copd, depression, anxiety, fibromyalgia, heart disease, and unexplained abdominal pain. research shows that hrv biofeedback can also be used to improve physiological and psychological wellbeing in healthy individuals. hrv data from both polyplethysmographs and electrocardiograms are analyzed via mathematical transformations such as the commonly-used fast fourier transform (fft). the fft splits the hrv data into a power spectrum, revealing the waveform's constituent frequencies. among those constituent frequencies, high-frequency (hf) and low-frequency (lf) components are defined as above and below .15 hz, respectively. as a rule of thumb, the lf component of hrv represents sympathetic activity, and the hf component represents parasympathetic activity. the two main components are often represented as a lf/hf ratio and used to express sympathovagal balance. some researchers consider a third, medium-frequency (mf) component from .08 hz to .15 hz, which has been shown to increase in power during times of appreciation. pneumograph a pneumograph or respiratory strain gauge uses a flexible sensor band that is placed around the chest, abdomen, or both. the strain gauge method can provide feedback about the relative expansion/contraction of the chest and abdomen, and can measure respiratory rate (the number of breaths per minute). clinicians can use a pneumograph to detect and correct dysfunctional breathing patterns and behaviors. dysfunctional breathing patterns include clavicular breathing (breathing that primarily relies on the external intercostals and the accessory muscles of respiration to inflate the lungs), reverse breathing (breathing where the abdomen expands during exhalation and contracts during inhalation), and thoracic breathing (shallow breathing that primarily relies on the external intercostals to inflate the lungs). dysfunctional breathing behaviors include apnea (suspension of breathing), gasping, sighing, and wheezing. a pneumograph is often used in conjunction with an electrocardiograph (ecg) or photoplethysmograph (ppg) in heart rate variability (hrv) training. biofeedback therapists use pneumograph biofeedback with patients diagnosed with anxiety disorders, asthma, chronic pulmonary obstructive disorder (copd), essential hypertension, panic attacks, and stress. capnometer a capnometer or capnograph uses an infrared detector to measure end-tidal co2 (the partial pressure of carbon dioxide in expired air at the end of expiration) exhaled through the nostril into a latex tube. the average value of end-tidal co2 for a resting adult is 5% (36 torr or 4.8 kpa). a capnometer is a sensitive index of the quality of patient breathing. shallow, rapid, and effortful breathing lowers co2, while deep, slow, effortless breathing increases it. biofeedback therapists use capnometric biofeedback to supplement respiratory strain gauge biofeedback with patients diagnosed with anxiety disorders, asthma, chronic pulmonary obstructive disorder (copd), essential hypertension, panic attacks, and stress. rheoencephalograph rheoencephalography (reg), or brain blood flow biofeedback, is a biofeedback technique of a conscious control of blood flow. an electronic device called a rheoencephalograph is utilized in brain blood flow biofeedback. electrodes are attached to the skin at certain points on the head and permit the device to measure continuously the electrical conductivity of the tissues of structures located between the electrodes. the brain blood flow technique is based on non-invasive method of measuring bio-impedance. changes in bio-impedance are generated by blood volume and blood flow and registered by a rheographic device. the pulsative bio-impedance changes directly reflect the total blood flow of the deep structures of brain due to high frequency impedance measurements. hemoencephalography hemoencephalography or heg biofeedback is a functional infrared imaging technique. as its name describes, it measures the differences in the color of light reflected back through the scalp based on the relative amount of oxygenated and unoxygenated blood in the brain. research continues to determine its reliability, validity, and clinical applicability. heg is used to treat adhd and migraine, and for research. pressure pressure can be monitored as a patient performs exercises while resting against an air-filled cushion. this is pertinent to physiotherapy. alternatively, the patient may actively grip or press against an air-filled cushion of custom shape. applications urinary incontinence mowrer detailed the use of a bedwetting alarm that sounds when children urinate while asleep. this simple biofeedback device can quickly teach children to wake up when their bladders are full and to contract the urinary sphincter and relax the detrusor muscle, preventing further urine release. through classical conditioning, sensory feedback from a full bladder replaces the alarm and allows children to continue sleeping without urinating. kegel developed the perineometer in 1947 to treat urinary incontinence (urine leakage) in women whose pelvic floor muscles are weakened during pregnancy and childbirth. the perineometer, which is inserted into the vagina to monitor pelvic floor muscle contraction, satisfies all the requirements of a biofeedback device and enhances the effectiveness of popular kegel exercises. contradicting this, a 2013 randomized controlled trial found no benefit of adding biofeedback to pelvic floor muscle exercise in stress urinary incontinence. in another randomized controlled trial the addition of biofeedback to the training of pelvic floor muscles for the treatment of stress urinary incontinence, improved pelvic floor muscle function, reduced urinary symptoms, and improved of the quality of life. in 1992, the united states agency for health care policy and research recommended biofeedback as a first-line treatment for adult urinary incontinence. in 2019, the national institute for health and care excellence recommended against the routine use of use biofeedback in managing urinary incontinence in women who can actively contract the pelvic floor. it may be considered though, to aid motivation and adherence to therapy. fecal incontinence, constipation and anismus biofeedback is a treatment for anismus (paradoxical contraction of puborectalis during defecation). this therapy directly evolved from the investigation anorectal manometry where a probe that can record pressure is placed in the anal canal. biofeedback therapy is also a commonly used and researched therapy for fecal incontinence, but the benefits are uncertain. biofeedback therapy varies in the way it is delivered. it is also unknown if one type has benefits over another. the aims have been described as to enhance either the rectoanal inhibitory reflex (rair), rectal sensitivity (by discrimination of progressively smaller volumes of a rectal balloon and promptly contracting the external anal sphincter (eas)), or the strength and endurance of the eas contraction. three general types of biofeedback have been described, though they are not mutually exclusive, with many protocols combining these elements. similarly there is variance of the length of both the individual sessions and the overall length of the training, and if home exercises are performed in addition and how. in rectal sensitivity training, a balloon is placed in the rectum, and is gradually distended until there is a sensation of rectal filling. successively smaller volume reinflations of the balloon aim to help the person detect rectal distension at a lower threshold, giving more time to contract the eas and prevent incontinence, or to journey to the toilet. alternatively, in those with urge incontinence/ rectal hypersensitivity, training is aimed at teaching the person to tolerate progressively larger volumes. strength training may involve electromyography (emg) skin electrodes, manometric pressures, intra-anal emg, or endoanal ultrasound. one of these measures are used to relay the muscular activity or anal canal pressure during anal sphincter exercise. performance and progress can be monitored in this manner. co-ordination training involves the placing of 3 balloons, in the rectum and in the upper and lower anal canal. the rectal balloon is inflated to trigger the rair, an event often followed by incontinence. co-ordination training aims to teach voluntary contraction of eas when the rair occurs (i.e. when there is rectal distension). there is some research that shows the effects of biofeedback on irritable bowel syndrome. however, there may be some adverse effects when using these devices. in 2010 and 2017, the national institute for health and care excellence recommended against the use of biofeedback in managing constipation in children. eeg main articles: electroencephalography and neurofeedback caton recorded spontaneous electrical potentials from the exposed cortical surface of monkeys and rabbits, and was the first to measure event-related potentials (eeg responses to stimuli) in 1875. danilevsky published investigations in the physiology of the brain, which explored the relationship between the eeg and states of consciousness in 1877. beck published studies of spontaneous electrical potentials detected from the brains of dogs and rabbits, and was the first to document alpha blocking, where light alters rhythmic oscillations, in 1890. sherrington introduced the terms neuron and synapse and published the integrative action of the nervous system in 1906. pravdich-neminsky photographed the eeg and event related potentials from dogs, demonstrated a 12–14 hz rhythm that slowed during asphyxiation, and introduced the term electrocerebrogram in 1912. forbes reported the replacement of the string galvanometer with a vacuum tube to amplify the eeg in 1920. the vacuum tube became the de facto standard by 1936. berger (1924) published the first human eeg data. he recorded electrical potentials from his son klaus's scalp. at first he believed that he had discovered the physical mechanism for telepathy but was disappointed that the electromagnetic variations disappear only millimeters away from the skull. (he did continue to believe in telepathy throughout his life, however, having had a particularly confirming event regarding his sister). he viewed the eeg as analogous to the ecg and introduced the term elektenkephalogram. he believed that the eeg had diagnostic and therapeutic promise in measuring the impact of clinical interventions. berger showed that these potentials were not due to scalp muscle contractions. he first identified the alpha rhythm, which he called the berger rhythm, and later identified the beta rhythm and sleep spindles. he demonstrated that alterations in consciousness are associated with changes in the eeg and associated the beta rhythm with alertness. he described interictal activity (eeg potentials between seizures) and recorded a partial complex seizure in 1933. finally, he performed the first qeeg, which is the measurement of the signal strength of eeg frequencies. adrian and matthews confirmed berger's findings in 1934 by recording their own eegs using a cathode-ray oscilloscope. their demonstration of eeg recording at the 1935 physiological society meetings in england caused its widespread acceptance. adrian used himself as a subject and demonstrated the phenomenon of alpha blocking, where opening his eyes suppressed alpha rhythms. gibbs, davis, and lennox inaugurated clinical electroencephalography in 1935 by identifying abnormal eeg rhythms associated with epilepsy, including interictal spike waves and 3 hz activity in absence seizures. bremer used the eeg to show how sensory signals affect vigilance in 1935. walter (1937, 1953) named the delta waves and theta waves, and the contingent negative variation (cnv), a slow cortical potential that may reflect expectancy, motivation, intention to act, or attention. he located an occipital lobe source for alpha waves and demonstrated that delta waves can help locate brain lesions like tumors. he improved berger's electroencephalograph and pioneered eeg topography. kleitman has been recognized as the "father of american sleep research" for his seminal work in the regulation of sleep-wake cycles, circadian rhythms, the sleep patterns of different age groups, and the effects of sleep deprivation. he discovered the phenomenon of rapid eye movement (rem) sleep with his graduate student aserinsky in 1953. dement, another of kleitman's students, described the eeg architecture and phenomenology of sleep stages and the transitions between them in 1955, associated rem sleep with dreaming in 1957, and documented sleep cycles in another species, cats, in 1958, which stimulated basic sleep research. he established the stanford university sleep research center in 1970. andersen and andersson (1968) proposed that thalamic pacemakers project synchronous alpha rhythms to the cortex via thalamocortical circuits. kamiya (1968) demonstrated that the alpha rhythm in humans could be operantly conditioned. he published an influential article in psychology today that summarized research that showed that subjects could learn to discriminate when alpha was present or absent, and that they could use feedback to shift the dominant alpha frequency about 1 hz. almost half of his subjects reported experiencing a pleasant "alpha state" characterized as an "alert calmness." these reports may have contributed to the perception of alpha biofeedback as a shortcut to a meditative state. he also studied the eeg correlates of meditative states. brown (1970) demonstrated the clinical