Human skin offers resistance to electric current just like a resistor do in an electronic circuit. This transient change in the electrical conductivity of the skin followed by an arousal or oriented response is referred to as Galvanic skin response or GSR. In a fully relaxed person, the skin offers very high resistance and the skin’s surface is around 40mV with respect to the inner tissue. Sweating in the high emotional state or stress causes increased blood flow to the skin and reduces the skin resistance and increases its electrical conductivity. This phenomenon is widely used in Medical and Forensic sciences to assess the mental state of a person.
The GSR was originally termed as Psychogalvanic reflex and Electrodermal response (EDR). The GSR measurement is used in studies of emotion and stress, conditioning, habituation, cognitive processing etc. Skin conductance can be classified into Tonic skin conductance and Phasic skin conductance. The Tonic skin conductance varies with psychological arousal and increases further with activity or stress. Phasic skin conductance is a wavelike response in the skins conductivity followed by a stimulus. Aroused persons show spontaneous skin conductance due to mental or internal stimuli and the frequency increases as the level of arousal increases.
About the Skin
Skin is the largest sense organ of the body richly supplied with blood capillaries and sensory nerve endings and pressure (Pacinian corpuscles) and touch (Tactile corpuscles) sensors. Skin is porous and has many openings of Sebaceous and Sweat glands. The sebum secreted by the sebaceous glands make the skin oily while the sweat from the sweat glands remove water , salts and other waste materials from the blood. Skin also plays an important role in thermoregulation by removing heat from the body through sweating. The sweat evaporates on the skin surface by taking heat from the body. This causes the cooling effect.
The peripheral blood flow to the skin is greatly affected by the stimulation of the hypothalamus of brain, sympathetic nervous system and adrenal hormones. The thermoregulatory centre in the hypothalamus always monitors the body temperature and stimulates sweating to remove excess heat. High emotional state causes stimulation of hypothalamus and limbic system of the brain followed by the stimulation of sympathetic nervous system and adrenal hormone release.
Fight and Flight.
This is the response of the body in high emotional state followed by Fear or Stress. The fight and flight mechanism originates in the brain by sensory stimuli or thought. This activates the sympathetic nervous system and causes release of Adrenaline, the Emergency Hormone from the Adrenal glands located on the upper surface of kidneys. Secretion of Adrenaline follows the stimulation of sympathetic nervous system which causes a number of physiological changes to boost the body functions to face the emergency situation.
1. Heart beat increases to increase the cardiac output.
2. Respiratory rate increases to provide more oxygen to cells
3. Gastric functions increase to burn food to provide more energy
4. Peripheral blood flow to the skin increases and blood capillaries in the skin becomes more permeable to water and ions. This increases sweating to eliminate heat and wastes.
5. Skin tone changes, resistance of skin decreases and electrical conductivity increases.
These changes are meant for preparing the body to face the emergency situation and results in either Fight (Attack) or Flight (Escape). If the mind is restless and the emotional state of the person is fluctuating especially in conditions like over stress and mental illness like Anxiety disorder or Depression, brain feels that the body is in danger and stimulate the Fight and Flight mechanism.
Brain to Skin response
The skin acts as a resistive layer and readily passes electric current. In normal Skin tone, the resistance varies from 25 Kilo Ohms to a few Mega Ohms. This permits little current to pass through it. This is why the burns occur in electric shock. The high current in AC (230 volt) or DC (above 50 volt DC) causes heat in the skin, just like a low value resistor heat up when high current passes through it. Insulation of hand and feet during electric work prevents the flow of current through the skin to the earth. Electrical safety measures advice this to prevent shock hazard. More over keeping the left hand behind the body during electric work reduces the risk of passing the current through the left side in which heart is located.
If the body is in stress or in a state of high emotion, the flight and fight mechanism causes a reduction in the Skin resistance. From Mega ohms, the skin reduces to 5 kilo ohms or less depending on the person or stress level. In addition to reduction in the skin resistance, vascular flow to the skin also increases. The blood vessels become more leaky and water, ions etc spreads over the skin through sweating. This also facilitates current flow. These effects are more prominent in fingers, hands etc especially in the left hand. The most advanced layers of the brain cortex, link to the thumb and forefinger and complex physiological response occurs when the forebrain is aroused. The best point at which to measure skin resistance is the thumb and forefinger because this part of the body is most heavily represented neurologically in the evolutionarily advanced thought centers of the brain used to manipulate objects. Changes in Alpha rhythms of brain causes blood capillaries to enlarge, and this too affects resistance.
The history of Psychometry dates back to 1906 when Carl Gustav Jung referenced the use of GSR method in psycho analysis. In his paper “Studies in Word Analysis”, he described the use of GSR using finger electrodes. The method involves connecting two electrodes in the hands to measure the skin resistance. After connecting the electrodes, a list of words was a read to the person under examination. He noticed that if a word in the list is emotionally important, the needle in the GSR instrument moved in response to skin resistance. Any word that produced the response indicates the possible areas of conflicts in the person.
The Psycho galvanometer is a simple analog meter with two electrodes connected to it. A small DC current passes from one electrode to the other through the skin. Depending on the skin resistance, amount of current passing varies which can be observed through the reflections of the needle.
Using GSR technique, it is easy to detect depths of hypnotic trance. This helps to suggest the therapy in conditions of stress and behavior changes.GSR can also be used in behavior modification therapy. Assessing the problems like fear complex can be easily done with GSR method.
Biofeedback is the technique in which the relaxation of the body is achieved through self regulation and awareness. The persons level of awareness lies in the arousal of the cortical region of the brain. So by measuring the state of mind through galvanic response it is easy to assess the level of awareness so that biofeedback techniques can be adopted. Many studies have shown that
1. Low level cortical arousal is necessary in relaxation, hypnosis.
2. High state of cortical arousal leads to focused concentration, increased reading speed , increased power of reflection etc.
3. Increased power of recall also is the effect of cortical arousal.
4. Cortical arousal and Skin galvanic response are directly related. That is, high cortical arousal causes increased skin conductivity and vice versa.
5. Relaxation and biofeedback techniques reduce stress which can be expressed in low skin conductivity.
6. If an EEG is used simultaneously with GSR, the increase of brain arousal corresponding to the changes of measured skin resistance can be detected.
Click to see 2 circuits of skin resistance measurement published in Electronics For You Magazine during Sept 2005 and June 2009
SKIN RESPONSE METER-EFY JUNE 2009
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