Galvanic skin potential (GSP) refers to the voltage measured between two electrodes without any externally applied current. It is measured by connecting the electrodes to a voltage amplifier. This voltage also varies with the emotional state of the subject. A painful stimulus such as a pinprick elicits a sympathetic response by the sweat glands, increasing secretion. Although this increase is generally very small, sweat contains water and electrolytes, which increase electrical conductivity, thus lowering the electrical resistance of the skin. These changes in turn affect GSR. Another common manifestation is the vasodilation (dilation) of blood vessels in the face, referred to as blushing, as well as increased sweating that occurs when one is embarrassed. EDA is highly responsive to emotions in some people. Fear, anger, startled response, orienting response, and sexual feelings are among the reactions that may be reflected in EDA. These responses are utilized as part of the polygraph or lie detector test. EDA in regular subjects differs according to feelings of being treated fairly or unfairly, but psychopaths have been shown to manifest no such differences. This indicates that the EDA record of a polygraph may be deceptive in a criminal investigation. EDA is a common measure of autonomic nervous system activity, with a long history of being used in psychological research. Hugo D. Critchley, Chair of Psychiatry at the Brighton and Sussex Medical School states, "EDA is a sensitive psychophysiological index of changes in autonomic sympathetic arousal that are integrated with emotional and cognitive states." Many biofeedback therapy devices utilize EDA as an indicator of the user's stress response with the goal of helping the user to control anxiety. EDA is used to assess an individual's neurological status without using traditional, but uncomfortable and expensive, EEG-based monitoring. Oftentimes, EDA monitoring is combined with the recording of heart rate, respiratory rate, and blood pressure, because they are all autonomically dependent variables. EDA measurement is one component of modern polygraph devices, which are often used as lie detectors. The E-meter used by the Church of Scientology as part of its practice of "auditing" and "security checking", is a custom EDA measurement device. External factors such as temperature and humidity affect EDA measurements, which can lead to inconsistent results. Internal factors such as medications and hydration can also change EDA measurements, demonstrating inconsistency with the same stimulus level. Also, the classic understanding has treated EDA as if it represented one homogeneous change in arousal across the body, but in fact different locations of its measurement can lead to different responses; for example, the responses on the left and right wrists are driven by different regions of the brain, providing multiple sources of arousal; thus, the EDA measured in different places on the body varies not only with different sweat gland density but also with different underlying sources of arousal.