A polygraph, also known as a lie detector, is a device that monitors key indicators of autonomic arousal such as heart rate, blood pressure, respiration rate, and galvanic skin response. It is used to determine if a person is being truthful or deceptive in their responses to questioning. A sonograph is a device that uses sound waves to create images of the body's internal structures. An electrocardiograph (EKG) is a device that measures the electrical activity of the heart. An electroencephalograph (EEG) is a device that measures the electrical activity of the brain.