Skin Conductance / Heart Rate Monitor







Particularly challenging was building a Skin Conductance Amplifier device that could be used in close proximity to a Nuclear Magnetic Resonance (NMR) scanner in use at the Medical College of Wisconsin. Subjects are placed inside the NMR scanner and then attached to the skin conductance device. The device monitors the absolute (SCL) and relative (SCR) electrical conductance of the skin and outputs a corresponding voltage. Using electrodes placed on the fingers or forearm the device monitors a small electrical current that corresponds to the amount of sweat gland activity in the skin. Sometimes referred to as a 'lie detector' the skin conductance amplifier is used to gauge anxiety level and study Human Fear Conditioning and Learning.

For electronics that connect directly to a human subject extreme care must be taken to assure that no shock hazard is present or possible. To this end the device employs many safeguards to insure this can never happen. Isolated amplifiers and power supplies are used where contact with the human subject is made. The entire unit is double isolated from any A.C. power source.

In addition to the skin conductance circuitry a Plethysmograph based Heart Rate Monitor circuit was included inside the same electronics case. The plethysmograph uses a light emitter and photodetector to monitor blood flow near the surface of the skin. An analog signal is then generated from the minute changes in the light reaching the photodetector. Both circuits were my design.

Skin conductance and heart rate are monitored inside the scanner and the analog data is sent outside to a nearby computer system for recording. Brain activity is monitored and recorded by the scanner at the same time. The device data is then compared to the recorded brain activity. This technique has been used by many graduate students for their PhD dissertations including Christine Smith, "Network Analysis of Human Fear Conditioning: Differential Acquisition, Extinction, and Reversal" (2003) and Dominic Cheng, "Neural Correlates of Response Expression During Fear Learning: Conditioning and Awareness" (2005).



Daniel Shurilla
Electronic Research Technician

Christine N. Smith, Ph.D.
Department of Neurosciences
University of California, San Diego

Dominic T. Cheng, Ph.D.
Postdoctoral Fellow
Johns Hopkins University School of Medicine