Galvanic skin response (GSR)
The galvanic skin response (GSR) can be used for capturing the autonomic nerve response as a parameter of the sweat gland function. Due to relatively simplicity of measurement, and a quite good repeatability GSR can be considered to be useful and simple method for examining autonomic nervous system function, specifically the peripheral sympathetic system.
Physically GSR is a change in the electrical properties of the skin in response to different kinds of stimuli. In GSR changes in the voltage measured from the surface of the skin are recorded. The main origin of the signal has suggested to be the activation of the sweat glands. The most commonly used stimuli are an electrical shock delivered to a peripheral nerve or auditory stimuli. However, any stimulus capable of an arousal effect can evoke the response and the amplitude of the response is more dependent on the surprise effect of the stimulus than on the physical stimulus strength.
In history GSR is also known as, or closely related to, the sympathetic skin response (SSR) and skin conductance response (SCR). In clinical neurophysiological literature, the response is known also as the peripheral autonomic surface potential (PASP). Most of the GSR studies in last decades are concerned with the normal values of response amplitude and latency. Also the habituation of response amplitudes during repeated stimulations has been studied. Response amplitudes vary substantially, depending on the experimental conditions.
GSR responses for healthy control (left) and for patient suffering from psychotic disorder. Repeated measurements are plotted from bottom to top. Amplitude scale is the same in both axes.
Analysis methods for GSR
In the figure above are shown set of typical GSR responses for healthy person and for person suffering from psychotic disorder. The wave shape is usually biphasic or triphasic and lasts several seconds. Because GSRs are such long lasting waveforms inter-stimulus intervals (ISIs) should be long enough. When using short ISIs response overlapping should be considered by decomposing the overlapped responses.
Normally reproduced within-subject GSRs have common features in waveshapes. Amplitudes tend to habituate, latencies might increase slightly and waveshapes remain fairly unaltered in repetitions. The decrease in amplitudes and increase in latencies is affected by the weakening of the surprise effect of stimulation and by the weakening of alertness of the subject during the experiment.
In recording situations some low pass filter can be used to avoid high frequency noise. As the signal-to-noise ratio (SNR) of the GSR signal is high individual responses are usually studied without any signal processing. In some studies several responses are averaged but this can lead false interpretation due to variability of the response. Thus, it is generally suggested that averaging should not be used.
We have presented an advanced method for analyzing the patterning of successive galvanic skin responses. The method is based on principal component analysis (PCA) in which the stochastic sample is presented as a weighted sum of orthogonal basis vectors. The method was then tested using measurements from 20 healthy controls and 13 psychotic patients. As seen in figures below responses between healthy and psychotic persons differ noticeably. For most of the healthy controls there is a clear pattern in reproduced GSRs, whereas within psychotic patients the lack of time-locking of GSRs seemed to be characteristically. The two groups of test persons could be separated by clustering algorithm based on the largest eigenvalues
GSR measurements (left) and clusterization (right) for healthy controls (+) and for psychotic patients (o).
Click images to enlarge.
This kind of result, where all patient could be ranked correctly giving the described method sensitivity of 100%, is promising. It is possible that in future this kind of GSR based methods may offer a new and potential tool for screening risk groups and populations in evaluating drug effects by revealing also the early and subtle alternations.