The skin conductance is one of the fastest responding measures of stress response and has been previously used to measure the difficulty of driving tasks[Hel78]. It has been found to be one of the most robust and non-invasive physiological measures of autonomic nervous system activity[CT90]. Selye and others have linked skin conductance response to stress and autonomic nervous system arousal[Sel56].
A characteristic orientation or ``startle'' response occurs in the signal whenever a person is forced to attend to a change in their environment, either external or internal[Dam94]. Signal processing algorithms were developed to detect these responses. First the raw GSR signal, sampled at 20Hz, was convolved with a smoothing filter, then the first forward difference was calculated and a threshold was applied. The feature triggered by this threshold indicated the steep slope associated with the rising edge of the startle response. An example of this algorithm is shown graphically in Figure 7.
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