The PsychLab SC amplifier.
The Psychlab TG-WN Auditory stimulator.
EL122 8mm diameter electrodes
Normal position for SC electrodes.
Typical SC response.
Habituation is a measure of the 'fight or flight' response to discrete stimuli. Typically a tone stimulus is presented up to 12 times, with variable Inter Trial Interval (ITI), and Skin Conductance response is measured. The subject is said to have habituated when 3 consecutive stimuli fail to exhibit a response.
Skin Conductance is measured with the PsychLab SC device. Electrodes are applied to the medial phalange of the index and middle fingers.
The subject may be positioned behind a screen or in another room.
A PCC script is devised which causes the TG-WN Tone/Noise generator to operate at given pseudo randomised inter trial interval (ITI). True random intervals may alternatively be used. The ITI should be at least 10 seconds, to allow for a full SC response wave to occur and recover. Ideally longer intervals with adequate variation of ITI will be used to prevent the subject anticipating the stimulus, however there is a trade off when choosing ITI as very long periods of say 25 seconds will risk the subject becoming bored or even going to sleep. Typically the stimulus tone will be 1 second duration.
Best results are achieved if the subject is positioned at a distance from the computer. The SC Amplifier and the TG-WN stimulator may be placed near to the subject using extension USB cables if required.
Equipment required is:
Set of SC electrodes
Two 8mm diameter measuring electrodes (part no EL122) may be positioned on the medial phalange of the index and middle fingers of the same hand. Peel off the backing from one side of the collar and stick it onto the electrode so that it does not obscure the active area of the electrode. Then fill the electrode with SC jell, so that the jell is flush with the electrode/collar surface. Wipe away excess jell, then peel off the other surface of the collar, taking care not to get jell on the sticky surface. Apply the electrode and collar to the skin, pressing firmly. The skin should not be prepared with alcohol or any type of electrode prep or abrading, because this will change conductivity. However, some experts recommend to use a standard procedure with every subject, for example:
1. Always wash your hands with soap and water before applying electrodes to someone else. If you have any breaks in the skin of your hands, or any “weeping” rashes or lesions, wear examination gloves.
2. Always have the subject wash his/her hands with soap and water and dry them thoroughly before applying electrodes. This helps to equate the degree of skin hydration across subjects.
3. If the subject has any breaks in the skin or weeping lesions on his/her hands near the recording sites, DO NOT RUN THAT SUBJECT.
During the experiment, data are continuously recorded to disk. This allows flexibility when analyzing data, as any part of the record can be accessed should it be decided to modify the analysis after the recording has been made. Specific SC response (SR) to each stimulus is obtained by sectioning data into blocks which begin at stimulus onset and have duration 10 seconds, or a period which does not overlap with any subsequent stimulus (i.e. less than the minimum ITI). The PsychLab Wave detection algorithm, which produces the black markers on the onset and peak of a positive going wave, is used to evaluate each response. The response is considered valid if the following conditions are met:
1) Its amplitude must be greater than the minimum response criterion (typically 0.02 micro Siemens).
2) Its onset must lie between 0.8 seconds and less than 4 seconds after stimulus onset.
3) The peak must lie within the time period chosen for the block.