Human skin is a complicated multi-layered barrier that helps facilitate several vital functions. In general, the electrical resistance of the skin has variable behaviour depending on whether AC or DC voltages are applied, and what state the skin is in when presented with stimulus. DC voltages offer higher impedance than AC, and higher voltages reduce the overall impedance, along with time exposed to that stimulus. 


Although there is a complex picture surrounding skin resistance, patterns of behaviour in it can be drawn as the biology and chemistry alters.       


In psychology, the delay in reaction time that is seen between congruent and incongruent stimuli is known as The Stroop Effect (1935). A change in the skin resistance is an effective monitor of delayed reaction times: when the colour of a printed word is not consistent with that named colour (e.g., "Green" printed in the colour yellow as opposed to green), this can be observed. There are several theories that have been proposed to explain this including: Selective Attention Theory, Speed of Processing Theory, Automaticity and Parallel Distributed Processing. This test, monitored by skin resistance, can demonstrate the level of concentration needed when information is displayed in different formats.      


Skin resistance (from sweat), along with heart rate, blood pressure, respiratory rate are used together in the polygraph - to help form a picture of a person's reaction to questions in the well-known "lie detector".     

Usage with EasySense

1. Turn on the sensor.

2. Clean the surface of the skin that is to be measured.

3. Connect the sensor to EasySense, using Devices.

4. Using Setup, select Continuous, Interval 1s.

5. With the above recommendations, carefully locate the electrodes.

6. Prepare to use a dedicated Stoop test card or plan your experimental regime. 

7. Press Start to initiate data collection and collect when prompted. 

8. Press Stop when complete.

Practical Considerations

Please present clean and dry skin to for the electrodes to make use of.


A clearly defined surface area of the electrode that will be used is recommended. It is important to note that the location of the electrodes will have some impact on the absolute reading (see below, Additional Information). 


Whilst in use, we do not recommend electrode skin contact, if the sensor is connected via USB to a mains connected collection device. 


Make sure that where the electrodes are located, there can be little chance of sweat build up.

Additional Information

Skin consists of several distinct layers. The prime function of skin is to provide a protective layer for underlying tissue, to provide a barrier against foreign material, to aid the elimination of water and waste, and to regulate tissue moisture. The skin does, however, have permanent openings (mainly eccrine glands). The barrier that the skin provides also changes with injury and imperfections that result from day-to day events.


The skin is composed of three layers – the epidermis, the corium, and subcutaneous tissue. The epidermis is made up of the stratum corneum (horny layer). This porous fibrous layer is permeable to a range of substances from ions to large molecules. In the palms and soles lies the stratum lucidum which gives a fluid resistant barrier. It provides most of wet skin resistance but is permeable to cations. The malphigian layer offers little resistance to materials that have penetrated to it.


Loose connective tissue in the corium (internal dermis) helps support many of the skin's other components, such as sweat glands. Most sweat glands are of the eccrine variety: they are greater in number and widely spread compared with their apocrine relative. As sweat glands become more active, then excretion of water and salt will increase and the skin resistance falls. Factors that influence increased activity include pain and anxiety.

      

When one places electrodes against the skin, resistance and capacitance properties can be measured. These are distributed across two layers - stratum lucidum and the stratum corneum. When the latter layer is dry it will offer greater than 1 MΩ cm.2. Removing the stratum lucidum from the model, one can arrive upon the following representation:  



The electrical impedance, Z, can be calculated from the above (simplified) circuit at a frequency, f, by the following. The impedance of the skin varies with the frequency applied: the higher the frequency then the lower the impedance observed. This implies that a capacitive element to the electrical performance is present. At higher frequencies, the voltage drop in the corneum can be considered low enough to be discounted. 


         |Z| = {[(2Rsc-2 + (-2π f Csc)2)-1/2 + (Rit-2 + (-2π f Cit)2)-1/2]-1 + Rs-1}-1 


When f = 0, (DC), then: 


          Z = Rs(2Rsc + Rit)/(Rs + 2Rsc + Rit)    

    

Since this sensor measures in the DC mode, then what we can reliably evaluate is the value of Z. This can also be applied comparatively.


The variations in skin resistance are dependent on temperature, whether the stimulus is AC or DC, bodily area considered, stress, electrode size, size of the current and how long the current remains constant. Sweat glands will deposit sodium chloride (> 0.05 molar) that will greatly influence the measurement of skin resistance. 

Care of the Unit

From time to time, the sensor housing may need cleaning. Please use a gentle cloth. 


Please refrain from using plastic solvents such as acetone.


Do not store the unit long-term in a high humidity environment.


Refrain from stressing the unit outer casing beyond atmospheric pressure.


Carefully remove the electrodes when experiments are concluded.  


Please do not use the unit whilst connected to the skin and being powered by USB from a electrical mains-driven acquisition station.  


Please follow the guidance for the advice in Notices Section.