EDA (Electrodermal Activity)

Electrodermal Activity : is a term used to describe changes in the skin's ability to conduct electricity.  It is a useful, fast, cheap, well established (since 1880's), low-tech and relatively non-invasive psychophysiological measure, which can be used to study many issues including: cognition, affect and individual differences.  It can also be used in ambulatory studies.  There are also many dependent measures that may be observed within EDA and these different aspects of the EDA appear to reflect different kinds of cognitive or affective response. For EDA a measurement is taken of the function of the eccrine sweat glands.  The function of these glands can be influenced by two things: 

"sympathetic innervation via acetylcholine and activity of myoepithelial cells which are controlled by the endocrine system via epinerphrine levels in the blood stream."

The preferred way of reporting EDA tends to be as a measure of conductance rather than resistance. This is because of the true nature of the skin (it is not a single resistor, but instead a series of resisters that act in parallel). Therefore, the most common unit of measurement in EDA is the microsiemen.

EDA is linked to changes in hydration in the sweat glands.  For example when the sweat glands fill with salt-water the skin then shows less resistance to electricity or conversely greater conductance.  

"Following sympathetic excitation of the eccrine gland sweat is forced into the sweat channel. Once enough sweat has passed into the channel, a valve at the top of this channel opens and sweat is released onto the skin surface.  Both an increase in the amount of sweat in the channel and an increase in the amount of sweat on the skin surface probably affect skin conductance."

There are various causes to account for fluctuations in skin hydration including: emotional sweating and thermoregulation.  The specific process by which this process of sweat release affects the measured EDA is not yet completely understood.

Ways in which EDA can be measured include: endosomatic measurement and exosomatic measurement.  Endosomatic measurement is the more invasive of the two techniques and involves microneurography, i.e., the application of tiny electrodes directly onto the 'sympathetic' skin neurons. This yields a direct measurement of the electrical activity of the skin's neurons.  This technique records both uniphasic and biphasic responses, making the data collected similar in type to that produced by an EEG. The exosomatic measurement involves two electrodes that are placed on the skin's surface and an electrical signal of tiny magnitude is passed over this surface between the two electrodes.  Although both AC and DC signals can be used DC is the most common.  It does not matter which type of current is used because the data is uniphastic (the current measured at the receptor is either increased or decreased as compared to the baseline).  Exosomatic measures are possibly just as good as endosomatic measures in that there is such a strong correlation between skin neuron firing rates and skin conductance.  The data that is used comprises the amount and pattern of electricity recorded at the receptor electrode. 

The size of the EDA response varies depending upon whether or not you are looking at tonic or phasic changes.

  • Tonic changes: 1-30 microsiemens
  • Phasic changes: .05 - 5 microsiemens

The rise time for the SCR tends to be between 1-2 seconds, the response peak tends to be at around 2-4 seconds and the rec t/2 tends to take about 4-8 seconds.

Following is a list (extract from page about EDA University of Kansas website), of Electrodermal dependent measures that can be recorded.  Like many other psychophysiological techniques, with EDA measures of general responsiveness can be obtained and related responses examined. 

General responsiveness:

  • we can look at general tonic levels of skin conductance as a way to compare different groups of individuals
  • we can calculate the average number of spontaneous fluctuations in an individual
  • we can compare levels of tonic or SF rate in a single individual across multiple long-lasting situations (for example, we could compare tonic EDA when a person is sleeping to when the person is awake).

Event related responses: phasic EDA that is shown after some specific event, to analyze this data we must "time-lock" the EDA data recording to the specific event

  • average response magnitude or amplitude
  • response peak (the size of the largest response recorded for any event)
  • response frequency (the average number of response over a specific period of time)
  • response probability (the likelihood of a response to a type of event)
  • latency measures
  • rise time (how long does it take to reach response peak)
  • recovery time or rec t/2 (how long it takes to return to 1/2 baseline)

 Recording EDA: polygraphy

There are not only lots of different ways to use EDA data and lots of different fluctuations in the EDA response that can be measured, it also the case that people record EDA and report EDA data in a number of different ways.

Different measures reported:

  • SCL or SRL
  • SCR or SRR
  • SF
  • NSSCR

In order to set up an EDA or polygraphy recording it is usual to place electrodes (generally silver-silver chloride electrodes), on the first and second fingers of either one or both hands. One electrode emits an electrical signal whilst the other primarily acts as a receiver, thus a circuit is established which passes across the skin.  The skin in turn acts like a series of resistors.  Electrodes are applied using a paste containing both a concentration of salt similar to the concentration found in sweat and silver chloride.  It is important to note that one hand can be more responsive than the other and the more responsive hand can be task specific.

An interesting alternative technique to the polygraph is the palmar sweat index or PSI. This is a measure of the number of active sweat glands. It is measured through the use of a chemically sensitive film that is pressed to the hand. This technique is even easier and more mobile than the polygraph and seems to correlate nicely with SCR.

CNS influence on EDA

Much research has been done to try to determine what brain structures influence the EDA response, including:

Comparative psychology animal data:

*  anterior hypothalamus (based on input from the amygdala)
*  frontal lobe (primarily prefrontal areas)
*  RAS

Patient population research:

*  ventromedial frontal cortex
*  anterior cingulate cortex
*  inferior parietal lobe

There is still debate surrounding the exact workings of the system.  However it has been suggested that one good model is the idea that there are three separate systems that influence the EDA.

Locomotor system:

*  controlling brain structures = frontal lobe motor processing areas and motor areas in the brain stem (medulla & pons)
*  function = an anticipatory increase in EDA that allows for a response to potentially dangerous stimuli that requires a motor response

Orienting-arousal system:

*  controlling brain structures = prefrontal areas of the frontal lobe, hippocampus, amygdala, RAS *  function = part of the orienting response produced by the limbic system that is seen in response to dangerous or novel stimuli

Thermoregulatory system:

*  controlling brain structures = hypothalamus
*  function = not so related to cognitive functions, a response by this system is generated either in order to maintain normal homeostasis or in response to physical trauma

Source: Information and extracts (in italics) for this article  - a page about EDA, University of Kansas.

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