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Galvanic Skin Response Sensor:
Electrodermal Response EDR is actually the medically preferred term for changing of electrical skin resistance due to psychological condition.  The term Galvanic Skin Response or GSR remarkably dates back to about 1900.  The change is caused by the degree to which a person’s sweat glands are active.  Psychological stress tends to make the glands more active and this lowers the skin's resistance.

You can't get more simple than the Galvanic Skin Response GSR sensor. It is just a cut 9V LEGO motor wire and some aluminum foil wrapped around your fingers with tape. I was inspired by talks by Mindfest panelists Karen Wilkinson and Mike Petrich who talked about using this type of sensor. I've also found out that the Media Lab at MIT has a program called the Affective Computing Research Project that also uses this sensor.

It is popularly known as a lie detector or psychogalvanometer, but is also used in Biofeedback conditioning. The theory is that; the more relaxed you are, the dryer your skin will be and so the higher the skin’s electrical resistance. When you are under stress your hand sweats and then the resistance goes down. The range is reported to be 5K to 25K Ohms, but I'm not sure where the measurement was taken.


If you are using the newer LEGO MINDSTORMS NXT, then I suggest you buy a NXT converter cable from LEGO Education Store and simply cut the RCX connnector off.  The two wires are electrically equivalent to the old RCX sensor signals.  I tend to get RAW sensor values in the range of 1000 to 850.
The magazine Electronics Now had an nice article on building a Biofeedback Monitor in the December 1996 issue. I have modified their plans for finger probes slightly. You use "Sticky Back Velcro" and Aluminum Foil to make them. An assembly diagram and photo of the finished product are shown below. I used the Loop side of the Velcro (looks like carpet) for the part that wraps around the finger, and small square of the Hook side (stiffer plastic) to act as a catch. The stripped and frayed end of the LEGO connector wire is sandwiched between the Loop sticky side and the Aluminum Foil that covers the rest of the probe.  These are known as a dry type electrodes.  Professional  electrodes would employ a conductive paste that would make the readings more stable and repeatable.
Radio Shack sold a Lie Detector Kit (left) that used similar dry electrodes.  The great thing about the instruction manual for this product is that it included 18 physiological and psychological experiments and are repeated below.  All of these experiments could be carried out using the MINDSTORMS version.  I think this same basic product was also sold for Biofeedback (right) and you can still get the instructions for that product from the Radio Shack support page.
Experiments from the Science Fair Electronic Lie Detector Project Kit copyright 1970 by Radio Shack.

PHYSIOLOGICAL EXPERIMENTS

EXPERIMENT 1 - Breathing
Ask the subject to take a very deep breath. You will observe the sound increases in pitch. This indicates his skin resistance has dropped due to the activity of the sweat glands. The slight muscular effort of taking a deep breath causes the sympathetic nervous system to send impulses to the sweat glands which in turn reduces the skin resistance. The change in skin resistance is called a GALVANIC SKIN RESPONSE. It is usually abbreviated to GSR by psychologists. If you are acting as the subject, can you detect your normal breathing cycles?
There are great individual differences which will be observed in these experiments. Some people produce large responses just by talking. Conversely, others will produce only a slight response even it they are kicked in the shins.

EXPERIMENT 2 - Muscle Tension
Direct the subject to squeeze a rubber ball hard for several seconds, or curl his toes and release them. Next, tense the stomach muscles, or same similar activity requiring muscular exertion.  Which activity produces the most response? In doing these experiments the subject should keep the hand with the electrodes very still if possible. Allow at least 15 to 20 seconds for rest between tests

EXPERIMENT 3 - The Effect of Temperature on the Galvanic Skin Response
Request the subject to stand outside for a few minutes if it is cold and repeat experiment 1. What results do you get? Give him a few minutes to warm up and try the experiment again. Are the results any different? If possible have the subject remain in a very warm place for a while and try Experiment 1 on him. Have him hold some ice cubes for a short interval; re-test him.

EXPERIMENT 4 - Position of the Electrodes
By now you have probably discovered that a deep breath will usually give a good galvanic response. Try placing one electrode on one hand and one electrode on the other hand. Do you notice any difference? If you are careful, you can try holding the electrodes firmly to other places of the skin. Can you still get good results? As a result of this experiment, what conclusions con you draw about the distribution of sweat glands in the skin?

EXPERIMENT 5 - Experiments With More Than One Person
Put one electrode on one person and the other electrode on another person. Have the two people hold each others hands which are not connected to the electrodes. Try any of the previous experiments on each person separately and on both together. Or place several persons in a chain with the electrodes at the end of the chain, and try experiments on these subjects tied together in series like batteries or other electric elements.

EXPERIMENT 6 - The Effects of Relaxation
Place the electrodes on a subject as previously described. Instruct him to relax as much as possible. Have him shut his eyes and sit or lie quietly. What happens? When a person relaxes his skin resistance increases The instrument reflects this change. Perhaps you can see how a device might be made which would wake a person up whenever he fell asleep.

EXPERIMENT 7 - The Effects of Pain
Ask the subject to pinch himself or bite his lip or finger. Any painful stimulus will produce large galvanic skin responses.

PSYCHOLOGICAL EXPERIMENTS
In general the instrument will indicate strong, pleasant or unpleasant emotions. It will also react when a subject is startled, under tension, nervous, or exerting unusual mental effort. The experiments outlined below are just some of the many possible ones that can be tried

EXPERIMENT 8 - The Startle Response
When the electrodes have been placed on a subject and he has remained quiet for 30 seconds, suddenly clap your hands loudly together, or strike your fist on the table. You will observe a large galvanic skin response.
You will notice that the instrument does not respond until some time after the strong stimulus. This time delay is typical with the galvanic skin response and is designated as the LATENCY of the response. It may vary from one to three seconds. Part of the reason for the time delay or latency of response is the addition of a chemical reaction with the electrical reaction.  A chemical reaction is usually slower in response than electrical reactions.
Try different methods of startling the subject, such as a flash of light, a slight electric shock. etc.  A pronounced reaction may result from a very mild stimulus.

EXPERIMENT 9 - Adaptation and Recovery
Select a standard method of startling the subject which can be regulated and repeated at approximately the same strength every time. Repeat this stimulus at intervals of not less than 30 seconds. What happens to the galvanic skin response? You will note that they rapidly decrease until practically no response can be observed. This reaction is called ADAPTATION by psychologists
Next, do not startle the subject for several minutes. It is advisable to loosen or remove the electrodes occasionally to allow the blood to flow and the skin to breathe.
Suddenly repeat the startle stimulus to the subject. You will probably observe a large galvanic skin response. This is called RECOVERY. How many trials does it take for adaptation to re-occur now as compared with the first time?

EXPERIMENT 10 - Word Association Tests
Most sensory stimuli so far considered have acted directly, or nearly so, on the nervous system to produce a galvanic skin response. If abstract stimuli such as words are presented to the subject, the effect on the nervous system is usually indirect and due to the meanings associated with the words. Pictures or music may also produce galvanic skin response.
Psychologists have often tested the effect of words on subjects by presenting them one at a time and noting their reactions. There ore some words which nearly all people will react to and others which will produce no noticeable reaction. Below is a list of words which you con read to a subject and observe his galvanic skin responses. Read the words slowly and clearly. Pause at least three seconds between each word to allow time for a reaction to occur; otherwise you will not be able to tell which words are associated with large responses. The words with the asterisk have been found to produce galvanic skin responses in most subjects. You can make up your own:

carrot   kiss*   can   white   give   ball   gross   subject’s name*
paper   woman*   sand   mother*   pencil   love*
read   afraid*   walk   proud*   flower   fry   pond   marry*

You can easily observe an adaptation effect by repeating those words to which the subject gave large responses.

EXPERIMENT 11 - Apprehension
Most subjects will give a galvanic skin response if they are apprehensive or if they expect that something is about to happen to them. You may startle a subject by a loud noise or a sudden light blow, or you may pinch him but all this is not necessary. Merely remark casually to on onlooker - “Let’s see what a good pinch will do.” and reach out a hand toward the subject. This will often give nearly as large o response as on actual pinch. You may pretend to touch electric wires to the subject to give him a shock and also obtain a galvanic skin response.
These results depend on many factors, such as your personality, the subject’s personality, how well the subject knows you, how many people are present and how many of them are strangers to the subject. It is difficult to control all these factors and you should not be surprised if you do not obtain consistent results.

EXPERIMENT 12 - The Effect of Touch
It is possible to use very weak stimuli with a subject who is apprehensive or otherwise a bit frightened of this instrument. Merely reach out and touch the subject lightly on the hand. You may after, obtain a large galvanic skin response with this very slight stimulus. How do you account for this?

EXPERIMENT 13 - Mental Effort
Any difficult mental task will often produce a galvanic skin response. Ask a subject to solve an arithmetic problem in his head such as, what does 6+3-5 x 4+7+836 = ? You may also try asking him to solve a difficult problem, define a hard word, spell a difficult word, read small print at a great distance, etc.

EXPERIMENT 14 - A Game
Ask the subject to write a number from 0 to 10 on a piece of paper. No one should watch him or look at the number. Then make repeated guesses of the number. When you guess the correct number, the instrument should show it. The subject may try to fool the device by laughing, tensing, coughing, or relaxing as much as possible,
You may try other variations of this by using playing cords or by having the subject hide an abject while others are out of the room. The subject may either deny everything or may say nothing

EXPERIMENT 15 - A Lie Detector
Ask the subject a few simple questions for which you know the correct answer, and observe his response. A lie will be indicated by a galvanic skin response - a lowering of skin resistance. This will occur from one to four seconds after the answer has been given. For this reason, it is important to space the questions at least ten seconds apart.  A simple question such as “how tall are you?” will usually produce a large response it the person lies about it Sometimes a response will be observed before the question is actually asked. This is a result of the emotional activity which the person undergoes when he anticipates a revealing question.
One simple procedure is to show the subject a playing card and ask him to name it. After a few preliminary trials where he tells the truth (by request) he may then be permitted to lie. The first lie will probably be clearly indicated.  After catching a subject once in a lie, drop this method because most people quickly became practiced at lying about such a simple thing. A similar technique can be attempted by using a coin and asking the subject to name it after it is flipped and shown to him
Note:  People vary greatly in their responses.  Some are very practiced liars, and a simple, non-emotional or unimportant lie can readily be told without being detected by this device. Also, nearly everyone can learn, in the process described above, to lie in a relaxed manner. These factors require that the operator be both skilled and practiced in the measurement of the galvanic skin response

EXPERIMENT 16 - Conditioning
This is a difficult experiment to do, even for professional psychologists, so do not be concerned if you do not obtain good results. Some subjects are better for this experiment than others and you may have to try two or three.
Instruct the subject to assume a relaxed position. Get two stimuli ready for presentation to the subject.  One of the stimuli should be very weak so that by itself it will not produce a galvanic skin response. This may be a small light, a quiet doorbell, or a spoken word. The second stimulus should be strong enough to definitely produce a good galvanic skin response. It may be a very loud sound, such as that made by a hammer on a pan, or it may be a light blow on the arm or a pinch. etc.
First demonstrate that the weak stimulus alone produces no galvanic skin response. Then present both stimuli at nearly the same time to the subject. (You may need two people to do this.) For best results the second, strong stimulus should be given about one-half second after the weak stimulus. Repeat this at intervals of about thirty seconds for three to six times. Then, without forewarning the subject, do not give him the second, strong stimulus. Do you observe that he gives a good galvanic skin response anyway? If he does, he is said to be CONDITIONED. He has responded to the weak stimulus as if it were the strong stimulus. He is apparently unable to distinguish the two stimuli. The response resulting from the weak stimulus which formerly did not produce a response is said to be a CONDITIONED RESPONSE.

EXPERIMENT 17 - Extinction of a Conditioned Response
Repeat Experiment 16 to obtain a strong conditioned response. His conditioned response will probably be less.  Repeat the weak stimulus again at intervals and you will observe that his galvanic skin response will gradually decrease and soon became unobservable. (This is similar to the adaptation experiment.) This process is called EXTINCTION.

EXPERIMENT 18 - Spontaneous Recovery
Repeat Experiment 17. Give the subject a ten or fifteen minute rest and then attach the electrodes to him again. Do not tell him what you are going to do, and present to him the first weak stimulus again. You may observe that he again gives a conditioned response. This is called SPONTANEOUS RECOVERY. The initial weak stimulus has again recovered its power to produce a strong response. You can again extinguish this spontaneous recovery and after a further rest observe a second recovery. You can recondition him at any time by presenting both stimuli together as in Experiment 16. Try variations of this experiment and others described in this manual and see what you can discover Psychology is still an open science.

GSR References:

Schwartz, Mark S., and Andrasik, Frank,
Biofeedback, A Practitioner’s Guide, Guilford Press.

The GSR2 is a product that has been around since the late 70s.  Over 500,000 were sold through the Sharper Image store alone.  It also uses dry electrodes, and  I'm guessing they are either chrome covered brass or stainless steel.  It works by simply laying two fingers across the electrodes.   I suspect that the pressure with which you press against the electrodes also effects the measurement.   However, if you don't want to build your own, this is about the least expensive alternative.
Below is a very simple NXT program that produces a tone whose frequency corresponds to skin resistance.  The greater the skin resistance, the greater the RAW sensor value.  Higher skin resistance indicates a more relaxed condition, so this produces a lower frequency tone.  Sweating lowers the skin resistance and in turn produces a lower RAW sensor value.  This then produces a higher frequency tone.  If the electrodes are not connected, the RAW value is 1023 which results in a negative frequency and the NXT produces no sound.  Click HERE to download a more complex version that displays the RAW sensor value and allows you to turn the sound on and off
MIT Media Affective Computing Project

Galvanic Skin Response Meter

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I also wrote a simple data logging program that just records the RAW sensor value every 10 seconds into a file.  The file is then uploaded from the NXT to the PC and plotted using Excel.  Below is a graph of my GSR while playing an intense video game.  Lower values indicate lower skin resistance and hence higher stress.  Near the end I was trapped in a dark room and was being munched to death by aliens.  The rapid drop in skin resistance reflects the increased tension of being killed.  I kept recording for a few minutes after the game to capture the emotional recovery.