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Author
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Topic: Plunging EDR
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Fed Employee
Member
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posted 08-29-2008 11:03 AM
  
Had an exam involving a woman suspected in a theft of $. I use the LX 4000 and decided to run an AFMGQT. First chart, minimal EDR responses, excellent cardio and pneumos. Chart two, EDR starts plunging. Changed the position of the plates, nothing. Had her wash her hands with warm water - no change. Switched to the EDR pads - no change. Ran a total of four charts. Manual scoring, I have her NDI since cardio and pneumos are kicking in on the controls. Ran Polyscore....DI. Used Polyscore without EDR, INC. Comments, suggestions IP: Logged |
rnelson
Member
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posted 08-29-2008 11:52 AM
It is my suspicion that there is still a lot that we don't know about EDA measurement.Physiologically, a change in EDA data should represent a change in activity. Explanations about sweating seem inadequate, when you can lightly touch the arm of a subject attached to the EDA, and prompt large reactions (which are not from sweating). Explanations about cholinergic activity in the postganglionic sympathetic neurons in the skin are more parsimonious with the range of phenomena observed. Plunging EDA is troubling, because we are asked to imagine that data to represent a continuous change in either resistance or conductance. However, the continuous duration of the plunging condition casts doubt on that assumption. It seems possible that plunging EDA conditions are caused by some neurophysiological condition that is being recorded without our understanding or consent. It is interesting that we observe more problems with this on some instruments. That suggests that certain circuit designs are more or less vulnerable to the phenomena. It is possible to mechanically replicate a conductance or resistance effect to test an EDA circuit. Has anyone ever tried to create a circuit that could replicate plunging EDA? I wonder if some engineer somewhere already knows the answer.
r
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"Gentlemen, you can't fight in here. This is the war room."
--(Stanley Kubrick/Peter Sellers - Dr. Strangelove, 1964)
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ckieso
Member
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posted 08-29-2008 12:19 PM
I also use the LX-4000. I have occasionally had this problem as well. For the majority of my exams I have beneficial data that is obtained by the EDA. However, occasionally I have an examinee, that even when I adjust the electrodes, have them wash their hands, etc. there simply is no EDA response. In those cases I use the Pneumo and cardio channels to make a decision. I am not sure what would cause no response in the EDA and am as puzzled as you are. ------------------
"Truth Seekers" [This message has been edited by ckieso (edited 08-29-2008).] IP: Logged |
ebvan
Member
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posted 08-29-2008 01:49 PM
If it happens on the next subject, it is your instrument. I had this happen years ago when I was using an analog instrument
Try swapping the plates from lead to lead.If it is the subject, you might try different locations for the plates, 1st and 4th finger, different hand, etc. I like the adhesive pads that I bought from Limestone they really improved my EDR tracings in all aspects. ------------------
Ex scientia veritas IP: Logged |
Barry C
Member
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posted 08-29-2008 02:41 PM
I routinely had problems with a plunging baseline with the LX4000. That is one of the reasons they came up with the detrending feature. Record both and see what you get. I think it's the circuit. I don't see it half as much with the Stoelting. I think Lafayette is still recording skin resistance, which Honts showed works, but it requires centering about twice as often as skin conductance.One of these days I want to compare instruments, but I haven't found the time. Stoelting records skin potential as well as SC, and even when I see a dropping SC tracing, I still get good SP tracings. Sometimes a dropping tracing means the subject is out of the range of the circuit / software. Putting the electrodes closer or farther apart (depending on which side of "out" the signal is) might remedy the problem. IP: Logged |
Taylor
Member
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posted 08-30-2008 10:21 AM
You said she was flat lined on the first chart. Was she on medications?IP: Logged |
rnelson
Member
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posted 08-30-2008 11:01 AM
quote:
Sometimes a dropping tracing means the subject is out of the range of the circuit / software. Putting the electrodes closer or farther apart (depending on which side of "out" the signal is) might remedy the problem.
That is what I was trying to get at, but you've said it better. Plunging EDA does not mean that someones resistance or conductance is actually plunging. It's more likely that plunging data illustrate some other condition that we are not fully aware of. Limestone has a nice on-screen ohm-meter. It's a gizmo, but helps to understand the data. What do we know about the range for the Lafayette EDA? Limestone used to be 100 to 9meg. Perhaps its time to hear from the instrument manufacturers on this. r ------------------
"Gentlemen, you can't fight in here. This is the war room."
--(Stanley Kubrick/Peter Sellers - Dr. Strangelove, 1964)
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Fed Employee
Member
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posted 08-30-2008 03:48 PM
During the first chart I had some EDR, but it was minimal, at best. The next couple of charts were "down hill." To my knowledge the examinee was not on medication. There was no problem with the next examinee. The EDR worked like a charm. I'll try the detrending feature. ThanksIP: Logged |
pal_karcsi
Member
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posted 08-30-2008 09:51 PM
I have the same problem with a woman.
Possible causes : the subject doesn´t care about the evaluation , is bored , tired , hungry or as someone said her EDR values are out of range. I understand the LX 4000 EDR range are 100 to 2 megas.
Maybe here the answer.
More research is needed in this area.
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Mad Dog
Member
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posted 09-02-2008 06:47 AM
The LX 400 uses a constant current circuit and thus measures resistance as is displayed on the chart screen in Ohms. I think they limit the current to 10 micro amps. I have attached the electrodes to several people who are known "plungers" and observed their tonic level resistance steadily increase in a relatively stimulus free environment. It is if the particular person's skin is acting as a capacitor and building a charge. These people are quite capable of producing a phasic response and then repeat the increasing tonic resistance cycle.Lafayette uses the services of a small group of brilliant engineers in Ohio. They developed the detrending filter to address this phenomenon (which appears to be a combination of the measuring model and certain individual's skin property). The filter constantly measure slower wave changes and adds a correction to maintain a neutral slope baseline. Essentially what it does is correct for constant tonic changes by using a short term and long term review of data input. Essentially it is a software fix to a combined hardware and physiological phenomenon. Constant current models and constant voltage models have advantages and disadvantages. Professor Boucsein reports several studies comparing the two methods and reports the method of measurement is not as important as the units in which they are reported. He is referring to scientists interested in standardized reporting units without having to convert units. Next time you have such a person try attaching the electrodes and monitor the Ohmic display. I am confident you will see the numbers steadily increasing and the tracing continues to plunge. For now, the fix is the detrending filter. We only score phasic changes anyway. cheers
mark handler IP: Logged |
Barry C
Member
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posted 09-02-2008 08:06 AM
Keep in mind Lafayette recommends you record both - since the detrend filter can, sometimes, filter out a reaction, or a portion of a reaction. You should use the "raw" as a means to determine what you are (or aren't) seeing with the filtered "detrended" tracing.IP: Logged |
Poly761
Member
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posted 09-02-2008 12:18 PM
Anyone know of any valid studies/research that indicate the race of an examinee (may) affect the EDA?END.....
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Barry C
Member
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posted 09-02-2008 05:12 PM
What do you mean? EDA has been studied extensively, and I think race has been a variable, but what do you mean as far as polygraph is concerned? Even if one race reacts more or less than another (and I don't know that is the case), we're still looking at differential reactivity.IP: Logged |
Barry C
Member
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posted 09-02-2008 05:15 PM
Okay, I Googled it. Do the same and you'll get dozens of hits. Here's a link to the first abstract:
http://www3.interscience.wiley.com/journal/119675380/abstract?CRETRY=1&SRETRY=0
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detector
Administrator
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posted 09-02-2008 05:54 PM
Hey Barry,I'm just guessing, but I think poly may be referring african americans tending to show less eda response on a whole than other races. Although its anecdotal, when I was in polygraph school, there were about ten students, of which two were black, one male and one female. In school, everyone tests everyone multiple times and there were consistent issues with those two students showing no or very little reaction on eda regardless of placement of the leads. We finally brought this up to chuck and he stated that in his experience, there seemed to be something to that theory. by the way, that link didn't work for me because I think the link line contains specific data for your session. ------------------
Ralph Hilliard
PolygraphPlace Owner & Operator Be sure to visit our new store for all things Polygraph Related
http://store.polygraphplace.com IP: Logged |
Barry C
Member
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posted 09-02-2008 06:16 PM
The link, which should have worked, confirmed your findings.I googled "skin conductance race" to get it. IP: Logged |
Barry C
Member
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posted 09-02-2008 06:34 PM
Okay, I got it too. Some type of cookie error. They must try to track how you get to them.IP: Logged |
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