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Author
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Topic: Plethysmograph
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Barry C
Member
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posted 11-11-2005 03:49 PM
  
I just got a finger plethysmograph for my Lafayette 4000 system. I know how to score reactions, but I don't know all the technical aspects of the thing. (I just opened my latest APA Journal, and I'll be reading the reprinted article on the plethysomograph in just a few minutes - and some say there is no God!)When I tested it out (on myself) the time constant produced better looking readings on 5 seconds, but the very brief instructions said to set it to .1 second. (It has options of .1, 1 sec. and 5 sec.) Is there a way to easily determine what is optimal? What else do I need to know before I just start experimenting with it? IP: Logged |
J.B. McCloughan
Administrator
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posted 11-12-2005 12:34 AM
I would set it to the manufacture's recommended sample rate of .1.What are you using it for?
IP: Logged |
Barry C
Member
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posted 11-12-2005 07:35 AM
Lafayette just says to start there, but it doesn't say under what conditions to move it up to the next setting. I read the APA article, and I've got a better grip on it - I think, but I'm probably just going to have to try it out.I run Utah tests now, for the most part, and they use and score blood volume changes, which is what I'll be doing. IP: Logged |
Eric Fiander
Member
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posted 11-28-2005 09:26 AM
Hi Barry;
I have used the PLE for 20 years. I have experimented with the analog and computerized models, both.
The optimum setting, the one that produces the best and timliest tracing is in my opinion the .1 that is recommended. I have used all the others over the years but always come back to the comfort setting of .1IP: Logged |
Barry C
Member
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posted 11-28-2005 04:25 PM
Thanks Eric. I talked to Charles Honts, and he told me the other two settings are not needed, and to keep it at .1. The problem I'm finding is that I get great tracings on myself, but lousy tracings on real examinees.Lafayette has told me the device allows the finger to go in past an optimal point, so I've toyed with it somewhat. Today I tested a guy (pre-employment), and I tried evey micron of his finger as far a placement goes and the tracings stunk for the most part. I got tired of playing around with it after the first chart or so and decided to just see what happens. When the test was over, I kept the charts running, and when the guy relaxed (between charts) the tracings got better. During the test I was getting tracings that were about 1/4" in height (or smaller) at times - on the highest sensitivity! The guy's hand was cold (very nervous), which is common when I have these problems (almost 100% of the time). What can I do to get better tracings? I'm going to send Lafayette a copy of todays charts when I get a few minutes to see what they can offer. I think they did all their tests on somebody standing around the office and worked the sensitivity settings around that data. If I could turn it up beyond 10, I'd be all set. Do you use Lafayette or Stoelting? Any advice would be appreciated. IP: Logged |
Eric Fiander
Member
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posted 11-29-2005 06:59 AM
Hi Barry;
I use a Lafayette. My tracings generally run just over a 1/2 inch at a sensitivity setting of 4-5, using the middle finger of the left hand. I find that indviduals with callouses on their hands are a challenge. I usually sit the finger so that the end of the PLE lies over the first knuckle. I had the same problem in 1985 when I started using the device, but playing around with the placement showed me the optimum place to put it. Generally, its trial and error with the placement until you find the "sweet spot."
Speaking of Hontz,I ran into him at a conference on "Understanding the wrongfully convicted" last month. He is doing some interesting work on false confessions. I hope I'm still alive by the time he finishes his research!IP: Logged |
Barry C
Member
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posted 11-29-2005 08:22 AM
I'll keep trying. I'll try the middle finger too. I've been using the index finger. I can't remember if Honts told me to use it or if I read it in the Stoelting manual, but that's what I've been using. (I use Lafayette, but I have the Stoelting software, which discusses their PLE.)IP: Logged |
Eric Fiander
Member
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posted 11-29-2005 09:46 AM
At polygraph school, those many years ago, I was introduced to the PLE by Raskin. He was the one who suggested the middle finger and I've had great luck with it ever since.
Keep on trying, as I find it effective. I usually only get 2-3 scorable reactions from a few series of charts, but that's 2 or 3 points I wouldn't have had.IP: Logged |
Eric Fiander
Member
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posted 11-29-2005 09:49 AM
Barry;
Just a point that probably hasn't escaped you, but it still should be made. If you put the PLE on the same limb as the cardio cuff your tracing is severely diminished.
I put mine on the middle finger between the GSR spoons, opposite the cuff arm.IP: Logged |
Barry C
Member
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posted 11-29-2005 12:59 PM
I just ran a test, and I tried it on both the middle and index finger (of the non-cuff arm), and the tracings were about the same. I tried it on my middle finger, and they seemed to be slightly stronger. Once I got it set today, I never touched it between charts, and the tracings varied about 50% in size. They usually got small just as I said, "The test is about to begin...." Of course, I expect that, but they get too small at times to do anything with. I've got to look over the charts - I let them run a lot between tests. I'll keep playing with them, but I'm not sure it's me yet. Even as poor as the tracings are, I'm able to get a few points here and there, so maybe I'm being too picky? Thanks for the info. IP: Logged |
Barry C
Member
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posted 12-01-2005 08:23 PM
Okay, here's the update.I talked to Chris at Lafayette today, and he thinks I've got a bad PLE, so I might have been going crazy for nothing, but the upside is it will eventually work. Secondly, Lafayette is still working on the PLE sensor. It works as Eric has pointed out, but they're still trying to improve - what exactly I'm not sure. Lafayette is selling a lot of them, and the word is, the feds may be buying some, so there should be more and more out there soon. IP: Logged | |
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