Signal detection
In this example, we are performing the EDA analysis on the Stroop task. There are 3 focus areas:
Color naming
Word reading
Color-word interference
To start, go to Analysis → Electrodermal activity → Preferences
In the preferences box, use the following settings:
THE SCR THRESHOLD LEVEL FROM THE BIOPAC CONFERENCE WAS 0.02; they recommend this but the threshold can also be lower.
BIOPAC ACTUALLY RECOMMENDS THE PHASIC EDA CONSTRUCTION CALLED “SMOOTHING BASELINE REMOVAL“
→ SECOND CHOICE IS HIGH PASS FILTER
Go back to Analysis → Electrodermal activity → Construct phasic EDA from tonic
If you see the following error, you need to adjust the threshold:
ADD SCREENSHOT OF THE SIGNAL WITH MARKERS
EPOCH WIDTH IS A CHOICE. NEED TO FIND OUT WHAT CHOICE IS THE BEST.
Epoch analysis
Event type: EDA → Skin Conductance Response
EPOCH WIDTH OF 10 SEC SOUNDS CORRECT, THAT’S WHAT BIOPAC SUGGESTED
In the next window, you can the select what measures you want exported: Event count: how many SCRs were found in the focus area Value: the signal value at the marker location (or epoch onset location); this should theoretically be the same as Maximum Maximum: peak value of the SCR Minimum: this is important! You cannot interpret the amplitude of the SCR without considering the baseline! For example, your baseline might be 8.5 and the maximum 10.8 microsiemens. 10.8 microsiemens is NOT the amplitiude of the SCR! the amplitude is 10.8 minus 8.5 microsiemens. So we have to either request an amplitude measurement or subtract the minimum from the maximum. |
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The output will look like this:
Subtract Minimum from Maximum to compute the SCR amplitude.
Each epoch is one SCR and the values you requested in the previous step are listed in order.
WHY DOES IT SAY EVENT COUNT 2 for epoch 2?
Find cycle analysis (alternative)
This analysis is part of a Biopac video tutorial. What it does is, it asks you to select an area around the SCR (from the onset to the peak but that can be widened) and then it exports summary measures for this selected period. The problem with that is that we do not care about the mean and standard deviation of the signal between onset and peak of the SCRs. Using the minimum, maximum and number of SCRs is still useful. Instead of this analysis, I recommend the epoch analysis.
Adjust the Measurements pane as follows:
Click the cogwheels icon, click Settings → Measurements → change the numbers to 3 rows by 5 columns.
Then, select the EDA channel, such that the small drop-downs to the left switch to the EDA channel and select the following measurements.
When you select Event amplitude, the following window pops up. We select “Amplitude at first event only”. This might cause the event amplitude and count to be zero.
When you select Event count, select where you want to look for the events and select your EDA channel that contains the water droplet markers.
For Event count, use the following:
Once your measurement list is complete, click the down-arrow again and then New Preset. You can name your measurement preset and use it easily for each subject.
Next, we export all these measurements for each SCR into an excel spreadsheet.
Analysis → Find cycle
Tab Cycles/Peaks: Select Events
Start event: General → Waveform onset on your EDA channel
End event: EDA → Skin conductance response; located on EDA channel
Selection tab:
Starting event: -3
Ending event -1
Adjust if necessary; you want the onset of the EDA response (i.e. the dips before each rise) to be included in the selection (blue range).
Output tab: check “Save measurements into Excel spreadsheet file” and click “Find all in Focus Areas”.
The output looks as follows:
