EDA cleaning and options

There are 2 types of EDA:

·      Tonic: absolute signal coming from the electrode

·      Phasic: transformation of tonic waveform applied to events → proper filtering of waveform important to get a proper phasic signal

Detailed background and analysis details can be found here, please read!

Here Biopac EDA presentation slides including setup, data quality considerations and further information:

Biopac EDA presentation slides

In order to establish a baseline against which responses will be compared/measured, we need a time window of resting right before responses start. Below you see the “EDA Preferences” window (Analysis → Electrodermal activity → Preferences) and the software asks how many seconds (or desired time unit) you want to use to establish the baseline. Let’s say, we are looking at the Stroop task (see below): instead of having regular individual events, we are looking at time periods that last up to several minutes. You can see that this person’s EDA starts to peak even before the beginning of the first Stroop block. Therefore, we need the time scale to start at least one minute, if possible more, before the task begins.

Open

The EDA signal increases even before the beginning of the first block, i.e. the focus area. We therefore have to make sure we capture the signal at rest.

Processing

·      The little ‘I’ (info) button on the right hand bar of a channel is not there right after data collection, it only appears after something has been done to the data. You can check what has been done. This one shows that the data has already been resampled from 2,000Hz to 100Hz (Biopac samples everything at 2,000Hz by default)

·      remove high-frequency noise, happens if contact between electrode and skin is poor

·      FIR: finite-impulse response à use this one, nice and sharp cutoff without much data distortion

·      IIR: … impulse response

·      Usually creates a new channel with filtered data to preserve original data but this can be turned off if desired

Preferences

·      The first time you use the EDA option, you are asked to set your preferences; set those and then use these presets for ALL participants!

o   Recommended option: “Smoothing baseline removal”

o   Second option: high-pass filter →

·      Stick with smoothing baseline, high-pass filter can sometimes take a while to settle down

·      Threshold as low as you can to identify stimulus responses; recommended .02 micro siemens but can be lower

o   Baseline: baseline = the moment of stimulus onset and then compared to peak → baseline can shift over time

o   Opening parentheses “(“: beginning of SCR crossing our .02 threshold; as the signal on the phasic waveform drops down, it adds a closing parenthesis “)”

o   If you click on one, it places the cursor at the exact position of threshold crossing

o   Check lightbulbs: timing when stimuli were presented and investigating signal there

o   Channel 1 tonic; after SCR analysis, the phasic waveform was created (here channel 5)

o   A response needs to happen AFTER the stimulus presentation; at least 1 second (signal transmission) and less than 4 seconds. If you see an immediate response, that is likely due to something that happened previously; i.e. exclude responses too close (minimum) or too far away (max) from your stimulus presentation

o   Using the time period option, we get a summary of all different types of analyses; by event would be better for block design

Summary measures

o   How many responses occur within an area?

o   Select area; use channel info bar and select Event count → number shown is number of events (droplets)

o   Can also do amplitude (mean amplitude)

o   Can also use find cycle or even better: Analysis → Epoch analysis