What Does This Channel
Measure?
EDA measures the subject’s skin conductance (sweat gland activity,
sometimes called galvanic skin response). ANSLAB identifies
significant changes in skin conductance level – defined as
drops or rises greater than .02 microSiemens above a zero slope
baseline within a
defined time interval. These are called SCRs (skin conductance
responses) and depending on the experimental context can be specific or
non-specific (without known stimulus). SCRs can be mapped to
experimental events to measure subject reactivity to various
stimuli. Non-specific SCRs can be counted per unit
of time (e.g., 1 min), and are called
non-specific fluctuations (NSFs).
ANSLAB displays five windows for EDA. Figure 2, “Detected SCR
onsets” labels all qualifying onsets (baseline
points) of specific or nonspecific SCRs with small green X’s. Figure 4
displays bivariate distributions of SCR pattern characteristics.
Outliers typically identify SCRs that are due to technical or movement
artifact. These need to be inspected and excluded, if necessary.
The following variables are extracted from the electrodermal
activity channel:
What Kinds of Artifacts are
Common in this Channel?
ANSLAB almost always correctly identifies significant SCRs, so this
channel does not require heavy editing. The most common artifacts
are technical errors in hookup or data collection. These
artifacts will appear as large spikes in the EDA graph:
Another irregularity that must be noted but cannot be edited is the
nonresponder or person with little electrodermal lability. A
small percentage of the general population do not
respond electrodermally and will produce a near flat line in figure 2
of the editing windows. A flatliner can be recognized by
restricted range on the vertical axis and the
lack of identified changes >.02 uS. One needs to decide a-priori
whether to
exclude the data from these analyses (typically data is included).
Finally, ANSLAB will occasionally fail to identify significant changes
in
SCL or will ‘misidentify’ non-significant changes. Scan the graph
and note on the vertical axis the height of all rises and dips.
If you suspect that ANSLAB has made a mistake, follow the instructions
below to add or remove X’s.
How Are Artifacts Removed?
You mainly need to look out for movement artifacts that become apparent
in the bivariate plots, or for instances when the program has missed or
misdetected an SCR. Before every wave (>0.02 uS) in the signal
there should be a green cross marking that the program has detected
this event. If there is none, it might be that the rise was too
slowly or because the amplitude was less than 0.02 uS, or the signal
was very noisy at this point. If you don't agree, you need to select
“editing of SCR
onset”. Then, you can mark a suspicious period by clicking
twice in the signal, and then hitting “0”. Now you are in
the editing window, and if you type “0” again, you’ll get directly to
the marked interval. You will then be able to insert events
(onsets of SCRs) with
<@> and a mouseclick in the area you want to insert the
event. You can deleted events with <#> and clicking near the
event.
Another way of editing many events quickly is to choose in the menu
“exclude SCR
onset”. You will then be able to draw an exclusion box. Or you
can select “exclude event” on the menu. You can then
type in the number of the SCR you want to exclude. If you can’t
see if the wave is higher than 0.02 mS, you can measure it by typing
<shift-d>, and then click on the minimum and maximum of that
wave. The Y-value is the one that should be larger than 0.02 for
there to be a valid SCR. The X-value (time) should be within the
qualifying time window. You can repeat these measurements. After you're
done, hit <esc> twice.
To exclude a segment that is clearly a technical artifact, mark the
area by clicking twice in the signal, and then hitting “0”.
Type “0” again to display the marked interval. Mark the beginning
of the artifact by typing ‘b’ and clicking just before the
artifact. Mark the end of the artifact by typing ‘e’ and clicking
on the other side of the artifact. Two lines should be drawn
through the artifact interval showing you what data you are
excluding.
When you are done, select “exit editing” and then select "Save reduced
data (existing files will be overwritten)" if you want to save your
changes.
What Qualities Must Be
Preserved In Editing?
The most important editing goal is to remove obvious technical
artifacts that distort a mean. The second goal is to identify
electrodermally stable individuals so that they can be considered
separately or in some cases excluded from
data analysis.