EDA Analysis: Preprocessing, Decomposition, and SCR Detection • PhysioEDA

Introduction

PhysioEDA provides a complete pipeline for electrodermal activity (EDA) signal analysis built on the PhysioExperiment data model. This vignette walks through the core analysis workflow: generating or loading EDA data, preprocessing (filtering, artifact correction), tonic/phasic decomposition, SCR peak detection, and feature extraction.

EDA, also known as galvanic skin response (GSR), reflects sympathetic nervous system arousal through changes in skin conductance. The signal can be separated into a slowly varying tonic component (skin conductance level, SCL) and rapid phasic responses (skin conductance responses, SCR).

Creating or Loading EDA Data

PhysioEDA provides convenience functions for generating simulated EDA data. For real data, use the I/O functions from PhysioIO.

library(PhysioEDA)

# Generate a simulated EDA signal (10 minutes at 10 Hz, 1 channel)
x <- make_eda(n_time = 6000, n_channels = 1, sr = 10)
x

# Generate EDA with embedded stimulus events
x_event <- make_eda_with_scr(n_time = 6000, sr = 10, n_events = 4)

Signal Quality Assessment

Before processing, it is important to check signal quality. The edaQuality() function computes metrics including flatline detection, artifact percentage, signal-to-noise ratio, and an overall quality score.

quality <- edaQuality(x)
quality

The quality score ranges from 0 to 100. Signals scored as “good” (>= 70) can proceed directly to analysis, while “acceptable” (>= 40) signals may benefit from artifact correction. “Poor” signals (< 40) should be examined manually.

Preprocessing

Filtering

EDA signals are typically lowpass filtered to remove high-frequency noise. The edaFilter() function uses FFT-based filtering with smooth frequency-domain transitions.

# Lowpass filter at 1 Hz to remove high-frequency noise
x <- edaFilter(x, type = "lowpass", cutoff = 1.0)

# The filtered signal is stored in a new assay
SummarizedExperiment::assayNames(x)

Downsampling

If the original sampling rate is higher than needed (e.g., 256 Hz), downsample to a rate suitable for EDA analysis (typically 4-10 Hz). Anti-aliasing filtering is applied automatically.

# Downsample to 4 Hz (only needed if original rate is higher)
# x <- edaDownsample(x, target_sr = 4)

Artifact Detection and Correction

The edaArtifact() function detects artifacts using threshold, gradient, and flatline methods, and can correct them via linear interpolation.

# Detect and correct artifacts
x <- edaArtifact(x,
  methods = c("threshold", "gradient", "flatline"),
  threshold_range = c(0.001, 60),
  correct = "interpolate",
  output_assay = "cleaned"
)

# View artifact summary
S4Vectors::metadata(x)$eda_artifacts$summary

Tonic/Phasic Decomposition

The central step in EDA analysis is separating the signal into tonic (SCL) and phasic (SCR) components. PhysioEDA supports four decomposition methods.

Highpass Method

The simplest approach uses FFT-based frequency separation. Suitable for quick analyses when physiological accuracy of the tonic component is not critical.

x_hp <- edaDecompose(x, method = "highpass", cutoff = 0.05)

Median Filter Method

Uses a sliding median window to estimate the tonic component. More robust to outliers than the highpass method.

x_med <- edaDecompose(x, method = "median", window_sec = 4)

Continuous Decomposition Analysis (CDA)

Implements the method of Benedek and Kaernbach (2010). Deconvolves the signal with a biexponential (Bateman) impulse response function to recover the underlying sudomotor nerve activity driver signal.

x_cda <- edaDecompose(x, method = "cda", tau1 = 0.75, tau2 = 2.0)

cvxEDA

Implements a simplified version of the convex optimization approach by Greco et al. (2016). Uses iterative ADMM with Wiener deconvolution and L1 sparsity on the driver signal.

x_cvx <- edaDecompose(x, method = "cvxeda", tau1 = 0.75, tau2 = 2.0,
                       alpha = 0.01, gamma = 0.1)

Visualizing the Decomposition

plotDecompose(x_cda, channel = 1)

SCR Peak Detection

After decomposition, SCR peaks can be detected in the phasic component using either a gradient-based or amplitude threshold method.

# Detect peaks using the gradient (zero-crossing) method
peaks <- edaPeaks(x_cda, method = "gradient", amplitude_min = 0.01)
peaks

# Visualize detected peaks
plotPeaks(x_cda, peaks = peaks, channel = 1)

Feature Extraction

The edaFeatures() function computes summary statistics per channel, including SCR count, rate, mean amplitude, mean SCL, and area under the phasic curve.

features <- edaFeatures(x_cda)
features

You can also restrict feature extraction to a specific time window:

features_win <- edaFeatures(x_cda, window = c(100, 400))
features_win

Data Transformation

For statistical analyses that assume normality, EDA data can be transformed using log, square root, z-score, or range normalization.

# Log transform to reduce skewness
x_log <- edaTransform(x, method = "log")

# Z-score standardization
x_z <- edaTransform(x, method = "zscore")

# Reverse the transformation
x_orig <- edaUntransform(x_log)

Plotting the Raw Signal

# Plot the full EDA time series
plotEda(x, main = "Raw EDA Signal")

# Plot a specific time window
plotEda(x, time_range = c(100, 300), main = "EDA Signal (100-300 s)")

Complete Pipeline Example

library(PhysioEDA)

# Step 1: Load or simulate data
x <- make_eda(n_time = 6000, sr = 10)

# Step 2: Check quality
quality <- edaQuality(x)

# Step 3: Artifact correction
x <- edaArtifact(x, correct = "interpolate")

# Step 4: Decompose into tonic and phasic
x <- edaDecompose(x, method = "cda")

# Step 5: Detect SCR peaks
peaks <- edaPeaks(x)

# Step 6: Extract features
features <- edaFeatures(x, peaks = peaks)

# Step 7: Visualize
plotDecompose(x, channel = 1)
plotPeaks(x, peaks = peaks, channel = 1)

References

Boucsein, W. (2012). Electrodermal Activity. 2nd ed. Springer.
Benedek, M., & Kaernbach, C. (2010). A continuous measure of phasic electrodermal activity. Journal of Neuroscience Methods, 190(1), 80-91.
Greco, A., et al. (2016). cvxEDA: A convex optimization approach to electrodermal activity processing. IEEE Transactions on Biomedical Engineering, 63(4), 797-804.

Session Info

sessionInfo()