Clinical EEG Analysis with PhysioEEG

Clinical EEG Analysis with PhysioEEG

This vignette demonstrates the clinical EEG analysis capabilities of PhysioEEG, covering epileptic spike detection, quantitative EEG (QEEG) band power analysis, hemispheric asymmetry indices, burst-suppression detection, and EEG slowing assessment. These tools are designed to assist clinical neurophysiologists in routine EEG interpretation but should always be used in conjunction with clinical judgment.

Setup

Load PhysioEEG and generate simulated clinical EEG data containing known epileptiform discharges:

library(PhysioEEG)

# Create 60 seconds of 19-channel EEG with 15 embedded spikes
pe <- make_eeg_spikes(n_time = 30000, n_channels = 19, sr = 500, n_spikes = 15)

# Inspect the data
dim(SummarizedExperiment::assay(pe, "raw"))
# [1] 30000    19

# Ground-truth spike locations are stored in metadata
spike_truth <- metadata(pe)$spike_locations
head(spike_truth)

For preprocessing before clinical analysis, apply filtering to remove low-frequency drift and powerline noise:

# Bandpass filter (0.5-70 Hz) with 50 Hz notch
pe <- eegFilter(pe, lowcut = 0.5, highcut = 70, notch = 50)

Spike Detection

The eegSpikeDetect() function identifies epileptiform spike discharges using either morphology-based derivative analysis or template matching. The morphology method detects sharp transients by thresholding the first derivative, while the template method cross-correlates a canonical spike waveform with each channel.

Basic Spike Detection

# Morphology-based detection (default)
spikes <- eegSpikeDetect(pe, method = "morphology",
                          assay_name = "filtered")
head(spikes)
# Returns: channel, sample, time_sec, amplitude, duration_ms, confidence

Parameter Tuning

The detection sensitivity depends on the threshold_sd parameter. Lower values increase sensitivity (more detections) but may produce false positives. Higher values are more conservative:

# High sensitivity (more detections, more false positives)
spikes_sensitive <- eegSpikeDetect(pe, method = "morphology",
                                    threshold_sd = 3,
                                    min_amplitude = 30,
                                    assay_name = "filtered")

# Conservative detection (fewer false positives)
spikes_conservative <- eegSpikeDetect(pe, method = "morphology",
                                       threshold_sd = 5,
                                       min_amplitude = 80,
                                       assay_name = "filtered")

cat("Sensitive:", nrow(spikes_sensitive), "detections\n")
cat("Conservative:", nrow(spikes_conservative), "detections\n")

Template Matching

The template method uses cross-correlation with a canonical spike waveform, which can be more robust for stereotyped spike morphologies:

spikes_template <- eegSpikeDetect(pe, method = "template",
                                   min_amplitude = 50,
                                   assay_name = "filtered")
cat("Template detections:", nrow(spikes_template), "\n")

Sensitivity Analysis

Compare detections against known spike locations to evaluate performance:

# Compare with ground truth
truth_times <- spike_truth$time_sec
detected_times <- spikes$time_sec

# Count hits (within 100ms tolerance)
tolerance <- 0.1
hits <- sum(sapply(truth_times, function(t) {
  any(abs(detected_times - t) < tolerance)
}))

sensitivity <- hits / length(truth_times)
cat(sprintf("Sensitivity: %.1f%% (%d/%d spikes detected)\n",
            100 * sensitivity, hits, length(truth_times)))

Quantitative EEG (QEEG)

eegQEEG() computes absolute and relative spectral band powers for each channel using Welch’s method (windowed FFT averaging). Standard EEG frequency bands are delta (1-4 Hz), theta (4-8 Hz), alpha (8-13 Hz), beta (13-30 Hz), and gamma (30-50 Hz).

# Compute QEEG with standard bands
pe_qeeg <- eegQEEG(pe, assay_name = "filtered")

# Access results
qeeg_info <- metadata(pe_qeeg)$qeeg

# Relative power per channel (channels x bands)
print(round(qeeg_info$relative_power, 3))

# Absolute power is also available
print(round(qeeg_info$absolute_power, 2))

Custom Band Definitions

You can define custom frequency bands for specific clinical questions:

# Fine-grained alpha sub-bands
custom_bands <- list(
  delta      = c(1, 4),
  theta      = c(4, 8),
  low_alpha  = c(8, 10),
  high_alpha = c(10, 13),
  beta1      = c(13, 20),
  beta2      = c(20, 30)
)

pe_custom <- eegQEEG(pe, bands = custom_bands, assay_name = "filtered",
                      output_assay = "qeeg_custom")
custom_info <- metadata(pe_custom)$qeeg
print(custom_info$band_names)

Normative Comparison

Compare band power ratios against typical normative ranges:

# Compute theta/alpha ratio (elevated in cognitive impairment)
rel_power <- qeeg_info$relative_power
theta_alpha_ratio <- rel_power[, "theta"] / pmax(rel_power[, "alpha"], 1e-10)
cat("Theta/Alpha ratio per channel:\n")
print(round(theta_alpha_ratio, 2))

# Channels with ratio > 1.5 may indicate abnormal slowing
abnormal <- which(theta_alpha_ratio > 1.5)
if (length(abnormal) > 0) {
  cat("Potentially abnormal channels:", paste(abnormal, collapse = ", "), "\n")
}

Asymmetry Indices

eegAsymmetry() computes hemispheric asymmetry indices from paired electrode sites. The standard metric is log(right power) - log(left power) in the alpha band. Positive values indicate relatively greater right-hemisphere alpha power, which is typically associated with greater left-hemisphere cortical activity.

# Compute frontal alpha asymmetry (standard F4-F3, F8-F7 pairs)
asym <- eegAsymmetry(pe, assay_name = "filtered")
print(asym)

Custom Electrode Pairs and Bands

# Parietal asymmetry in theta band
parietal_asym <- eegAsymmetry(pe,
                               pairs = list(c("P4", "P3"), c("T6", "T5")),
                               band = c(4, 8),
                               assay_name = "filtered")
print(parietal_asym)

Clinical Interpretation

Frontal alpha asymmetry is used in research on emotional processing and depression. A consistently negative asymmetry index (greater left alpha = less left activity) has been associated with withdrawal-related affect:

# Evaluate asymmetry direction
for (i in seq_len(nrow(asym))) {
  direction <- if (asym$asymmetry_index[i] > 0) {
    "greater right alpha (left activation)"
  } else {
    "greater left alpha (right activation)"
  }
  cat(sprintf("Pair %s: AI = %.3f -> %s\n",
              asym$pair[i], asym$asymmetry_index[i], direction))
}

EEG Suppression

eegSuppression() identifies periods of burst and suppression by computing the root mean square (RMS) amplitude in sliding windows. This is relevant for monitoring depth of anesthesia, coma assessment, and neonatal EEG interpretation.

# Detect burst-suppression patterns
bs <- eegSuppression(pe, threshold = 10, min_duration_ms = 500,
                      assay_name = "filtered")

# View segments
head(bs)

# Burst-Suppression Ratio (percentage of time in suppression)
bsr <- attr(bs, "bsr")
cat(sprintf("Burst-Suppression Ratio: %.1f%%\n", bsr))

Interpreting BSR

The Burst-Suppression Ratio (BSR) provides a single summary metric:

if (bsr < 5) {
  cat("BSR < 5%: Normal or mild suppression\n")
} else if (bsr < 40) {
  cat("BSR 5-40%: Moderate burst-suppression\n")
} else {
  cat("BSR > 40%: Severe burst-suppression\n")
}

# Segment durations
burst_durations <- bs$duration_ms[bs$type == "burst"]
supp_durations <- bs$duration_ms[bs$type == "suppression"]
cat(sprintf("Mean burst duration: %.0f ms\n", mean(burst_durations)))
if (length(supp_durations) > 0) {
  cat(sprintf("Mean suppression duration: %.0f ms\n", mean(supp_durations)))
}

EEG Slowing

eegSlowing() detects pathological EEG slowing using spectral analysis. Three methods are available:

• TDR: Theta/delta ratio per channel
• DTAR: (Delta + theta) / (alpha + beta) ratio per channel
• Peak frequency: Dominant frequency per channel

# DTAR method (most comprehensive)
slowing <- eegSlowing(pe, method = "dtar", assay_name = "filtered")
print(slowing)

# Summarize classifications
table(slowing$classification)

Peak Frequency Method

The peak frequency method identifies the dominant oscillation in each channel. Peak frequency below 8 Hz is considered abnormal:

slowing_pf <- eegSlowing(pe, method = "peak_frequency",
                          assay_name = "filtered")
print(slowing_pf)

# Channels with abnormal peak frequency
abnormal <- slowing_pf[slowing_pf$classification != "normal", ]
if (nrow(abnormal) > 0) {
  cat("Channels with slowing:\n")
  print(abnormal)
}

Focal vs. Generalized Slowing

Compare slowing across hemispheres to distinguish focal from generalized patterns:

slowing_all <- eegSlowing(pe, method = "dtar", assay_name = "filtered")

# Separate left and right channels
col_data <- SummarizedExperiment::colData(pe)
labels <- as.character(col_data$label)

left_ch <- grep("1|3|5|7", labels)
right_ch <- grep("2|4|6|8", labels)

left_mean <- mean(slowing_all$value[left_ch])
right_mean <- mean(slowing_all$value[right_ch])

cat(sprintf("Left hemisphere mean DTAR: %.2f\n", left_mean))
cat(sprintf("Right hemisphere mean DTAR: %.2f\n", right_mean))

if (abs(left_mean - right_mean) > 1.0) {
  cat("Possible focal slowing detected (hemispheric difference > 1.0)\n")
} else {
  cat("Slowing pattern appears generalized\n")
}

Full Clinical Workflow

A complete clinical EEG assessment combines all tools in a systematic pipeline:

# 1. Generate or load clinical EEG data
pe <- make_eeg_spikes(n_time = 30000, n_channels = 19, sr = 500, n_spikes = 15)

# 2. Preprocessing
pe <- eegFilter(pe, lowcut = 0.5, highcut = 70, notch = 50)

# 3. Spike detection
spikes <- eegSpikeDetect(pe, method = "morphology", assay_name = "filtered")
cat(sprintf("Detected %d spike events\n", nrow(spikes)))

# 4. QEEG band power
pe <- eegQEEG(pe, assay_name = "filtered")
qeeg <- metadata(pe)$qeeg

# 5. Asymmetry assessment
asym <- eegAsymmetry(pe, assay_name = "filtered")

# 6. Burst-suppression
bs <- eegSuppression(pe, assay_name = "filtered")
bsr <- attr(bs, "bsr")

# 7. Slowing detection
slowing <- eegSlowing(pe, method = "dtar", assay_name = "filtered")

# 8. Summary report
cat("\n=== Clinical EEG Summary ===\n")
cat(sprintf("Recording duration: %.1f seconds\n", 30000 / 500))
cat(sprintf("Channels: %d\n", 19))
cat(sprintf("Spike events: %d\n", nrow(spikes)))
cat(sprintf("Burst-Suppression Ratio: %.1f%%\n", bsr))
cat(sprintf("Channels with slowing: %d/%d\n",
            sum(slowing$classification != "normal"), nrow(slowing)))
cat(sprintf("Asymmetry (F4-F3): %.3f\n", asym$asymmetry_index[1]))

Clinical Interpretation Guidelines

When interpreting results from PhysioEEG clinical analysis functions, keep the following considerations in mind:

Spike Detection Limitations

• Automated spike detection has high sensitivity but moderate specificity. All detections should be confirmed by visual inspection.
• False positives commonly occur at eye blink artifacts, electrode artifacts, and steep sleep transients (vertex sharp waves, K-complexes).
• The confidence score provides a relative ranking but is not calibrated as a probability.

QEEG Interpretation

• Band power values depend on recording montage, impedances, and electrode placement. Normative comparisons require matched recording conditions.
• Relative power is more robust to inter-individual amplitude differences than absolute power.
• Age-appropriate normative databases should be used for comparison (not included in this package).

When to Combine with Clinical Judgment

• These tools are intended to augment, not replace, clinical expertise.
• Abnormalities detected by automated analysis should always be verified against the raw EEG trace.
• Clinical context (medications, state of consciousness, age, clinical history) is essential for proper interpretation.
• Findings from automated analysis should be integrated with other clinical data (imaging, neuropsychological testing, clinical presentation) for a complete assessment.