Lateralization Index

Lateralization and Symmetry

Initialize NeuroAnalyzer 
using NeuroAnalyzer
eeg = load("files/eeg.hdf")

The Lateralization Index (LI) quantifies the difference in power between the right and left hemispheres at a specific frequency or frequency range. It is calculated as:

\[ LI = \log\left(\frac{A}{B}\right) \]

where:
\(A\) is the average power at a given frequency (e.g., 10 Hz alpha) for the right hemisphere,
\(B\) is the average power at the same frequency for the left hemisphere.

Interpretation:

  • LI > 0 → right-dominant
  • LI < 0 → left-dominant
  • LI = 0 → symmetric

Lateralization index for the range 8.0-12.5 Hz:

lat_idx(eeg,
        frq=(8.0, 12.5))
0.1229164837346802

Asymmetry Index

The Asymmetry Index quantifies the difference in power between the right and left hemispheres at a specific frequency or frequency range.

asy_idx() computes the log-ratio and normalized difference of mean band power between two sets of channels (typically left vs right hemisphere):

  • asi = log(mean_power(ch1)) - log(mean_power(ch2)): positive when ch1 has more power than ch2
  • nasi = (mean_power(ch1) - mean_power(ch2)) / (mean_power(ch1) + mean_power(ch2)): bounded on (-1, 1), analogous to a contrast ratio

Interpretation:

  • Positive Asymmetry Index: Indicates greater power in the right hemisphere relative to the left. This is often associated with withdrawal-related emotions (e.g., anxiety, depression) when measured in the frontal alpha band.
  • Negative Asymmetry Index: Indicates greater power in the left hemisphere relative to the right. This is linked to approach-related emotions (e.g., happiness, motivation).
  • Zero Asymmetry Index: Indicates balanced activity between the hemispheres.

Calculating frontal alpha (8.0 to 13.0 Hz) asymmetry index:

# left
ch_left = channel_cluster(eeg,
                          cluster = :f1)
# or
ch_left = channel_pick(eeg,
                       pick=[:left, :frontal])
# right
ch_right = channel_cluster(eeg,
                           cluster=:f2)
# or
ch_right = channel_pick(eeg,
                        pick=[:right, :frontal])
# alpha frequency
alpha = band_frq(eeg,
                 band=:alpha)
# asymmetry index
asy_idx(eeg,
        flim=alpha,
        ch1=ch_left,
        ch2=ch_right)
(asi = 1.0887768827817115, nasi = 0.4963026613913089)

Symmetry Index

Channel Symmetry Index measures the balance between positive and negative amplitudes in the EEG signal between two channels. A symmetry value of 1.0 indicates a perfectly balanced signal.

Interpretation:

  • Symmetry index > 1.0: More positive amplitudes.
  • Symmetry index < 1.0: More negative amplitudes.

Calculating channel symmetry index between F3 and F4 channels:

sym_idx(eeg,
        ch=["F3", "F4"])
2×1 Matrix{Float64}:
 0.9878786466947764
 1.0676924319521848

Applications

  • Emotional Processing: The frontal alpha asymmetry is widely used to study emotional regulation. For example, greater left frontal alpha power is associated with positive emotions, while greater right frontal alpha power is linked to negative emotions.
  • Cognitive Functions: Asymmetry in other frequency bands (e.g., beta, gamma) can reflect differences in attention, memory, and language processing.
  • Clinical Research: Used to investigate neurological and psychiatric disorders, such as depression, anxiety, and schizophrenia, where hemispheric imbalances are observed.

Practical Considerations

  • Frequency Bands: The Asymmetry Index can be calculated for any frequency band (e.g., delta, theta, alpha, beta, gamma), depending on the research question.
  • Channel Selection: Ensure that homologous channels (e.g., F3 and F4, C3 and C4) are selected for accurate comparisons.
  • Artifact Removal: EEG signals should be cleaned of artifacts (e.g., muscle activity, eye movements) to avoid misleading results.