7.4.1. Power Computation #
For each electrode and frequency band:
- Filtering in the selected band is applied.
- Power is computed in a sliding window:
window_samples = window_seconds * Fs
for i = 1:(length(signal) - window_samples)
power(i) = 1 + log10(sum(signal(i:i+window_samples-1).^2) / window_samples)
end
Power formula:power = 1 + log10(mean(signal_window.^2))
7.4.2. Arousal #
Formula:Arousal = (Electrode1_beta + Electrode2_beta) / (Electrode1_alpha + Electrode2_alpha)
Theoretical rationale: The beta/alpha ratio reflects the activation level of the cerebral cortex. Beta activity is associated with active thinking and stress, while alpha activity is associated with relaxation.
Physiological explanation of arousal:
Arousal is the general activation level of the nervous system, ranging from deep sleep to severe stress.
Neurophysiology of arousal:
Low arousal: alpha rhythm dominates (8–13 Hz)
- relaxed wakefulness
- eyes closed
- meditation
- calm state
Medium arousal: mixed activity
- eyes open
- normal activity
- alpha/beta balance
High arousal: beta rhythm dominates (13–30 Hz)
- active thinking
- problem solving
- stress
- anxiety
- physical activity
Interpretation:
- Low values (<0.5): low arousal, relaxation, possible drowsiness
- Medium values (0.5–1.5): normal arousal, wakefulness
- High values (>1.5): high arousal, stress, active mental work
Clinical significance:
- Persistently high arousal may indicate chronic stress or anxiety.
- Very low arousal may indicate depression, fatigue, or drowsiness.
- Optimal arousal depends on the task (low for rest, medium to high for work).
7.4.3. Valence #
Formula:Valence = (Electrode2_alpha / Electrode2_beta) - (Electrode1_alpha / Electrode1_beta)
Theoretical rationale: Alpha/beta asymmetry between the left and right hemispheres reflects emotional valence. The left hemisphere (AF3) is associated with positive emotions; the right hemisphere (AF4) with negative emotions.
Physiological explanation of valence:
Valence is the evaluation of an emotion as pleasant or unpleasant. Research shows that the left and right hemispheres process emotions differently.
Theory of hemispheric asymmetry (Davidson, 1992):
Left hemisphere (AF3):
- processing of positive emotions
- approach behavior
- activity, enthusiasm
- high alpha = low activation = relaxed state with a positive emotional orientation
Right hemisphere (AF4):
- processing of negative emotions
- withdrawal behavior
- caution, anxiety
- high alpha = low activation = relaxation, but with possible negative valence
Interpretation:
Positive values:
- the right hemisphere is more relaxed (high alpha/beta)
- the left hemisphere is more active (low alpha/beta)
- positive emotions, pleasure, joy
Negative values:
- the left hemisphere is more relaxed
- the right hemisphere is more active
- negative emotions, sadness, anxiety, discomfort
Close to zero: neutral emotions, balance
Important to understand:
- Alpha activity is inversely proportional to cortical activation.
- High alpha/beta = low cortical activation in that hemisphere.
- Low alpha/beta = high cortical activation in that hemisphere.
Practical significance:
- Positive valence during relaxation is a good sign.
- Changes in valence reflect emotional responses to stimuli.
7.4.4. Outlier Handling #
Values are clipped at the 95th percentile to remove artifact outliers:quantile_arousal = quantile(abs(arousal), 0.95)
arousal(abs(arousal) > quantile_arousal) = sign(arousal) * quantile_arousal
The same is done for valence.
