Understanding Frequency, Amplitude, Phase, Phase Lag, and Coherence in EEG

 

Understanding Frequency, Amplitude, Phase, Phase Lag, and Coherence in EEG and Neurofeedback

Electroencephalography (EEG) is a non-invasive method used to measure electrical activity in the brain. It records voltage fluctuations resulting from the ionic current within neurons. EEG data is commonly analyzed in terms of frequency, amplitude, phase, phase lag, and coherence, which are critical for understanding brainwave patterns. These measures can be observed across different frequency bands that represent various cognitive and emotional states. Here’s an in-depth explanation of these concepts, along with the role of microvolts (μV) in EEG analysis.

 

1. Frequency: The Speed of Brainwave Oscillations

Definition: Frequency refers to the number of oscillations or cycles that a brainwave completes in one second, measured in Hertz (Hz). In EEG, different frequencies correspond to distinct types of brain activity.

Relevance: Frequency is essential for distinguishing between brainwave patterns, as different cognitive and emotional states are associated with specific frequency ranges.

Frequency Bands:

1. Delta (0.5–4 Hz): Associated with deep sleep and unconscious states. High delta activity in waking states can indicate neurological conditions.

2. Theta (4–8 Hz): Linked to daydreaming, creativity, and meditative states. Excessive theta is often observed in conditions like ADHD.

3. Alpha (8–12 Hz): Occurs during relaxed and calm states, often seen in wakeful rest or meditation.

4. Beta (12–30 Hz): Represents alertness, concentration, and cognitive processing. High beta activity may indicate anxiety or stress.

5. Gamma (30–100 Hz): Associated with higher-order cognitive functions, such as attention, memory, and conscious awareness.

2. Amplitude: The Strength of Brainwave Activity

Definition: Amplitude refers to the magnitude or strength of the brainwave signal, typically measured in microvolts (μV). It reflects the power or intensity of the electrical activity generated by neurons.

Relevance: Amplitude gives an indication of how active certain brain regions are. Higher amplitudes usually represent more synchronized neuronal firing, whereas lower amplitudes may reflect less synchronized activity.

Amplitude in Different Frequency Bands:

Delta waves tend to have higher amplitudes, especially during deep sleep.

Theta waves show moderate amplitude in tasks requiring creativity or introspection.

Alpha waves often show moderate-to-high amplitude in relaxed but alert states.

Beta waves usually have low-to-moderate amplitude, indicating active mental processing.

Gamma waves have low amplitudes, as they are fast oscillations related to complex cognitive processes.

3. Phase: Timing of Brainwave Cycles

Definition: Phase refers to the position of a brainwave within its oscillatory cycle at a given moment in time. Each cycle has points corresponding to the beginning, middle, and end of the wave.

Relevance: Phase helps determine how different brain areas synchronize with each other. Two brainwaves can be in phase (peaks and troughs align) or out of phase (peaks align with troughs).

Phase in Different Frequency Bands:

Low-frequency waves (like delta and theta) tend to have longer cycles, so phase shifts occur more gradually.

High-frequency waves (like beta and gamma) have shorter cycles, so phase changes rapidly, allowing for faster processing and communication between brain regions.

4. Phase Lag: Delays in Synchronization

Definition: Phase lag refers to the delay between the phases of two brainwaves recorded from different brain regions. It indicates how long it takes for one region's activity to catch up with another's.

Relevance: Phase lag is crucial for understanding communication delays between brain regions. A small phase lag may indicate efficient communication, while a large phase lag can reflect poor coordination or neurological dysfunction.

Phase Lag in Different Frequency Bands:

Slow waves (delta and theta) naturally have longer phase lags due to their low frequency.

Fast waves (beta and gamma) exhibit shorter phase lags, which facilitates quick information transfer and cognitive synchronization between brain areas.

5. Coherence: Synchronization Between Brain Regions

Definition: Coherence refers to the degree of synchronization or phase consistency between brainwaves from two different locations in the brain. It measures how similarly two regions oscillate over time.

Relevance: High coherence indicates well-coordinated communication between brain areas, while low coherence suggests that different areas are working more independently or are poorly synchronized.

Coherence in Different Frequency Bands:

Delta coherence: High delta coherence during wakefulness can suggest brain damage or neurological issues.

Theta coherence: High theta coherence is often seen in creative thinking or meditative states but may be elevated in conditions like ADHD.

Alpha coherence: Increased alpha coherence often indicates relaxation and calm focus, while low alpha coherence may be associated with disconnection or anxiety.

Beta coherence: High beta coherence is typical during cognitive tasks but can be excessively high in conditions like OCD or anxiety.

Gamma coherence: High gamma coherence suggests high-level cognitive functioning and synchronous information processing across brain regions.

 

Understanding Microvolts (μV) and Their Role in EEG

Definition: A microvolt (μV) is a unit of electrical potential equal to one-millionth of a volt. In EEG, brainwave activity is recorded as small voltage fluctuations in microvolts.

Relevance: EEG readings are measured in microvolts because the electrical activity generated by neurons is very small. The strength or amplitude of brainwaves is reported in microvolts, allowing researchers and clinicians to understand how powerful the oscillations are.

How Microvolts Relate to Frequency and Amplitude:

Amplitude in EEG is directly measured in microvolts (μV) and reflects the voltage change produced by brain activity.

Frequency is determined by counting how many cycles (oscillations) occur per second within the signal. It is not measured in microvolts but can be extracted from the same EEG data that provides amplitude values.

Example:

A high-amplitude alpha wave would appear as a strong oscillation with a frequency of 8–12 Hz and an amplitude of around 20–50 μV. In contrast, a low-amplitude gamma wave might have a much higher frequency (30–100 Hz) but a lower amplitude, often under 10 μV.

Using Microvolts to Determine Amplitude:

The recorded EEG data show voltage changes over time. By analyzing the size of these fluctuations, researchers can calculate the amplitude in microvolts. Large amplitude means strong neural activity, while smaller amplitudes reflect less synchronized or weaker neural activity.

 

Summarizing the Relationships Between Frequency, Amplitude, Phase, and Coherence

Frequency defines the speed of brainwave oscillations.

Amplitude measures the strength of these oscillations, reflected in microvolts.

Phase shows where in its cycle a brainwave is at any given time.

Phase lag reveals how long it takes for one region’s activity to align with another’s.

Coherence measures the degree of synchronization between two brain regions.