Comprehensive Study Guide: Principles and Clinical Applications of Electroencephalography (EEG)

Theoretical Foundations of the Electroencephalogram (EEG)

• Definition and Scope: The electroencephalogram (EEG) is a non-invasive diagnostic and research method used to explore electrical potential changes in the brain by recording them at the scalp level. The recorded activity reflects neuronal activity closely linked to changes in the membrane potential.
• Types of Membrane Potential Changes:     * Action Potential: A brief and complete depolarization (nerve impulse) that propagates unchanged along the length of the axon.     * Postsynaptic Potentials: These are formed at the level of the synapses and propagate electrotonically through the dendrites.         * Excitatory Postsynaptic Potential (EPSP): Occurs when the modification depolarizes the membrane.         * Inhibitory Postsynaptic Potential (IPSP): Occurs when the modification hyperpolarizes the membrane, leading to inhibition.

Recording Methods for Brain Electrical Activity

• Intracellular Recording:     * Uses microelectrodes with the tip inserted directly into the cell to measure transmembrane potential.     * Pros/Cons: It is highly precise but very laborious. It records only a single cell's behavior.     * To study neuronal networks (cellular "conversation"), multiple electrodes are needed, but even two electrodes only provide data on those specific interconnected cells.
• Extracellular Recording:     * Microelectrodes are placed in the extracellular space between cells.     * Records electrical field changes generated by the depolarization of neighboring cells.     * Limitations: The electric field attenuates with distance, so only a small group of nearby cells can be recorded.
• Electroencephalography (EEG):     * Mainly used for clinical purposes and human studies because the previous methods are too complex and invasive for routine use.     * Allows for the simultaneous recording of massive neuronal populations, providing high clinical utility despite being a more "coarse" method.

Physiological Basis and Origin of the EEG Signal

• Signal Origin: The EEG records the cooperation of millions of neurons located beneath the scalp electrodes. It does not reflect a single cell but the vector sum of tiny electrical fields.
• Role of Pyramidal Cells:     * Pyramidal cells are the main output cells of the cortex and are situated parallel to each other.     * Their apical dendrites are oriented perpendicular to the cortical surface, while their axons point toward the brain's interior.     * Dendritic zones are closer to the recording electrodes than the axons.
• Role of Interneurons: These cells lack a specific orientation; consequently, the vector sum of their activity usually cancels out and does not contribute significantly to EEG waves.
• Signal Nature: Because action potentials are very brief and cause minimal charge transfer, the EEG primarily reflects the sum of postsynaptic potentials from synchronously activated pyramidal cells.
• General Characteristics:     * Frequency Range: Typically varies between $1-50\,Hz$ (most commonly $1-30\,Hz$).     * Amplitude Range: Generally between $20-100\,\mu V$.     * Wave Form: Usually appears completely irregular in a normal recording.

Classification of Basic EEG Rhythms

• Beta (\beta):     * Frequency: $13-27\,Hz$.     * Amplitude: Very low ($5-10\,\mu V$).     * Location/Condition: Most easily recorded in the frontal region; spreads over larger areas during mental exertion.
• Alpha (\alpha):     * Frequency: $7-13\,Hz$.     * Amplitude: $20-100\,\mu V$.     * Pattern: Occurs as spindles lasting $1-3\,seconds$.     * Location/Condition: Recorded mostly in parietal and occipital regions. It is associated with a relaxed awake state and diminishes or disappears when the eyes are opened.
• Theta (\theta):     * Frequency: $4-7\,Hz$.     * Amplitude: $30-70\,\mu V$.     * Location/Condition: Recorded in temporal and frontal zones. Specifically dominant in children up to $5\,years$ old and in adults during early sleep stages.
• Delta (\delta):     * Frequency: $0.5-4\,Hz$.     * Amplitude: Highest at $50-200\,\mu V$.     * Location/Condition: Present during deep sleep in adults and in children younger than $2-3\,years$.
• General Note: An awake adult will have waves across all frequency bands in all regions, though specific rhythms dominate depending on the mental state and brain area.

Main Applications of EEG

• 1. Study of Consciousness:     * Consciousness is maintained by the "cross-talking" of small functional modular units (approx. $100\,\mu m$ diameter) that activate asynchronously (alpha and beta rhythms).     * Synchronized neuronal activity results in slow, large amplitude waves (theta and delta).     * Consciousness is typically lost when the rhythm falls below $4-5\,Hz$. EEG is also used to guide narcosis (anesthesia).
• 2. Sleep Studies:     * Slow Wave Sleep (Non-REM): Characterized by four electrophysiological stages (clinically merged into $N1$, $N2$, and $N3$). Marked by a progressive decrease in frequency and increase in amplitude. Parasympathetic activity dominates; muscles are relaxed.     * REM Sleep (Rapid Eye Movement/Paradoxic): Characterized by sudden EEG desynchronization (low amplitude, high frequency). Sympathetic activity dominates. Muscle tone is lost (except for respiratory and eye muscles).     * REM Duration: Accounts for $20-25\%$ of total sleep, appearing discontinuously $5-7\,times$ per night, predominantly toward the morning.
• 3. Diagnosis of Epilepsy:     * Affects $1-1.5\%$ of the population. Caused by abnormal synchronous discharge of a large number of neurons, manifesting as high-amplitude sharp waves (spikes).     * Partial (Focal) Epilepsy: Abnormal activity remains in a specific focus; spikes appear only on certain electrodes.     * Generalized (Non-focal) Epilepsy: Activation involves large brain areas; spikes in all leads.         * Petit Mal (Absence): Transient loss of consciousness without motor involvement. Pathognomonic EEG shows spike-wave complexes (spike + slow wave, $0.2-0.5\,s$, $150-300\,\mu V$).         * Grand Mal: Loss of consciousness, tonico-clonic movements, salivation, and loss of sphincter control due to vegetative center involvement.
• 4. Cerebral Death: Indicated by a flat EEG trace, signifying extensive and irreversible neuronal death.

The Electroencephalograph and Recording Techniques

• Major Components:     * Signal Acquisition System: Electrodes, usually recording at least $8$ channels simultaneously.     * Amplification and Filtering System.     * Signal Charting System.
• Electrode Placement (The 10-20 System):     * Electrodes are placed symmetrically at intersections of a grid based on anatomical landmarks.     * Nasion: Root of the nose.     * Inion: Occipital protuberance.     * The distance between nasion and inion is considered $100\%$.     * Labels: Fp (prefrontal), F (frontal), C (rolandic/central), P (parietal), O (occipital), M (median), T (temporal).
• Recording Modes:     * Bipolar: Two recording electrodes connected to the amplifier.     * Unipolar: One recording electrode vs. an indifferent electrode (usually at the earlobe with $0$ potential).
• Patient Preparation: Hair must be washed the day before or degreased with alcohol/ether. Mental preparation for stillness is essential.
• Environment: Spacious room, noise-protected, away from high-interference equipment (X-rays, etc.).

Biopac Laboratory Protocols (EEG I and II)

• Equipment: BIOPAC MP35/36 Data Acquisition Unit (DAU), SS2L Electrode Lead Set, adhesive electrodes, and conducting gel.
• Crucial Setup Steps:     * Gently abrade skin and move hair for scalp contact.     * Third electrode (ground) is placed on the Mastoid region (behind the ear).     * Wait $5\,minutes$ after placement for electrode contact stabilization.
• Experimental Conditions (EEG II):     * Recording 1: Relaxed with eyes open.     * Recording 2: Mental math (exertion) with eyes closed.     * Recording 3: Recovery from hyperventilation with eyes closed.     * Recording 4: Relaxed with eyes open.
• Warnings and Hints:     * Hyperventilation: Can lead to dizziness; subject must be seated and supervised. Stop if they feel sick.     * Movement: Blinking, talking, and muscle movement (facial muscles) introduce significant artifacts.

Data Analysis and Measurement Parameters

• Channel Mapping:     * CH 1: Raw EEG.     * CH 40: Alpha band.     * CH 41: Beta band (or Alpha RMS in EEG II).     * CH 42: Delta band.     * CH 43: Theta band.
• Key Measurement Values:     * Stddev (Standard Deviation): Measures variability of wave amplitudes. Preferred because it is less influenced by extreme artifacts or single spikes.     * Freq (Frequency): To calculate accurately, the I-beam cursor must select exactly one cycle/period of a wave.     * Mean: Displays the average value for the selected segment.     * Alpha RMS: Root-mean-squared of the alpha rhythm, used as an index of activity level.
• Calibration: Lasts $8\,seconds$; the baseline should be stable around $0\,\mu V$.