L71. Physiology - EEG

EEG Defined

• EEG measures voltage fluctuations on the scalp's surface, reflecting brain electrical activity.
• These fluctuations are termed brain waves, recordings of voltage differences over time.
• EEG electrodes consist of small metal disks attached to the scalp using conducting gel.
• Multiple recording electrodes and a reference electrode are used.
• The signal from each recording electrode is compared to the reference, creating the EEG signal.
• A recording electrode is called a channel.
• Modern EEG recordings use 8-40 channels for multichannel recording.
• Electrode placement is standardized via the 10-20 system, ensuring consistent EEG acquisition.
• The "10" and "20" indicate distances between electrodes as 10% or 20% of skull distance.
• Scalp voltage is in microvolts (10⁻⁶ volts).
• EEG amplifiers boost the signal by hundreds of thousands (10⁵) for detection.
• Data is now digitized via analog-to-digital processors.
• Digitized data can be visualized or analyzed using Fourier Transforms (FT).

Reading EEGs

• Three key aspects to reading EEGs include: Brain wave frequencies, specific wave patterns, and artifacts.
• Artifacts are disturbances from technical issues, such as electrode movement, muscle activity, head movements, scratching, or sweating.
• Brain waves are categorized by frequency from highest to lowest.
• Specific wave patterns are not discussed.
• Fourier Transform (FT) analyzes wave data.
• FT converts voltage vs time to intensity vs frequency.
• Area under the curve is 100%.
• FT indicates the percentage of waves at each frequency.
• This reveals which frequency ranges predominate in the EEG signal.

Brain Wave Categories

• Gamma (>40 Hz) involves widely spread brain areas active simultaneously, displaying gamma oscillations.
• Gamma activity reflects "binding," unifying awareness, and requires Fourier analysis to detect.
• Beta (13-40 Hz) is usually symmetrical and evident frontally.
• Beta waves have low amplitude and high frequency.
• They may be reduced in areas of cortical damage or when alert, anxious, or with eyes open.
• Alpha (8-13 Hz) is best seen in posterior regions, with higher amplitude on the dominant side. Alpha waves are enhanced by closing eyes and relaxation, and diminished by eye opening or mental activity.
• It is the major rhythm in relaxed adults, especially after the 13th year.
• Theta (4-8 Hz) is abnormal in awake adults but normal in children up to 13 years old and during sleep.
• Theta is normal in the hippocampus, representing its activation, but not detectable on surface EEG.
• It can indicate focal subcortical lesions or diffuse disorders.
• Delta (<4 Hz) has a broad distribution, may be bilateral and widespread.
• Delta waves are high in amplitude.
• They are not present in normal adult EEGs; if present, it may indicate disease.
• Delta waves are associated with NREM stages 3 & 4, indicating low movement and arousal, and are present in newborns.

EEG Brain Topography

• EEG brain topography comes from digital acquisition of many EEG channels, and uses 64-256 electrodes in an array for signal acquisition.
• Software codes frequency, intensity, or voltage into colors, similar to a weather map, with purple as negative and red as positive voltages.
• Points between electrodes are interpolated, visualizing whole-brain electrical activity, which is mainly used experimentally.
• Clinically approved applications include:
• Monitoring for acute complications in the operating room or ICU.
• Evaluating cerebrovascular disease if neuroimaging and routine EEG are inconclusive.
• Evaluating dementia and encephalopathy when the diagnosis remains unclear.
• Screening for epileptic seizures in high-risk ICU patients.
• Screening for epileptic spikes or seizures in long-term monitoring.
• Performing topographic voltage and dipole analysis for pre-surgical evaluations of intractable epilepsy.

Causes of EEG

• Thalamocortical loops are feedback connections between cerebral cortex layer VI and the thalamus that generate anatomical and functional loops.
• The lateral geniculate nucleus (LGN) receives only a minority of input from retinal afferents.
• Retina ganglion cells provide only 20% of afferent synapses to the LGN. Extrinsic inputs account for 40% of input:
• Brainstem systems (ACh, 5-HT, norepinephrine, histamine) modulate thalamic relay and interneuron activity.
• The thalamic reticular nucleus (nRt) regulates thalamic relay cell excitability.
• Inhibitory LGN interneurons regulate relay excitability.
• Cortical feedback (40% of afferents from layer VI of area 17) is the largest input source to the LGN.
• Thalamocortical loops are in all thalamic nuclei across the cerebral cortex.
• Action potentials cycle between the thalamus and cortex in synchrony.

Thalamus Functional Modes

• Tonic mode conveys excitation from sensors to the cortex, is involved in EEG desynchronization (REM, waking), and occurs with thalamic relay cell depolarization.
• Burst mode causes rhythmic bursting with spikes, alters information flow, involved in EEG synchronization (NREM stage 4), and occurs with thalamic relay cell hyperpolarization.
• The shift between modes is controlled by:
• The reticular nucleus of the thalamus (nRt) uses inhibitory neurons to hyperpolarize thalamic relay cells; active nRt leads to burst mode.
• The reticular activating system uses Ach to inhibits nRt neurons and activates thalamic relay cells; increased activity at PPT-LDT inhibits nRt and depolarizes thalamic relay cells, causing tonic mode.
• Associations with brain states:
• Tonic mode is associated with waking, facilitating faithful sensory input to the cortex, and REM sleep.
• Burst mode is associated with NREM sleep.

Thalamocortical Loop Conduction

• Brain waves are not causal but reflect activity at the cellular and circuit levels.
• Brain waves are generated through synchronized depolarization of cell populations.
• Beta waves are involved in task-dependent synchronization in active brain areas, fast, low amplitude, indicate coordinated activity in smaller populations, and occur in tonic mode.
• Delta waves occur during burst mode, relatively large synchronized populations conduct through thalamocortical loops.
• EEG recordings mainly detect activity in the cerebral cortex's gray matter.
• Voltage changes in EEG signals represent the summed voltages of extracellular currents in radially arranged dendrites of pyramidal cells in layers II, III, V, and VI of the cerebral cortex.
• EPSPs and IPSPs, not action potentials, generate the EEG signals.

Evoked Potentials

• Evoked potentials (EPs), also called event-related potentials (ERPs), are EEG signals triggered by a specific activity, which can be sensory, cognitive, or motor.
• Sensory evoked potentials involve repetitive stimuli and recording the EEG.
• Resulting EEGs are averaged to cancel out unrelated activity, revealing the ERP form.
• ERP deflections are called components and are named P (points down) or N (points up), followed by the latency in the ERP (e.g., P1 = P200, P3 = P580, N400).
• Components indicate stimulus processing.
• Early components (<150 ms) often relate to subcortical processing.
• Late components (>150 ms) often relate to cortical processing.
• EPs can diagnose subcortical or cortical damage.

Clinical EEG Usage

• EEG is an important diagnostic tool with high (1ms) time resolution, better than PET scans (30 seconds).
• EEG detects pathology-related changes in brain electrical conduction, including:
• Epilepsy, sleep disorders (detecting changes in sleep stages like NREM and REM), stroke (detecting altered electrical conduction due to tissue death), head trauma (detecting changes to electrical activity from tissue damage), and brain tumors (detecting pressure-related electrical conduction changes).
• EEG is not effective for conditions not affecting brain electrical activity like psychiatric illnesses (depression, schizophrenia).
• EEG is used to diagnose epilepsy.
• EEG is the main test for seizures because it shows abnormal electrical function even with normal MRI/CT scans.
• A focus is a seizure start site, and is generally detected by EEG.
• EEG of epilepsy patients can be interictal or ictal.
• Interictal (routine) EEG is recording when a patient is not having a seizure.
• Interictal epileptiform abnormalities indicate abnormal activity in epilepsy patients between seizures, revealing foci locations.
• A common interictal abnormality is a spike, which are like delta waves (around 4 Hz) but are produced by pathological mechanisms.
• Spikes can be focal, multifocal, or generalized, determining the type of epilepsy.
• Ictal EEG is recording during a seizure.
• Ictal EEG in generalized epilepsy shows widespread brain involvement.
• Ictal EEG is often less useful than interictal EEG for determining foci locations.