Electroencephalography - Fundamentals and Normal Patterns

Electroencephalography (EEG): A Comprehensive Guide What is EEG and Why Do We Use It? Electroencephalography (EEG) is a non-invasive technique that records the spontaneous electrical activity of the brain using electrodes placed on the scalp or, in some cases, directly on the brain's surface through surgical implantation. The procedure is widely used in clinical practice to diagnose neurological disorders—especially epilepsy—and in research to study how the brain functions during different mental states and tasks. The key advantage of EEG is that it is non-invasive, relatively inexpensive, and provides real-time information about brain activity with excellent temporal resolution (you can see changes in activity occurring in milliseconds). However, as you'll learn, it has important limitations in terms of spatial precision—that is, pinpointing exactly where in the brain the activity is coming from. What Does EEG Actually Measure? This is a crucial question, and the answer is more specific than you might initially think. EEG does not measure all electrical activity in the brain equally. Instead, it primarily reflects postsynaptic potentials (the electrical changes that occur when neurotransmitters bind to receptors on the receiving end of a synapse) generated by the dendrites of pyramidal neurons in the cortex. Why pyramidal neurons specifically? Pyramidal neurons are shaped like pyramids with a long apical dendrite (a long branch extending upward) that is oriented perpendicular to the skull surface. When thousands of these neurons fire synchronously with their dendrites aligned in the same direction, they generate electrical fields that add together and project to the scalp surface. This alignment is essential—if neurons fired randomly in different directions, their electrical fields would cancel each other out and produce no detectable signal. Another critical point: EEG measures the summed activity of thousands to millions of neurons firing together. A single neuron's electrical activity is far too weak to detect at the scalp. Therefore, EEG is blind to sparse or unorganized activity and only picks up signals when large populations of neurons are synchronized. Deep brain structures contribute minimally to scalp EEG. Structures like the thalamus, hippocampus, and brainstem are either too far from the scalp or their neuronal geometry is unfavorable (their dendrites don't align perpendicular to the skull). This is an important limitation: EEG primarily tells you about cortical (surface brain) activity, not what's happening deep inside the brain. It's also worth noting that axonal action potentials—the rapid electrical spikes that neurons use to fire—are largely invisible to EEG. Action potentials generate electrical fields that decay very quickly with distance (quadrupole fields), whereas postsynaptic potentials from dendrites generate stronger fields that reach the scalp (dipole fields). This is why EEG shows you what neurons are "receiving" (via postsynaptic potentials) rather than what they are "sending" (via action potentials). How Are EEG Electrodes Placed? EEG electrodes can be placed in two main ways, depending on the clinical need. Scalp electrodes (the standard, non-invasive approach) are positioned according to the International 10-20 system, an internationally standardized system that assigns names and locations to each electrode based on percentages of the distance between anatomical landmarks on the head. This standardization allows clinicians and researchers around the world to use consistent terminology and compare recordings across different subjects and labs. Intracranial electrodes are surgically implanted electrodes placed directly on or inside the brain. This includes electrocorticography (ECoG), where electrodes sit on the surface of the cortex, and stereotactic EEG, where thin electrodes are inserted deep into the brain tissue. Intracranial recording provides much higher spatial resolution—you can pinpoint brain activity to smaller regions than scalp EEG allows—but it is invasive and typically reserved for special clinical situations, such as evaluating a patient for epilepsy surgery. Physical Barriers: Why Can't We Perfectly Locate Brain Activity? This is where we encounter one of EEG's fundamental limitations. The brain is surrounded by multiple tissue layers: the meninges (protective membranes), cerebrospinal fluid, and the skull. All of these structures are electrical conductors, but they conduct electricity at different rates. As electrical currents generated deep in the brain travel outward to the scalp, these tissue layers "smear" and blur the signal, much like looking at an object through frosted glass. Moreover, different electrical currents generated at different locations in the brain can cancel each other out (a phenomenon called volume conduction). Because of this cancellation effect, it is mathematically impossible to uniquely determine where a scalp EEG signal originated in the brain. This is known as the inverse problem—given a set of measurements at the scalp, there are multiple possible brain configurations that could have generated those measurements. Advanced source-localization techniques can estimate an equivalent current dipole (essentially, a single location and orientation that best approximates the source), but these estimates are not unique and can be misleading. The bottom line: EEG tells you when activity occurs with millisecond precision, but where activity occurs is much less certain, especially for deep brain structures. EEG Frequency Bands: The Language of Brain States Clinical EEG is primarily described using frequency bands—ranges of oscillation speeds measured in cycles per second (Hertz, or Hz). The majority of clinically relevant EEG activity falls between 1 and 30 Hz, though certain conditions may show activity outside this range. Activity outside the 1-30 Hz range is usually considered artifactual (noise from muscle movement, eye movement, electrical interference, etc.). The five main frequency bands are delta, theta, alpha, beta, and gamma. Each is associated with specific brain states and locations on the scalp. Understanding these bands is essential for interpreting EEG recordings. Delta Waves (0.5–4 Hz) Delta waves are the slowest and highest-amplitude oscillations in normal EEG. In healthy adults, delta activity dominates during deep, slow-wave sleep. However, in infants, delta waves are common even during wakefulness—this is normal and expected at that developmental stage. Abnormal delta activity in awake adults is clinically significant. Focal (localized) delta may indicate a subcortical lesion or structural abnormality beneath that region of scalp. Diffuse (widespread) delta activity can suggest metabolic encephalopathy (brain dysfunction due to systemic metabolic problems), hydrocephalus (excess cerebrospinal fluid), or other diffuse brain disorders. Theta Waves (4–7 Hz) Theta activity is normal in young children and typically appears in adults during drowsiness, meditation, or deep relaxation. Like delta, theta can be subdivided into normal and abnormal categories: Excess theta for a given age in a waking patient suggests abnormality Focal theta may signal a subcortical lesion Diffuse theta can occur with metabolic disorders, hydrocephalus, or encephalopathy A key insight: The threshold for "normal" theta depends on the person's age. A child's baseline EEG naturally contains more theta activity than an adult's. Alpha Waves (8–13 Hz) The alpha rhythm is the posterior dominant rhythm of the normal, relaxed, awake human. You will see prominent alpha activity when a person is seated comfortably, relaxed, and has their eyes closed, especially over the parietal and occipital regions (the back of the head). Here's a classic and clinically useful finding: Alpha activity attenuates (decreases) when the person opens their eyes or engages in mental effort. This "alpha blocking" or "alpha suppression" is a normal, healthy response and is regularly used in clinical EEG assessment. One important developmental note: In young children, the dominant rhythm may be slower than 8 Hz, technically falling into the theta range. As the brain matures, the dominant rhythm speeds up into the alpha range. Beta Waves (13–30 Hz) Beta activity is typically symmetrical (present equally on both sides of the head) and maximal frontally (over the front part of the scalp). Beta activity is generally associated with active mental engagement, alertness, or anxiety—the kind of busy, focused thinking you engage in when solving a problem or feeling tense. Low-amplitude, variable-frequency beta is normal and reflects the brain's active processing state. Gamma Waves (30–100 Hz) <extrainfo> Gamma rhythms are thought to reflect the binding of distributed neuronal populations during cognitive or motor tasks—essentially, the synchronization of different brain areas as they work together to accomplish a task. Gamma is less commonly discussed in basic clinical EEG but is important in research settings. The Mu Rhythm (8–13 Hz) The mu rhythm is a specialized oscillation seen over motor cortex (the area controlling voluntary movement). It reflects synchronized firing of motor neurons at rest. A fascinating discovery is that mu suppression—a decrease in mu rhythm—occurs when someone observes another person performing an action. This suppression indicates activation of the mirror-neuron system, a set of neurons that fire both when we act and when we observe others acting. This phenomenon is thought to underlie our ability to understand and empathize with others' actions. </extrainfo> Normal EEG: What Does Healthy Brain Activity Look Like? A healthy adult EEG recording has several characteristic features: Voltage amplitude: Most activity ranges between 20–100 microvolts (µV), with some normal variation depending on the electrode location and individual factors Frequency range: Predominantly between 1 and 30 Hz in awake, relaxed adults Symmetry: Activity is generally symmetric (similar on left and right sides of the head) Responsiveness: The brain shows appropriate changes in response to stimuli—most notably, alpha blocking with eye opening A particularly useful clinical marker is the change in frequency when a person transitions from relaxed rest to active engagement: Alpha activity (8–13 Hz) decreases and beta activity (13–30 Hz) increases. This normal response demonstrates that the brain is functioning appropriately. The following image shows a typical clinical EEG display with multiple electrode channels recording simultaneously: Key Takeaways EEG measures postsynaptic potentials from pyramidal neurons, not all brain activity equally Only synchronized activity from large neuronal populations produces a detectable EEG signal EEG has excellent temporal resolution but poor spatial resolution due to volume conduction and the inverse problem Frequency bands (delta, theta, alpha, beta, gamma) describe the brain's electrical state and correlate with different levels of arousal and mental activity Normal EEG shows age-appropriate frequencies, symmetry, and appropriate responsiveness to behavioral changes like eye opening Abnormal EEG—whether focal or diffuse, slow or fast—often signals underlying neurological dysfunction, but EEG alone cannot pinpoint exactly where in the brain the problem originates