An electroencephalogram, commonly known as an EEG, measures the electrical activity produced by the brain. This non-invasive technique observes the brain’s function and its response to stimuli. It offers insights into the nervous system, revealing patterns that reflect different states of consciousness and cognitive processes. Sensors are placed on the scalp to detect these electrical signals.
The Brain’s Electrical Language
The brain communicates through electrical signals generated by neurons. While individual neurons produce action potentials, the EEG primarily measures the collective activity of neuron populations. It detects the summation of postsynaptic potentials, which are slower electrical changes occurring when neurons receive signals at synapses. When occurring synchronously across many neurons, these potentials create measurable electrical fields.
These electrical currents originate in the cerebral cortex, the brain’s outer layer, and propagate outwards. They travel through tissues like cerebrospinal fluid, skull bones, and the scalp. Their strength diminishes as they travel, but they remain detectable on the head’s surface. This organized electrical activity from large groups of neurons forms the basis of the EEG signal.
Capturing Brain Activity
EEG recording involves placing electrodes onto the scalp. A conductive gel is applied to each electrode site to reduce electrical resistance and ensure optimal contact with the skin. 19 to 256 electrodes are positioned using a standardized system, such as the 10-20 system, to cover different regions of the brain. These electrodes detect minute voltage fluctuations representing the brain’s electrical signals.
Once detected, these raw electrical signals are extremely small, often in the microvolt range, requiring amplification. Amplifiers boost these faint signals, making them strong enough to be processed. The amplified analog signals are converted into a digital format by an analog-to-digital converter, allowing them to be displayed on a computer screen and stored for later analysis. This procedure is non-invasive and causes no discomfort.
Unlocking Brain States: Different Waveforms
EEG signals are characterized by their frequency and amplitude, creating distinct patterns known as brain waves, each associated with different states of consciousness or mental activity.
Delta waves, typically ranging from 0.5 to 4 Hertz (Hz), are the slowest and highest amplitude waves, predominantly observed during deep, dreamless sleep. Their presence indicates profound relaxation and reduced awareness.
Theta waves (4-8 Hz) are seen during drowsiness, light sleep, deep meditation, or daydreaming. They are also linked to memory formation and emotional processing, suggesting reduced alertness but heightened internal focus.
Alpha waves (8-13 Hz) are prominent when awake but relaxed, particularly with eyes closed. These waves decrease in amplitude when eyes open or during mental effort.
Beta waves (13-30 Hz) characterize an alert, actively thinking state, reflecting focused attention, problem-solving, and conscious thought. These waves dominate during complex cognitive tasks or anxiety.
Gamma waves (above 30 Hz) are the fastest, associated with higher-level cognitive functions like learning, memory recall, and conscious perception. Their presence indicates active information processing and binding of sensory inputs into a unified perception.
Where EEG Signals Make a Difference
EEG signals are widely used in clinical settings for diagnosing various neurological conditions. For instance, they are instrumental in identifying epilepsy, as epileptic seizures are characterized by abnormal, synchronized electrical discharges that appear as distinct patterns on an EEG recording. The technique also helps in diagnosing sleep disorders, such as narcolepsy and insomnia, by analyzing the specific brain wave patterns associated with different sleep stages.
Beyond diagnosis, EEG plays a role in assessing brain function following injuries or medical events. It can help evaluate the extent of brain activity after concussions, strokes, or traumatic brain injuries, providing information about recovery progress or potential complications. During certain surgical procedures, EEG monitoring helps surgeons ensure adequate blood flow to the brain and detect any signs of distress. Furthermore, EEG is a valuable tool in scientific research, allowing scientists to study cognitive processes like attention, memory, and language processing in real-time.