Hello, all! From the last post about neurons, you might remember I talked about electrical impulses generated by neurons in the brain. Neurons communicate with one another by initiating inhibitory or excitatory activity by means of neurotransmitter release or change in membrane potential at synapses, the connections between neurons. Because our brain is always on, processing sensory information, forming memory, and regulating movement and bodily function, neurons are constantly firing in electrical bursts!
The Brain…It’s Electric!
The voltage is pretty low on a cellular level, but groups of hundreds or thousands of these cells firing in sync are enough to generate a detectable electrical field that can propagate through tissue and to the surface of the head. Surface electroencephalography, or EEG, is a bioinstrument that can record the electrical activity in the brain by placing electrodes on the scalp. Now that we’ve a better understanding of where electrical activity in the brain comes from, we can discuss how EEG works and its different uses.
How EEG Works
The electrodes are mounted on the scalp using elastic caps and are either dry or wet. Wet electrodes are made so with the use of conductive gel prior to the application of electrodes in specific positions on the cap. The electrodes measure very small signals which are digitized and amplified, then shown on a screen as amplitudes of voltage. These electrodes measure the electrical activity with reliable time resolution, meaning that you can detect spikes of activity in cortical areas at very specific times. Therefore, EEG is a useful tool for studying brain activity at the onset of an event. We call these fluctuations “event-related potentials” or ERP’s.
How To Read EEG
The electrodes are arranged on the cap in very specific locations mapping the areas of the cortex that process information. The recorded signals allow us to understand which areas are active in processing this information at specific time points. These are the areas of interest:
Not only can the signals be read from specific areas in the brain at a specific time, but the frequency of the “brain waves” can give us information about cognitive processes regarding an event or any other activity. So, the frequency patterns from an EEG may change when you’re asleep to when you’re awake and walking on a treadmill. Depending on the kind of study, each wave has its own association to cognitive processes.
- Delta (less than 4 Hz): low frequency wave pattern, sleep & memory
- Theta (4-7 Hz): Memory, difficult cognitive tasks
- Alpha (7-12 Hz): Relaxation, focus/attention
- Beta (12-30 Hz): Motor processing, movement
- Gamma (30-50 Hz): Sensory/information processing
Based on its excellent temporal (time) resolution, non-invasive EEG is used as a clinical tool in monitoring and diagnosing patients with ailments such as epilepsy and other neurological disorders. Outside of clinical use, dry, surface EEG systems are popular for brain-computer interfaces and controlling prosthetic devices using brain waves for control! Lastly, EEG is a common tool in neuroscience and neuromechanics research.
In most human neuromechanics research, EEG is used as a tool to record brain activity during some task, likely involving movement. Unfortunately, EEG can also record a lot of noise such as shifting of the electrodes or wires (movement artifact), line noise, eye movement, and electric potential from head muscles (jaw clenching is a good example). So, before we can interpret the data, a lot of filtering and pre-processing is required and is typically done with the help of Python or toolboxes in Matlab such as EEGLab. Stay tuned for EEG Part 2, where I will dive into how EEG data is handled!