WHAT IS EEG AND WHAT DOES IT TELL US ABOUT BRAIN FUNCTION
The Electroencephalogram (EEG) is a non-invasive method for measuring brain activity. It is perhaps one of the most popular and best utilized methods for capturing cortical information processing in modern neurophysiology. The information we can obtain from an EEG testing differs in a number of ways from the information delivered by other brain imaging methods, such as magnetic resonance imaging (MRI), or positron emission tomography (PET). The most distinctive EEG feature is its high temporal resolution; while MRI provides us with neuro-anatomical images of the brain as a snapshot in time, EEG captures the ongoing brain dynamics as it happens.
EEG- a window into the real time neural processing
The EEG signal reflects changes in the neural oscillations occurring within milliseconds. This makes it an invaluable tool for investigation of the real time cortical information processing and for the development of fast and responsive brain-computer interfaces (BCI) that can capture, classify and feed back relevant characteristics of the signal collected from the brain. Neurofeedback is one instance of such a brain computer interface.
You may wonder how exactly the tiny voltage fluctuations collected from the scalp (the EEG signal) represent the neural activity in the brain and connect to our thoughts and feelings. Below we try to provide an outline of the neurophysiological origins of the EEG signal.
NEUROPHYSIOLOGICAL ORIGINS OF THE EEG SIGNAL
Human emotions, thoughts and behaviours are encoded in the brain through precisely coordinated multidimensional patterns of synchronous neural activation and deactivation. The EEG captures the activation dynamics of the pyramidal cells of the Cerebral Cortex (grey matter). Although EEG cannot record the activity of many sub-cortical and cerebellar structures, our knowledge of cortico-cortical and cortico-sub-cortical connections and of their rhythmical wave characteristics, allows us to assess the functioning of areas out of the scope of the direct EEG recording, such as the limbic system, the brain stem and the thalamus.
THE PYRAMIDAL CELLS
The cortex can be divided in six distinct cortical layers, each of which contains different type of neurons and distinct connections with other sub-cortical and cortical areas. It is the activation (excitatory and inhibitory postsynaptic potentials) of the giant pyramidal cells in cortical layer V that is reflected in most of the EEG.
FROM THE CORTEX TO THE HEAD SURFACE
The electrical field produced by the neural activation travels from the cells to the scalp and decreases in voltage with every layer it passes through (dura mater, skull, skin) it needs to overcome to reach the scalp. Therefore there are some necessary conditions a neural signal needs to meet to be detected correctly on the scalp:
- Large numbers– large populations of pyramidal cells need to be activated in the same fashion
- Timing or Synchrony- this activation needs to occur in synchrony
- Orientation – the orientation of the active neural population needs to be the same or the electrical fields would cancel each other out. In addition, the dipole orientation needs to be perpendicular to the scalp surface for the voltage with correct polarity to be recorded
The electrical fields produced by whole population of pyramidal neurons resembles the field generated by a single dipole (charged entity with a positive and negative ends). The dipole orientation and excitation state determines the negative and positive deflections that give the wavy appearance of the EEG.
OSCILLATIONS, RHYTHMS AND BRAINWAVES
The EEG signal consists of electrical oscillations with different rhythmical characteristics referred to as brainwaves. The brainwave spectrum reflects the individual’s state of arousal, the ‘tone’ of the nervous system and the dynamics of cortical relaxation and activation cycles. The qEEG spectrum analysis reveals the patterns of individual’s cortical information processing. Although operating in a continuous spectrum, brainwaves are commonly divided into frequency bandwidths to describe their function. These functional characteristics of the brainwaves, alongside with the loci of their generation lay in the core of the analysis we conduct as part of your qEEG brain assessment.
More detailed information on the individual brain frequencies can be found here.