Quantitative EEG analysis

The significance of QEEG

Quantitative EEG analysis (QEEG) and interpretation has become more important as technology and techniques have turned what was once a highly specialised and subjective approach into something potentially more robust.

QEEG application can be controversial (as it is not that yet widely understood) but knowledge of how EEG waves are generated and their functional meaning is advancing in conditions such as ADHD, PTSD and traumatic brain injury.

A particular topical interest is whether we can distinguish brain injured persons from "normals" based on their QEEG ? A nice step forward from this would be then to use this information to guide neurofeedback and biofeedback approaches to improve function in brain injured persons.

Amongst it's advantages, modern EEG hardware is extremely inexpensive compared with methods of brain investigation such as MRI and PET. It also boasts high temporal resolution (detailed knowledge of how the EEG signals vary over time) which is not possible with other techniques which have a temporal resolution of seconds at best.

Quantitative EEG (QEEG) analysis

In an earlier article, "Quantitative EEG and normative databases" we looked at a QEEG approach which is to compare the EEG recorded from one individual with a database of matched norms. This approach is intended to identify (Quantify) significant differences in the EEG pattern in an individual which then provides the basis for neurofeedback training or other intervention.  That this works at all is based on the fact that alterations in the EEG reflect a critical observation that these patterns are indicative of the maintenance of brain function and therefore can provide a key for diagnosis of brain dysfunction.

Quantitative EEG must be approached with care however. If we treat this too much as a "Black box" technique we run the risk of misinterpreting the signals.  EEG is a very sensitive parameter of a subject's state and the EEG patterns can vary significantly for example in stages of sleep or with eye's open or closed. In a damaged brain, the normal mechanisms of EEG pattern may be impaired and changes can occur in the magnitude of signals, in their frequency or the locations in which they are observed.

Electrode positioning

 Standard 10-20 Electrode positions

Standard 10-20 Electrode positions

An essential component of quantitive EEG methods is the ability to have consistent and standardised electrode placements. In 1959, the 10-20 International System of Electrode Placement was established. The electrode position and codes are identified by the first letter being associated with the area of electrode placement and the number indicating the side (left/right) and placement within this area.  The EEG signal set itself is derived by measuring the potential between an electrode site on the scalp and a reference electrode.

So called Brodman's areas are also commonly used today to describe locations on the cortex. In 1909 German anatomist K Brodman described 52 distinct areas which he numbered in sequence - although the numbering sequence doesnt relate to any particular brain function

The EEG signals are extremely small in magnitude and and care must be taken in the choice and preparation of electrodes.  A modern EEG system such as the NeXus 4, NeXus 10 or NeXus 32 units are designed to provide high fidelity data capture with the NeXus 32 providing for the full qEEG electrode array of 24 electrodes with some additional channels for peripheral signals.

The NeXus 32 and most other modern EEG capable systems taoday will be digital in design although the phsyiological signals they must deal with are inherently analogue.  In other words they must sample the raw analog signals meaured from the electrode sites at a suitable sample ate and resolution. The NeXus 32 analogue to digital converter offers up to 2048 Hz sample rates with 24 bit resolution (higher than most if not all on the market). It also offers extremely high input impedance so that measurement fidelity is high.

Typical processing relies initially on Fourier Transform techniques which are described in detail other articles on this site such as "Quantitative EEG and normative databases"

 Research has shown how massively interconnected areas of the brain are

Research has shown how massively interconnected areas of the brain are

EEG Coherence

An interesting approach to QEEG is to consider what is referred to as EEG coherence.

As illustrated very drammatically by the Human Connectome Project, the brain is a structure with massive interconnections between neurons. This connectivity is reflected in functional terms by the fact that connected neurons tend to fire synchronously.

Data illustrates that this synchronicity is performed in bursts which repeat at different frequencies with the frequency defining the functional meaning of connectivity. 

Disrupted synchronisation may be a sign of neurological or psychiatric dysnfunction.  Coherence is an engineering term that refers to the measurement of similarity between two different signals. We can imagine in relation to the EEG signal, that coherence is the measurement of similarity between two electrode sites on the scalp and this can be calculated as a mathematic process.

NeXus 32 with EEG cap and cap adaptor

Kirtley Thornton described how he was able to identify mTBI (mild traumatic brain injury) with 100% accuracy amongst a group of subjects with a coherence measurement approach. 

This technique of quantitative EEG is significant as currently mTBI in particular is very difficult to identify with other technques such as medical imagining.  This is a problem as mTBI may remain undiagnosed and asymptomatic in the early days post injury only to return with a vengence later. Quantitative EEG analysis may allow much earlier intervention.

EEG, Methods, TBIDerek Jones