EEG

Intro

Electroencephalography (EEG), which primarily measures neurophysiological changes related to postsynaptic activity in the neocortex [26], has proven to be a powerful tool for studying complex neuropsychiatric disorders [27-30]. EEG has been the primary measure used to capture and characterize epileptiform and abnormal paroxysmal activity through the detection of focal spikes, which occur with increased frequency in ASD [31,32].

Medical Utility

Resting EEG studies have shown that 20% of individuals with ASD show epileptiform discharges at rest, typically without the presence of clinical seizures [33,34]. Higher rates of epileptiform activity have also been reported in sleep studies; for example, Chez, et al. [35] reported that 61% of individuals with ASD and no clinical history of seizures displayed epileptiform abnormalities.

Frequency Band Classification

The most common way to characterize resting EEG is by breaking down the oscillatory patterns into bands of frequencies that share physiological properties. The typical clinically relevant frequency bands of EEG range from 0.3 to 100 Hz. Within the scope of the current paper, we focus on five frequency bands ranging from 1 to 100 Hz:

• delta (1 to 3 Hz),
• theta (4 to 7 Hz),
• alpha (8 to 12 Hz),
• beta (13 to 35 Hz), and
• gamma (>35 Hz).

These historically documented frequency bands have attracted rapidly growing interest in clinical and cognitive neuroscience fields, and are believed to govern different cognitive processes [36].

Frequency Band Use Cases

• Delta dominates deep sleep, and is thought to underlie the event-related slow waves seen in tasks for detection of attention and salience [37].
• Theta is most commonly studied in relation to memory processes [38].
• Alpha waves are present in relaxed awake individuals, and are associated with precise timing of sensory and cognitive inhibition [39].
• Beta waves are associated with alertness, active task engagement, and motor behavior [40].
• gamma waves, present during working-memory matching [41] and a variety of early sensory responses, are believed to facilitate feature binding in sensory processing [42,43].

source: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3847481/

need: add links to citations

SPECIFICITY AND SENSITIVITY OF ROUTINE EEG

Sensitivity and Specificity of EEG

Epileptiform activity is specific, but not sensitive, for diagnosis of epilepsy as the cause of a transient loss of consciousness or other paroxysmal event that is clinically likely to be epilepsy. EEG has relatively low sensitivity in epilepsy, ranging between 25–56%. Specificity is better, but again variable at 78–98%. These wide ranges can be explained partly by diverse case selection and differences in clinical requirements for diagnosis of epilepsy in population studies of EEG specificity and sensitivity. Secondly, correlation between different EEG patterns and epilepsy varies, and only IED are associated with seizure disorders at a sufficiently high rate to be of clinical use. Abnormalities of background cerebral rhythms, focal slow activity or regional attenuation are much less specific than epileptiform activity, although they can indicate localised structural pathology underlying the seizure disorder, or diffuse cortical dysfunction as in symptomatic generalised epilepsies. Some types of epileptiform phenomena—3 per second spike wave discharge, hypsarrhythmia, and generalised photoparoxysmal response—are strongly correlated with clinical epilepsy, whereas focal sharp waves in centro-temporal or occipital regions have moderate association with clinically active epilepsy. Of children with centro-temporal or rolandic EEG discharges, only about 40% have clinically expressed seizures. Spikey or rhythmic phenomena such as 14 and 6 Hz spikes, phantom spike and wave, rhythmic mid temporal theta (θ), psychomotor variant and subclinical rhythmic epileptiform discharge in adults (SREDA), have low or zero predictive value for epilepsy. Misinterpretation of such non-epileptogenic phenomena, or overinterpretation of non-specific EEG abnormalities and spiky/paroxysmal variants of normal cerebral rhythms, are a common reason for over-diagnosis of epilepsy

EEG Application

This problem stems from the fact that EEG recording procedures remained very similar than that of the early EEG days. The EEG is recorded using Ag/AgCl electrodes that are in contact with the skin (i.e., scalp) through electrolytic gel (Webster, 1997). The electrolyte performs dual task, it bridges the ionic current flow from the scalp and the electron flow in the Ag/AgCl electrode and ‘glues’ the electrode to the scalp. To further improve signal quality the scalp is frequently cleaned and, especially in clinical applications, skin on the scalp is abraded. The abrasion process, as well as the usage of conductive gel (electrolyte) makes the whole EEG setup inconvenient for practical application, especially for consumer applications. The application of electrolyte and the electrodes, even when typical EEG caps are used, cannot be performed by a user, requiring expert assistance. The setup process is lengthy as it includes preparing the skin, applying the gel, positioning the electrodes (or the cap) and ensuring that the EEG signal quality level is acceptable. Additionally, the user (or expert) has to remove the electrolyte and clean the user’s head afterwards, and also clean and dry the electrodes (and the cap) that were used. This also takes time and requires additional effort