Hereafter, this theoretical result has been thought true for actual human head as “ it is important to note that the dipolar nature of ERP components means that every component is actually positive over some parts of the head and negative over other parts, summing to zero over the entirety of the head” (Luck 2014). The assumption of ‘sum to zero’ behind AR was partly buttressed by the demonstration that the surface potential integral of a dipole in a layered spherical surface is zero (Bertrand et al.
REFERENTIAL MONTAGE DEFINITION OFFLINE
And it is now implemented by offline re-referencing instead of the original online recording setup. The AR is currently one of the most widely adopted references. Experience with the average monopolar reference electrode shows that this is usually approached in practice” (Offner 1950). In 1950, the first clinical use of AR was reported (Goldman 1950), and it stated that “ if the EEG sources consist of a large number of randomly placed and randomly oriented dipoles, a rather constantly zero average will be obtained over the surface of the scalp. This suggested the average reference (AR) by connecting all EEG electrodes through high resistances in order and then taking the common junction as a reference. The Wilson EKG common terminal reference sought for a zero-potential reference by combining leads from three limbs.
In 1940s, a better reference was inspired by the EKG technology. These attempts were similarly problematic due to the contamination from EMG and EKG and the difficulty of interpreting the field maps. Many other body sites have been explored, such as the angle of the jaw, the chin, the tip of the nose, and the neck, etc. It illustrates that the intuitively appealing reference may be fraught with difficulties. Furthermore, this reference near the neck tends to pick up electromyography (EMG) and electrocardiogram (EKG) artifacts (Luck 2014). 1988), confounding the interpretation of results (Shaw 1984 Travis 1994). Subsequently, it was shown that LM seriously biases EEG power (Niedermeyer 1987) and coherence spectra (Fein et al. However, the authors failed to accurately localize the origin of psychomotor seizures as the LM distorted the field maps (Faux et al. This work spurred international interest in the role of EEG in clinical epilepsy and firmly linked the term “psychomotor epilepsy” to a specific EEG pattern. In 1930s, EEG recordings with LM were used by Gibbs and Lennox to study grand mal and psychomotor (partial complex) seizures (Gibbs et al. A case in point is the ever popular linked-mastoids/ears (LM) reference. A thorough discussion of the advantages and limitations of references is provided with recommendations in the hope to clarify the role of each reference in the ERP and EEG practice.Įarly, there are misleading examples of poor references. 2019) that there is a general form for the reference problem, the ‘no memory’ property of the unipolar references, and a unified estimator for the potentials at infinity termed as the regularized REST (rREST) which has more advantageous statistical evidence than AR. We show that each reference is derived with a different assumption and serves different aims. Examples of unipolar references are the reference electrode standardization technique (REST), average reference (AR), and linked-mastoids/ears reference (LM) (2) non-unipolar references that include the bipolar reference and the Laplacian reference. In this paper current popular references are classified into two categories: (1) unipolar references that construct a neutral reference, including both online unipolar references and offline re-references.
) is thus necessary to provide application-oriented principled recommendations. A comprehensive review accompanied by a brief communication with rigorous derivations and notable properties (Hu et al. This diversity seriously undermines the reproducibility and comparability of results across laboratories. Consequently, more than ten references are used in the present EEG-ERP studies. The ideal reference should be the one with zero or constant potential but unfortunately it is well known that no point on the body fulfills this condition. Which reference is appropriate for the scalp ERP and EEG studies? This unsettled problem still inspires unceasing debate.
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