The EEG Split Alpha Peak: Phenomenological Origins and Methodological Aspects of Detection and Evaluation

Abstract

Electroencephalographic (EEG) patterns were analyzed in a group of ambulatory patients who ranged in age and sex using spectral analysis as well as Directed Transfer Function, a method used to evaluate functional brain connectivity. We tested the impact of window size and choice of reference electrode on the identification of two or more peaks with close frequencies in the spectral power distribution, so called "split alpha." Together with the connectivity analysis, examination of spatiotemporal maps showing the distribution of amplitudes of EEG patterns allowed for better explanation of the mechanisms underlying the generation of split alpha peaks. It was demonstrated that the split alpha spectrum can be generated by two or more independent and interconnected alpha wave generators located in different regions of the cerebral cortex, but not necessarily in the occipital cortex. We also demonstrated the importance of appropriate reference electrode choice during signal recording. In addition, results obtained using the original data were compared with results obtained using re-referenced data, using average reference electrode and reference electrode standardization techniques.

Keywords: average reference; directed transfer function; functional brain connectivity; reference electrode standardization technique (REST); spectral analysis; split EEG alpha peaks.

Figure 1

(A) EEG recording in the first patient, a 27-year-old woman; (B) The power spectral density calculated for 2-, 4-, and 8-s windowed segments of the EEG record; (C) The relative power spectra for five frequency bands (theta: 4–7 Hz; alpha: 7–8, 8–10, 10–13 Hz; beta: 13–25 Hz) for a segment of data.

Figure 2

(A,B) EEG recording and the power spectral density calculated for the 2-s segment of the EEG record in the second patient (28-year-old woman), recorded using two different montages: bipolar and monopolar (S1); (C) The relative power spectra for five frequency bands (theta: 4–7 Hz; alpha: 7–8, 8–10, 10–13 Hz; beta: 13–25 Hz) for a segment of data.

Figure 3

(A) Superimposition of a 1-s segment of the EEG record in the same patient as in Figure 2; (B) Spatiotemporal map of this segment; (C) Six cycles in the EEG recording marked with the letters a-f in (A,B). Individual map corresponds to the relative numbers from 1 to 21 in (A,B).

Figure 4

Relative power spectrum calculated for a 2-s segment of the EEG record in the third patient (54-year-old woman), recorded using two reference electrodes: linked earlobes (A1–A2) and neck (NK). The relative power spectrum is presented in five frequency bands (theta: 4–7 Hz; alpha: 7–8, 8–10, 10–13 Hz; beta: 13–25 Hz).

Figure 5

Power Spectral Density (PSD) for the same EEG segment as in Figure 4 (A), and for the re-referenced data to: (B) linked earlobes reference (A1–A2), (C) average reference electrode (AVERAGE), and (D) reference electrode standardization techniques (REST).

Figure 6

Directed Transfer Function (DTF) strength of outward links for the same EEG segment as in Figure 4 (A), and for the re-referenced data to: (B) linked earlobes reference (A1–A2), (C) average reference electrode (AVERAGE), and (D) reference electrode standardization techniques (REST). Each chart contains graphs that represent the strongest 60% of connections between EEG channels, determined by the magnitudes and directions of the DTF calculated for three frequencies (7, 8, and 10 Hz).

Figure 7

Impact of volume conduction on the identification of split alpha. (A) Directed Transfer Function (DTF) strength of outward links for the transformed data using the CSD transformation of the original EEG with the reference electrode placed at NK. Graphs represent the strongest 60% of connections between EEG channels, determined by the magnitudes and directions of the DTF calculated for three frequencies (7, 8, and 10 Hz). (B) Power Spectral Density (PSD) for the same data as in (A). (C) Pearson correlation coefficients between CSD and four reference electrodes (NK, A1A2, AVERAGE, REST) at every EEG derivation. (D) Comparison of the average strength of outward links calculated for the transformed data using the CSD transform and for each of the four reference electrodes (NK, A1–A2, AVERAGE, REST).