Hearing the Sound in the Brain: Influences of Different EEG References

Abstract

If the scalp potential signals, the electroencephalogram (EEG), are due to neural "singers" in the brain, how could we listen to them with less distortion? One crucial point is that the data recording on the scalp should be faithful and accurate, thus the choice of reference electrode is a vital factor determining the faithfulness of the data. In this study, music on the scalp derived from data in the brain using three different reference electrodes were compared, including approximate zero reference-reference electrode standardization technique (REST), average reference (AR), and linked mastoids reference (LM). The classic music pieces in waveform format were used as simulated sources inside a head model, and they were forward calculated to scalp as standard potential recordings, i.e., waveform format music from the brain with true zero reference. Then these scalp music was re-referenced into REST, AR, and LM based data, and compared with the original forward data (true zero reference). For real data, the EEG recorded in an orthodontic pain control experiment were utilized for music generation with the three references, and the scale free index (SFI) of these music pieces were compared. The results showed that in the simulation for only one source, different references do not change the music/waveform; for two sources or more, REST provide the most faithful music/waveform to the original ones inside the brain, and the distortions caused by AR and LM were spatial locations of both source and scalp electrode dependent. The brainwave music from the real EEG data showed that REST and AR make the differences of SFI between two states more recognized and found the frontal is the main region that producing the music. In conclusion, REST can reconstruct the true signals approximately, and it can be used to help to listen to the true voice of the neural singers in the brain.

Keywords: EEG; brainwave music; reference electrode; reference electrode standardization technique (REST); scale-free.

Figure 1

Music materials for simulation in MIDI and waveform format (10 s for example). (A) MIDI and waveform of Bach, BWV 772, with one voice. (B) MIDI and waveform of Bach, BWV 772, with two voices. (C) MIDI and waveform of Mozart, K387, with four instruments.

Figure 2

Comparison of the three reference electrode methods for the one source of BWV772 shown in Figure 1A. (A) The location of the source is (−0.65674, −0.06352, 0.56552). The red dot indicates the projected position of the source on the scalp surface. (B) Relative errors of the music signals with three references compared to the standard signals. (C) Correlation coefficients of three references with the standard signals.

Figure 3

Comparison of three reference electrode methods for the two sources of BWV 772 shown in Figure 1B. (A) The location of the sources are (−0.264, 0.352, 0.750) and (0.278, 0.457, 0.689). The red dots indicate the projected positions of the source on the scalp surface. (B) Relative errors of the music signals with three references compared to the standard signals. (C) Correlation coefficients of three references with the standard signals.

Figure 4

Comparison of the three reference electrode methods for the two sources of BWV 772 shown in Figure 1B. (A) The location of the sources are (0.254, 0.124, 0.822) and (−0.678, −0.267, 0.473). The red dots indicate the projected positions of the source on the scalp surface. (B) Relative errors of the music signals with three references compared to the standard signals. (C) Correlation coefficients of three references with the standard signals.

Figure 5

Comparison of the three reference electrode methods for the four sources of K387 shown in Figure 1C. (A) The location of the sources are (0.087, 0.859, 0.097), (0, 0, −0.076), (0.342, 0.643, 0.473) and (0.495, −0.638, 0.320). The red dots indicate the projected positions of the source on the scalp surface. (B) Relative errors of the music signals with three references compared to the standard signals. (C) Correlation coefficients of three references with the standard signals.

Figure 6

Comparison of the three reference electrode methods for the four sources of K387 shown in Figure 1C. (A) The location of the sources are (0.087, 0.859, 0.097), (−0.865, −0.030, −0.076), (0.495, −0.638, 0.320) and (0.342, 0.643, 0.473). The red dots indicate the projected positions of the source on the scalp surface. (B) Relative errors of the music signals with three references compared to the standard signals. (C) Correlation coefficients of three references with the standard signals.

Figure 7

The scale-free exponents of the brainwave music pitch. The scale free exponents of the music pitch were calculated by the Zipf's law. Group 1 was the brainwave music group, while group 2 was the control group. For REST and AR, there were significant differences between the groups. (t-test, ^*p < 0.05).

Figure 8

The topographic map of the probability of electrodes being represented for REST, AR and LM references. The topographic map was the average probability for 24 subjects, group 1 (12 subjects) was the brainwave music group, group 2 (12 subjects) was the control group.