A New Recognition Method for the Auditory Evoked Magnetic Fields

Magnetoencephalography (MEG) is a persuasive tool to study the human brain in physiology and psychology. It can be employed to obtain the inference of change between the external environment and the internal psychology, which requires us to recognize different single trial event-related magnetic fields (ERFs) originated from different functional areas of the brain. Current recognition methods for the single trial data are mainly used for event-related potentials (ERPs) in the electroencephalography (EEG). Although the MEG shares the same signal sources with the EEG, much less interference from the other brain tissues may give the MEG an edge in recognition of the ERFs. In this work, we propose a new recognition method for the single trial auditory evoked magnetic fields (AEFs) through enhancing the signal. We find that the signal strength of the single trial AEFs is concentrated in the primary auditory cortex of the temporal lobe, which can be clearly displayed in the 2D images. These 2D images are then recognized by an artificial neural network (ANN) with 100% accuracy, which realizes the automatic recognition for the single trial AEFs. The method not only may be combined with the source estimation algorithm to improve its accuracy but also paves the way for the implementation of the brain-computer interface (BCI) with the MEG.

The effect of harmonization on cortical magnetic responses evoked by music of rapidly changing tonalities

The act of shifting from one key to another is termed tonal modulation, which has been used to articulate emotion expressions and formal structures in Western music. The present study recorded cortical activity to examine how the auditory-evoked magnetic fields are affected by harmonizing music of rapidly changing tonalities. Participants were asked to covertly sing the pitch names of well-learned modulating melodies along with the harmonized or unharmonized melodies. In our musical stimuli, three flats were added to the key signature for every four beats. Such a rapid modulation is achieved by a chromatic inflection of the submediant tone between the third and fourth beats. Tonal modulations with such chromatic progressions are termed chromatic modulations. A major finding was that the amplitude of N1m (neuromagnetic response at approximately 110 ms after the onset of a stimulus) was significantly reduced by harmonization only when a modulation occurred. We also observed that harmonization enhanced the P2m (neuromagnetic response at approximately 200 ms after the onset of a stimulus) amplitude. The results provide evidence of the impacts of harmonization on attention efforts and pitch categorization.