Neurochemical regulation of auditory information ... - Helda - Helsinki.fi

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Neurochemical regulation of auditory information ... - Helda - Helsinki.fi

1.2. Non-invasive methods for studying neural correlates

of information processing

Electroencephalography (EEG) and magnetoencephalography (MEG)

The EEG and MEG are non-invasive neurophysiological techniques, which allow

detection of changes in human brain activity with millisecond temporal resolution. The

EEG response represents spontaneous electrical activity, produced by large populations

of neurons, and is usually recorded by means of silver-chloride electrodes placed on

the scalp. Because of the electrical conductive features of the brain tissues, electrical

signals, even from the deep brain structures, can be recorded at the surface of the human

head. Hans Berger, who was the first to record the non-invasive EEG in humans in

1926, aimed to use the EEG for the diagnosis of psychiatric disorders; however, at the

present time, the EEG is used as the most powerful tool in identifying brain epileptic

activity. Both the EEG and MEG are generated with the same neuronal mechanism;

however, they can detect some different aspects of the electromagnetic field. Electric

currents produced by synchronized neuronal currents are measurable with the EEG,

whereas MEG measures magnetic fields produced by electrical currents. These produced

magnetic fields are very weak. As a result, in order to record the brain’s magnetic

responses, special superconductive devices must be used. Specially constructed

electrically shielded rooms, specifically designed gradiometers measuring the magnetic

field gradient, advanced superconducting quantum interference devices (SQUIDs), and

the markedly increased sensitivity of the MEG method, led to the start of MEG research

in the late 1970s (for review, see Makela et al. 2006). The localization of the MEG brain

activity sources is less affected by the distortions caused by the skull and tissues than

the EEG source localization (Banaschewski and Brandeis 2007). Simultaneous EEG/

MEG recording has more advantages than single EEG or MEG recordings (Sharon et al.

2007). For example, Liu et al. (2002), in their Monte Carlo stimulation study, showed

that combination of the EEG/MEG provides better localization accuracy than the use

of the EEG or MEG alone. Clinically oriented studies with epilepsy patients confirm

these results (Bast et al. 2007; Yoshinaga et al. 2002). At the moment, the application

of the MEG, as is the case with the EEG (brain-stem auditory evoked potentials –

BAEPs, somatosensory evoked potentials – SEPs, visual evoked potentials – VEPs, and

others), is related to the neurological evaluation; however, the MEG is most popularly

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