Electromagnetic Measures Of Brain Activity

The recording of the group electrical activity of neuronal assemblies is possible in human subjects with the techniques of EEG and MEG. In these techniques, electrical activity is recorded extracranially from the scalp of the individual in a noninvasive way (Bearden 2007). The appeal of these techniques is the extremely high temporal resolution (~msec), which allows one to follow the measurement of global patterns of neural activity in real time. Also, the electrical signal is directly related to the neural activity (in contrast to methods such as PET and fMRI, which rely on indirect assessment of neural activity through vascular effects), although the recorded electrical signals still represent an average over extended regions of the brain. This high temporal resolution makes them ideal candidates for studying synchrony phenomena (Tononi and Edelman 1998; Varela et al. 2001). Although considerable limitations and constraints exist in regard to spatial localization of activity across brain regions, these have mostly been overcome through technical and mathematical means. In spite of this, the spatial resolution, which depends also on the number of electrodes at one's disposal, remains quite poor in comparison with that achievable with fMRI and is on the order of 1 centimeter. Also, the signals are quite weak, so the task has to be repeated a large number of times (tens or even hundreds).

The electromagnetic techniques record the scalp distribution of the field produced by the sum of mainly postsynaptic potentials of the neurons, each of which can be conceptualized as an electric dipole. EEG detects the electrical potential of the field, and MEG detects the magnetic component. The two techniques have very different technical requirements. EEG uses relatively simple equipment, basically a multielectrode helmet, an amplifying and filtering device, and a computer (Ebner et al. 1999). MEG, by contrast, employs cutting-edge technology (Ioannides 2006), because the detection of a magnetic field intensity as weak as the one produced by the brain (~20,000 billion times weaker than the intensity of the Earth's magnetic field) requires the use of superconducting coil units based on superconducting quantum interference device (SQUID) technology. These units need to be specially cooled to a temperature near absolute zero. To avoid the intrusion of electromagnetic interference from the environment, the recording takes place in a room that has been appropriately shielded.

A minimum of 32 scalp electrodes is needed to localize the sources of the recorded potential EEG, but high-density arrays of 128 or 256 electrodes are now common. MEG commonly employs arrays of 100-300 detectors. The intensity of the EEG or MEG response is typically very low, and it takes generally a large number of repetitions (~100) of the task/stimulus presentation (trial) to achieve a sufficient power. The time series from each of the recording units is then segmented according to the stimulus delivery sequence, and these temporally realigned segments are averaged together to obtain a waveform that represents the mean time course of the response to the trial for that recording location (evoked potentials). By combining these waveforms from all of the detectors, one can obtain a map of the surface field distribution at each time interval inside the trial window. Both EEG and MEG are essentially surface techniques: because the intensity of the electromagnetic field decreases rapidly with distance, the detection of neural signals is restricted to the sources closest to the detectors—that is, the neocortex. The activity of subcortical regions is very difficult to detect.

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