Electroencephalography recording

Surface EEG recordings (chapters 3-6) were performed using 9 mm Ag-AgCI electrodes attached to the scalp with collodion (SUE diagnostics, Surrey, U.K.). As with EMG electrode placement, the skin was prepared beforehand with isopropyl alcohol and gentle abrasion. Once placed, electrodes remained fixed for the duration of an experiment.

For the experiments in chapters 3-6, surface electrodes were positioned with reference to the International 10-20 System (Misulis, 1997). Electrode montages varied slightly between experiments in terms of EEG recording sites. However, the method of positioning an electrode at each site remained constant. C3 and C4 were positioned using single pulse TMS to locate the left and right motor hand areas, respectively, and electrodes placed at these sites. Further electrodes were placed 2.5cm apart with reference to C3 and C4. These sites were termed, from anterior to posterior, F3/F4, FC3/FC4, and CP3/CP4, in correspondence (+/- 1 cm) with the 10-20 system. EEG electrodes were also sited directly at FCZ, CZ, P3, P4, P03 and 01 (figure 2.1). F3/4, FC3/4, C3/4, and FZ are likely to overlie the dorsolateral prefrontal and lateral premotor areas, primary sensorimotor cortices and medial prefrontal cortex, respectively, whereas FCZ and CZ were considered to overlie mesial motor areas, particularly the cingulate and supplementary motor areas (Homan et al., 1987; Steinmetz et al., 1989; Andres et al., 1999; Asada et al., 1999).

In chapter 7, a different technique of electrode placement was used. Scalp EEG was recorded from 64 (10 mm Ag-AgCI) surface electrodes mounted on a cap (Easy Cap, Falk Minow Services, Germany) with linked mastoid references (figure 2.2). Conducting gel (Abralyt 2000, Falk Minow Services, Germany) was applied to the skin after gentle abrasion with isopropyl alcohol. Electrode impedance was kept below 5 kü, monitored using Neuroscan software (SCAN 4.2, Neurosoft Inc. U.S.A.). In addition, eye movements were recorded with vertical electro-oculogram (VEOG) electrodes.

F 4 # F3 # F C 4 # F C 3 # CZ # C 4 # C3 • GP4# CP3 # P 4 « $ P03

Figure 2.1. The position of EEG electrodes used for experiments in chapters 3-6. Electrode locations are based on the International 10-20 system.

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Figure 2.2. The EEG electrode array used in chapter 7. The 64 surface electrodes were mounted on a cap worn by the subject. VEOG = vertical electro-oculogram electrodes.

Electrical currents can spread throughout the conducting tissues of the brain. Scalp EEG therefore reflects intracranial electrical potentials and thereby the activity of neural generators within the brain, although high frequencies may be filtered out by the dura, skull and scalp. Scalp EEG is measured against specific reference electrodes. The underlying assumption is that the reference should be electrically quiet, which is not always the case (Nunez et al., 1997). It has become apparent that the effects of volume conduction (Nunez et al., 1997) and the use of common reference electrodes (Fein et al., 1988; Rappelsberger and Petsche, 1988; Classen et al., 1998) may distort coherence estimates and thus the interpretation of such analysis.

Volume conduction and common reference inflate coherence estimates (Nunez et al., 1997). In addition, they may lead to misleading changes in coherence during dynamic conditions. For example, alpha and beta frequency EEG oscillatory activity over the premotor and primary sensorimotor areas in humans typically decreases in power during motor tasks (event-related desynchronisation; Pfurtscheller et al., 1994,1997; Salmelin and Hah, 1994; Toro et al., 1994a,b; Stancak and Pfurtscheller, 1996a,b,c; Leocani et al., 1997). This might cause a relative increase of the common reference signal and/or occipital alpha (volume-conducted) activity contamination over the areas of interest. This increase in the ratio of common signals, either volume-conducted or reference, might result in an apparent increase of coherence without a change in functional coupling (Andrew and Pfurtscheller, 1996).

There are several different methods that can be used to derive EEG: standard common reference, averaged reference, linked earlobe reference, balanced non-cephalic electrode reference, bipolar recording and current source density estimation (Laplacian derivatives). However, there is considerable debate over which is the most appropriate way to avoid inflated EEG-EEG coherence estimates (Nunez et al., 1997; Nunez et al.,

1999). Laplacian derivatives, which remove the common reference effect, may, on the contrary, lead to an underestimate of inter-regional EEG coherence especially for low frequencies including alpha (Mima et al., 2000a).

In chapters 3 and 4, a bipolar derivation was adopted. Electrodes were linked to form a chain of three bipolar leads on each side (F3-FC3, FC3-C3, C3-CP3, F4-FC4, FC4-C4, C4-CP4) and a single ipsilateral occipital bipolar lead (P03-01). By using bipolar leads without common electrodes for coherence analysis, the use of a common reference was avoided, but this technique also avoided excessive spatial filtering. The effects of volume conduction were further limited by looking for a change in EEG-EEG coherence following rTMS. In chapters 5 and 6, linked earlobe electrodes were used as a

reference, and in chapter 7, linked mastoid electrodes. This procedure might have introduced a common signal to all other channels, leading to inflated coherence estimates. However, this was limited by using a subtractive approach, with the assumption that movement-related activity is not picked up by the reference electrodes. Therefore, in order to separate the task-related coherence from the background coherence, the values of the resting state were subtracted from those of the active state. This subtraction method also reduces between-subject differences.