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This discussion of some of the issues relevant to the recording of the EEG, and the discussion of the issues involved in signal extraction and analysis are derived from only a few of the many papers written on the subject (Coles, Gratton, Kramer, & Miller, 1986; Coles & Rugg, 1995; Garnsey, 1993; Picton, et al., 1994; Rugg & Coles, 1995). The EEG is a record of the potential difference recorded between two points on the skin. Contact with the skin is made through recording electrodes. Prior to electrode attachment the skin must be prepared so as to reduce the impedance of the electrode connection. Picton et al (1994) suggest that typically the unprepared skin has an impedance of some 50 KO. Such a high impedance results in an

unacceptable level of electromagnetic artifact in the EEG. Generally, this impedance is reduced to 5 KQ or below, either by using an abrasive paste or by puncturing the skin. Electrodes are

generally held in place either with an adhesive or by mechanical pressure exerted by means of t an elasticated cap.

The connection between skin and electrode is made indirectly through an electrode jelly.This indirect connection acts to stabilise the skin-electrode interface and thus prevents the

introduction of artifacts as a result of changes in the characteristics of the interface. An exchange of ions occurs between the electrode and the jelly (and between the jelly and the skin), the characteristics of which depend upon the metal used to make the electrodes. By acting as a low frequency filter these ion-exchanges can significantly affect the characteristics of the EEG recorded. This filtering effect can be minimised by using 'reversible' electrodes. EEG is recorded using a 'differential amplifier' having an input channel for each site from which EEG is to be recorded. A differential amplifier amplifies the difference between two signals. Hence any signal which is common to both signals should not be amplified. The degree to which the amplifier is successful at removing such common signals is measured by the Common Mode Rejection Ratio. Each 'active' electrode is connected to one side of one channel of the differential amplifier.

Typically, the other side of each amplifier channel is connected to the 'reference' electrode. The reference electrode may be 'virtual' in that it may consist of the average of two electrodes one

placed on each of the mastoid processes (a 'linked mastoid' reference), or on the the chest and i lower neck (a 'non-cephalic' reference), or the reference may be taken to be the average

potential of all the active sites (an 'average' reference). The potential at each 'active' site is measured with respect to the potential at the reference. The position of the reference therefore determines the absolute potential difference measured between two points. The potential measured at an active site may be positive with respect to one reference, but negative with

respect to a second reference. For this reason the locations at which a change in absolute polarity occurs are not informative. Such a location may change whenever the location of the reference changed. However the rate of change of potential over space is constant regardless of the absolute potential difference recorded between the active and reference electrodes. The choice of any reference has consequences for both the signal recorded and its interpretation. A linked mastoid reference, as used in the present experiments, may allow the 'shunting' of current from one side of the head to the other and consequently distort the signal being measured. It has been argued that this rarely happens in practice (Miller, Lutzenberg and Elbert, 1991, cited in Picton et al, 1994, p.434).

In addition to the EEG recorded from each active site, an electro-oculogram (EOG) is also recorded. When the EOG channel exceeds a specified voltage limit, the EEG for that trial may be rejected on the grounds that the EEG will contain too large a degree of EOG artefact, and, as a result, the signal-to-noise ratio would be unacceptably low. Alternatively, an attempt may be made to assess the nature of the EOG artefact in the EEG and remove it. In the experiments reported here, experimental trials contaminated with EOG were excluded from further analysis. Generally one electrode on the scalp is attached to a ground connection. This acts as the centrepoint of the differential amplifier.

The signals emanating from the differential amplifier are usually subject to analogue filtering. Both low-pass and high-pass filters are generally applied; the former attenuates components of the waveform with frequencies present in the waveform above those of interest, whilst the latter attenuates components of the wavefom with frequencies below those of interest. The

characteristics of analogue filters are often specified by reference to the frequency which the filter attenuates to some 70% of its unfiltered value. Low pass filters reduce the amount of muscle activity in the signal, generally derived from the face and neck muscles, which otherwise reduces the signal-to-noise ratio. A low pass filter also attenuates interference derived from the electrical mains. Such interference may also be removed using a 50 Hz 'notch' filter. Because analogue filters do not have discrete cut-off frequencies they may also attenuate frequencies in the waveform that result from brain activity. Furthermore because they respond differently to different frequencies, analogue filters may also distort the temporal relationship between different components of the waveform.

Following amplification and filtering, the EEG signal is generally passed to an analogue-to- digital converter (ADC). The ADC converts the analogue EEG voltage into a digital value. The nature of this conversion depends upon the resolution of the ADC in respect of the frequency with which the ADC samples the incoming voltage and the minimum voltage difference between two adjacent samples that can be discriminated. If the sampling frequency is not at least twice the highest frequency in the waveform being sampled (the Nyquist frequency), then the waveform will be distorted due to an aliasing bias in which higher frequency components

will appear as a lower harmonic. The ability of the ADC to discriminate different voltages depends upon the number of'bits' available for the conversion. In a typical 12 bit converter, the voltage may take on any of 2^^ or 4096 different values. With a typical dynamic range of +/- 5V, this means that the ADC discriminates input voltages that differ by 2.4mV. If the signal has been amplified by a factor of 20000, then the digitised waveform discriminates between

voltages which differ by 0.122 pV at the scalp.