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Digital recording and review techniques have evolved in parallel with the electronics and data-processing revolution of the past 30 years. Digital recording gained popularity over analog EEGs because of several advantages.13 Digital recording takes advantage of modern microprocessor costs and general flexibility. More specifically, it allows electroencephalogram (EEG)-record review with user-selected montages, filters, vertical scaling (gain or sensitivity), and horizontal scaling (e.g., time resolution or compression). It also replaced warehousing or microfilming paper records and allows for electronic exchange of EEGs. Finally, digital EEG makes possible the routine application of a variety of complex signal-processing tasks, such as frequency analysis, automated seizure detection, statistical quantitative analysis, and dipole source localization.

Amplification and Analog-To-Digital Conversion

Recording EEG aims to accurately capture the EEG signals and avoid contaminants. The first step is to boost the microvolt signals recorded at the scalp or occasionally from intracranial electrodes. Amplification by a million-fold or more requires high-quality electronics so as to avoid contamination by other unwanted signals.

Electrode impedances should be kept below 5000 ohms. The amplifier input impedances must be more than 100 megaohms. This allows the amplifiers to work efficiently. Interchannel crosstalk must be <1%, also measured in decibels as 40 dB down or better. Additional electronic noise in the recording should be less than 1.5 μV peak to peak at any frequency from 0.5 to 100.0 Hz, including 50–60 Hz.4

EEG amplifiers are differential: they amplify separately (1) the voltage between one recording site and the ground electrode and (2) the voltage between a second recording site and the ground electrode. Then the electronics subtract one from the other (producing the difference between the two). The subtraction process removes other signals (e.g., 60 Hz line noise) that is present and common to both recording electrodes. The subtraction process itself is imperfect, so that some of the contaminants remain. The perfection of removing the contaminants common to both channels is measured by the common mode rejection ratio (CMRR). The higher the CMRR, the better the amplifier system excludes unwanted contaminants.5 CMRR is measured in decibels of attenuation; a higher value indicates a cleaner signal. CMRR > 100 dB (attenuation by 100,000) are typical of today's amplifier systems.

Many systems now record EEG using a single reference electrode as the second recording channel in the differential system. This allows the subsequent software to change montages at will, a process described further below. In such a system, it is critical that the ground and reference electrode remain well connected.

This initial amplified signal is an analog signal, that is, continuous in time and amplitude like most traditional electrical signals. The signal is filtered to remove further unwanted contaminants. The high filter often is set at 100 Hz and a low filter at 0.16 Hz. These reduce some contaminants, such as muscle ...

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