Eeg Montages

In the recording of the EEG, electrodes are typically placed on the scalp using the 10-20 system (Fig. 5). In this standardized method, contacts are named by their location (frontopolar, frontal, central, parietal, temporal, occipital, and auricular). They are also numbered with odd numbers over the left hemisphere, even numbers over the right, and z referring to the midline. The particular sequence in which the EEG data is displayed is called the montage. Montages may be bipolar or referential. Bipolar montages involve a comparison of voltages recorded from (usually adjacent) active electrodes in a chain-like fashion, (e.g., front-to-back or side-to-side). By contrast, referential recordings involve a comparison of each electrode to an (ideally) inactive electrode. Examples of reference sites include the ear, the mastoid, the vertex, or an average of many active leads ("average reference").

Bipolar montages visually map the peak of voltage negativity over the scalp owing to the property of phase reversal that emerges from the serial comparisons between adjacent electrodes along a chain. As one ascends and descends the underlying voltage "hill" with each bipolar comparison along a chain, the sign of the comparison flips from positive to negative (Fig. 6). Referential recordings do not exhibit phase reversals, but may show a truer picture of the relative amplitudes of voltage at each electrode. Negativity at the "active" lead is defined as an upward deflection in the display. In addition to displaying the channels based on comparisons among the electrodes, other channels are often used to permit comonitoring of cardiac rhythm, eye movement, respiration, and EMG activity as needed for the study at hand.

Sphenoidal Electrode Placement

Fig. 5. The International 10-20 system of electrode placement. This figure depicts an overhead view of the most commonly used array of standardized electrodes in EEG. Odd numbers refer to the left, even numbers to the right. Electrodes are named according to location: F, frontal; C, central; P, parietal; T, temporal; Fp, frontopolar; O, occipital; and A, auricular. Sphenoidal leads (Sp) and supplemental temporal (T1, T2) leads are not shown.

Fig. 5. The International 10-20 system of electrode placement. This figure depicts an overhead view of the most commonly used array of standardized electrodes in EEG. Odd numbers refer to the left, even numbers to the right. Electrodes are named according to location: F, frontal; C, central; P, parietal; T, temporal; Fp, frontopolar; O, occipital; and A, auricular. Sphenoidal leads (Sp) and supplemental temporal (T1, T2) leads are not shown.

Eeg Montages

Fig. 6. Bipolar EEG recordings and phase reversal. A hypothetical region of scalp negativity is illustrated topologically, as a voltage "hill" (A). The higher the hill, the greater the negative field potential at that point along the anterior-posterior axis, with electrode A most anterior, and electrode E most posterior. The hill peaks at C. In (B), the waveforms derived from bipolar comparisons between adjacent electrodes are illustrated, as in a bipolar recording. By convention, if contact 1 is more negative than contact 2, the deflection is upward. Because these comparisons are performed serially along the anterior-posterior axis, the difference between adjacent channels flips polarity as one traverses the peak of the hill. The flip in polarity leads to the highly visible phase reversal on a bipolar tracing, denoted by the star in (b). The shared contact of the phase reversal maps the voltage peak, electrode C in this case.

Fig. 6. Bipolar EEG recordings and phase reversal. A hypothetical region of scalp negativity is illustrated topologically, as a voltage "hill" (A). The higher the hill, the greater the negative field potential at that point along the anterior-posterior axis, with electrode A most anterior, and electrode E most posterior. The hill peaks at C. In (B), the waveforms derived from bipolar comparisons between adjacent electrodes are illustrated, as in a bipolar recording. By convention, if contact 1 is more negative than contact 2, the deflection is upward. Because these comparisons are performed serially along the anterior-posterior axis, the difference between adjacent channels flips polarity as one traverses the peak of the hill. The flip in polarity leads to the highly visible phase reversal on a bipolar tracing, denoted by the star in (b). The shared contact of the phase reversal maps the voltage peak, electrode C in this case.

Fig. 7. Analog vs digital. The upper curve depicts an analog signal with variable amplitude over time. Digitization relies on using discrete intervals along both the x- and y-axes and making sampling measurements of the analog signal at discrete time intervals and assigning discrete amplitude values. The ability of a digitized output signal to faithfully resemble its analog input depends on the fineness of these discrete steps (resolution) along these axes. Sampling at greater than twice the Nyquist frequency ensures an adequate degree of digital sampling to minimize aliasing.

Fig. 7. Analog vs digital. The upper curve depicts an analog signal with variable amplitude over time. Digitization relies on using discrete intervals along both the x- and y-axes and making sampling measurements of the analog signal at discrete time intervals and assigning discrete amplitude values. The ability of a digitized output signal to faithfully resemble its analog input depends on the fineness of these discrete steps (resolution) along these axes. Sampling at greater than twice the Nyquist frequency ensures an adequate degree of digital sampling to minimize aliasing.

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