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Clinical Neurophysiological Recording Methods

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Figure 7–2. Illustration of the cortical mantle in the coronal plane.Electrical dipoles (dashed arrows) and the magnetic fields (circular arrows around two such dipoles) generated by cortical columns are illustrated. Radially oriented (gyral) electric dipoles project to the scalp surface, but their magnetic fields remain tangentially oriented with respect to the scalp surface and appear at some distance from the dipole generating them. Tangentially oriented (sulcal) electric dipoles do not project to the scalp surface directly overlying them, but their magnetic fields do. Electrical dipoles are attenuated and diffused by the tissues through which they must pass before appearing at the scalp surface; magnetic fields do not suffer this attenuation and diffusion, but their strength falls off at 1/r2, where r = radius from the dipole source.

Figure 7–3. The 10–20 International System of Electrode Placement.Electrodes are labeled according to their approximate locations over the hemispheres (F = frontal, T = temporal, C = central, P = parietal, and O = occipital; z designates midline); left is indicated by odd numbers and right by even numbers. A parasagittal line running between the nasion and inion and a coronal line between the preauricular points is measured. Electrode placements occur along these lines at distances of 10% and 20% of their lengths, as illustrated. In most clinical laboratories, the Fpz and Oz electrodes are not placed, but are instead used only as reference points. Fp1 is placed posterior to Fpz at a distance equal to 10% of the length of the line between Fpz- T3-Oz; F7 is placed behind Fp1 by 20% of the length of that line. O1 is placed anterior to Oz at a distance equal to 10% of the length of the line between Oz-T3-Fpz; T5 is placed anterior to O1 by 20% of the length of that line. F3 is placed halfway between Fp1 and C3 along the line created between Fp1-C3-O1; P3 is placed halfway between O1 and C3 along that same line. Right hemisphere electrodes are placed in similar fashion. Reference electrodes, in this case placed on the ears, are labeled A1 and A2.

Figure 7–4. An example of spectral mapping.This map describes relative power (percentage of total power) in the right hemisphere across several frequency ranges in a 25-year-old male with diffuse intermixed slowing on visual inspection of the electroencephalography record.

Figure 7–5. P30, P50, and N100 evoked potentials to a short-duration, moderate-intensity, broad-frequency binaural stimulus in a 34-year-old male control subject.The actual latencies of these evoked potentials vary from their stated latency by about 10 ms; this degree of variability is normal and is expected in most recordings. The low-amplitude N100 in this tracing is "split," meaning that two definable but partially overlapping waveforms contribute to the evoked potential observed in this tracing.

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