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A. Valencia-Estrada, C. Balachandran, A. Klistorner, S.L. Graham; Effect of Electrode Position on the Multifocal VEP Amplitude and Signal to Noise Ratio . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4127.
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Purpose: To determine the location of best macular signal in an individual. To compare the effects on m-VEP amplitude and signal to noise ratio, of electrodes centered over the inion and over the position giving largest macular signal (best macular position; BMP). Methods: m-VEP was recorded from 20 normal subjects using 4 occipital electrodes ( positioned at the inion,10% of the nasio-inion distance below, 10% and 20% of the nasio-inion distance above the inion) and one nasion electrode (Veri-Science® Software). After determining the position associated with the largest macular amplitude, m-VEP (Acumap®) was re-recorded with a 4 channel bi-occipital straddle electrode cross configuration (Acumap® system) at this BMP and compared with the responses obtained with the cross centered over the inion. Differences in amplitudes and signal to noise between "On Inion" and BMP were analyzed. Results: All subjects but 1 had the best macular signal with the occipital electrode at 10% of the nasio-inion distance above the Inion. With the electrode cross centered over this BMP there was a significant improvement in full field m-VEP amplitude (6.7%, p=0.03). 60% of the subjects showed an 11% improvement in amplitude. In the inferior hemifield there was a significant improvement in amplitude of 12% (p=0.001), both centrally (8% ± 4%; eccentricity 1° to 12°) and peripherally (18% ± 5%; 12° to 26°). In the superior hemifield there was no significant improvement in amplitude (2%, p=0.6). However, while the central superior field did not show any significant change with electrode position (-4%± 4%; eccentricity 1° to 12°) there was a marked improvement the peripheral field amplitude (18% ± 4%; eccentricity 12° to 26°). Conclusions: Changing the position of the electrodes to the BMP improves the full field amplitude of m-VEP signals. The improvement in the inferior hemifield is probably the result of the electrodes being positioned closer to the superior visual cortex. The selective improvement in the superior peripheral field however, is probably the result of a change in the direction/orientation of the dipoles.
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