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Multifocal Topographical Evoked Potential Mapping

1Gockeln R., 1Riegert S., 1Tutschke I., 2Winter R.,
1Medizinische Hochschule Hannover, Augenheilkunde, Elektrophysiologisches Labor (Hannover)
2Medizinische Hochschule Hannover, Augenheilkunde (Hannover)

Purpose: This study illustrates how multifocal visual evoked components are derived from multichannel bipolar recordings and are analysed topographically. In addition to the potential wave shapes, the latency (ms) and the wavelengths (mV) of the components will also be influenced by the cortical site of the recordings.
Methods: Multifocal VEPs were recorded from different parts of the visual field (up to 30° of eccentricity) of both eyes of fifty normosensoric subjects; stimulating each eye separately. The pseudorandom alternated stimulus array was produced with a multichannel Roland Consults system (Retiscan, Wiesbaden/Brandenburg, Germany). The stimulus consisted of 61 sectors, each with 16 checks, 8 white ( > 130 cd/m2 ) and 8 black ( < 2 cd/m2). For electrode placement we used a bipolar (occipito-occipital) arrangement. The visual evoked wave shapes were recorded simultaneously from a row of four scalp electrodes placed along the sagital midline and four scalp electrodes placed along the transversal axis of the occipital cortex. Each of the eight electrodes could be used as the negative or the positive electrode.
Results: The data of these study show, that different recording channels display different average peak latencies and amplitudes ranging over the occipital cortex between 79,5 ms / 0,086 mV (4 cm inferior to 4 cm superior the inions) and 106 ms / 0,143 mV (2 cm inferior to 2 cm superior the inions). During recording of the mVEPs along the transverse axis one could observe a reduction of the amplitude maximum compared to the sagital axis. On the other hand, we found differences in the electrode location of the evoked components when different retinal areas are stimulated.
Conclusions: The multifocal VEP demonstrates a good correlation with the topography of the visual field by bipolar occipital recordings along the sagital midline. The bipolar recording site 2 cm inferior (negative electrode) and 2 cm superior to the inions seems to lead to more compatible results. For more accurate statements concerning objective visual field defects, one should evaluate the superior, inferior and the lateral hemifields separately.