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A. Llinas, X. Zhang, J. M. Liebmann, R. Ritch, R. Llinas, D. C. Hood; A Comparison of Multifocal Magentoencephalographic (mfMEG) and Multifocal Visual Evoked Potential (mfVEP) Recordings From Glaucoma Patients With Monocular Visual Field Defects. Invest. Ophthalmol. Vis. Sci. 2007;48(13):220.
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To assess whether the mfMEG can be used to study the cortical consequences of glaucomatous damage, we compared a novel technology, multifocal visually evoked magentoencephalography (mfMEG) to multifocal visual evoked potentials (mfVEP) recorded from patients with asymmetric visual field defects.
mfMEG and mfVEP recordings were obtained from both eyes of 4 patients with unilateral glaucomatous field defects. The patients had glaucomatous damage (abnormal disc and abnormal 24-2 field) and a mean deviation of at least -4db in the affected eye. The better eye had a normal 24-2 field. The same 60-sector, pattern-reversal dartboard array with a diameter of 44.5 degrees was used for both mfMEG and mfVEP recordings. These recordings consisted of 2 runs of either 4 minutes (mfMEG) or 8 minutes (mfVEP) per eye. For the mfVEP, recording electrodes were placed at the inion (I) and I+4 cm, and also at two lateral locations up 1 cm and over 4 cm from I. For the mfMEG, 274 detectors were used. For the mfVEP, the recordings from the best channel (i.e. highest signal/noise [SNR]) was used for each location. For the mfMEG, independent component analysis (ICA) was applied and the best combination of ICs taken for each location. For both mfVEP and mfMEG recordings, the SNR at each location was obtained by comparing the responses in the signal (40 to 150 msec) and noise windows (325 to 430 msec).
The SNR of the mfMEG was about the same as that of the mfVEP. On average, the SNR of the mfMEG was greater than the SNR for the mfVEP in half of both the better and affected eyes. Overall, the mean SNR was 4.1 (mfMEG) and 4.0 (mfVEP) for the better eyes and 2.7 (mfMEG) and 3.0 (mfVEP) for the more affected eyes. Both the mfVEP and mfMEG showed the characteristic decrease in SNR with decrease in local 24-2 visual field sensitivity. The correlation (Spearman r) between the mfVEP and mfMEG for the better eye ranged from 0.58 to 0.35.
On average, the SNR of the mfMEG was about the same as that of the mfVEP, even though the recording time for the mfMEG was only one-half as long. Given that the sources of the mfMEG can be better localized than those of the mfVEP, the mfMEG may prove valuable in better understanding the cortical consequences of glaucomatous damage.
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