Purchase this article with an account.
Vaegan, J. Zheng; Optimised Magnocellular Targeted Steady State Sweep VEPs Thresholds in Early Glaucoma Detection. Invest. Ophthalmol. Vis. Sci. 2007;48(13):213.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
To maximize steady state (SS) swept contrast visual evoked potential (VEP) signal noise ratios (SNRs) to magnocellular targeted stimuli. To compare how estimated VEP and corresponding subjective thresholds detect early glaucoma.
We chose electrode type, temporal frequency, modulation mode and channel selection methods giving best SNR. Both eyes of 5-9 young normals were used per study. Vertical 0.25 cy/° sine profile gratings in 34x20°, were reversed sinusoidally in time at 7.5, 12.5, 18.5 or 37.5 RPS or appeared/disappeared at the same rate in Hz. Monocular VEPs (15 sweeps, 64-1% contrast, octave steps, 4x1sec traces) were averaged from 4 almost orthogonal channels (Fpz, Oz, MTR, MTL, (inion-1cm) reference). SNR was (FFT amplitude at signal frequency)/(mean of the 2 adjacent frequencies). T2 circ and a Pythagorean sum channel were also calculated. VEP thresholds were estimated by exponential fits to the rising phase of the amplitude by contrast curve and subjective thresholds to the same stimuli by QUEST in a 2AFC task. Both eyes of 58 subjects, blindly classified by an independent clinician as normal (49), OHT (8), suspect (21), mild (7) or moderate POAG (30) were tested with 18RPS VEPs and psychophysics.
Gold cups were more tractable and had a non significant advantage at high contrasts. Noise was the same across contrasts and channels, increased in the sum channel and decreased exponentially with frequency. Reversal, 18RPS, and the sum channel all significantly increased SNR (p<0.05). VEP contrast functions were flat to ≥2% then exponential. In individuals, even small deviations from the exponential perturbed the threshold severely. There were no clinical differences by VEPs or psychophysics,
Optimal VEP parameters and threshold measurement methods are now better defined. They do not suggest an My generator. The Pythagorean sum of orthogonal channels gives higher SNR and avoids source selection with repeated measures by condensing all data to one representation of the underlying dipole. SS sweep VEPs can be used to compare parameters over groups but not to reliably classify patients, unlike adaptive methods that put all tests near threshold.
This PDF is available to Subscribers Only