Purchase this article with an account.
M.D. Menz, M.K. Menz, E.E. Sutter; Methods to Improve the Spatial Coverage of the Multifocal VEP . Invest. Ophthalmol. Vis. Sci. 2005;46(13):5679.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Purpose: The coverage of the central visual field that can be achieved with the mfVEP is limited by the convoluted cortical representation in at least two ways: (a) By the locally varying angle of the focal dipole moment relative to the electrode pair and (b) by cancellation of the response from a stimulus patches straddling a cortical fold such as patches extending to both sides of the calcarine fissure. Problem (a) can be addressed using multiple electrode placements. Here we compare two different methods that address problem (b). Methods: (1) The likelihood of signal cancellation within a stimulus patch is reduced by splitting each of the 60 elements in the radial direction thus increasing the resolution of the stimulus array from 60 to 120 sectors (high resolution method). (2) Using a second recording with a stimulus array of 60 sectors rotated such that its sectors straddled the borders of those used in the first recording (rotation method). For a fair comparison of the two methods we used the same total recording time. The focal stimulation was contrast reversal of a check pattern. Recording and analysis were performed using the VERIS pro software package (EDI, San Mateo, CA). Three normal subjects participated in the study. Results: Analysis was performed by using the results from both eyes to produce a measure of signal–to–noise for each sector. If this was below a criterion value, the sector is considered unusable. Isolated unusable sectors are not considered a serious problem because the surrounding good sectors can be used to fill–in the bad area by either spatial averaging and/or interpolation. However, many contiguous unusable sectors are very undesirable. The different methods are evaluated with this criterion: which method produces the least amount of contiguous unusable sectors. In this respect, the rotation method was better. However, the high–resolution method performed better than we expected; it's primary weakness is in the outer two rings of eccentricity. Perhaps a modified form of the high–resolution stimulus may yet be found that is better. Conclusions: By performing two recordings, one conventional and one with a rotated stimulus, the total area of useful signal–to–noise can be expanded, thus making the mfVEP a more useful clinical and scientific tool.
This PDF is available to Subscribers Only