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The Influence of Defocus on Multi-focal Visual Evoked Potentials (mfVEP)
1Pieh C., 2Bach M.,
1Albert-Ludwigs-Universität, Augenklinik (Freiburg)
2Albert-Ludwigs-Universität Freiburg, Augenklinik, Elektrophysiologisches Labor (Freiburg)
Purpose: The multifocal VEP (mfVEP) may become a useful objective perimetric technique. We compared mfVEPs obtained under best-corrected visual acuity to those under various degrees of dioptrical defocus, in order to assess the influence of reduced visual acuity due to optical factors on the mfVEP response.
Methods: Monocular mfVEPs were recorded from the right eye in eight normal subjects. With a VERIS system we generated dartboard-like stimuli with 60 sectors arranged in 6 concentric annuli on a computer monitor. At a distance of 26 cm the stimulus spanned ±30°. Two sets of mVEP records were obtained, one with best-corrected visual acuity and another when the stimulus was defocused by a plus lens between +0.5D and +3.0D. Two pairs of electrodes were placed 3 cm above/below and 3 cm to the right/left of the inion. Total recording time was 20 min; the VEP response appeared in the second order kernel.
Results: In spite of interindividual differences, most effect was seen in the central stimulus area, where defocus reduced the mfVEP amplitude to 63%±20. With increasing eccentricity (annuli 2 to 6), the influence became progressively less and the response amplitude amounted to 76%±17, 89%±13, 90%±13, 91%±11 and 95%±9, respectively. Visual acuity after defocus varied from 0.6 to 0.1 depending on the additional plus lens.
Conclusions: Compared to the multifocal ERG, the mfVEP is more sensitive for defocus. In the centre, where the stimulus has finest detail, the effect was strongest, as could be expected. We conclude, that already moderate reduction of the visual acuity can affect the mfVEP response. This should be taken into consideration, when recording and interpreting an mfVEP examination.