Effect of optical defocus on multifocal ERG responses

Background: Multifocal electroretinography (mfERG) is a sensitive technique to probe retinal function topographically. Various conditions such as macular degeneration decrease the first-order kernel (K1) response magnitude of mfERG. Previous studies have given inconsistent results on the effect of optical defocus due to poor controls. This study investigated the effect of optical defocus on the K1 response of the central retina using a well-controlled method. Methods: Twenty subjects were recruited to undergo mfERG measurement using the VERIS 4.0 system. A four millimetre artificial pupil was fitted before each fully-dilated right eye, optically corrected for the viewing distance. The implicit times and response amplitudes of n1 (first negative trough) and p1 (first positive peak) under three different optical defocus conditions (zero dioptres, +1.00 D and +3.00 D) were measured. Results: The implicit times of n1 and p1 did not demonstrate any significant variation from the central macula to para-macula under different optical defocus conditions. The response amplitude of n1 did not show any changes from the central macula to para-macula but the response amplitude of the central macular p1 showed a significant reduction by 12 per cent under +1.00 D defocus and +3.00 D defocus. Conclusions: Optical defocus causes mild reduction in mfERG at the central macula but there are no significant changes in the periphery. A full optical correction is recommended for mfERG measurement to minimise the reduction of the macular response due to optical defocus.