Assessing the Contribution of Focal Retinal Areas to the Full Field ERG

Abstract

Purpose: : In theory, the ganzfeld-evoked ERG is said to represent the weighed average of the electrical contribution generated from all the retinal areas activated by the light stimulus. We generated focal ERGs (FERG) from different retinal eccentricities to examine how they compared to the full-field response (FFERG).

Methods: : Photopic ERGs (DTL electrode, background 30 cd.m^-2; Flash 1 log cd.sec.m^-2; average of 100 flashes) were recorded from six normal subjects. FERGs were obtained with the use of a spherical eye patch pierced with a 5 mm hole (stimulating approximately 5^o of retina) either centrally or at 20^o or 40^o nasally respectively, as confirmed at perimetry.

Results: : The FERG waveforms were of very low voltage (largest component 80 Hz frequency components as determined with discrete wavelet analysis) compared to the FFERGs. We identified 8 components with peak times between 8 and 95 ms and time-locked FFERGs features. Of interest, the amplitudes of the post-b-wave peak components (i-wave and post i-wave oscillations) were significantly larger than the pre-b-wave components while the reverse is seen in the FFERGs. Also, the timing of the a-wave was fastest in FERGs (8 vs 15 ms; p<0.05) and corresponded to a notch seen on the descent of the FFERG a-wave. None of the FERG parameters were eccentricity dependent.

Conclusions: : Despite the limited number of retinal eccentricities and stimulus sizes tested, the different FERG and FFERG waveform composition and the lack of significant changes with eccentricities cannot support the above mentioned definition of the FFERG. That is of course unless the full field stimulus also allows for lateral (or otherwise) interactions of focal retinal areas that would add to the making of the FFERG signal. Finally, it is of interest to note that FERG-like FFERG waveforms were also observed in patients with advanced degenerative retinopathies (and minimal visual field), such as Retinitis Pigmentosa, suggesting that a better understanding of FERG genesis could have a diagnostic value. Funded in part by FFB (USA).

Keywords: electrophysiology: clinical • retina