Abstract
Purpose: :
Clinical ERGs are normally recorded with a single electrode placed directly on the cornea (contact lenses) or close to it (DTL fiber electrodes). Single electrode recording measures the sum of the bioelectric activity of the entire retina and thus cannot evaluate local function unless a specialized stimulus (such as mfERG) is used. The purpose of this study was first to examine if the photopic ERG varied substantially in amplitude when the eye and electrode positions are changed and secondly if these variations can give enough information about the local function of the retina.
Methods: :
Photopic ERGs (1 log cd.sec.m-2; background 30 cd.m-2) were recorded from 5 normal subjects with 4 active electrodes namely, a DTL fiber positioned in the inferior conjunctival bag, and 3 skin electrodes positioned on the lower lid (LL) and external (EC) and internal canthi (IC) respectively, all referenced against the same indifferent electrode placed on the forehead. Prior to each ERG recording, subjects were instructed to look at one of 11 fixation LEDs positioned either centrally or at 8, 16, 24, 32 and 40o nasal or temporal to the central LED.
Results: :
Irrespective of the direction of gaze, there was little variation (mean: 66.1±5.5uV) in amplitude of DTL ERGs. In primary position, LL were 37.6% smaller than DTL ERGs compared to 6.2% and 24.5% for the EC and IC ERGs, respectively. In nasal gaze position, LL were 28.2% smaller than DTL ERGs compared to 56% and 37.4% for the EC and IC ERGs, respectively. In temporal gaze position, LL were 33.3% smaller than DTL ERGs compared to 31.8% and 20.8% for the EC and IC ERGs, respectively. Inversion of ERG polarity recorded with the EC electrode occurs at about 5o nasally as gaze direction moved from nasal to temporal.
Conclusions: :
Non-significant variations on the amplitude of the DTL fiber electrode demonstrate that the retina was still fully stimulated as the direction of gaze changed. Furthermore, as the eye and electrode positions are changed we found significant variations in ERG amplitudes recorded with skin electrodes, particularly with the EC electrode. We are currently feeding this data to a 3-D finite element model of the human eye (based on geometry and conductivity of major ocular tissues and structures outside the eye) to see if the distribution of the sources at any point on the retina could be predicted (inverse problem) using an ERG recording technique that combines multiple eye and electrode positions. The variations in ERG morphology reported above suggest that it does.
Keywords: electroretinography: non-clinical • electroretinography: clinical