Normal change in the foveal cone ERG with increasing duration of light exposure

Foveal cone electroretinograms (ERG) were elicited with a stimulator-ophthalmoscope from 24 normal subjects with a 4 degrees stimulus flickering at 42 Hz and centered within a 12 degrees steady surround. The stimulus and surround were presented at retinal illuminances of 4.8 log td and 5.5 log td, respectively, to facilitate visualization of the fundus. Several consecutive averaged responses were evaluated to determine whether increasing duration of light exposure causes an increase in amplitude, as previously found for the full-field cone ERG. On average, amplitude increased by 27% over time, and the linear regression of amplitude on recording number accounted, on average, for 42% of the amplitude variability between consecutive responses. Two subjects had amplitudes that were initially subnormal, based on previously published norms, but that value increased to within the normal range in subsequent recordings. These findings show that a significant change in the cone ERG occurs in the fovea with increasing duration of light exposure at these retinal illuminances, and suggest that, when the stimulator-ophthalmoscope is used, consecutive foveal cone ERGs should be obtained from patients with suspected macular disease to avoid a false diagnosis of retinal malfunction.