The contribution of human cone photoreceptors to the photopic flicker electroretinogram.

We aimed to noninvasively estimate the relative contribution of the cone photoreceptors to the photopic flicker electroretinogram (ERG) in humans. Responses to ganzfeld square-wave flicker (15, 30, and 60 Hz) were recorded in the presence of a rod-saturating background. We used a modified paired flash technique in which a bright probe flash was presented at different times during a flicker cycle and the ERG response was recorded. The cone photoreceptor response was estimated by comparing the relative amplitude of the response during a flicker cycle to the response in the absence of flicker, at early times after the presentation of the probe flash. The normalized responses were then scaled to absolute values based on the assumption that the response to 15-Hz flicker is dominated by the photoreceptor response at early times in a cycle (<15 ms). The postreceptoral contribution was obtained by subtracting the absolute photoreceptor response from the flicker response. The derived cone response showed a peak around 25 ms, from the start of the flicker cycle for 15-Hz and 30-Hz flicker with no clear peak for 60-Hz flicker. In humans, the postreceptoral components dominate the photopic flicker ERG at 15, 30, and 60 Hz. The findings are comparable to those from pharmacological studies in monkeys.