Human cone photoreceptor responses measured by the electoretinogram a-waveduring and after exposure to intense illumination

1. We recorded the a-wave of the electroretinogram from human subjects with normal vision, using a corneal fibre electrode and ganzfeld stimulation un der photopic conditions, so as to extract the parameters of cone phototrans duction. The amplitude of bright flash responses provided a measure of the massed circulating current of the cones, while the amplitude of dim flash r esponses provided a measure of the product. of the fraction of cone photopi gment present, and the amplification constant of transduction within the co nes. 2. In the presence of steady background illumination, the cone circulating current declined to half at 3000 photopic trolands, and to a quarter at 20 000 photopic trolands. 3. At very early times after the delivery of a near-total bleach, we could not determine the level of circulating current as our bright flashes did no t appear to saturate the a-wave (presumably because so little pigment was p resent). However, by 20-30 s after a total bleach, the cone circulating cur rent had returned to its dark-adapted level. 4. Following smaller bleaches (when ca 50% of the pigment remained present) the bright flashes were able to saturate the a-wave even at very early tim es. Within 3 s of extinction of the illumination, the cone circulating cur rent had returned to its dark-adapted level. 5. This is at least a factor of 300 times faster than the period of ca 15 m in required for full recovery of rods exposed to the same level of bleach, and indicates a major difference between rods and cones in the way that the y cope with the photoproducts of bleaching. 6. Despite the very rapid recovery of circulating current after bleaches, t he recovery of dim-flash sensitivity was much slower, with a time constant of ca 1.5 min after a near-total bleach. This time course is very similar t o previous measurements of the regeneration of cone photopigment, and it se ems highly probable that the reduction in dim-flash sensitivity results fro m pigment depletion.