Extremely rapid recovery of human cone circulating current at the extinction of bleaching exposures

We used a conductive fibre electrode placed in the lower conjunctival sac to record the a‐wave of the human photopic electroretinogram elicited by bright white flashes, delivered during, or at different times after, exposure of the eye to bright white illumination that bleached a large fraction (∼90%) of the cone photopigment. During steady‐state exposures of this intensity, the amplitude of the bright‐flash response declined to ∼50% of its dark‐adapted level. After the intense background was turned off, the amplitude of the bright‐flash response recovered substantially, for flashes presented within 20 ms of background extinction, and fully, for flashes presented 100 ms after extinction. In addition, a prominent ‘background‐off a‐wave’ was observed, beginning within 5–10 ms of background extinction. We interpret these results to show, firstly, that human cones are able to preserve around half of their circulating current during steady‐state illumination that bleaches 90% of their pigment and, secondly, that following extinction of such illumination, the cone circulating current is restored within a few tens of milliseconds. This behaviour is in stark contrast to that in human rods, where the circulating current is obliterated by a background that bleaches only a few percent of the pigment, and where full recovery following a large bleach takes at least 20 min, some 50 000 times more slowly than shown here for human cones.