In rat rods it has recently been shown that prolonged exposure to moderately bright illumination causes a massive relocation of the G-protein transducin from the outer to inner segment of the cell, and a reduction of ~5 X in the amplification constant of transduction measured from the a-wave of the electroretinogram (ERG) (Sokolov et al. 2002). We have now looked for a similar change of amplification constant in human rod photoreceptors.

We recorded the a-wave of the ERG in response to ganzfeld illumination (Thomas & Lamb, 1999), in two subjects (T.D.L. and E.N.P.). Ethical approval was given by the Cambridge Human Biology Research Ethics Committee. We first recorded responses to a family of flashes under dark-adapted conditions and then, after exposure of the eye to bright steady illumination for 30 min, we measured the recovery of responsiveness to a repeated series of dim flashes, interspersed at 2 min intervals with bright flashes. The adapting light had an intensity of 1700 cd m^-2, and with the dilated pupil diameter of ~7 mm gave a retinal illuminance of ~67 000 scotopic trolands, which would have bleached around 75 % of the rhodopsin.

For subject T.D.L., the normalized recoveries after one such exposure are plotted in Fig. 1, for bright flashes ([opencircle]) and dim flashes ([fillsquare]). If the amplification constant were reduced following the exposure, then the filled symbols would be expected to be lower than the open symbols. Although a small effect is apparent at 20Ð30 min after the exposure, the change is only ~20 % (rather than the ~5 X expected by analogy with the rat recordings), and might be explained by incomplete regeneration of visual pigment. Results for the second subject were similar.

A limitation of these experiments is that the rod current is eliminated for 5 min after an exposure of this intensity, and the amplification constant cannot reliably be determined during the first 10 min or so. Our interpretation of the results is that for these observers any reduction in amplification constant following a 30 min exposure to bright light is either very small or else recovers almost completely within the first 10 min of darkness.

All procedures accord with current local guidelines.