Light adaptation of the electroretinogram. Diminished in retinitis pigmentosa.

The cone electroretinogram has been examined at different levels of ganzfeld adaptation in normals and subjects with retinitis pigmentosa (RP). As light adaptation increases, the amplitude and time course (measured as b-wave implicit time) of the cone ERG decrease in both normal and RP subjects. The same level of light adaptation, however, decreases the amplitude and the implicit time more in normal than in RP subjects. The ineffectiveness of light for adapting the ERG of RP subjects is most easily explained by assuming that RP cones absorb less light than normal cones. By comparing these parameters between normal and RP subjects at different levels of light adaptation, it is possible to estimate this ineffectiveness of cone absorption in RP subjects. The results imply that RP cones can transduce and adapt but fail to absorb light as effectively as normal cones. The quantitative relationship between cone b-wave implicit time and retinal illumination provides a unique method for examining cone function as well for standardizing ganzfeld backgrounds in ERG laboratories.