Light adaptation and dark adaptation of human rod photoreceptors measured from the a-wave of the electroretinogram

1. We recorded the a-wave of the human electroretinogram from subjects with normal vision, using a corneal electrode and ganzfeld (full-field) light s timulation. From analysis of the rising phase of rod-isolated flash respons es we determined the maximum size (a(max)) of the a-wave, a measure of the massed circulating current of the rods, and the amplification constant (A) of transduction within the rod photoreceptors. 2. During light adaptation by steady backgrounds the maximal response was r educed, as reported previously. a(max) declined approximately as I-o(I-o I-B), where I-B is retinal illuminance and I,is a constant. In different su bjects I-o ranged from 40 to 100 trolands, with a mean of 70 trolands, corr esponding to about 600 photoisomerizations s(-1) per rod. (1 troland is the retinal illuminance that results when a surface luminance of 1 cd m(-2) is viewed through a pupil area of 1 mm(2).) The amplification constant A decr eased only slightly in the presence of steady backgrounds. 3. Following a full bleach a(max) recovered along an S-shaped curve over a period of 30 min. There was no detectable response fbr the first 5 min, and half-maximal recovery took 13-17 min. 4. The apparent amplification constant decreased at early times after large bleaches. However, upon correction for reduced light absorption due to los s of pigment, with regeneration of rhodopsin occurring with a time constant of 9-15 min in different subjects, it appeared that the true value of A wa s probably unchanged by bleaching. 5. The recovery of a(max) following a bleach could be converted into recove ry of equivalent background intensity, using a 'Crawford transformation' de rived from the light adaptation results. Following bleaches ranging from 10 to > 99%, the equivalent background intensity decayed approximately expone ntially, with a time constant of about 3 min. 6. The time taken for a(max) to recover to a fixed proportion of its origin al level increased approximately linearly (rather than logarithmically) wit h fractional bleach, with a slope of about 12 min per 100% bleach. Similar behaviour has previously been seen in psychophysical dark adaptation experi ments, for the dependence of the 'second component' of recovery on the leve l of bleaching.