The a-wave of the human electroretinogram recorded with a minimally invasive technique

A minimally invasive technique is described for recording the a-wave of the human ERG and extracting the parameters of transduction in the rod and cone photoreceptors. A corneal DTL fibre electrode is used, but the pupil is not dilated and the cornea is not anaesthetized. Although the amplitude of the signal collected by the DTL electrode varies from session to session, this is not a problem, as the photoreceptor fractional circulating current is obtained by normalization of the response family. A method is described for varying the effective flash intensity over a wide range, by controlling the duration of the xenon flash. In order to fit the kinetics of the responses, an analytical equation is derived for the convolution of the previous 'delayed gaussian' expression with the cell's capacitive time constant. This equation provides a good description of both the rod and the cone response families. For rods, the capacitive time constant was found to be τ(rod) ≃ 1 msec as reported previously, but for the cones a considerably longer time constant of τ(cone) ≃ 4-5 msec was needed. For rods, the amplification constant (A(rod) ≃ 5 sec^-2) was close to previous estimates, but for cones the sensitivity (expressed in terms of corneal illuminance) was higher than in previous work. Calculation of the amplification constant of transduction within the cones requires knowledge of their light collection properties, and the absence of hard information makes this estimate somewhat speculative. However, when account is taken of the larger diameter of the inner segments of cones in the peripheral retina, then our estimated amplification constant for the cones (A(cone) ≃ 3-7 sec^-2) is of a similar order of magnitude to that obtained for the rods.