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

Abstract

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 tau rod approximately 1 msec as reported previously, but for the cones a considerably longer time constant of tau cone approximately 4-5 msec was needed. For rods, the amplification constant (Arod approximately 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 (Acone approximately 3-7 sec-2) is of a similar order of magnitude to that obtained for the rods.

Time course of the flash response of dark‐ and light‐adapted human rod photoreceptors derived from the electroretinogram

“…The methods used for recording the ERG, for presenting stimuli and for calculating bleaching levels were closely similar to those described by Smith & Lamb (1997) and Thomas & Lamb (1999). In brief, a conductive thread electrode (DTL, UniMed Electrode Supplies, Farnham, UK), placed loosely in the lower fornix of one eye, was used to record the ERG from three adult subjects (the authors) with normal vision apart from minor errors of refraction.…”

“…as derived in eqn (5) of Smith & Lamb (1997). The cell's response is the change in this circulating current from its resting level, given by:…”

Normal Light Response, Photoreceptor Integrity, and Rhodopsin Dephosphorylation in Mice Lacking Both Protein Phosphatases with EF Hands (PPEF-1 and PPEF-2)

“…a(t) is the a-wave amplitude at time t, a max is the maximum amplitude of the a-wave, A is a factor proportional to the gain of phototransduction, ⌽ is the number of photoisomerizations/rod elicited by the test flash, and t d is an intrinsic delay time. Data were fit using a variation of this equation that takes into account the capacitive time constant of the photoreceptor (39). A corrected value for ⌽ was applied at each time point that took into account the reduced amount of rhodopsin present following the conditioning bleach.…”

Demonstrations

“…We demonstrate the apparatus developed in this laboratory for recording the a-wave of the electroretinogram (ERG) from human subjects (Smith & Lamb, 1997). The equipment (Fig.…”

“…Cone-isolated responses are obtained using the red filter, in the presence of a blue background that saturates the rods. The parameters of transduction within the living photoreceptors can then be extracted by fitting the predictions of a quantitative model of the transduction cascade (Lamb & Pugh, 1992), as described by Breton et al (1994) and Smith & Lamb (1997).…”