Abstract
Abstract: :
Purpose: The parameters of rod and cone phototransduction can be derived relatively easily from full–field ERG a–waves [1]. However, several conditions, such as Stargardt disease and age–related macular degeneration, selectively involve photoreceptors in the central retina. Based in part on previous work [2], here we present an approach for obtaining focal rod and cone transduction parameters from the central 40 degrees of retina. Methods: We recorded dark– and light–adapted full field and focal rod and cone a–wave responses to high intensity white flashes in 6 normal subjects. Four flash intensities were used for all conditions ranging from approximately 3.2 to 4.4 log scot td–s (2.83 to 4.03 log phot td–s). The full field responses were obtained in a Ganzfeld dome as described previously [1]. To record a–waves from the central retina, the subject–to–dome distance was increased so that the opening of the dome subtended 40 degrees at the viewing distance. To obtain focal rod a–waves, the four flash intensities were presented in the dark followed by a double flash series. Subtraction of the double–flash responses eliminated the stray light and the cone component. To record focal cone a–waves, the single flashes were presented on a background of 3.2 log td–s. The stray light was masked with a retina–wide 3.6 log td–s surrounding light field. Results: The maximum receptor response (RmP3) of the central receptors was, on average, 20.5%±5.3% (rods) and 44.1%±6.5% (cones) of the full–field values. The mean central receptor sensitivity (S) was 48%±16.3% (rods) and 17.6%±8.9% (cones) of the full field values. Conclusions: Our RmP3 percentages agree with anatomical estimates indicating that 25 % of the rods and 40 % of the cones are within the central 40 degrees of retina [3]. The rod parameters are also in agreement with data obtained on an optical bench [2]. The lower central, as compared to full–field, cone S values are consistent with the smaller diameter, more rod–like structure of the central cones and may help explain why the cone S values are low in retinitis pigmentosa patients, where only the central retina is intact [4].1. Hood & Birch (1997) Doc. Ophthal; 2. Nusinowitz et al. (1995) JOSA; 3. Curcio et al (1990) J. Comp. Neurol; 4. Hood & Birch (1996) Vis. Res. CR: none Support: EY09076 Maximum allowed length: 2436 characters Current length: 2503 characters with spaces 2104 characters without spaces
Keywords: clinical research methodology • electrophysiology: clinical • photoreceptors