Comparing the Photopic ERG Luminance-Response Function Among Species: Putting the Photopic Hill to Use

Abstract

Abstract: : Purpose: In humans, the photopic ERG b-wave luminance-response function curve adopts a unique shape where, with progressively brighter flashes, the amplitude of the b-wave first increases and then saturates following which, further increments in flash intensities will yield a gradual reduction in amplitude. This phenomenon is often referred to as the Photopic-Hill. The aim of this study was to investigate if this feature only characterises the human ERG. Methods: Photopic ERGs were recorded from Guinea pigs (N=5), albino (N=5) and pigmented (N=3) rats and albino (N=3) and pigmented (N=6) mice. The responses were recorded with a DTL fibre electrode and evoked to flashes of light of 20 µs in duration and of intensities ranging from -2.14 log cd.sec.m_-2 to 2.8 log cd.sec.m_-2 that were presented against a rod desensitising background of 17 cd.m_-2. Data analysis included: maximum b-wave amplitude (V_max) and corresponding flash intensity, b/a-wave amplitude ratio and retinal sensitivity (k). Results: A Photopic Hill similar to that seen in humans was observed only with the Guinea pigs and pigmented rats and mice. The maximum b-wave amplitude (V_max) was of 250 µV for the pigmented rats and mice and 60 µV for the Guinea pigs compared to 90 µV for the human subjects. The intensity needed to reach V_max varied as follows: 0.35 log cd.m_-2.sec (humans), 0.9 log cd.m_-2.sec (Guinea pigs), 1.9 log cd.m_-2.sec (pigmented rats) and 2.4 log cd.m_-2.sec (pigmented mice). At maximal amplitude, the shape of the resulting ERG also varied among the species studied. This is best exemplified with the b/a wave amplitude ratio which was 8 for the pigmented rats and mice due to a near absent a-wave. In comparison, the b/a wave ratio is of 1.4 for the Guinea pigs and 3.5 for humans. Conclusion: The previously demonstrated Photopic Hill does not appear to be restricted to the human ERGs only. It would thus appear that the photopic V_max, along with the intensity at which this amplitude is obtained, could be used to further describe the cone pathway of animal models and thus facilitate comparisons among species. Although the exact origin of this effect remains to be determined, it was previously suggested that the OFF retinal pathway might be at the origin of the down phase of the Photopic Hill. Our results would also suggest that the latter retinal pathway is deficient in albino rodents.

Keywords: 396 electroretinography: non-clinical • 554 retina