Proceedings of The Physiological Society

Europhysiology 2018 (London, UK) (2018) Proc Physiol Soc 41, PCB306

Poster Communications

Investigating the effect of scotopically or photopically matched pre-adapting backgrounds on the "Light Adaptation Effect" of the human electroretinogram

Z. Xu1, X. Jiang1, C. J. Hammond1, O. A. Mahroo2,1,3

1. Ophthalmology, King's College London, London, United Kingdom. 2. Institute of Ophthalmology, University College London, London, United Kingdom. 3. Physiology, Development and Neuroscience, University of Cambridge, Cambridge, United Kingdom.

The retina adapts over a large range of light intensities. The summed electrical response of the retina to light can be recorded non-invasively, and is termed the electroretinogram (ERG). If a light background is switched on after a period of dark adaptation, the ERG response to flashes increases in amplitude over several minutes. This has been termed the "Light Adaptation Effect" (LAE). It is unclear whether this is driven by rod or cone stimulation. We explored relative contributions from rod and cone systems by evaluating the effect of pre-adapting backgrounds that were either matched scotopically (for rod stimulation) or photopically (for cone stimulation). In three healthy subjects (ages 21, 24, 39), the LAE was measured as follows. Subjects dark-adapted for 20 minutes, and then were exposed to a standard white background (30 photopic cd m-2). White flashes (3 cd m-2 s) were delivered over 20 minutes (36 flashes at 0.5 s intervals, repeated every 2 min). Flash and background intensities were chosen to correspond with international clinical standards. Pupils were pharmacologically dilated. ERGs were recorded from both eyes with conductive fibre electrodes placed in the lower conjunctival fornices. The study had research ethics committee approval. B-wave amplitudes were plotted against time following background onset. In subsequent experiments, following dark adaptation, prior to the white background, the subject viewed a "pre-adapting" background (red or blue) for 10 minutes. Background intensities were chosen to yield pairs that were photopically or scotopically matched. Scotopically matched backgrounds were 2.27 scotopic cd m-2 (photopic intensities were 30 and 0.15 cd m-2); photopically matched backgrounds were 5 photopic cd m-2 (scotopic intensities were 0.38 and 146 cd m-2). With no pre-adapting background, the b-wave amplitude was initially 55-57 microvolts, increasing over several minutes, reaching a plateau by c.10 minutes, after which increase in amplitude was minimal. Amplitudes at 20 min ranged from 101 to 112 microvolts. Initial amplitudes were 50-54% of the final amplitude; amplitudes at 10 minutes were 94-97% of the final amplitude. Pre-adapting backgrounds reduced the LAE to varying degrees, such that the initial amplitude was closer to the final amplitude. However, photopically matched backgrounds did not produce identical effects on the LAE, and the same was found for scotopically matched backgrounds. Our findings suggest that both rod and cone input may play a role in the LAE. Also, the observing of a plateau at 10 min (with no pre-adapting background) suggests arrival at a steady state, supporting the clinical recommendation of a minimum 10 min light adaptation prior to assessment of cone system function.

Where applicable, experiments conform with Society ethical requirements