Proceedings of The Physiological Society

Physiology 2014 (London, UK) (2014) Proc Physiol Soc 31, PCB076

Poster Communications

The effect of pre-adaptation scotopic/mesopic backgrounds on the growth in amplitude of the cone-driven flash electroretinogram after onset of a photopic background: evidence for rod system involvement

R. J. Piper1, M. H. Qureshi1, S. Siddiqui1, J. J. Lam1, C. J. Hammond2,3, O. Mahroo2,1

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

The electroretinogram (ERG) represents the summed electrical response of the retina to light, and can be recorded non-invasively from human subjects in vivo. Previous studies have noted that the ERG amplitude elicited in response to flashes delivered following onset of a photopic background becomes larger over the course of several minutes following onset of the background (termed by some the "light adaptation effect" (LAE)). One proposed explanation is that inhibition to the cone system from the rod system, present in the dark, progressively weakens over this time period. We tested this hypothesis by preceding exposure to the photopic background with exposure to backgrounds of varying scotopic intensity, and exploring the effect on the LAE. Full-field flash ERG responses were recorded using a conductive fibre electrode from the eyes of 3 healthy human subjects, following pharmacological pupil dilatation. White flashes (1.3 photopic cd m-2 s) were delivered 0.5 s apart in series of 15 (the series was repeated every 10 s), in the presence of a white background (c. 30 photopic and c. 50 scotopic cd m-2). Prior to the white background subjects were dark adapted for 20 min. Responses to flashes delivered on the white background were averaged over 100 s time windows and amplitudes measured at a fixed time following flash delivery (25.5-26.5 ms, close to the peak of the b-wave). In subsequent experiments, subjects were exposed for 20 min to one of two "pre-adaptation" backgrounds following the 20 min dark adaptation and prior to onset of the white background. The pre-adaptation backgrounds were either a red (0.7-1.9 scotopic cd m-2) or blue (47-55 scotopic cd m-2); both had the same photopic intensity (4.2-4.5 cd m-2). The pre-adaptation backgrounds were expected to have different effects on rod stimulation (the red would be expected to shut off c.30-50% of rod circulating current, whilst the blue would saturate the rods), whilst having the same effect on the cones (as photopic intensities were similar). Following 20 min dark adaptation, response-amplitudes (as measured at fixed times post-flash) increased approximately three-fold over the course of 10-15 min after onset of the white background. However, following pre-adaptation to the red background, only a 1.5 to 2-fold increase was seen, and this was complete within 5 min. Following pre-adaptation to the blue background, no increase was seen on the white background. The two pre-adaptation backgrounds had different effects on the LAE; the backgrounds were expected to stimulate the rod system differently, but produce the same overall stimulation of the cone system. Our findings are thus consistent with involvement of the rod system in modulating the light adaptation effect of the photopic ERG.

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