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Avinash Aher, Declan J McKeefry, Neil Robert Alan Parry, John Maguire, Ian J Murray, Tina I Tsai, Cord R H Huchzermeyer, Jan J Kremers; Rod versus cone driven ERGs at different stimulus sizes. Invest. Ophthalmol. Vis. Sci. 2017;58(8):5341. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To study how rod and cone driven responses depend on stimulus size in normal subjects and patients with retinitis pigmentosa (RP).
The triple silent substitution technique was used to isolate L- and M-cone and rod driven ERGs with 19%, 18%, and 33% photoreceptor contrast resp. Mean luminance was 284 cd/m2; CIE coord: (0.60, 0.39). Experiments were conducted on five normal subjects (age: 27-55 years) and three RP patients (age: 41-53 years). The ERGs in control subjects were recorded at nine different temporal frequencies (between 2 and 60 Hz) and with five different stimulus sizes: full-field (FF) and 70°, 60°, 50° and 40° diameter circular stimulus. In the experiments with the RP patients, only rod and L-cone driven ERGs were measured with FF and 40° diameter stimuli at 8 and 48 Hz. Amplitudes of the responses were defined as those of the 1st harmonic component after Fourier analysis of the ERGs.
In normal subjects, the rod driven responses displayed a fundamentally different behavior than the cone-driven responses. At low temporal frequencies (2-4 Hz), FF rod and cone driven responses were barely above noise. Up to 12 Hz, rod-driven signals increased by about a factor of 4 when measured with smaller stimuli. In contrast, L and M cone-driven responses in this frequency region did not change substantially with stimulus size. On the other hand, at high temporal frequencies (24 Hz and higher), rod and cone driven response amplitudes decreased with decreasing stimulus size. In contrast with responses measured in the normal subjects, 8 Hz rod driven and the 48 Hz L-cone driven ERGs to the 40° stimuli were not larger than those to FF stimuli in RP the patients.
The increased responses with smaller stimuli in normal subjects with the rod isolating conditions indicates that a fundamentally different mechanism drives the ERGs in comparison with the cone driven responses and validates the rod isolation. We propose that the increased responses are caused by stray light stimulating the peripheral retina, indicating that it is possible to isolate and study rod driven ERGs using triple silent substitution technique at photopic luminances. In RP patients both cones and rods are affected in the peripheral retina.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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