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Dingcai Cao, Joel Pokorny; Isolated mesopic rod and cone electroretinograms realized with a four-primary photostimulating methodology. Journal of Vision 2009;9(14):32. doi: 10.1167/9.14.32.
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The electroretinogram (ERG) is a commonly used non-invasive clinical tool to detect abnormalities in rod and cone function. The standardized diagnostic protocols recommended by the International Society for Clinical Electrophysiology of Vision (ISCEV) measure rod and cone ERGs separately at difference adaptation levels. Due in part to long testing time and bright flashes, the ISCEV protocols may lead to poor patient cooperation and inaccurate results. Taking advantage of the fact that both rods and cones are active at mesopic light levels, the purpose here is to establish an ERG protocol that allows measurement of isolated rod and cone responses at a single adaptation level. We implemented a four-primary colorimetric system that allows independent control of the stimulation of the four receptor types in the human eye, using a commercially available ERG system (Diagnosys ColorDome) to generate three types of stimuli at mesopic light levels, modulating rods alone, cones alone, and both rods and cones. ERGs were recorded at three mesopic light levels (0.02, 0.16, 1.26 cd/m2) for 2, 4, 8 and 16 Hz stimuli. ERG response amplitudes and phases were derived by Fourier transformation. Two healthy observers and one patient with Retinal Pigmentosa (RP) participated. The RP patient showed no measurable rod response to conventional ERG stimuli. For the normal observers, ERG waveforms show a clear periodic pattern mirroring the sinusoidal stimuli. The analysis of the amplitudes and phases of the ERG waveforms indicate that at the lowest light level, cone response is minimal and rods dominate the response to the combined stimuli. With increases in light levels, cone responses increased. Interestingly, the amplitude of the response to the combined stimuli was intermediate between that of the isolated cone and the isolated rod stimuli for all light levels. A vector sum model based on the amplitudes and phases of the responses to the isolated rod and cone stimuli predicts these results. ERGs for the RP patient show responses to cone isolating stimuli but minimal or no response to rod isolating stimuli at all three light levels, confirming good receptoral isolation. We concluded that with the four-primary stimulating methodology, under the same adaptation conditions rod and cone ERGs can be measured separately.
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