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H. Jaegle, L.T. Sharpe; mfERG Estimates of L– and M–Cone Topography . Invest. Ophthalmol. Vis. Sci. 2005;46(13):3431.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: The relative number of long–wavelength (L–) and middle–wavelength (M–) sensitive cones in the human retina varies greatly. Psychophysical and electroretinographic (ERG) estimates of foveal L:M cone ratios range from ca. 0.2 to 10. In vitro studies in man and in monkey retinas suggest an L:M ratio increase with increasing eccentricity and a higher ratio in temporal than nasal retina. Multi–focal ERG (mfERG) in human report a lower foveal L:M ratio for the central foveal (5° dia.) than for an annular ring centred at 40° eccentricity. However, the correlation between the mfERG data and psychophysical estimates in the same subjects was best for more peripheral ratios than for the central foveal estimate. Some of this variation is probably due to noise in the ERG data. Therefore, the aim of this study was to estimate changes in the topography of L– and M–cone driven mfERG responses in a large group of normals after applying individual response signal–to–noise (SNR) analysis. Methods: Subjects were 51 trichromats between 20 and 62 years of age. Their mfERGs were measured with a DTL fiber electrode. A "silent substitution" technique was used to generate L– and M–cone–isolating stimuli. The maximum contrast for both cone isolating conditions was 47% with a mean luminance of 19.2 cd/m2 for the L– and 33.8 cd/m2 for the M–cones. The stimulus consisted of 103 hexagonal elements which subtended 84° x 75° of visual angle at a viewing distance of 18 cm. Response signal quality was estimated by means of a signal window RMS / noise window RMS ratio and noisy data sets were rejected. The individual and the combined mfERGs were analyzed according to ring averages. Results: When all 51 subjects were analyzed individually, the mean L:M response ratio was 1.59±0.81 (sd) for the central (fovea) hexagon and 1.96±1.07 for the peripheral ring centred at ca. 40° eccentricity. In contrast, for their combined responses, the L:M response ratios were 1.97 and 1.88, respectively. However, if only those subjects (N = 17) for whom the log(SNR) for any ring was > 0.15 were included, then the agreement between the individual –– 1.69 ± 0.67 (central) and 1.68 ± 0.80 (peripheral) –– and combined –– 1.74 (central) and 1.56 (peripheral) –– L:M response ratio estimates was very good. Conclusions: Applying SNR analysis significantly improves the accuracy of mfERG L:M response ratio topography estimates. On average, the topography of the L– and M–cone signals shows a slightly higher L:M response ratio in the mid periphery (20°), but a similar ratio at 40° eccentricity compared with the central 5° L:M response ratio.
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