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Omar A. Mahroo, Vinaya Srirangam, Rosina Zakri, Naser Ali, Christopher J. Hammond, Trevor D. Lamb; Full-field Photopic Electroretinogram Responses to Flashes are not Consistent with Correction for Stiles-Crawford effect. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2465.
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The Stiles-Crawford directional effect (Proc R. Soc. Lond. B 1933; 112:428-50) describes reduced retinal sensitivity to light entering through the periphery of the pupil compared to the centre, and is particularly marked for cone photoreceptors. Thus, with dilated pupils, calculating retinal illuminance (the product of corneal luminance and pupil area) using the actual pupil area can over-estimate stimulation of the cone system, as the additional light through the peripheral pupil will be less effective; hence a "corrected" pupil area is often used to take account of this effect. We investigated the effect for the full-field photopic electroretinogram (ERG).
ERG responses to red or white full-field flashes (between 0.4 and 4000 photopic cd m-2 s) were recorded from two normal subjects with natural, and later pharmacologically dilated, pupils. Photopic isolation was achieved either by a rod-saturating blue background (c. 1500 scotopic Td), or by a prior rod-saturating bright flash (c. 20 000 scotopic Td).
Responses recorded with the natural and dilated pupil were very similar when the retinal illuminances of the test flashes were made equivalent according to calculations using the actualpupil area, rather than the corrected pupil area, suggesting that the Stiles-Crawford correction was inappropriate.
In some subjects at least, when using full-field illumination, the Stiles-Crawford correction is not appropriate, and interpretation of ERG studies that use this adjustment may need to take this into account.
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