A patient with a damaged optic nerve may complain that the world seems 'darker' with the bad eye. Can this awareness of a difference in brightness between the two eyes be measured and serve as an indicator of asymmetry of optic nerve function? If so, it might be possible to estimate the relative afferent pupillary defect in a patient whose pupils have already been dilated. Efforts to compare brightness sense with the relative afferent pupillary defect have not met with clear success. One study found a false-positive rate as high as 51%, and in another study only rough comparisons were made. Brightness comparison testing provides, at best, only a very rough estimate of the relative afferent defect. We only turn to brightness testing in patients in whom it is not possible to measure the relative afferent pupillary defect, and we only trust the results in those who can give a crisp end point. It seems that less neutral density filter is needed to balance the apparent brightness between the two eyes than to balance the pupillary responses.
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