Lighting Spectral Effect on Landolt C Performance is Enhanced by Blur and Abolished by Mydriasis

When pupil size is changed by varying the surround spectrum, there is a perceived color shift of the task towards the complementary hue of the surround. This occurs even though none of the surround light falls on the task, and the task illumination is unchanged. This induced color effect is a neural process. To investigate whether such a mechanism is an alternative explanation of our results on the effects on visual performance of spectrally controlled pupil sizes, we studied visual performance both with and without mydriasis (pharmacologically dilated and fixed pupil). If the induced color hypothesis is valid, then it should occur with both fixed and light-responsive pupils. In addition, we studied whether the pupil size effect on visual performance can occur in accurately refracted subjects, or if it is enhanced by the addition of a small amount of optical blur (+0.50 DS). We studied 12 subjects, 21 to 35 years of age, correctly refracted and with added blur, under each of two conditions: normal pupils and mydriasis. We compared Landolt C recognition, with a fixed task luminance but variable contrast, for two different surround spectra, each at 50 cd/m². The two different surround spectra controlled subjects pupil size. For normal pupils, performance was better with smaller pupils, and the improvement in performance due to switching to a scotopically enhanced surround was greater with added blur, even though blursreduced overall performance. Under mydriasis, change of the surround spectrum had no effect on performance, whether there was blur or not. However, the added blur reduced performance under the mydriasis condition, showing our measures are sensitive to these parameters. These experiments rule out the induced color hypothesis and demonstrate the benefits of smaller pupils on Landolt C contrast sensitivity even when subjects are correctly refracted. Further, the results indicate that the measured improvement due to pupil size change is greater when there is imperfect refraction (blur).