A Quantitative Investigation Into the Effects of Pupil Diameter and Defocus on Contrast Sensitivity for an Extended Range of Spatial Frequencies in Natural and Homatropinized Eyes

Abstract

Contrast sensitivity was measured in 12 subjects for different spatial frequencies of sinusoidal grating patterns, generated by oscilloscope, for pupil diameters 2-8 mm and for defocus of +1-4 D, following homatropine eyedrops. Changes in pupil diameter, without correction for the change in retinal illumination, had no significant effect on contrast sensitivity, except at 0.5 and 1 c deg-1 when a significant reduction occurred with the 2 mm pupil. Defocus caused a large reduction in contrast sensitivity at spatial frequencies higher than the peak of the contrast sensitivity function (3 c deg-1) and a smaller reduction below the peak. In both individual and group results, there was no significant effect of defocus in causing a disproportionately greater reduction in contrast sensitivity at higher spatial frequencies nor were the zero mimina predicted by optical theory observed. The results were confirmed in eight subjects viewing with the natural eye, though the reduction in contrast sensitivity caused by +1 D defocus was not significant: this was attributed to the relaxation of accommodation in response to defocus. To predict the performance of the visual system, multiple regression equations were derived to incorporate terms for pupil diameter, defocus and spatial frequency. These equations reflected the lack of effect of pupil diameter, while defocus caused a 51% loss in contrast sensitivity per dioptre at higher spatial frequencies (3-38 c deg-1) and a 19% reduction at low spatial frequencies.

Pupil Response as an Objective Measure of Visual Acuity*

“…3). high spatial frequencies, in good agreement with previous reports (Bour, 1980;Kay and Morrison. 1987).…”

The Effects of Physostigmine Sulphate Eyedrops on Human Visual Function

“…This suggests that defocus of the retinal image, but not the miosis, caused by physostigmine contributed substantially to the loss of contrast sensitivity. This accords with previous studies which showed that changes in pupil diameter, without compensation for changes in retinal illumination, were without significant effect on contrast sensitivity and that defocus had least effect at very low spatial frequencies (Kay & Morrison, 1987b Alpern & Jampel (1959) who had additionally shown that CFF was not affected by defocus or by instillation of pilocarpine which caused otherwise similar actions to physostigmine. The decrease in laser interference fringe contrast sensitivity was not explicable by either the reduction in pupil diameter which had a more extended time course nor by defocus of the aperture in which the interference fringes were observed.…”

Are Normally Sighted, Visually Impaired, and Blind Pedestrians Accurate and Reliable at Making Street Crossing Decisions?

“…There were no significant differences in bias values between the conditions of VþH and V-only (Paired t test, t (1,17) ¼ -0.087, P ¼ 0.93). Bias values under the H-only condition, however, were found to be significantly worse than either the bias values under the sensory conditions of VþH (Paired t test, t (1,17) ¼ -3.50, P ¼ 0.003) or V-only (Paired t test, t (1,17) …”