Long-term renormalization of chromatic mechanisms following cataract surgery

Abstract

The optical density of the human crystalline lens progressively increases with age, the greatest increase in the visible spectrum being at short wavelengths. This produces a gradual shift in the spectral distribution of the light reaching the retina, yet color appearance remains relatively stable across the life span, implying that the visual system adapts to compensate for changes in spectral sensitivity. We explored properties of this adaptive renormalization by measuring changes in color appearance following cataract surgery. When the lens is removed, cataract patients often report a large perceptual shift in color appearance that can last for months. This change in color appearance was quantified for four cataract patients (63-84 years) by determining the chromaticity of stimuli that appeared achromatic before surgery, and at various intervals after surgery for up to 1 year. Stimuli were presented on a calibrated CRT as 9.5-deg spots, with 3-s duration and 3-s interstimulus intervals (ISIs). Chromaticity was adjusted by the subjects in CIE L*a*b* color space with luminance fixed at 32 cd/m2, on a dark background. We also estimated the optical density of the cataractous lens by comparing absolute scotopic thresholds from 410 nm to 600 nm before and after surgery. The results demonstrated that immediately following surgery there is a large increase in the short-wave light reaching the retina, mainly below 500 nm. The achromatic settings generally showed an initial large shift in the "yellow" direction after surgery that gradually (but never fully) returned to the original achromatic point before surgery. The shifts in the achromatic point occur over a number of months and appear to occur independently of the fellow eye.

Fig. 1

The top panel shows scotopic threshold measurements for a typical cataract patient before surgery (open circles) and after surgery (closed circles). The curves show model fits (see text for details). Error bars are ±1 S.E.M. The bottom panel shows the mean optical density difference (thick black line) between the presurgery and postsurgery absorption curve fits for four patients with cataracts rated as 2–3+. The thin lines show ±1 standard deviation.

Fig. 2

Achromatic settings made for observer A using the test eye over a period of about 1 year. Each setting is labeled according to the day the setting was made relative to the surgery date. The open circle shows the equivalent presurgery setting calculated using the lens-absorption difference results. The asterisk shows the chromaticity of a typical white point (CIE Illuminant D65). Error bars are ± 1 S.E.M. Some settings have been omitted for clarity.

Fig. 3

Achromatic settings made for all four observers using the test eye over a period of 1 year for observer A and 6 months for the other three observers. The presurgery setting is indicated by an arrow. The open circle shows the equivalent presurgery setting calculated using the lens-absorption difference results. The first setting postsurgery tends to be reasonably close to the equivalent presurgery settings but already shows some adaptation. Subsequent settings slowly migrate toward a typical white point (e.g. CIE Illuminant D65 shown by an asterisk). Error bars are ± 1 S.E.M.

Fig. 4

The shift in achromatic settings over time for the four observers. The x-axis shows the days from surgery on a log scale. Day 0 is the adjusted presurgery using the lens-absorption measurements. The y-axis shows the Euclidean distance in CIE xy space from D65. The bold curve shows the mean of the best-fitting decaying exponential functions for each observer.

Fig. 5

Achromatic settings made for observer A using the test (T) eye (closed circles) and the fellow (F) eye (open squares) on three different days relative to surgery. The open circle shows the equivalent presurgery setting calculated using the lens-absorption difference results. The asterisk shows the chromaticity of a typical white point (CIE Illuminant D65). Error bars are ± 1 S.E.M.

Fig. 6

Color-discrimination ellipses (using the Cambridge Colour Test) for observer C before surgery and at 1 day and 6 months after surgery. The lens-absorption difference results were applied to the presurgery settings to produce the “adjusted” ellipse in the top-right panel.