Margaret Lutze

Aging of the human lens

The optical density of the human lens changes during life. Literature concerning both the spectral density function and the rate of such changes is reviewed. Analysis indicates that two components govern the spectral lens density function, with one increasing gradually during life. The average lens density increases linearly at 400 nm by 0.12 density unit per decade between the ages of 20 and 60 and by 0.40 density unit per decade above age 60. A tabulation of the two components of the average 32-yr old lens is given, as are equations to derive the average spectral lens density functions for observers aged 20-80.

Heterochromatic modulation photometry

Heterochromatic modulation photometry is a method for obtaining equiluminance for a pair of heterochromatic lights presented in temporal alternation. A series of fixed standard luminance/test luminance ratios are presented, and at each ratio the modulation depth of the pair is reduced in tandem until the observer reports that flicker disappears. The data can be described by a luminance contrast template that appears V shaped when plotted on log-log coordinates. In the fitting of individual data, a free vertical scaling factor reflects the observers sensitivity to luminance modulation and a free horizontal scaling factor reflects the observers similarity in spectral sensitivity to the CIE standard observer. Data for red/green flicker photometric matches demonstrate the technique. Heterochromatic modulation photometry offers several advantages over flicker photometry: (1) a single fixed perceptual transition occurs on each trial series, namely, the transition from flicker to steady, and (2) luminance matches can be obtained at fixed frequencies at a number of luminance levels. The same procedure can be applied to the measurement of the minimally distinct border and to the identification of tritan pairs (stimulus pairs that differ only in their stimulation of short-wavelength-sensitive cones).

GENETIC STUDIES OF VARIATION IN RAYLEIGH AND PHOTOMETRIC MATCHES IN NORMAL TRICHROMATS

The inheritance of Rayleigh match midpoints and photometric matches (551-667 nm) was studied in observers with normal color vision. An analysis was performed to evaluate whether the measured interobserver variations in these two traits were consistent with single gene allelic variation, polygenic variation, or environmental factors. A bipartite 2 deg field and a computerized tracking method were used to obtain Rayleigh matches; a new photometric technique, termed heterochromatic modulation photometry (HMP), was used to obtain photometric matches. Data were collected from 72 nonrelated males to determine distribution characteristics for the normal population. The distributions were analyzed for evidence of multimodality and the results indicated that the distributions for Rayleigh match midpoints and HMP matches obtained in this study were unimodal and symmetrical. Data from 52 observers from 5 families were used to study the transmission of the two traits in pedigrees. Statistical analysis of the pedigree data suggested that the major source of variations for Rayleigh match midpoints and for HMP matches were each due to allelic variation at single gene loci; that is, each may be determined by a single gene. Results were inconclusive as to whether variation in the two traits could be determined by the same gene.

The color of night: Surface color perception under dim illuminations.

Several studies document rudimentary color vision under dim illumination. Here, hue perceptions of paper color samples were determined for a wide range of light levels, including very low light levels where rods alone mediate vision. The appearances of 24 paper color samples from the OSA Uniform Color Scales were gauged under successively dimmer illuminations from 10-0.0003 Lux. Triads of samples were chosen representing each of eight basic color categories; red, pink, orange, yellow, green, blue, purple, and gray. Samples within each triad varied in lightness. Observers sorted samples into groups that they could categorize with specific color names. Above 0.32 Lux, observers sorted the samples into the originally chosen color groups with few exceptions. For 0.1-0.01 Lux, the red and orange samples were usually correctly identified as either red or orange. The remaining samples tended to be grouped into two categories, associated with the scotopic sample reflectance. The lowest reflectance samples were below threshold and were named black. The higher reflectance group was named predominately as green or blue-green (three observers; the fourth observer used blue or achromatic). At the three dimmest levels (< or = 0.0032 Lux) there continued to be conspicuous color percepts. Color categories were reliably assigned based on relative sample scotopic lightness. Of the samples above threshold, those with lower reflectance were classified as red or orange (all observers) and the higher reflectance samples as green or blue-green (three observers) or achromatic or blue (the fourth observer). Rods and L-cones presumably mediated color percepts at the intermediate light levels used in the study. At the three lowest light levels there were distinct color appearances mediated exclusively by rods. We speculate that at these light levels the visual system estimates probable colors based on prior natural experience.

A Computer-Controlled Briefcase Anomaloscope

We designed a portable Rayleigh match anomaloscope suitable for automated clinical and field testing of color vision. The instrument uses high radiance light-emitting diodes and interference filters to yield narrow-band primaries of 551 nm, 588 nm and 667 nm. Computer-controlled automated protocols allow rapid evaluation of match midpoint and matching range. The Color Match Area effect can be measured for fields varying between 1° and 7°. Anomaloscope matches can be made with both a conventional split-field and with full-field substitution. The instrument can also be used to measure heterochromatic flicker photometric spectral sensitivity, with any two of the three primaries presented in temporal alternation.

Improved Clinical Technique For Wald-Marré Functions

We have described a computer-controlled technique to assess Wald-Marre functions. The background fields are similar in spectral composition to those used by previous authors, however flicker rather than discrete flashes was used for the test stimulus. For estimation of long- and middle-wavelength cone sensitivities, a flicker rate of 15 Hz was used; for the short, 2.4 Hz was used.

Achromatic parvocellular contrast gain in normal and color defective observers: Implications for the evolution of color vision

The PC pathway conveys both chromatic and achromatic information, with PC neurons being more responsive to chromatic (L-M) than to achromatic (L+M) stimuli. In considering the evolution of color vision, it has been suggested that the dynamic range of chromatic PC-pathway processing is tuned to the chromatic content of the natural environment. Anomalous trichromats, with reduced separation of their L- and M-cone spectral sensitivities, have diminished chromatic input to PC-pathway cells. Dichromats, with absent L or M cones, should have no chromatic input to PC-pathway cells. Therefore, the PC-pathway dynamic range of color defectives should be released from any constraint imposed by the chromatic environment. Here we ask whether this results in compensatory enhancement of achromatic PC-pathway processing in color defectives. This study employed a psychophysical method designed to isolate PC-pathway processing using achromatic stimuli. In a pulsed-pedestal condition, a four-square stimulus array appeared within a uniform surround. During a trial, one of the test squares differed from the other three, and the observers task was to choose the square that was different. A four-alternative, forced-choice method was used to determine thresholds as a function of the contrast of the four-square array to the surround. Seven color defective and four normal observers participated. Results showed no systematic differences between normals and color defectives. There was no enhancement of achromatic processing as compensation for reduced chromatic processing in the PC-pathway system in color defectives. From physiological recordings, PC-pathway achromatic contrast gains of dichromatic and trichromatic New World primates and trichromatic Old World macaques have also been shown to be similar to each other. Our study and the animal studies imply that PC-pathway contrast gain parameters were regulated by factors other than the environmental chromaticity gamut, and may have arisen in a nontrichromatic common ancestor to both Old and New World primates.

Critical Flicker Frequency in X-Chromosome Linked Dichromats

Critical flicker frequency (CFF) was investigated in dichromats as a function of luminance and field size. For each of six field sizes (0.7°–5.5°), CFF thresholds were obtained for a square-wave-modulated 580 nm stimulus of -0.35 to 4.65 log trolands. CFF increased linearly with luminance but reached an asymptote above 3.0 log trolands. Deuteranopes showed a higher asymptotic CFF than protanopes for all field sizes. The relative spectral sensitivity of deuteranopes and protanopes to the 580 nm stimulus cannot account for the differences in asymptotic CFF, although the ascending portions of the curves for the two groups could be aligned with a translation along the luminance axis. The differences in asymptotic CFF are not due to the edge conditions of the test stimulus. Thresholds were obtained for a 1.5° test with a 580 nm equiluminance surround. The relative difference in asymptotic values for the two groups for both surround and no surround conditions remained the same. A difference in the density of cone mosaics for deuteranopes and protanopes was analyzed as a possible explanation for the differential flicker sensitivity found in our study.

Assessment of #TheDress With Traditional Color Vision Tests: Perception Differences Are Associated With Blueness

Based on known color vision theories, there is no complete explanation for the perceptual dichotomy of #TheDress in which most people see either white-and-gold (WG) or blue-and-black (BK). We determined whether some standard color vision tests (i.e., color naming, color matching, anomaloscope settings, unique white settings, and color preferences), as well as chronotypes, could provide information on the color perceptions of #TheDress. Fifty-two young observers were tested. Fifteen of the observers (29%) reported the colors as BK, 21 (40%) as WG, and 16 (31%) reported a different combination of colors. Observers who perceived WG required significantly more blue in their unique white settings than those who perceived BK. The BK, blue-and-gold, and WG observer groups had significantly different color preferences for the light cyan chip. Moreland equation anomaloscope matching showed a significant difference between WG and BK observers. In addition, #TheDress color perception categories, color preference outcomes, and unique white settings had a common association. For both the bright and dark regions of #TheDress, the color matching chromaticities formed a continuum, approximately following the daylight chromaticity locus. Color matching to the bright region of #TheDress showed two nearly distinct clusters (WG vs. BK) along the daylight chromaticity locus and there was a clear cutoff for reporting WG versus BK. All results showing a significant difference involved blue percepts, possibly due to interpretations of the illuminant interactions with the dress material. This suggests that variations in attributing blueness to the #TheDress image may be significant variables determining color perception of #TheDress.