Joel Pokorny

Brightness of equal-luminance lights

The brightness of lights defined by heterochromatic flicker photometry as being of equal luminance was determined by direct comparison with a white standard. Stimuli included mixtures of white and colored lights. Eight different colorimetric purities were tested for each of eleven primaries. Adaptation to a known white was maintained. Resulting brightnesses were expressed in terms of the ratio of the amount of light required for a brightness match to the amount required for a flicker photometric match (B:F). B:F ratios varied with the primary, being largest for red and blue primaries. The variation of B:F ratio with colorimetric purity differed depending on the primary, although for any given dominant wavelength the highest-purity stimulus always had the largest B:F ratio. The data qualitatively exclude a number of simple models of brightness generation.

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.

Functional Loss in the Magnocellular and Parvocellular Pathways in Patients with Optic Neuritis

PURPOSEnTo evaluate contrast threshold and contrast gain in patients with optic neuritis under conditions designed to favor mediation by either the inferred magnocellular (MC) or parvocellular (PC) pathway.nnnMETHODSnAchromatic and chromatic contrast discrimination was measured in 11 patients with unilateral or bilateral optic neuritis and in 18 age-matched controls with normal vision, using achromatic steady- and pulsed-pedestal paradigms to bias performance toward the MC or PC pathway, respectively. In addition, L-M chromatic discrimination at equiluminance was evaluated using the steady-pedestal paradigm. A physiologically plausible model could describe the data with parameters accounting for contrast gain and contrast sensitivity in the inferred MC or PC pathway. The fitted parameters from the eye affected by optic neuritis were compared with those from the normal eye using generalized estimation equation (GEE) models that can account for within-subject correlations.nnnRESULTSnCompared with normal eyes, the affected eyes had significantly higher saturation parameters when measured with both the achromatic pulsed-pedestal paradigm (GEE: β [SE] = 0.35 [0.06]; P < 0.001) and the chromatic discrimination paradigm (β [SE] = 0.18 [0.08]; P = 0.015), suggesting that contrast gain in the inferred PC pathway is reduced; the affected eyes also had reduced absolute sensitivity in the inferred MC pathway measured with the achromatic steady-pedestal paradigm (β [SE] = 0.12 [0.04]; P = 0.005).nnnCONCLUSIONSnOptic neuritis produced large sensitivity losses mediated by the MC pathway and contrast gain losses in the inferred PC pathway. A clinical framework is presented for interpreting contrast sensitivity and gain loss to chromatic and achromatic stimuli in terms of retinal and postretinogeniculate loci contributions to detection and discrimination.

The Farnsworth-Munsell 100-hue test in cone excitation space

The Farnsworth-Munsell 100-hue test (FM 100-hue) has been subject to a variety of sophisticated analyses in recent years including conversion to uniform color space and Fourier analysis of error patterns. The majority of acquired color defects seen by clinicians, often assessed by the FM 100-hue or by arrangement tests, arise from retinal or optic nerve disorders. Chromatic discrimination at the retinal level is well described within cone excitation space. Retinal and optic nerve disorders should show chromatic discrimination loss along ‘critical axes’ in cone excitation space. We therefore calculated cap locations in the MacLeod-Boynton cone excitation space, where S/(L + M) cone excitation is plotted vs L/(L + M) cone excitation in a constant luminance plane. The FM 100-hue cap locations form an ellipse tilted from the S and L/(L + M) axes. Excitation on one or the other axis gives an approximately sinusoidal error pattern as a function of cap number, but the separation is not 90°. Error patterns for congenital and acquired defects can be predicted and compared with data. Normal FM 100-hue test error scores can be plotted on a Δl/l template. A similar approach can be taken for arrangement tests and confusion errors can be plotted on the template. The results are consistent with chromatic discrimination data as a function of test illuminance.

Aberrant flicker sensitivity revealed by heterochromatic modulation photometry

Heterochromatic modulation photometry is a method in which 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 expectation is a distinct minimum modulation sensitivity at the standard/test ratio representing the luminance match. At other luminance ratios, flicker sensitivity should vary with the luminance difference between standard and test. We have devised theoretical templates to describe modulation sensitivity as a function of standard/test ratio. The results of other flicker studies indicate that flicker sensitivity would be expected to fall within the domain between a linear system (sensitivity proportional to the amplitude of modulation) and a Weberian system (sensitivity proportional to percent modulation). Using ‘red’ and ‘green’ lights, at low photopic luminances this expectation is well realized. At higher luminances and when the ‘red’ light is of higher luminance than the ‘green’, observers are much less sensitive to modulation than the models predict.

Sensitivity of screening tests for S-cone discrimination

We evaluated tritan screening tests in color normals by producing S-cone discrimination loss with reduced illumination. The Tokyo Medical College plates, the Rabkin plates, the Velhagen plates, and the Minimalist test were compared to the Standard Pseudoisochromatic Plates, Part 2 (SPP-2) for which we have previously reported data. We used six light levels ranging from 0.1 to 63 lux. Seventeen observers with normal color vision served in the experiment; seven older observers (median age 49) and ten younger observers (median age 28). All performed all tests correctly at 63 lux. There were statistically significant differences between the younger and older groups on all tests. For pseudoisochromatic plates, the Rabkin plates were the least sensitive to reduced illumination while the Tokyo Medical College plates showed high sensitivity. The four plates of the Velhagen test, showed differing sensitivity. The error scores on the Minimalist test increased with decreasing illumination level. The error scores of the tritan series were always greater than those of the protan and the deutan series. The Velhagen and the Minimalist test allow estimation of graded sensitivity loss comparable to the SPP-2.