Examining color discrimination of anomalous trichromats using the Colour Assessment and Diagnosis test and the Rayleigh anomaloscope

Abstract

Purpose : The spectral sensitivity of the long (L) and middle (M) wavelength sensitive photopigments can vary greatly. Anomalous trichromacy arises from the presence of two spectrally distinct types of L cone (L and L’) or M cone (M and M’). Our goal was to assess the effect of spectral separation on chromatic discrimination in individuals with anomalous trichromacy using the Colour Assessment and Diagnosis (CAD) test and the Rayleigh anomaloscope.

Methods : Fifteen subjects with anomalous trichromacy (1 protan, 14 deutan) were recruited and genotyped. The CAD test was used to assess red-green (RG) color vision under photopic, ambient lighting using CIE Standard Illuminant D65 lights, and a Rayleigh matching range was obtained in all subjects. Spectral separation was used to divide anomalous individuals into two groups: those with spectral separation of 3nm or less (min SS) and those with spectral separation of above 3nm (max SS).

Results : There were 8 anomalous individuals with min SS and 7 with max SS. Individuals with min SS had significantly worse RG CAD thresholds (mean ± SD, 13.60 ± 5.42 units) than individuals with max SS (5.95 ± 2.30 units, p = 0.004), where CAD units are normalized to 1 for the normal observer. Likewise, min SS was associated with larger Rayleigh matching ranges (48 ± 26.01 Nagel units) than max SS (24 ± 12.16 Nagel units, p = 0.043), although there was notable overlap between the two groups. There was a stronger inverse correlation with spectral separation and CAD thresholds (R² = 0.497) than spectral separation and the Rayleigh match (R² = 0.277, p = 0.013, Fisher’s transformation).

Conclusions : As expected, smaller spectral separation between the L/L’ or M/M’ photopigments in anomalous trichromats was associated with larger Rayleigh matching ranges and higher CAD thresholds. The CAD test was more predictive of the size of spectral separation in anomalous trichromats than the Rayleigh match, although previous literature has shown that a more extensive Rayleigh protocol provides results that are sensitive to spectral separation.¹ The CAD test offers a more practical method for detecting small differences in chromatic discrimination.
¹PMC5017313

This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.