Cone letter charts: rapid color test using a range of letter sizes

Normal color vision (CVN) allows recognition of numerous surface colors, precise wavelength discrimination, and color information essential for optimal safety and performance in myriad settings [1]. CVN derives from three cones: blue (short wavelength, S), green (middle wavelength, M), and red (long wavelength, L). Hereditary color vision deficiency (CVD; 8% males, 1/200 females) is as follows: L cones absent (protanopia, 1% of males); M cones absent (deuteranopia, 1%); and shift in cone sensitivity (protanomaly: L shifted toward M, 1%; deuteranomaly: M shifted toward L, 5%) [1, 2]. Acquired CVD occurs in many eye diseases [3, 4] and can be a biomarker for cognitive impairment [5]. Cone contrast sensitivity (CS) quantifies type and severity of CVD using large letters [2‐5]. We expanded cone CS with more letter sizes (cone visual acuity, VA; small letter CS). A composite score was developed to quantify color vision across a range of letter sizes to improve matching of hereditary and acquired CVD abilities to visual demands. Cone-specific (L, M, S) CS functions were derived for potential research and clinical applications.

Computer-generated VA and CS charts were displayed on a calibrated Microsoft Surface computer presenting letters visible only to L, M, or S cones based on measurement of CIE chromaticity and luminance (CS-100 colorimeter, Konica Minolta) followed by transformation to cone excitations and cone contrasts [2, 3]. VA charts were as follows: 0.1 logMAR/row, five letters/row, and 0.02 logMAR/letter read correctly. L and M cone (LM) charts were as follows: 20/190 to 20/15 (0.98 to − 0.12 logMAR), 8% contrast. S cone charts were as follows: 20/240 to 20/50 (1.08 to 0.38 logMAR) and 64% contrast (contrasts derived from modified Innova Systems, Inc. test; larger and higher contrast letters were used for S cones due to their sparse retinal distribution) [2, 3]. CS charts were as follows: 10 rows, 5 letters/row, and logCS decreased 0.15/row yielding 0.03 logCS per letter correct. LM contrast range was as follows: 16–1%, S: 128–8%. Large letter CS was as follows: LM, 20/324, S, 20/430 [2, 3]. Small letter CS was as follows: LM 20/68, S 20/180. Testing was binocular with habitual correction in a dark room at 91.44 cm. Data were distributed normally (Jarque–Bera test). ANOVA and paired and unpaired t-tests with Bonferroni correction and regression analyses were used for descriptive and comparative analyses (Microsoft Excel version 2211). Fifteen CVNs (27 ± 6 YO) and 8 CVDs (32 ± 17), confirmed as CVN or CVD by Ishihara and anomaloscope testing, participated after written informed consent in accord with our IRB approved protocol.

Figure 1 shows cone VA and CS charts (details in figure caption). Figure 2a shows CVN means (± 2SE) for each test with individual results from CVDs for their defective cone type (5 deutans, 3 protans). Mean differences between CVNs and CVDs were significant on each test (P < 0.001). A composite score based on sum of log scores for each test (VA corrected for sign) revealed a significant difference between CVNs (mean 3.3) vs. CVDs (mean 1.6; mean difference = 1.7, 95% CI = 1.25–2.21, P < 0.001). Cone CS functions were derived by converting mean CVN values to CS vs. spatial frequency and applying best-fit logarithmic regression (Fig. 2b).

Computer-generated cone-specific VA and CS charts offer a new, rapid clinical method to quantify color vision. Color tasks requiring fine detail (electronics, signal light detection, seeing details with macular dystrophies, AMD) may benefit more from cone VA and/or small letter CS. CVD physicians discriminating skin tones, and patients with acquired CVD from glaucoma or MS, may benefit more from large letter CS. Composite scores provide a comprehensive metric which may, in some cases, exceed sensitivity of current cone CS tests. Cone-specific contrast sensitivity functions derived herein exemplify absolute differences between cones offering detailed information potentially useful for both research and clinical applications.