Aids for color vision deficiency: introduction to the feature issue.

Approximately 8% of Caucasian males and 0.5% of females have congenital red-green color vision deficiencies (CVD), and a number of eye diseases are accompanied by acquired CVD. This feature issue includes ten contributions regarding existing and proposed algorithms and devices intended to help CVD subjects compensate for their color deficiencies. It also addresses limitations in the effectiveness of CVD aids for subjects with different types and degrees of color vision deficiency.

Assessment of visual performance in the evaluation of new medical products.

When evaluating how a medical product affects vision, it is important to assess how that product affects the ability to function in real life, not only the ability to read letters on a vision chart. Nevertheless, the measurement of visual acuity with a vision chart remains the primary test of the effects of medical products on vision. Here, we review efforts to identify reliable, cost-effective clinical tests to serve as surrogate measures of functional visual performance.:

Achromatic and chromatic sensation as a function of color temperature and retinal illuminance.

Changes in color appearance with retinal illuminance were studied by scaling the achromatic, yellow, and blue sensation components for test lights with color temperatures from 3041 to 8650 K at 4.10, 2.18, and 0.33 log Td. At 4.10 log Td two observers showed similar pure whites (4823 and 5258 K) and narrow transition zones (1502 and 969 K) from yellow to blue chromatic response categories. The relative amounts of yellow, blue, and white varied with color temperature in a similar manner for both observers. One observer maintained similar absolute whites and transition zones for all illuminances. For the second observer the transition zone broadened and shifted to higher color temperatures as illuminance decreased. At color temperatures both above and below the transition zone chromatic saturation was greatest at the intermediate illuminance. The loss of saturation at 0.33 and 4.10 log Td is consistent with the observation that as the illuminance of a spectral color is raised above threshold, saturation increases to a maximum and then falls. Our findings reinforce the notion that at relatively low illuminances chromatic responses increase with increasing illuminance more rapidly than achromatic responses and that the opposite is true at high illuminances.

A whole-field scotopic retinal sensitivity test for the detection of early glaucoma damage.

We designed and evaluated a new test for the detection of early glaucoma damage in 133 persons who were classified as normal, suspected of having glaucoma, or having glaucoma. The test stimulates the entire visual field with a flashing white light under dark-adapted conditions. It can be performed easily by a high proportion of subjects and is as reproducible as present psychophysical tests. Glaucomatous eyes were discriminated from normal with a diagnostic power of 0.91 as judged by receiver operating characteristic analysis, and specificity and sensitivity were 91% and 86%, respectively. Glaucoma suspects with an abnormal response to the whole-field scotopic test were more likely to have other signs indicating early optic nerve injury, including a greater proportion of borderline field defects, nerve fiber layer defects, or glaucomatous fellow eyes. Assessment of whole-field scotopic sensitivity may be useful in glaucoma screening.

Saturation and purity of near-threshold spectral increments on achromatic backgrounds.

Vector models of color vision predict that the chromatic saturation of near-threshold incremental stimuli should grow with increasing achromatic adaptation. In contrast, colorimetric models predict that saturation should decrease with increasing adaptation in correspondence with excitation purity. An experimental test disagrees with both predictions, but is consistent with a two-process model of chromatic adaptation and contrast. Foveal monochromatic test stimuli 0.6 log unit above threshold were superposed on achromatic backgrounds with retinal illuminances of up to 5.3 log Td. Saturation and purity both decreased as background illuminance increased up to 2 log Td, but the saturation decrease was relatively small. As background illuminance increased above 2 log Td, purity remained nearly constant and saturation often increased, making the overall saturation function U-shaped. Thus, a purity of 0.1 at high background illuminance could sometimes elicit a higher saturation estimate than a purity of 0.9 at low background illuminance.

Pattern-evoked potentials and optic nerve fiber loss in monocular laser-induced glaucoma.

ERG and VEP responses to counterphase checkerboard stimuli were obtained from cynomolgus monkeys with monocular glaucoma induced by laser photocoagulation of the trabecular meshwork. The glaucomatous eyes showed reductions of PERG amplitude that were directly related to the histologically defined nerve damage. VEP amplitudes were also reduced in the glaucomatous eyes, but were more variable and less affected by damage than the PERG responses. An acute increase in eye pressure to 40 mm Hg in eyes without damage had no detectable effect on PERG amplitudes.

Hue signals from short- and middle-wavelength-sensitive cones.

Hue sensations resulting from the selective stimulation of short-wavelength-sensitive (S) and middle-wavelength-sensitive (M) cones were deduced from measurements of spectral unique green and unique blue under conditions of high or low S-cone sensitivity relative to M- and long-wavelength-sensitive-cone sensitivity. Selective reduction of S-cone stimulation shifted unique blue toward shorter wavelengths and unique green toward longer wavelengths, implying losses of perceived yellowness and short-wavelength redness relative to perceived blueness. The results imply that, under acromatic adaptation conditions, M-cone stimulation yields a sensation of predominantly bluish cyan and S-cone stimulation yields a sensation of predominantly reddish magenta. S-cone stimulation also appears to be indirectly responsible for yellowish sensations at long wavelengths and, by cancellation of the M-cone blueness signal, for greenish sensations at middle wavelengths.

Selective loss of pattern discrimination in early glaucoma.

A new perimetric pattern discrimination test was compared with conventional automated perimetry (Humphrey program 30-2 or Octopus program 32) in glaucoma patients, glaucoma suspects, and control subjects. The new test is based on the rationale that a greater percentage of retinal ganglion cells should be needed to detect a stimulus by its shape, or pattern, than by its brightness. The pattern discrimination stimulus was apatch of nonrandom dots embedded in a surrounding random dot field of the same average density. Pattern discrimination thresholds were measured by changing the degree of regularity, or coherence, of the stimulus dots. The fully coherent target was a static, 1-s duration, 20 x 20-dot checkerboard. Using a criterion-free relative operating characteristic analysis, we estimated the ability of both the pattern discrimination and conventional tests to distinguish the normal data distribution from the suspect and glaucoma distributions. The pattern discrimination test appeared to produce separations greater than conventional perimetry for glaucoma suspects and separations equivalent to conventional perimetry for glaucoma patients.

Hybrid perimetry: a blend of static and kinetic techniques.

A new hybrid perimetry technique is proposed that combines elements of kinetic and static methods. The stimulus is presented in stationary flashes, as in static perimetry, but successive flashes are moved progressively across the retina to map isopters, as in kinetic perimetry. Tests with a Tübingen perimeter show that the three methods provide similar estimates of sensitivity and variability for normal subjects. The hybrid method requires about the same time to determine an isopter point as the static method requires to determine a threshold. However, the hybrid method tests a larger retinal area in that time than the static method while providing an isopter measurement that is free of the reaction time artifacts characteristic of the kinetic method. Comparisons of photopic and scotopic hybrid fields show that one well-chosen scotopic isopter can provide as much information as three photopic isopters.

Scotopic sensitivity loss in glaucoma.

Photopic and scotopic increment thresholds were measured at selected visual field positions for 39 patients suspected of having glaucoma, 39 patients with diagnosed glaucoma, and 31 age-matched normal control subjects. Stimuli were presented at equal-eccentricity pairs of positions just above and below the nasal horizontal meridian. Both photopic and scotopic thresholds were elevated significantly for both the suspect and glaucoma groups. The average photopic and scotopic threshold elevations were the same for the suspect group, but scotopic threshold elevations were substantially greater than photopic threshold elevations for the glaucoma group. In a more detailed analysis of the glaucoma group data, local and diffuse components of the photopic and scotopic threshold elevations were operationally defined. The analysis revealed that localized photopic and scotopic scotomas were of equal depth, but diffuse scotopic defects exceeded diffuse photopic defects by a factor of 2:1 log units. These findings suggest that not all ganglion cell types are equally susceptible to glaucomatous damage and also point to scotopic perimetry as a potentially sensitive test for early glaucoma detection.

Cone response latency and log sensitivity: proportional changes with light adaptation.

Field adaptation causes proportional changes in cone response latency and log sensitivity for (1) psychophysical reaction times to threshold stimuli, (2) intracellular recordings of turtle cone responses, (3) local electroretinogram recordings of monkey cone responses and (4) theoretical impulse response functions derived from psychophysical flicker thresholds. To a good approximation, this relationship holds for all levels of cone adaptation except those that cause significant photopigment bleaching. For L and M cones, the proportionality constant between latency and log sensitivity appears to be cone-specific and largely independent of spatiotemporal stimulus parameters. For S cones, the proportionality constant depends to some extent on influences from other cone types.

Short-wavelength cones contribute to achromatic sensitivity.

Visual thresholds for flicker detection and light detection were measured for white and yellow flickering targets superposed on a white adapting field. The flicker frequency was set so that flicker detection thresholds always exceeded the light detection thresholds. The difference between the two types of thresholds was found to be smaller for yellow targets than for white targets. The most likely explanation of this result is that short-wavelength cones contribute to achromatic sensitivity at low temperal frequencies.

Summation of rod and cone responses at absolute threshold.

Visual thresholds were measured for rod stimuli, cone stimuli and mixtures of rod and cone stimuli. Comparisons of the rod and cone thresholds to the corresponding components of the mixed-stimulus thresholds revealed summation between subthreshold rod and cone responses. The amount of summation was less than linear addition but greater than either probability summation of orthogonal vector addition.

Rod-cone interaction in the dark-adapted fovea.

The foveal cone threshold was significantly lower after 45 min of dark adaptation than it was near the start of the cone plateau of the dark-adaptation curve. A concentric rod background subsequently raised the threshold by an amount correlated wit the difference between the cone plateau and the dark-adapted thresholds. Paradoxically, the rod background also lowered the cone threshold by an amount that differed from subject to subject. This sensitizing effect was identifiable by its relatively small variability across sessions. These results show that adaptation of parafoveal rods by either real light or dark light can change foveal cone thresholds.

Cone threshold vs. retinal eccentricity: changes with dark adaptation.

Detection threshold vs. retinal eccentricity functions measured during the cone plateau of the dark-adaptation curve differed from comparable functions measured on a uniform photopic background. Dark adaptation increased parafoveal sensitivity more than either foveal or peripheral sensitivity.

Relation of brightness to threshold for light-adapted and dark-adapted rods and cones: effects of retinal eccentricity and target size.

'Equal-brightness' functions of retinal eccentricity and target diameter were measured by a matching procedure, and compared with the corresponding threshold functions for four different adaptation conditions: light-adapted cones (LAC), dark-adapted cones (DAC), light-adapted rods (LAR) and dark-adapted rods (DAR). The separation between log brightness matches and log thresholds decreased with eccentricity and increased with target size for all adaptation conditions, but overall separation was substantially greater for the DAR condition than for the other three. A two-channel model of achromatic brightness is proposed to explain the results. The model assumes 'strong' and 'weak' channels, which contribute unequally to brightness. These channels are tentatively identified with tonic and phasic classes of retinal ganglion cells.