The effect of pupil size on binocular summation at suprathreshold conditions.

PURPOSE: To determine the influence of retinal illuminance on monocular or binocular visual reaction time (VRT). METHODS: On two normal subjects, uniform circular stimuli were presented with respect to a reference stimulus at the fovea under suprathreshold conditions, and the detection of positive and negative luminance variations was recorded. Binocular and monocular reaction times were measured (by the index finger pressing on a mouse key) for viewing with both natural and artificial pupils. RESULTS: Binocular reaction times were shorter than monocular reaction times; nevertheless, this binocular-summation effect was less marked in trials with the artificial pupil. Analyses of binocular-summation ratios for contrast changes for both pupil types indicated maximum and minimum binocular-saturation values depending on contrast variations in both positive and negative luminance changes. CONCLUSIONS: Binocular summation can be influenced by pupil size under suprathreshold conditions. Results are discussed in terms of retinal illuminance and cortical pupil response mechanisms.

Binocular summation of chromatic changes as measured by visual reaction time.

We determined visual reaction times to monocular and binocular changes in the luminance of isochromatic stimuli and to monocular and binocular changes in the color of isoluminant stimuli. Two isoluminant color changes were tested: chromatic variations along the red-green axis of Boynton's (1986) two-stage color vision model and chromatic variations along the yellow-blue axis of the same model. The results indicate a greater degree of binocular summation for luminance change than for color change. This result was largely independent of the motor component of reaction time.

Effects on visual function of approximations of the corneal-ablation profile during refractive surgery.

We offer an analysis that shows that the approximations made for the ablation depth during practical refractive surgery, in which the square-root terms are replaced by the first two terms of the series expansion, can limit the visual function of the observer by reducing the modulation transfer function (MTF). To simulate the refractive-surgical operation, we considered two groups of myopic patients with different ametropia who were emmetropized with different ablation profiles. We made the MTF calculations by taking the spherical aberration into account. In addition, a fuller analysis showed that these approximations limit the possibility of considering surfaces that are aspherical for reshaping the anterior cornea to optimize the observer's visual function.

Influence of the luminance and opponent chromatic channels on stereopsis with random-dot stereograms.

The present work examines the relationship between random-dot stereograms (via the disparity range parameter) and color-vision mechanisms (via the luminance channel and red-green and tritan directions at isoluminance). The results clearly indicate that the variations in the stereograms along red-green confusion lines contribute to stereopsis. Stereoscopic perception depends on spatial information for stereograms generated with variations along tritan confusion lines. For observers who perceive stereopsis via tritan directions, the results show a gradation in the disparity range, with the disparity range for stereograms generated by luminance variations being greater than for stereograms generated in red-green directions; the latter range is, in turn, greater than for stereograms generated along tritan directions.

Temporal evolution of the corneal curvature in contact lens wearers.

In the present work, we have measured the corneal radii in a group of 126 healthy subjects, studying the temporal evolution of these radii after adaptation to contact lenses of polyhydroxyethyl methacrylate with 55% hydration. We measured the horizontal and vertical radii before adaptation, as well as after 3, 6 and 9 months of wearing contact lenses. The decreases were not significant (P < 0.05) in the corneal radii of these subjects; maximum decreases were reached the day after wearing contact lenses, with subsequent recuperation, such that at 9 months the decreases, though persisting, were minimal.

Influence of the luminance signal and red-green and yellow-blue opponent chromatic signals in figural-stimuli stereograms.

The influence of color signals on stereopsis has been studied using figural-stimuli stereograms with variations introduced according to the opponent chromatic channels (red-green and yellow-blue), derived from Boynton's color-vision model. We used wallpaper stereograms, which enable the rank-order disparity ranges of the chromatic and luminance signals to be compared with the rank-order disparity range of proximity, a particular spatial configuration of the stereogram in which there are no variations in chromaticity and/or luminance. The results indicate that both chromatic signals contribute to stereopsis as does the luminance signal, contradicting the model of Hubel and Livingstone. The results also show there are no clear dependencies upon the kind of signal processed, as luminance and chromatic variations are processed with the same efficiency.

Appearance of color matchings under various chromatic adaptation conditions.

Three chromatic reference stimuli (red, green, and blue) seen under five adaptation conditions (red, green, blue, achromatic, and darkness) were appearance-matched to subsequent color stimuli seen under achromatic conditions. The experimental results have been used to verify the opponent-color color-vision model and also to check the validity of some of the published transformation equations for predicting the tristimulus values of colors matched to a reference stimulus seen under different chromatic-adaptation conditions. It is concluded that the extant transformation laws are not sufficiently general to be applied to all adaptation conditions.

Color-prediction discrepancies and differential chromaticity thresholds with photopigment bleaching.

The differential chromaticity thresholds of three color stimuli have been determined from metameric matches carried out during the cone-plateau period after photopigment bleaching. The results are compared with some recently published ones [E. Hita, L. Jiménez del Barco, and J. Romero, J. Opt. Soc. Am. A 3, 1203 (1986)] obtained under similar experimental conditions but without prior photopigment bleaching. The possible effects of rod intrusion on color-prediction discrepancies are discussed in the light of both sets of results. Under our experimental conditions, with foveal fields of 3 deg, luminance within the low photopic level, and sets of primary colors produced by filters of differing spectral bandwidth, the rod intrusion effect can be ruled out as a possible cause of color-prediction discrepancies, although it does appear to produce variations in the sizes of the discrimination ellipses and in their orientation.

Differential color thresholds from metameric matches: experimental results concerning failures of colorimetric additivity.

The differential chromaticity thresholds obtained by varying degrees of metamerism have been evaluated for five target chromaticities. Differences have been found between the chromaticities predicted by the CIE-1931 standard observer and those actually observed under our experimental conditions, with foveal fields of 3 degrees, luminance within the low photopic level, and sets of primary colors produced by filters of differing spectral bandwidths. Failures of colorimetric additivity under these experimental conditions are discussed. The results have also been analyzed by using other sets of color-matching functions, i.e., Judd's proposed modification at short wavelengths and also the supplementary standard observer, CIE-1964.

A comparative study of successive and simultaneous methods in colour discrimination.

Simultaneous and successive methods of comparison of stimuli are studied by comparing experimental results of colour discrimination experiments. In this way, colour differential thresholds for two normal observers and four different stimuli were measured by the two methods. In most cases, the capacity to discriminate colour decreased when the successive method was used, although no differences were found in qualitative aspects of discrimination. These results differ somewhat from previous reports in this field, probably because of experimental differences in the method of obtaining the thresholds.