Journal of the Optical Society of America最新文献

The offset of a pulsed conditioning field of light has recently been shown to produce enhancements of temporal resolution and brightness discrimination. These enhancements are similar to those that are due to imposing a high level of light adaptation on the visual system. Here we analyze whether there is a true equivalence of adaptive state between some level of steady light adaptation and the offset of a conditioning field. The enhancements of visual function at field offset were replicated by using a suprathreshold two-pulse discrimination task and a task requiring detection of an incremental probe stimulus superimposed upon a suprathreshold pulse. These effects are shown to be qualitatively but not quantitatively similar to those produced by an equivalent background selected on the basis of its ability to raise threshold to the same degree as field offset. We conclude that the equivalent-background principle cannot be supported for our measures of foveal visual function.

Three-dimensional discrimination ellipsoids are presented for a number of representative points in color space. These ellipsoids have been obtained not with the conventional split field but with flickering grating patterns. Thus our study extends the well-known results of Brown and MacAdam [J. Opt. Soc. Am. 39, 808-813 (1949)] to cases in which the image is structured in space and time. As expected, we find that the discrimination ellipsoids depend on the spatiotemporal structure of the stimulus. This has potential consequences for color-difference formulas as used in industry and commerce: no single formula will do when it is important to treat patterns with different structure. We present analytical descriptions, based on the Vos-Walraven [Vision Res. 12, 1327-1365 (1972)] line element augmented with spatiotemporal frequency-dependent coefficients that fit our results reasonably well. For coarse gratings (approximately 1 cycle per degree) or slowly modulated fields (approximately 1 Hz) our results prove to be compatible with the results of Brown and MacAdam obtained with a bipartite 2 degree field.

When balanced red and green lights are alternated more than 20 times per second, the perceived flicker can be reduced by advancing the green flicker about 10 degrees of the red-green cycle. The required advance for least flicker is greatest at retinal illuminances around 1000 td and frequencies between 30 and 35 Hz. A model that predicts tuning at this frequency exists, but the tuning curve that is predicted is broader than that observed. A modified model is left for future publication. Meanwhile, other empirical properties of the advance required by green over red are described. In addition to the intensity dependence of this phase shift, we describe its dependence on intensity balance between red and green. Also, the intensity balance turns out to depend on the frequency being used, in contrast to the independence expected by Ives, the inventor of heterochromatic flicker photometry.

For quantitative models of color vision, the R-cone contribution to the r-g channel is less than half of the R-cone contribution to the V lambda channel. There is currently no explanation of how this different contribution of R cones to the two channels comes about. We propose an asymmetrical receptive-field arrangement to explain the difference in weighting. Because cones in receptive-field surrounds are weighted less than cones in centers, placing R cones predominantly in surrounds and G cones in centers provides a simple differential weighting mechanism. Electrophysiological and psychophysical evidence substantiates such an asymmetry of simple-opponent fields.

A set of increment threshold data as a function of test-flash diameter, background luminance, and retinal eccentricity is presented. It is shown that for low background intensities the results can readily be described by simple transformations of flash diameter and background luminance: The threshold is independent of eccentricity if the quotient of diameter and eccentricity is constant and if the flash is presented on a background for which the product of background luminance and the square of eccentricity is constant. At an eccentricity of 50 deg, Ricco's law is violated: A small stimulus has a threshold 10 times as high as a large stimulus. On the basis of results found by other investigators for smaller eccentricities, it is concluded that the receptive field size at 50 deg of eccentricity is more than 10 deg (for low background luminances). For eccentricities smaller than 50 deg, a data analysis is given in order to derive an appropriate measure of the size of the sample units. This analysis shows that with increasing background luminance the decrease in the size of the sample unit is steplike rather than gradual.

The effects of spatial-frequency adaptation on spatial-probability summation were investigated by two techniques: the measurement of the dependence of cosine grating sensitivity on the number of cycles presented and the determination of the frequency-of-seeing curve for a cosine grating. Both measurements indicate that the contribution of spatial-probability summation to adapted grating thresholds is reduced in comparison with unadapted conditions. This reduction in the effects of spatial-probability summation, together with the decrease in local sensitivity, which depends on the spatial inhomogeneity of the visual system, is sufficient to account for the threshold elevation aftereffect measured when adapting and testing with cosine gratings.

There is a discrepancy between several studies that have shown the human luminous-efficiency function to vary with surround color and a recent study that failed to find this dependence. Data are presented that show that this discrepancy can be explained by differences in the matching techniques. Luminous efficiency measured by direct heterochromatic brightness matching does depend on surround color, whereas luminous efficiency measured by the flicker method does not. The independence of luminous efficiency as measured by flicker is evidence for an independent luminance channel.

Following Munsell's bisection procedure [J. Opt. Soc. Am. 23, 394 (1933)], we established a nine-step gray scale in which each step is an equal increment in lightness. We calculated retinal illuminances after intraocular scatter by using the point-spread function of Vos et al. [Vision Res. 16, 215-219 (1976)]. After this correction for intraocular scatter, we find a logarithmic relationship between retinal illuminance and achromatic lightness scales that are determined by the bisection method. Additional bisection experiments with a series of different backgrounds corroborate this result. We find that lightness depends linearly on the logarithm of scatter-corrected retinal illuminance, with different slopes for backgrounds of different lightness. This study also highlights the importance of using scatter-corrected illuminance in any quantitative model of lightness.

The summation-index technique was applied to flicker photometry to investigate the linearity law. Two test stimuli of different wavelengths were mixed, and the luminous efficiency of the mixture was measured under three different adapting conditions: no adaptation, 507-nm adaptation, and 650-nm adaptation. Most of the wavelength combinations and adapting conditions gave a summation index of 0.30, which is the linearity in the visual system that is responsible for the flicker perception, namely, the achromatic channel. An exception was the red and green combination under the no-adaptation condition, which gave a slightly smaller summation index than 0.30, indicating that these two stimuli do not add linearly. The subsummation was interpreted as cancellation of red and green responses in the red-versus-green opponent channel. The subsummation disappeared when the flicker frequency was increased from 5 to 12 Hz, confirming the interpretation.

Red-green and yellow-blue chromaticnesses were scaled for various monochromatic lights by a just-noticeable-difference method. The just-noticeable difference of each chromaticness, i.e., redness, greenness, yellowness, or blueness, was defined by the change of the canceling light intensity that was required to produce a just-noticeable difference in the amount of the opponent-hue attribute of each monochromatic light. The results showed that an approximately logarithmic transformation took place at the two opponent-color coding systems and that there existed an interaction between red-green and yellow-blue opponent-color coding systems in such a manner that the effective contribution of one opponent-color response to the perceived opponent-hue attribute was reduced by increasing the magnitude of the other opponent-color response. This interaction is considered to be responsible for the well-known veiling effect.