Thomas P. Piantanida

On seeing reddish green and yellowish blue.

Four color names—red, yellow, green, and blue—can be used singly or combined in pairs to describe all other colors. Orange, for example, can be described as a reddish yellow, cyan as a bluish green, and purple as a reddish blue. Some dyadic color names (such as reddish green and bluish yellow) describe colors that are not normally realizable. By stabilizing the retinal image of the boundary between a pair of red and green stripes (or a pair of yellow and blue stripes) but not their outer edges, however, the entire region can be perceived simultaneously as both red and green (or yellow and blue).

Temporal modulation sensitivity of the blue mechanism: Measurements made with extraretinal chromatic adaptation

Using the counterphase-modulated-tritan-metamers method of Wisowaty and Boynton [Vision Res. 20, 895-909 (1980)], we measured the modulation sensitivity function of the blue mechanism on dark backgrounds and on uniform yellow backgrounds that were produced either by illuminating the retina with yellow light or by filling-in of a stabilized retinal image. When measured on yellow backgrounds, temporal modulation sensitivity was markedly reduced whether the background was produced by retinal illumination or by filling-in. This sensitivity reduction has been attributed by other researchers to changes in the transmission characteristics of the blue/yellow pathway caused by yellow adaptation. Our results obtained with backgrounds that appeared yellow due to filling-in indicate that the site of adaptation must be extraretinal as there was no yellow light reaching the retina in the vicinity of the test probe.

Stereo hysteresis revisited

I have replicated the historic Fender and Julesz stereo hysteresis study [J. opt. Soc. Am. 57, 819-830 (1967)] but with a different method of image stabilization. My results generally support their findings that once fused, the two stabilized monocular images of a random-dot stereogram can be separated on the retinas by as much as 2 deg and still remain fused. However, there is one major exception: whereas they found it necessary to realign the images to within 6 min of arc for refusion to occur, I find that refusion of stabilized random-dot stereograms may occur well outside the classical Panums fusional area.

Studies of the field-of-view resolution tradeoff in virtual-reality systems

Most virtual-reality systems use LCD-based displays that achieve a large field-of-view at the expense of resolution. A typical display will consist of approximately 86,000 pixels uniformly distributed over an 80-degree by 60-degree image. Thus, each pixel subtends about 13 minutes of arc at the retina; about the same as the resolvable features of the 20/200 line of a Snellen Eye Chart. The low resolution of LCD-based systems limits task performance in some applications. We have examined target-detection performance in a low-resolution virtual world. Our synthesized three-dimensional virtual worlds consisted of target objects that could be positioned at a fixed distance from the viewer, but at random azimuth and constrained elevation. A virtual world could be bounded by chromatic walls or by wire-frame, or it could be unbounded. Viewers scanned these worlds and indicated by appropriate gestures when they had detected the target object. By manipulating the viewers field size and the chromatic and luminance contrast of annuli surrounding the field-of-view, we were able to assess the effect of field size on the detection of virtual objects in low-resolution synthetic worlds.

The Impact Of Boundaries On Color: Stabilized Image Studies

The NTSC standard for color television codes the chrominance signals at a lower spatial resolution than it codes the luminance signal. These differential resolutions result in a smearing of the colors in the scene relative to the edges that define objects, but television viewers are rarely aware of this degradation of the image because the human visual system also codes chrominance (i.e. hue) at a lower spatial resolution than it codes luminance (i.e. edges). Given the resolution difference for chrominance and luminance edges, a model of visual perception must explain why human observers do not perceive the color of objects flowing beyond the luminance edges of those objects. The internal chromatic aspects of objects, which may be determined at chrominance object-boundaries, may be constrained by the perceived spatial luminance boundaries of those objects. In the experiments that we will describe, a spatial chrominance and luminance boundary is prevented from moving on the viewers retina by moving it image synchronously with eye movements. When the edge is stabilized on the retina, the appearance of the image depends upon the enclosing boundaries that are not stabilized on the retina. We have found that the appearance of the image, independently of the images actual spatial energy distribution on the retina affects the ability of the human visual system to process information. For example, the viewers flicker sensitivity depends upon the perceived color of the image and not its actual spectral energy distribution. The same is true for the perceived color of a small spot imaged on the stabilized fields.

Color appearance of filled-in backgrounds affects hue cancellation, but not detection thresholds

A long-wavelength background can affect the appearance of an increment of light superimposed upon it in two ways. It can change the visual systems sensitivity to the increment, and it can change the appearance of the increment by directly adding redness to it. Through selective retinal-image stabilization, we evoked the filling-in phenomenon to change the appearance of 640- and 575-nm backgrounds. Either of these backgrounds could be made to appear red or yellow, depending upon whether it was viewed under stabilized or unstabilized conditions. When the appearance of the 640-nm background was altered by filling-in to appear less red, test probes superimposed upon it required less 540-nm component to achieve an equilibrium hue. Increment thresholds measured on the 640- and 575-nm backgrounds, however, did not change with the appearance of the backgrounds.

Methodology-specific Rayleigh-match distributions

Abstract Direct comparison of Rayleigh-match midpoint distributions obtained with different psychophysical methods reveals that unimodality is associated with the method-of-adjustment and multimodality is associated with the forced-choice method.

Molecular Genetics of Human Color Vision

The study of the inheritance of defective color vision began in 1777, when Huddart described the abnormal color vision of a shoemaker named Harris and his family. Subsequent development of genetic models of normal and defective human color vision can be traced through the research efforts of Dalton, Horner, Lord Rayleigh, Mendel, Franceschetti, DeVries, Pickford, Lyon, Rushton, Sperling, Smith and Pokorny, and many others. Using a genetic model that postulated an allelic series at two X-linked color-vision loci, a team of psychophysicists and molecular biologists recently examined the DNA of color-normal and color-defective human males, expecting to find a single copy each of the red- and green-cone photopigment genes on their subjects’ X-chro-mosomes, with the genes subtly mutated in their color-defective subjects. Instead, the DNA of their color-normal subjects harbored a single copy of the red-cone photopigment gene and a variable number of copies of the green-cone photopigment gene, and the DNA of their color-defective subjects showed strong evidence for gene hybridization rather than mutation. Both the variable number of color-vision genes and the presence of hybrid genes can be explained by anomalous recombination.

Separation Of Form Perception And Stereopsis

This study addresses the question of whether static and dynamic stereopsis require the perception of form. The retinal image requirements of the visual mechanisms subserving form perception and stereopsis are not only distinct but potentially antagonistic. Form perception requires the retinal image to have luminance gradients that are steep enough to produce suprathreshold temporal transients in the receptors during normal eye movements. Stereopsis, on the other hand, requires identification of corresponding luminance gradients in the two retinal images so that their retinal disparity can be calculated. Thus, while the motion of the retinal image caused by normal eye movements is essential to form perception, it may be detrimental to stereopsis. We eliminated the motion of the retinal image that would normally have occurred during eye movements by using a pair of SRI dual-Purkinje-image eyetrackers and stimulus deflectors to stabilize the retinal images of selected form elements. We examined the thresholds for perceiving motion in depth under stabilized and unstabilized conditions and found that the perception of motion in depth continues in the absence of monocular form perception. Likewise, when we stabilized the disparate images of a line stereogram, stereopsis persisted in the absence of form perception of those elements whose retinal disparity deter-mined their perceived depth. These results imply profound separation of the form-perception and stereopsis mechanisms.