Using a computerized test system, we compared binocular and monocular visual optotype acuity, varying both contrast and contrast disparity between the two eyes. When contrast was the same in the two eyes, binocular acuity was better than best monocular acuity by an average of 0.045 log minimum angle of resolution, or 11%. When contrast differed in the two eyes, binocular acuity in most but not all cases was still better than the monocular acuity of the eye that received the higher contrast. This binocular advantage became smaller but remained significant as contrast disparity became larger. These results are most simply explained by threshold contrast summation of high-spatial-frequency letter components.

BINOCULAR stereopsis provides information about the relative distance of objects from the differences in the horizontal position of their images on the two retinas. Because the size of the disparity between two points is inversely related to the square of the viewing distance, it is usually assumed that disparities have to be scaled according to distance using the vergence angle of the eyes, or by using the small vertical disparities that also exist between corresponding points of the two images. Here we present evidence that the visual system could extract information about the shapes of surfaces (without the need for scaling) by using the second spatial derivative of disparity--disparity curvature--which remains invariant with viewing distance. Rather than computing the second derivative, we suggest that an approximation to disparity curvature could be derived from the differences in curvature of corresponding line elements in the two eyes.

We measured stereoscopic slant detection thresholds for surfaces slanting about a horizuntai or a vertical axis. For randomdot covered s&aces, 1.25deg uf slant was required to detect slant about a ants axis, whereas 2.1 deg of slant was required tu detect sbmt abut 8 wrtic8l mds. T&3 8nisOtrOpy could be due t0 the fact th8t Orieltt8tiOO 4#k!iptitiwt Which ~311tim kki~8tiOn about mrf8ce slant, are generally smaller for surfaces slsnting about 8 verticad 8xis. To test this possibility, slant threshuids were measured for surfaces whose orientation disparity content was manipulated ~~~~tly of the other slant i~o~~n present. When tlie magm&ude of orientatiun disparity was thesame fOr ~8~~aUti~~~ta ho~zo~81~ 8 VerticaI 8X~~~~~~O~~~~~~~ about 1.5 deg of slant to be detected; thus the anisutropy became negiigiile. In contrast, when the o~en~tion disparity content of 8 surface slanting about a vertical axis was zeru, 3-4 deg of sknt was required fur detection; thus the anisutrupy became larger. Under the conditiuns of these experiments, it appears that the visuai system utilizes o~e~tation disparities.

The development of computerized typography has revolutionized our ability to create type designs, in facilitating both the rapid design of new fonts and the alteration of their characteristics almost infinitely[10, 13]. Although type designs vary for a variety of reasons, their primary purpose is to serve as the elements of text-coded communication. Legibility is a general term that refers to the effectiveness of typography in communicating the text code. It can be defined and measured in several ways, including direct judgment, reading speed[11, 3, 9], and visual acuity [16].

Although computerized visual acuity testing is psychophysically more flexible than “manual” testing using printed or projected optotype charts, the quality of currently available and affordable pixel-oriented computer images makes computers poorly suited for clinical testing. This is mainly because low spatial resolution in one or both dimensions limits the range of sizes available for testing at a single optical distance.