Jeffrey B. Mulligan

Sensitivity to shearing and compressive motion in random dots

The sensitivity of the visual system to motion of differentially moving random dots was measured. Two kinds of one-dimensional motion were compared: standing-wave patterns where dot movement amplitude varied as a sinusoidal function of position along the axis of dot movement (longitudinal or compressional waves) and patterns of motion where dot movement amplitude varied as a sinusoidal function orthogonal to the axis of motion (transverse or shearing waves). Spatial frequency, temporal frequency, and orientation of the motion were varied. The major finding was a much larger threshold rise for shear than for compression when motion spatial frequency increased beyond 1 cycle deg−1. Control experiments ruled out the extraneous cues of local luminance or local dot density. No conspicuous low spatial-frequency rise in thresholds for any type of differential motion was seen at the lowest spatial frequencies tested, and no difference was seen between horizontal and vertical motion. The results suggest that at the motion threshold spatial integration is greatest in a direction orthogonal to the direction of motion, a view consistent with elongated receptive fields most sensitive to motion orthogonal to their major axis.

RECIPROCITY BETWEEN LUMINANCE AND DOT DENSITY IN THE PERCEPTION OF BRIGHTNESS

Thresholds were measured for detecting perturbations in a regular lattice of dots by modulating local dot density, local dot luminance, or some combination of the two. For high mean densities (dot spacing less than or equal to 15 min of arc), perturbations in local density increase the perceived brightnesses of the individually resolved elements in the more densely filled regions, and appear (at near threshold levels) as modulations of brightness rather than density. This illusory brightness modulation may be nulled by applying a real luminance modulation to make the lattice elements appear equally bright. Once this is done, thresholds for detecting any nonuniformity in the array are elevated compared to thresholds for detecting uncompensated density modulation. This result suggests that uncompensated density modulation is detected via the illusory brightness variations. This interpretation suggests that dot brightness is determined on the basis of the space average luminance of an area a substantial fraction of 1 deg in diameter. To test this hypothesis, thresholds were measured for detecting luminance modulation in a regular array of dots viewed against a comparatively dim background, where the modulation was applied to the dots themselves, to the background alone, or to both the dots and the background in either reinforcing or cancelling relative phase. For small, closely spaced dots, the threshold for modulation of luminance can be predicted on the basis of the amplitude of the Fourier component at the modulation frequency, regardless of whether it is carried by dots, the background, or both. The threshold is greatly elevated when modulation in the dots cancels the background modulation, so that there is contrast modulation of the dots, but no net energy at the fundamental frequency (zero amplitude of the Fourier component). For large, coarsely spaced dots, on the other hand, thresholds for conditions which contain energy at the fundamental modulation frequency are higher. The threshold increase is much greater when the modulation is applied to the dots than when it is applied to the background. This result suggests that the coarsely spaced dots are saturating the response of spatially opponent units. This hypothesis was confirmed by tests using backgrounds with the same luminance as the dots; threshold elevations selective for dots or background were abolished.

Optimizing Stereo Separation in Color Television Anaglyphs

A method is described for minimizing the ghost images which normally appear when anaglyphs are presented on color television screens. This is done by careful adjustment of the phosphor levels in each of the anaglyph regions.

Visual sensitivity to spatially sampled modulation in human observers

Thresholds were measured for detecting spatial luminance modulation in regular lattices of visually discrete dots. Thresholds for modulation of a lattice are generally higher than the corresponding threshold for modulation of a continuous field, and the size of the threshold elevation, which depends on the spacing of the lattice elements, can be as large as a one log unit. The largest threshold elevations are seen when the sample spacing is 12 min arc or greater. These results are similar to those observed by Burr, Ross and Morrone [Vision Research, 25, 717-727 (1985)], who proposed an explanation based on a compressive point nonlinearity. Although their explanation is not consistent with the present data, the results may be explained in terms of nonlinear saturation of a spatially opponent stage early in the visual pathway. Theories based on response compression cannot explain the further observation that the threshold elevations due to spatial sampling are also dependent on modulation frequency: the greatest elevations occur with higher modulation frequencies. The idea that this is due to masking of the modulation frequency by the spatial frequencies in the sampling lattice is considered.