No abstract

Observers viewed either vertical or obliquely oriented vernier targets from either an upright position or with their heads tilted. Vernier acuity was consistently better for retinally vertical than for gravitationally vertical targets, even when presented against a background context of vertical stripes designed to aid veridical perception of gravitational orientation. These results indicate that vernier acuity depends on retinal image orientation rather than on perceived orientation. The high contrast of the vernier lines ensures that their gravitational orientation is clearly perceived. Thus the present results provide a stronger basis for ruling out the effects of perceptual orientation than previous studies involving grating contrast-threshold measurements. Since the vernier targets were presented as brief flashes, it is unlikely that the measured oblique effect is attributable to differences in eye-movement patterns.

Subjects were adapted to alternating upright red and oblique green checkerboards. After adaptation, vertical and horizontal gratings appeared pink and oblique gratings appeared green. The results show an orientation and spatial frequency selectivity predicted by two-dimensional Fourier analysis.

The axial elongation of high myopia is known to induce tangential stretching forces on the retina. Such forces might be expected to compromise retinal circuitry and thus affect the temporal and spatial contrast sensitivity function. In fact, we find that simple high myopes have normal contrast sensitivity for stationary gratings, moving gratings, and uniform field flicker. This suggests that, unlike many other retinal disease processes, the high myope's retina retains its normal integrity until the outer retina is compromised.

Judgments of the simultaneity of pairs of brief light flashes are not in general veridical, i.e., objective and subjective simultaneity relations are not identical. To explain this discrepancy, it was proposed that there exists a simultaneity center in the brain where paths of excitation from the visual system must coincide to produce the experience of simultaneity. The experiment tested an implication of the simultaneity center hypothesis, that simultaneity judgments are transitive. 3 experienced Os were monocularly presented with pairs of 5-msec light flashes. 3 stimulus lights, F, N, and E, were presented pairwise in 3 sessions: F vs. N, F vs. E, and N vs. E. Thresholds of simultaneity were obtained for each session in terms of the interstimulus interval required to produce subjective simultaneity. Within the experimental variability obtained, the transitivity relation was confirmed which supports the simultaneity center hypothesis.