Journal of the Optical Society of America最新文献

An afterimage method has been used to investigate the relative magnitudes of the nonmotor and motor components of the fusional response to vertical disparity in a complex visual stimulus of diameter 57 degrees consisting of 50 horizontal lines and a square of side 2.5 degrees in the middle. The largest vertical disparity that evoked a stable fusional response was found to be in the range 3-6 degrees, of which the nonmotor component amounted only to 8-15', i.e., 2-10% of the total. At these fusional amplitudes, binocular single vision was already disrupted in the foveola. When the 50 horizontal lines were omitted from the stimulus so that only the central square of side 2.5 degrees remained, the fusional amplitudes decreased by only 25% while the absolute level of the nonmotor components remained the same. The nonmotor components found here are much smaller than those (amounting to about 2 degrees, or 25-40% of the total response) reported recently in the literature.

The spatially localized threshold-elevation aftereffect of spatial-frequency adaptation was measured by using localized, aperiodic test patterns that have bandpass Fourier transforms. At a given retinal location, the threshold-elevation curves are consistent with the fatigue of size-turned mechanisms with center-surround sensitivity profile. Only a few different sizes of such mechanisms were required to fit the local results. The local aftereffect was also measured as a function of eccentricity near the fovea. The results indicate that the threshold-elevation aftereffect of spatial-frequency adaptation is not spatially homogeneous.

The problem of inferring local surface orientation from changing images is studied computationally by deriving conditions under which the motion information is sufficient for an information processing system, biological or otherwise, to infer unique descriptions of the local surface orientation. The analysis is based on shape-from-motion proposition, which states, that given the first spatial derivatives of the orthographically projected velocity and acceleration fields of a rigidly rotating regular surface, then the angular velocity and the surface normal at each visible point on that surface are uniquely determined up to a reflection. The proof proceeds in two steps. First it is shown that surface tilt and one component of the angular velocity are uniquely determined by the first spatial derivatives of the velocity field. Then it is shown that surface slant and the remaining two components of the angular velocity are uniquely determined if the first spatial derivatives of the acceleration field are also available.

It seems quite certain that the visual entoptic phenomenon called Haidinger's brushes is a consequence of dichroism of the macular pigment; this dichroism is usually ascribed to orientation of the pigment molecules. An alternative explanation of the dichroism, an explanation that requires no orientation of pigment molecules, investigated and shown to be feasible. Specifically, it is shown that form dichroism that is due to the structure of the Henle fiber layer can account for Haidinger's brushes if the macular pigment is almost entirely confined to the Henle layer, if individual fibers have an index about 5% higher than that of the interfiber medium, and if the fibers form a tightly packed array.

Thresholds of 1-deg, 200-msec, 641-nm foveal test flashes rise after an exchange of II5-equated 536- and 626-nm fields, taking about 30 sec to recover. Silent substitution (no rise of threshold after the exchange) occurs, however, if these fields are alternated during adaptation. Thresholds for a 1-deg, 20-msec test rise similarly after an exchange but recover in only 0.5 sec and are not influenced by alternation of the 536- and 626-nm fields. These results can be accounted for if the 641-nm tests are detected not through pathways controlled by long-wavelength cones alone but through nonopponent (20-msc) and opponent (200-msec) pathways whose sensitivities may be reduced by transient inputs from other cones.

The refractive index and the diameter of the fly rhabdomere were determined by comparing the experimental results derived from interference microscopy with the results of theoretical study on the scattering of plane waves by a homogeneous, isotropic cylindrical dielectric rod. It was found that the refractive index of the isolated rhabdomere of Calliphora erythrocephala is 1.363 +/- 0.003 in an area of the rhabdomere where its diameter is calculated to be 1.32 +/- 0.04 micrometers.

Rayleigh matches obtained from red-green color deficients with conventional methods show large individual differences within diagnostic categories. Similar matches obtained from the same observers with a large-field substitution method show much less variability and suggest that the differences observed among simple anomals, extreme anomals, and dichromats with conventional methods are probably not solely due to the visual pigments contained in the cones. A theory that attributes these differences to the relative number of abnormal cones present in the observer's retina is described.

When the thresholds for periodic spatial patterns containing two or more differently oriented components (e.g., crossed gratings) are measured under normal, unstabilized conditions, each component seems to be detected almost independently of the others if their angular orientations are sufficiently different. This psychophysical behavior has been attributed to anisotropic or orientation-tuned units in the visual cortex. Here we report that when the image of such a multicomponent pattern is stabilized on the retina, the independent-detection behavior vanishes. Under stabilized-image conditions, the contrast sensitivity is governed by the maximum local contrast at the retina. The number and relative contrast of individual components, even orthogonal ones, behave almost additively in making up the threshold contrast. We confirmed this conclusion with a variety of patterns that give orientation-tuning effects in unstabilized viewing. Controlled image motion (resembling the effect of the natural drifts of the eye) restores the independent-detection behavior in every case, as do other forms of temporal modulation (e.g., flicker or flash presentations). We infer (1) that orientation-tuned units in man do not respond to unchanging stimuli--they cannot function unless the pattern on the retina is temporally modulated, and (2) in the absence of temporal modulation, spatial patterns are detected by isotropic units of relatively low sensitivity.