Journal of the Optical Society of America最新文献

Observers detected and/or identified the spatial frequencies of grating stimuli. Spatial frequency varied from 3.8 to 5.5 cycles per degree, and contrast varied from 0.001 to 0.33. In nearly all cases, the psychometric functions that relate performance on the different tasks to contrast are multiples of one underlying function, provided that the functions are expressed in standard normal deviates. The underlying function is positively accelerated at contrasts less than 0.01 and levels off at contrasts greater than 0.05. A vector model interprets the results and relates them to the responses of individual spatially tuned mechanisms.

Optics began as a visual science, and the eye was the original optical instrument. Students of physiological optics, together with their clinical colleagues, were concerned mainly with the normal and pathological functioning of the eye as a receptor organ. In recent years, however, exciting developments have changed all that. Optics has taken off in many directions that have little immediate relation to the eye, such as x-ray astronomy, lasers, and photoacoustic spectroscopy. Vision research, in turn, has gone far beyond its sole preoccupation with the optics of the eye. Most exciting are new discoveries about the visual pathways and the specialization of individual brain cells for the processing of line orientation, stereoscopic depth, spatial frequency, motion, and color. Comparative studies reveal the functional architecture of the brain together with the genetically and chemically programmed cellular development that lays the groundwork for later modification by the visual environment. Stimulated by this neurophysiological progress, and by newly available optical concepts and techniques, visual scientists have greatly expanded their research beyond the traditional topics of physiological optics and color.

Moving the retinal image of a sinusoidal grating at a constant velocity (compensated for eye movements) provides controlled spatial and temporal frequencies at every point in the stimulus field. Using this controlled-velocity technique, we have measured the detection threshold for isoluminance, red/green gratings as a function of their spatial and temporal frequencies. The chromatic contrast-threshold surface obtained in this way is analogous to the achromatic contrast-threshold surface measured previously, but the results are quite different. For very low temporal frequencies (below 0.2 Hz), the chromatic sensitivity decreases steadily with decreasing temporal frequency. Below 0.01 Hz, chromatic patterns disappear completely even at maximum contrast (although achromatic or homochromatic patterns do not). In the region above 0.2 Hz, both achromatic and chromatic thresholds can be explained by the same receptive-field-like model. When the center and the surround components of this model are additively combined, they form the chromatic threshold surface; when the sign of either component is reversed, they form the achromatic one.

Contrast sensitivities to countermodulating gratings were measured with a two-alternative temporal forced-choice procedure following adaptation to a static grating of the same spatial frequency, a homogeneous flickering field of the same temporal frequency, or a countermodulating grating of identical spatial and temporal frequencies. At high spatial frequencies, the temporal-frequency content of the adaptation was not critical, that is, a countermodulating adaptation grating was only slightly more effective at raising threshold than was a static adaptation grating. At low spatial frequencies, the sensitivity to countermodulating test gratings could not be reduced by either a high-contrast stimulus matching the test in the spatial domain only or by one matching the test in the temporal domain only. Adapting to a high-contrast stimulus matching the countermodulating test grating in both spatial- and temporal-frequency domains was effective at reducing test sensitivity for one observer but not for another.

Time thresholds, i.e., the minimal durations necessary to just detect a change in brightness, were measured for light increments and decrements of a 1 degree test spot centered on a background of 20 degrees. Background luminance varied from -1 to 3 log td and retinal eccentricity from 0 degree to 50 degrees. Step size ranged from 0.04 to 1.5 log units and was the same in absolute units for both directions. Two types of stimuli were used: Type A, in which increments and decrements emerge from the same uniform background, and Type B, in which increments are the same as in Type A but decrements consist of a brief interruption of the test spot. Type A stimulation resulted in similar time thresholds for increments and decrements or, under some conditions, slightly shorter decrement thresholds. Type B stimulation resulted in similar thresholds for foveal vision. However, with increasing step size, decreasing background luminance, and increasing eccentricity, the time threshold for the decrement progressively exceeded that for the increment (up to 80 msec). This difference is attributed to different rise and fall times of the photoreceptor response as well as to Troxler's effect.

One of the criteria in ophthalmic spectacle lens design is the elimination of oblique astigmatism. For a range of equivalent powers, Seidel (primary or third-order) astigmatism can be eliminated, and the solutions of back- (or front-) surface power are commonly displayed graphically in the form of ellipses (Tscherning ellipses). The Tscherning ellipses apply only to lenses constructed from spherical surfaces. If one or both surfaces are made aspheric, the solutions for zero astigmatism are no longer in the form of ellipses. If one surface, usually the front surface, is made as a conicoid aspheric, the solutions for zero astigmatism can be presented graphically similarly to the Tscherning ellipses. For any given equivalent power, there are two or no solutions for spherical lenses. However, there is always one and up to three solutions for conicoid aspheric lenses.

The apparent contrasts of suprathreshold stationary gratings, countermodulated gratings, and homogeneous flickering fields were assessed with a contrast-matching procedure. Results show that, as stimulus amplitude is increased relative to threshold, variations in apparent contrast with spatiotemporal-frequency content become much less pronounced. In other words, the contrast-matching functions are more uniform across both spatial and temporal frequency at levels of contrast well above threshold. These data are interpreted in terms of a compensatory stage in the visual system that varies its gain characteristics according to the detectability of the stimulus.

Steady-state accommodation responses were measured in both eyes as a function of vergence angle and direction of lateral gaze. The measurements were made with a binocular laser optometer. Small speckle patterns were used as fusional stimuli in an otherwise dark field. These patterns have the advantage of providing no blur stimulus to accommodation. Convergence accommodation for vergence angles ranging from 0 to 25 deg was measured for lateral-gaze angles of +32, -32, and 0 deg. The average accommodation of the two eyes was linearly related to vergence angle over the observer's accommodation range but was independent of the angle of lateral gaze. The mean convergence accommodation/convergence ratio for three subjects, in diopters per meter-angle, was 0.9. Our measurements of convergence accommodation using laser-speckle targets are in good agreement with previous studies that used small pupils. Accommodation responses for binocular viewing of letters of a Snellen chart were also measured. When luminance was reduced, night myopia was observed. No similar effect was found for convergence accommodation. Accommodation to a dim target corresponded closely to the convergence accommodation.

For a moving observer it is essential to foresee the locomotor course with respect to structures in the environment. Optical flows that are available to a moving observer contain powerful information for visual kinesthesis. In general, optical flows consist of separable translational and rotational components. The information examined here is contained completely in the translational component and its time derivatives. Curved paths of observation are specified by different orientations of the translational components of optical velocity and acceleration fields. Obstacles and their temporal separation from a curvilinearly moving observer are specified in the optical flow, as is the angle of collision.