Retinal image quality for multifocal lenses with on- and off-axis annular zones.

Multi-focal intra-ocular or contact lenses, intended to increase depth of focus, conventionally have annular zones of additional refractive power, generating wavefront rings of coaxial spherical surfaces. It is, however, possible to influence depth of focus by changing not only the curvature of the wavefront, i.e., refractive power, in the annulus, but also the tilt, i.e., circularly symmetrical linear radial deviation imposed on the spherical wavefront. Employing the example of a single annulus bifocal, retinal image light distributions in the two regimes are calculated, using standard diffraction theory. Four measures of retinal image quality in through-focus scans show that plus power additions and wavefront tilts operate almost interchangeably. In testing these lenses, attention needs to be paid to the detailed operating characteristics of measuring devices of the Shack-Hartmann type to ensure that their grain and precision is compatible with the framework of the analysis.

Gestalt theory in 20th-century history.

During the century from the 1890 publication of Ehrenfels's proposition on Gestaltqualitäten to the 1989 dissolution of the European states governed by Marxist orthodoxy, Gestalt theory was drawn into the political fray in several ways. It was grotesquely misappropriated during the Nazi regime in support of race, territorial expansion, and war aims. On the other side, because it was seen as having a subjectivist taint, the Gestalt approach was anathema where dialectical materialist dogma reigned. In contrast, close reading of the seminal 1912 Wertheimer paper and the 1920 Köhler book reveals that the Gestalt founders' views accord well with current Gestalt research.

Multifocal contact lens myopia control: central and peripheral retinal image quality.

CLINICAL RELEVANCE: That myopic defocus, even if restricted to the peripheral retina, inhibits eye growth in young monkey eyes has motivated the therapy of myopia control through multifocal contact lens wear in children. BACKGROUND: To understand how eye-length regulating mechanisms are triggered by light requires knowledge of retinal light spread. That is largely lacking for the multifocal contact lenses used in the therapy because empirical methods identifying just the defocus in dioptres are inadequate. METHODS: "Through-focus" diffraction computations in contact lens/eye models with typical normal eye parameters, including polychromatic light, the chromatic aberrations and an M-cone phototransduction layer, offer estimates of retinal image spread for a range of viewing distances. RESULTS: Point- and edge-spread distributions of activation of phototransduction in the central retina show that the addition of multifocal zones produces some veiling for in-focus viewing and substantial improvement of image quality for near targets in the unaccommodated eye. These effects are much reduced in the retinal periphery. CONCLUSION: Whatever therapeutic value there is in prescribing multifocal contact lenses for myopia control, it is not particularly dependent on the precise configuration of the multifocal zones, nor can it be ascribed to changes in image quality specific to the retinal periphery; its origin is more likely less blur for near targets, reducing the stimulus to accommodation.

Topical Review: Pilocarpine-induced Miosis as Help for Early Presbyopes?

SIGNIFICANCE: The clinical utility of ophthalmic pilocarpine-induced pupil constriction to help overcome image blur of close-up targets in patients with failing accommodation is examined.Pilocarpine in low-concentration ophthalmic solution eye drops constricts the pupil to approximately 2 mm and thus reduces defocus blur. To gain regulatory approval of this drug for the treatment of presbyopia, clinical trials were conducted with 1.25% pilocarpine. Near vision was improved in a modest proportion of early presbyopes: between 12 and 22% more patients reached criterion near visual acuity than with a placebo, depending on conditions. The drug is well tolerated, and its effect has onset of only minutes and lasts several hours. Small pupils will cause diminished night vision and may have an impact on distance acuity to which possible minor drug-induced accommodative spasms could contribute. The therapy has a role for patients who want to postpone or briefly pause dioptric supplementation of their failing accommodation. No convincing case has been made for one version of ophthalmic pilocarpine over another.

Objective Measures of Retinal Image Degradation Due to Refractive Corrections.

SIGNIFICANCE: There is a need for a measure, as simple and yet as informative as possible, to describe objectively the retinal image quality when a patient views targets at various distances through spectacle, contact, or intraocular corrections with optics more elaborate than single vision. PURPOSE: The purposes of this study are to examine the current status of quantitative descriptions of retinal image blur and find optimal characterization of image degradation. METHODS: A variety of indexes of image degradation are computed for a typical eye and polychromatic light, in and out of focus, and as exemplars of sophisticated wave shaping, when the pupil transmission has been modified to a truncated Bessel amplitude function and to a "fractal" phase function. RESULTS: Figures are shown for the optical transfer, point- and edge-spread functions, and Koenig bar and optotype letter blur for the various imaging and defocus conditions, and the relative values of several blur indexes are compared graphically and in a correlation table. CONCLUSIONS: No single index captures the many ways in which the image can deviate from the diffraction-limited ideal. Among the incomplete descriptors of image degradation, the light distribution at a sharp edge stands out as optimally informative and economical, and, when condensed to just two values, one representing central image sharpness and the other outlying light spread, allows for a quick survey of the imaging deficit.

Focused and defocused retinal images with Bessel and axicon pupil functions.

Retinal image light distributions in a standard optical model of a diffraction-limited eye with round pupils are presented for several patterns of amplitude and phase modulation of the light admitted into the eye. Of special interest are circularly symmetrical configurations of truncated Bessel amplitude transmission functions, and of light subjected to axicon deviation. It is shown by several examples that this kind of beam shaping allows generation of retinal imagery, which can be more robust to defocus while maintaining minimal image degradation, and it points to situations of two separate zones simultaneously in sharp focus, several diopters apart.

Hering Hermeneutics: Supplement to Translation and Commentary of Hering (1899) by Strasburger et al.

Strasburger et al.'s welcome translation of Hering's seminal paper, and reminder of what Hering actually said about eye movements and spatial averaging in vernier acuity, is supplemented by references to further trends on how the subject has evolved to the present state of knowledge.

Preferential processing of cardinal over oblique orientations in human vision.

The oblique effect-poorer performance when contours are in oblique meridians-is here extended from the discrimination of line-orientation to the tilt illusion and to the detection and contextual induction of curvature. The distinction is made between a contour's susceptibility to contextual perturbation and its capacity to induce such perturbation, for which the oblique effect is only about one half. That the cardinal/oblique superiority is retained for the orientation of illusory borders and for the implicit orientation of shapes lacking explicit rectilinear delineation has implications for its neural substrate. To the extent that a geometrical-visual illusion, such as Poggendorff's or Hering's, depends on interaction in the domain of contour orientation, it manifests a corresponding orientational anisotropy. On the other hand, visual functions that govern whether and how well a boundary is visible are invariant with orientation.

Optotype recognition under degradation: comparison of size, contrast, blur, noise and contour-perturbation effects.

BACKGROUND: Visual acuity is measured by diminishing letter size till recognition threshold is reached, letters varying in legibility. In this experiment, size reduction was compared with other means of handicapping letter legibility. METHODS: In five normal observers, discrimination thresholds for 13 sans-serif capital letters in a 5 × 4 format were obtained by a staircase procedure for size reduction, as well as for 20 minutes of arc (logMAR = 0.6) letters subjected to four kinds of image degradation: reducing contrast, convolving with blurring spread, embedding in noise and perturbing contour smoothness. RESULTS: Threshold correlation and distribution of response errors show prominent differences and indicate the degree to which the visual processing of the various modes of image degradation is distinct. CONCLUSIONS: The validation of four other ways of impairing the recognition of optotypes in addition to size diminution reveals their potential in the differential diagnosis of defects in pattern detection, in evaluating therapeutic regimens and in developing concepts of form perception.

Optometric Trajectory--Gerald Westheimer.

A narrative starting with the author's entering the Optometry program at the Sydney Technical College in 1940, taking him through his days as an optometry and science student, an optometrist in private practice, and participant in organized optometry in Sydney. It described his steps to become an optometric scientist and his service as faculty member in three optometry schools in the United States. Finally, it follows him into a long career in vision science and neurophysiology at Berkeley.

The Road to Certainty and Back.

The author relates his intellectual journey from eye-testing clinician to experimental vision scientist. Starting with the quest for underpinning in physics and physiology of vague clinical propositions and of psychology's acceptance of thresholds as "fuzzy-edged," and a long career pursuing a reductionist agenda in empirical vision science, his journey led to the realization that the full understanding of human vision cannot proceed without factoring in an observer's awareness, with its attendant uncertainty and open-endedness. He finds support in the loss of completeness, finality, and certainty revealed in fundamental twentieth-century formulations of mathematics and physics. Just as biology prospered with the introduction of the emergent, nonreductionist concepts of evolution, vision science has to become comfortable accepting data and receiving guidance from human observers' conscious visual experience.

The law of equal innervation of both eyes: Thomas Reid preceded Hering by a century. An historical note.

The law of equal innervation of the two eyes, positing that conjugacy of movement of our two eyes is basic and innate, was enunciated in Hering's 1868 persuasively argued monograph. It has prevailed over Helmholtz's contrary view that conjugacy is learned. Yet 100 years earlier, Thomas Reid (1710-1796), Scottish clergyman and professor of philosophy, advanced exactly the same view as Hering, using almost identical arguments. Reid also considered whether the eyes' parallelism might have its origin in "custom" or "habit" rather than "nature" and reached the now accepted conclusion that it is innate.

Can perceptual learning compensate for optical image blur?

Tests of target recognition under blur mostly fail to separate factors of resolution and contrast from the influences of pure blur, i.e., shallow luminance edge gradients. In experiments designed to single out blur, patterns of fixed size were convolved with a Gaussian spread function whose parameter was the variable. In addition, contrast invariance was ensured. The spread parameter was varied to measure form discrimination thresholds for simple geometrical shapes. Following determination of baseline values, observers trained for 7days, 1000 form discrimination responses with error feedback per day in a staircase procedure of the blur parameter. For four observers, thresholds improved an average of 5% (range -11% to +14%) equally for trained and untrained patterns and remained stable during subsequent training with the same targets in a related form discrimination task not involving blur. Because it transferred across target sets, the very slight improvement was indeed in the perceptual capacity to compensate for optical image degradation and not in form discrimination, but its defocus equivalent was quite minor, well less than » diopter. Previous claims for blur adaptation must therefore rest on more complex factors that are not fully excluded in clinical settings.

Retinal light distributions, the Stiles-Crawford effect and apodization.

Analysis of retinal image formation for beams of coherent and incoherent radiation emphasizes the role of the Poynting vector's inclination with respect to the retinal image plane. Coherent beams interfere and give rise to a single Poynting vector that highlights the unique direction of incidence of energy flow, whereas multiple incoherent beams, especially incoherent extended sources in the pupil, generate electro-magnetic disturbances in the image plane each characterized by Poynting vectors of their own. As a result, the Stiles-Crawford diminution of luminant efficiency adds differently depending on the coherence of the entering light. Two practical considerations follow: first, in performing diffraction calculations for the retinal image from known wavefronts in the pupil plane, apodization terms should not be factored in, and, second, in principle, for perfect imaging in standard target viewing, Stiles-Crawford integration with increasing pupil diameter is not expected.

Perceptual learning of orientation judgments in oblique meridians.

Fourteen daily training sessions in orientation discrimination of foveal lines in the 45-deg meridian improved thresholds in the trained meridian by an average of 25 % in five observers. A substantial amount of training transferred to the other obliques, but none to the cardinal meridians, with a consequent reduction in the oblique effect. The data were interpreted as showing perceptual learning at two levels: performance facilitation specific to the trained orientation and improved proficiency globally. The failure of the cardinal orientations to share in the benefit is likely to have its origin in the fact that contour orientation in these meridians is so well established that it had already reached maximum hyperacuity thresholds. The judgment of obliques depends much more than the judgment of cardinals on whether the comparison and test stimuli are shown simultaneously or in succession, but this effect is not changed by perceptual training.

Measuring visual form discrimination with blur thresholds.

A method is described for gauging the discriminability of spatial forms. Rather than challenging form discrimination by size reduction as is done in testing visual acuity, the maximum image degradation by blurring is determined that still allows shape recognition. The procedure has the advantage that tests are substantially independent of optical (resolution) and retinal (light-processing) stages of vision and concentrate on the perceptual demands of distinguishing form. Candidate spread functions are analyzed with respect to both their spatial and spatial-frequency properties and compared with dioptric defocus. Form discrimination thresholds, in terms of the parameter of the imposed Gaussian spread, were determined for several classes of patterns and compared with contrast reduction, where target size can also be kept constant but retinal sensitivity issues predominate. The technique has utility for experiments in ordering shape difference hierarchies, in examining rules of Gestalt properties, and in identifying progress in perceptual learning. Diagnostic potential in patients with spatial visual dysfunction such as amblyopia remains to be explored.

Clinical evaluation of stereopsis.

Principles of the design and administration of clinical stereopsis tests are outlined. Once the presence of the distinct sense of the third dimension by binocular vision alone and without help from monocular cues has been established in a patient, the examination can proceed to the measurement of stereoscopic acuity. Best results are obtained with high-contrast, sharp, well-articulated and uncrowded elements from easily-recognized target sets, displayed with no time constraints. Polarization is the preferred method of right/left eye separation; time-sharing at a minimum of 60 Hz on computer displays with counterphase occluding goggles is a feasible procedure. Random-dot stereograms are problematic because not all observers can disentangle the coherent global disparity on a first view.

Spatial and spatial-frequency analysis in visual optics.

BACKGROUND: In the specification of visual targets and their transmission through the eye's optics to form retinal images, the spatial distribution of energy and its Fourier transform, the spatial-frequency spectrum, are equivalent, so long as linearity constraints are obeyed. The power spectrum, in which phase has been discarded, is an insufficient descriptor; it does not enable the original object to be reconstituted. PROCEDURE: Not so well known, and explored here, are joint representations in the space and spatial-frequency dimensions. Their properties are outlined for some sample targets and for transforms of the Gabor, Difference-of-Gaussians and Wigner types. A related approach is one in which other kernel functions, such as the Gaussian or its derivative, are substituted for the cosines in the Fourier transform; here also graphs can be generated which jointly display properties both of the target and of its point-by-point representation in a size-tuned domain. APPLICATIONS: This kind of study has application in matching the performance characteristics of optical devices to the eye's, in optical superresolution, and in the analysis of the demands placed on neural processing in, for example, visual hyperacuity.