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Biomedical subjects

L N Thibos

Publications and source records attributed to L N Thibos.

At least 37 records · Page 2Linked to original sources
PubMed1997

Use of liquid-crystal adaptive-optics to alter the refractive state of the eye.

We evaluated the potential of wavefront shaping with liquid-crystals for modulating the eye's refractive state. A spatial light modulator with 127 liquid crystals cells was imaged in the entrance pupil of the eye and programmed to induce prismatic, spherical, and astigmatic refractive changes. Psychophysical evaluation of these optical effects was in agreement with expectations for prisms up to approximately 0.08 D and for lenses up to approximately 1.5 D. These maximum dioptric values represent wavefront retardation of about 3 to 4 wavelengths of 584 nm light across a 3-mm diameter pupil. Optical aliasing of high-power prisms was traced to spatial undersampling of the wavefront retardation function by the discrete array of liquid crystal cells. Undersampling may also be the factor which limits the useful dioptric range of the technique.

Eyeglasses↗
PubMed1997

Effects of refractive error on detection acuity and resolution acuity in peripheral vision.

PURPOSE: To evaluate the effect of refractive error on detection acuity and resolution acuity in peripheral vision. METHODS: Detection acuity, defined as the highest spatial frequency for which luminance gratings can be discriminated from a uniform field, and resolution acuity, defined as the highest spatial frequency for which spatial patterns are perceived veridically, was determined for vertical and horizontal gratings located at 20 degrees, 30 degrees, and 40 degrees of eccentricity. Resolution was also measured for tumbling-E discrimination at these locations. Refractive state of the eye for test targets was manipulated by introducing an ophthalmic trial lens into the line of sight for the stimulus while holding accommodative state fixed. RESULTS: Detection acuity in the periphery varied significantly with the amount of optical defocus, whereas acuity for grating resolution or letter discrimination was unaffected by defocus over a large range (up to 6 D). These results are consistent with the working hypothesis that detection acuity in the periphery is limited by contrast insufficiency under normal viewing conditions, but resolution is limited by ambiguity because of neural undersampling. CONCLUSIONS: The large depth of focus for resolution acuity measured for peripheral vision indicates that spatial resolution is likely to remain sampling-limited even when peripheral refractive errors are not fully corrected, thus relaxing the methodologic requirements for obtaining noninvasive estimates of neural sampling density of the living eye in a clinical setting.

Contrast Sensitivity↗
PubMed1996

Characterization of spatial aliasing and contrast sensitivity in peripheral vision.

Psychometric performance was measured for contrast detection and spatial resolution tasks in foveal and peripheral vision. Objective evidence was obtained for a quantitative difference between resolution acuity and detection acuity in the peripheral field. These two types of spatial acuity differed by up to an order of magnitude (3 vs 30 c/deg at 30 deg eccentricity) and they varied with stimulus contrast in distinctly different ways. Contrast sensitivity at the resolution limit was an order of magnitude above the absolute threshold of unity and the shape of the contrast sensitivity function was significantly different from that measured for foveal vision. The results suggest that current models of eccentricity scaling of contrast sensitivity be re-evaluated to take account of the extensive aliasing zone of spatial frequencies which becomes functional in peripheral vision when the retinal image is well focused.

Contrast Sensitivity↗
PubMed1996

Undersampling produces non-veridical motion perception, but not necessarily motion reversal, in peripheral vision.

We investigated motion perception in peripheral vision (10-40 deg eccentricity) for drifting gratings above and below the Nyquist limit set by neural sampling of the retinal image. We found that psychometric functions for motion discrimination rarely exhibited worse-than-chance performance indicative of motion reversal. A series of control experiments indicated that failure to demonstrate motion reversal could not be attributed to: (1) failure to detect the contrast of the stimulus; (2) failure to detect the motion of the stimulus; (3) use of an inappropriate range of spatial frequencies. Although consistent motion reversal was not observed, additional experiments demonstrated that motion perception was nevertheless non-veridical for spatial frequencies above the Nyquist limit. These seemingly contradictory results were reconciled by the observation that aliased patterns could appear to move in several different directions, all of which were different from the direction of stimulus, but only of which was opposite to the stimulus direction. Nyquist limits inferred from motion discrimination lie near the predictions for P-ganglion cells in human retina and well above M-cell predictions, which implies the M-cell array is too sparse to account for the limits to verdical motion perception in peripheral vision.

Contrast Sensitivity↗
PubMed1996

Effect of window size on detection acuity and resolution acuity for sinusoidal gratings in central and peripheral vision.

Detection and resolution of square patches of sinusoidal gratings were measured in central and peripheral vision (30 degrees horizontal temporal visual field) for high-contrast gratings as a function of the number of cycles in the stimulus. We determined performance in a forced-choice paradigm for a fixed number of stimulus cycles by arranging for stimulus diameter to vary inversely with spatial frequency. For both psychophysical tasks and for both target locations, the psychometric function relating performance to log spatial frequency shifted to higher frequencies without changing slope significantly as the number of cycles in the stimulus was increased. Thus the entire effect could be captured by an analysis of spatial acuity, which increased with increasing number of grating cycles over the range 0.5-6 cycles but remained constant over the range 6-14 cycles. In the central field, resolution acuity and detection acuity were equal regardless of the number of cycles in the stimulus. In the peripheral field, detection acuity exceeded resolution acuity and perceptual aliasing occurred for stimuli in the range 1-14 cycles. From this result we conclude that resolution acuity is sampling limited in the periphery, provided that the stimulus contains at least one full cycle of the grating. Essential features of the results could be accounted for by Fourier analysis of the stimulus.

Fovea Centralis↗
PubMed1996

Subjective refraction of the peripheral field using contrast detection acuity.

BACKGROUND: Although peripheral resolution acuity is relatively unaffected by uncorrected refractive error, recent investigations of peripheral vision indicate that contrast detection is optically limited. It should be possible, therefore, to perform a subjective refraction in the peripheral visual field using a contrast detection task. METHODS: For a range of trial lenses, contrast detection acuities for vertical and horizontal gratings were measured with a two-interval forced-choice paradigm. Lens powers that maximized detection acuity were taken as the subjective refraction estimates. These powers were compared with objective refractions determined with retinoscopy and autorefractometry. RESULTS: Contrast detection acuity varied significantly with lens power at all retinal locations tested. Defocusing by one diopter from the optimum lens power reduced detection acuity by about a factor of two at 20 degrees eccentricity, and slightly less in the far periphery. Objective retinoscopy and autorefractometry agreed with subjective measurements for most conditions tested. CONCLUSIONS: Contrast detection acuity in the peripheral visual field varies with refractive blur, demonstrating the feasibility of performing subjective refraction in the periphery for a contrast detection task. Results suggest that visual fields measured with standard perimetry, which is based on contrast detection, may be affected by uncorrected peripheral refractive errors.

Adult↗
PubMed1995

Reversals of the colour-depth illusion explained by ocular chromatic aberration.

Although many colour-depth phenomena are predictable from the interocular difference in monocular chromatic diplopia caused by the eye's transverse chromatic aberration (TCA), several reports in the literature suggest that other factors may also be involved. To test the adequacy of the optical model under a variety of conditions, we have determined experimentally the effects of background colour on perceived monocular chromatic diplopia and perceived depth (chromostereopsis). A Macintosh colour monitor was used to present red, blue, and green test stimuli which were viewed monocularly or binocularly (haploscopically) through 1.78 mm artificial pupils. These apertures were displaced nasally and temporally from the visual axis under controlled conditions to induce a variable degree of TCA. Monocular chromatic diplopia and binocular chromostereopsis were measured for red and blue targets, and also for red and green targets, presented on either a black background or on a background which was composed of the sum of the targets' spectral composition (e.g. red and blue presented on magenta; red and green presented on yellow). In all cases, chromatic diplopia and chromostereopsis were found experimentally to reverse in sign with this change in background. Furthermore, we found that a given coloured target could be located in different depth planes within the same display when located on different background colours. These seemingly paradoxical results could nevertheless be explained by a simple model of optical TCA without the need to postulate additional factors or mechanisms.

Adult↗
PubMed1995

Useful variations of the Badal Optometer.

The simple Badal Optometer consists of a movable target and a fixed positive power lens placed at its focal distance away from the eye. The perceived angular size of the target is independent of target position and the power scale is linear. Limitations of the simple Badal Optometer include restriction of negative (myopic) ocular vergence range, the need for targets to be small, and the problem of "proximal" accommodation. We describe two modifications to the Badal system in which these limitations may be overcome by the use of a movable auxiliary optical system. In one modification, the movable auxiliary system consists of a target and positive lens which together may provide a virtual "target" for the Badal lens and thus increase the negative range. In the second modification, the Badal lens is positioned as it would normally be, but the target is an image of a distant stimulus created by the auxiliary lens. The target position (and hence the ocular vergence) is changed by moving the auxiliary lens. The distant target eliminates the proximal accommodative stimulus and allows spatial detail near the resolution limit to be displayed.

Accommodation, Ocular↗
PubMed1995

Statistical distribution of foveal transverse chromatic aberration, pupil centration, and angle psi in a population of young adult eyes.

Subjective transverse chromatic aberration (sTCA) manifest at the fovea was determined for a population of 85 young adults (19-38 years old) by means of a two-dimensional, two-color, vernier alignment technique. The statistical distribution of sTCA was well fitted by a bivariate Gaussian function with mean values that were not significantly different from zero in either the horizontal or the vertical direction. We conclude from this result that a hypothetical, average eye representing the population mean of human eyes with medium-sized pupils is free of foveal sTCA. However, the absolute magnitude of sTCA for any given individual was often significantly greater than zero and ranged from 0.05 to 2.67 arcmin for the red and the blue lights of a computer monitor (mean wavelengths, 605 and 497 nm, respectively). The statistical distribution of the absolute magnitude of sTCA was well described by a Rayleigh probability distribution with a mean of 0.8 arcmin. A simple device useful for population screening in a clinical setting was also tested and gave concordant results. Assuming that sTCA at the fovea is due to decentering of the pupil with respect to the visual axis, we infer from these results that the pupil is, on average, well centered in human eyes. The average magnitude of pupil decentration in individual eyes is less than 0.5 mm, which corresponds to psi = 3 deg for the angle between the achromatic and the visual axes of the eye.

Accommodation, Ocular↗
PubMed1993

New methods for discriminating neural and optical losses of vision.

Visual acuity, the premier test of visual function, is analyzed as a three-stage, hierarchical sequence beginning with the detection of spatial contrast, followed by the resolution of that contrast into spatially separate elements, and ending with the identification of a particular arrangement of elements as one of several possible test objects, e.g., a letter in the Snellen alphabet. Each stage in this process has its own inherent limitations which are associated with different stages of the visual pathway. New methods are described for testing each stage of the hierarchy individually for the purpose of identifying the weak link in the chain, thus paving the way for differential diagnosis of optical and neural losses of vision.

Contrast Sensitivity↗
PubMed1993

Diffractive properties of the Diffrax bifocal contact lens.

We have verified that the Pilkington Diffrax lens behaves optically as a diffractive bifocal contact lens. The longitudinal chromatic aberration of the lens is similar to that predicted by theory, both on and off the eye. The over-refraction with the lens on the eye is similar to the distance subjective refraction, which is also as predicted.

Contact Lenses↗
PubMed1993

Experimental determination of the chromatic difference of magnification of the human eye and the location of the anterior nodal point.

Theory predicts that retinal image size will vary with wavelength. However, this chromatic difference of magnification (CDM) is likely to be very small (< 1% between 400 and 700 nm) under natural viewing conditions. There has been only one attempt to measure CDM experimentally, and the results were inconsistent with optical theory. Using a technique described by Ogle [Research in Binocular Vision, Hafner, New York (1964)], which is sensitive to even small interocular differences in retinal image size, we measured the apparent tilts in the frontoparallel plane induced by interocular differences in wavelength. We obtained the ocular CDM by determining the afocal lens magnification necessary to cancel the apparent frontoparallel plane tilt caused by interocular differences in wavelength. We show that (1) the ocular CDM can be considerably less than theoretical model predictions, (2) the relationship among delta Rx (wavelength-dependent refractive error), CDM, and pupil position is consistent with our theoretical model, (3) CDM increases considerably when an artificial pupil in front of the eye is used, (4) the location of the anterior nodal point of the eye may be inferred from the data, and (5) unlike in the case for delta Rx, large intersubjective differences may exist for CDM. The results suggest caution in the use of artificial pupils experimentally with polychromatic stimuli because of amplification of CDM and concomitant losses of image contrast.

Color Perception↗
PubMed1992

The effect of pupil size on chromostereopsis and chromatic diplopia: interaction between the Stiles-Crawford effect and chromatic aberrations.

Several studies have reported that the magnitude of chromostereopsis changes as the pupil size changes. Einthoven's theory, that chromostereopsis is determined by interocular differences of monocular transverse chromatic aberration, can not easily explain this change. Therefore, several alternative hypotheses have been introduced, most notably by Vos [(1960) Vision Research, 6, 105-107], who argues that shifts in chromostereopsis with pupil size are due to decentration of the peak of the Stiles-Crawford effect (SCE) with respect to the pupil. We tested this hypothesis by measuring chromostereopsis under both scotopic (no SCE) and photopic conditions with centered and decentered artificial pupils. The results show that the SCE plays an important role in the effect of pupil size on chromostereopsis. Similar changes were also measured in monocular chromatic diplopia which supports the hypothesis that the effect of pupil size on chromostereopsis is due to monocular mechanisms.

Adult↗
PubMed1992

Psychophysical localization of the human visual streak.

In a topographical study of the human retina, Curcio and Allen documented the presence of the human visual streak, which they described as a prominent nasotemporal asymmetry in ganglion cell density. This asymmetry could also be expected to be measurable in any visual function limited by ganglion cell density. By using an interferometer we sought to test the hypothesis of Thibos et al. that peripheral resolution acuity is limited by the spacing of ganglion cells and should therefore reflect the anatomical asymmetry of the visual streak, once the attenuating effects of the eye's optics have been removed. This proved to be the case and differences predicted by a ganglion cell density of 2:1 were easily measurable. This study has potential clinical implications for the detection of disease or abnormalities of the visual system that cause death or dysfunction of retinal ganglion cells.

Cell Count↗
PubMed1992

Electronic submission of abstracts for the annual meeting of the American Academy of Optometry.

Recently the American Academy of Optometry conducted a 2-year trial in which abstracts submitted for presentation at the Annual Meeting could be transmitted to the Academy in electronic form. Electronic submission has many advantages for the author as well as for the Academy and therefore has now been endorsed as the preferred method of submission for future meetings. Step-by-step procedures for electronic submission are described.

Abstracting and Indexing↗
PubMed1992

Chromatic aberration and optical power of a diffractive bifocal contact lens.

Although diffractive contact lenses have been well documented in theory, no definitive experimental data have been reported which confirm that the near image is in fact created by diffraction rather than by refraction. We have tested the diffraction hypothesis for one type of diffractive contact lens (the Hydron Echelon bifocal) experimentally by measuring its longitudinal chromatic aberration in isolation and when worn on the eye. The basis of this test is that, according to theory, diffractive lenses should have chromatic aberration which is opposite in sign to that measured for the eye. Objective measurements of chromatic aberration were made with a focimeter when the lens was in a wet cell. Subjective measurements were made with a Badal optometer when the lens was worn on the eye. Four control experiments were conducted to provide baseline measurements of the eye's chromatic aberration, against which we compared the results obtained for the diffractive contact lens. The data were also compared with conventional measurements of refractive error obtained by standard subjective techniques and by an automated infrared refractor. Our results showed that the longitudinal chromatic aberration of the diffraction image of the Echelon bifocal lens was about one-half that obtained under the four control conditions: for the naked eye, for the nondiffraction image of the Echelon lens, or for either image of a refractive bifocal contact lens (CIBA Bisoft). These results are consistent with the theoretical prediction that the negative chromatic aberration of a diffractive contact lens should partially cancel the positive chromatic aberration of the human eye.(ABSTRACT TRUNCATED AT 250 WORDS)

Color Perception↗
PubMed1992

Use of interferometric visual stimulators in optometry.

Professor Fergus Campbell and colleagues pioneered the use of the interferometric visual stimulator as a tool for separating optical from neural factors which limit spatial vision. The device is now commercially available as a clinical instrument which 'bypasses the optics of the eye' to measure the potential visual acuity of the neural portion of the visual system. However, the clinician needs to be aware of limitations of current instrument designs and recent advances in our understanding of peripheral vision, to correctly interpret patient data gathered with these devices.

Humans↗