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An assessment of the retinal nerve fiber layer (RNFL) provides important information on the health of the optic nerve. There are several non-invasive technologies, including spectral domain optical coherence tomography (SD OCT), that can be used for in vivo imaging and quantification of the RNFL, but often there is disagreement in RNFL thickness between clinical instruments. The purpose of this study was to investigate the influence of scan centration, ocular magnification, and segmentation on the degree of agreement of RNFL thickness measures by two SD OCT instruments.RNFL scans were acquired from 45 normal eyes using two commercially available SD OCT systems. Agreement between RNFL thickness measures was determined using each instrument's algorithm for segmentation and a custom algorithm for segmentation. The custom algorithm included ocular biometry measures to compute the transverse scaling for each eye. Major retinal vessels were identified and removed from RNFL measures in 1:1 scaled images. Transverse scaling was also used to compute the RNFL area for each scan.Instrument-derived global RNFL thickness measured from the two instruments correlated well (R(2) = 0.70, p0.01) but with significant differences between instruments (mean of 6.7 μm; 95% limits of agreement of 16.0 μm to -2.5 μm, intraclass correlation coefficient = 0.62). For recentered scans with custom RNFL segmentation, the mean difference was reduced to 0.1 μm (95% limits of agreement 6.1 to -5.8 μm, intraclass correlation coefficient = 0.92). Global RNFL thickness was related to axial length (R = 0.24, p0.01), whereas global RNFL area measures were not (R(2) = 0.004, p = 0.66). Major retinal vasculature accounted for 11.3 ± 1.6% (Cirrus) or 11.8 ± 1.4% (Spectralis) of the RNFL thickness/area measures.Sources of disagreement in RNFL measures between SD-OCT instruments can be attributed to the location of the scan path and differences in their retinal layer segmentation algorithms. In normal eyes, the major retinal vasculature accounts for a significant percentage of the RNFL and is similar between instruments. With incorporation of an individual's ocular biometry, RNFL area measures are independent of axial length, with either instrument.

The glaucomas are a group of potentially blinding optic neuropathies that are characterized by progressive pathological losses of the retinal ganglion cells (RGCs) and their axons that form the optic nerve. The causes are unknown and, therefore, the diagnosis and assessment of progression of disease depends on ophthalmic testing to identify and quantify clinical characteristics of glaucomatous neuropathy, such as the pattern of visual field defects and/or thinning of the retinal nerve fiber layer. To relate these clinical measurements to the basic pathology of glaucoma, a neuron doctrine for glaucoma has been proposed to correlate data from standard tests (standard automated perimetry and optical coherence tomography) to the loss of RGCs. The doctrine was derived through initial laboratory studies of experimental glaucoma in macaque monkeys and, then, modified and refined though patient-based clinical investigations. The final formulation of the doctrine produced concordance between subjective and objective measurements when the results were translated to their common parameter of RGCs, for both normal vision and defective vision from glaucoma. Thus, it was concluded that for individual patients, alterations in structure-function relationships usually should be in agreement for the degree and location of visual field defects caused by glaucoma.

Purpose. In normal vision, stereoscopic cues are combined with perspective cues to provide veridical depth perception. The relative strengths of these depth cues, however, may be dependent upon context effects. We investigated the role of stimulus context on the interactions of binocular disparity, contrast, and size. Methods. The subjects, four observers with normal stereoacuity and one stereo-amblyope, discriminated far vs. near perceived depth of Gabor patches; feedback was based on the sign of binocular disparity. Depth discrimination functions were measured under conditions in which depth cues were consonant or in conflict. Three stimulus contexts were used: (1) variable disparity with fixed spatial frequency and contrast; (2) variable contrast with fixed spatial frequency and disparity; and (3) variable spatial frequency with fixed contrast and disparity. The effects of stimulus context were derived from comparisons of discrimination rates for identical stimuli across the three sets of conditions. Results. In subjects with normal stereopsis, for disparities less than 2 arcmin, depth perception was dominated by contrast in contrast-varying sessions, or by size in spatial frequency-varying sessions. With larger disparities, depth perception became dependent on disparity, regardless of the contrast or spatial frequency of the test stimulus. The results for the stereo-amblyope showed much greater dependence on perspective cues and, in most cases, the transition from perspective- to disparity-based depth perception did not occur. Conclusions. These investigations demonstrate strong stimulus context effects and have important implications for the combination rules of stereoscopic and perspective cues in depth perception of normal and stereo-deficient subjects.

A stable form of interocular suppression, referred to as "permanent suppression," can be produced by presenting a contoured field to one eye and a spatially homogenous field to the fellow eye, a viewing condition analogous to the classic "hole-in-the-hand" illusion. We examined the relation between permanent suppression and binocular rivalry suppression by comparing the changes in the increment-threshold spectral sensitivity function produced by these two forms of suppression. Permanent suppression produced a reduction in spectral sensitivity; however, in contrast to binocular rivalry suppression, the sensitivity alterations associated with permanent suppression were independent of the test-probe wavelength. The different patterns of sensitivity loss observed during binocular rivalry and permanent suppression indicate that different neural mechanisms mediate these two forms of interocular suppression.

PURPOSE To determine whether a structure-function model developed for normal age-related losses of retinal ganglion cells also models the retinal ganglion cell losses in glaucomatous optic neuropathy. METHODS The model to relate age-related loss of retinal nerve fiber layer thickness and reduced sensitivity for standard automated perimetry was evaluated with data from 30 glaucoma patients and 40 normal individuals. Perimetry thresholds were translated into separate retinal ganglion cell body estimates for test locations in the superior and inferior visual fields. The retinal nerve fiber layer thickness from optical coherence tomography was also divided into regions representing the superior and inferior hemifields to obtain estimates of the axons in each hemifield. The 2 estimates of retinal ganglion cell populations were compared for corresponding regions. RESULTS Agreement between neural estimates was good for normal individuals and patients with early glaucomatous damage. Results for individuals with advanced glaucoma showed disparities between neural estimates that were proportional to the stage of disease. A correction factor for the stage of disease was introduced for the derivation of ganglion cell populations from the nerve fiber layer measurements, which produced agreement between the optical coherence tomography and perimetric estimates for all patients. CONCLUSIONS The modified structure-function model provided well-correlated relationships between the subjective measures of visual sensitivity and the objective measures of retinal nerve fiber layer thickness when parameters for the patient's age and the severity of the disease were included. The results suggest constitutive relationships between structure and function for the full spectrum of normal-to-advanced glaucomatous neuropathy.

PURPOSE To investigate the relationship between intraocular pressure (IOP) and the progression of visual field defects caused by experimental glaucoma in Macaca mulatta monkeys. METHODS Perimetric fields were measured by behavioral methods in 18 rhesus monkeys during the course of unilateral glaucoma produced by argon laser treatment of the trabecular meshwork. The monkeys' IOPs were measured by applanation tonometry. Visual field defects were quantified by the mean deviation perimetric index from Humphrey Field Analyzer C24-2, model 630 (Humphrey Allergan, San Leandro, CA, U.S.A.), full-threshold data. RESULTS The monkeys' eyes demonstrated considerable variability in their susceptibility to pressure-induced neural damage. For 10 of the monkeys, significant field defects were correlated with the increases in their IOPs and the defects progressed monotonically to end-state glaucoma. For the other monkeys, the mean deviation index was not well correlated with IOP; some eyes withstood pressures in excess of 35 mm Hg for several months before significant reduction in visual sensitivity. However, once they began, the rate of progression of field defects was similar across subjects. CONCLUSIONS Laser ablation of the trabecular meshwork in monkeys provides a model for investigations of the effects of IOP that are not confounded by other ocular or visual disorders. Behavioral perimetry showed the same intersubject variability in the effects of elevated IOP on visual field sensitivities of monkeys that are common with high-tension glaucoma or ocular hypertension in patients. Thus, these investigations provide additional support for the use of the model for a wide variety of clinical investigations on glaucoma.

Psychophysical investigations of the effect of amblyopic process on the sustained and transient channels in the afferent visual pathway are reported. The experiments on photopic luminosity, increment-threshold spectral sensitivity, spatial contrast sensitivity, and reaction-time measures of suprathreshold grating detection provide converging evidence that in naturally occuring amblyopia of humans, the sustained neural channels are more severely affected than the transient channels. However, it appears that the sustained channels are not totally nonfunctional, but rather have a higher intensity requirement than normal.

Steady-state visual evoked potentials were elicited by the appearance/disappearance of sinusoidal gratings of varying spatial frequency for both eyes of amblyopic subjects at temporal frequences from 2--32 Hz. The amblyopic eyes showed reduced responses relative to those of the nonamblyopic eyes across a wide range of spatial frequencies, the difference between the 2 eyes being most marked for high spatial frequencies and low temporal frequencies. Both the nonamblyopic and the amblyopic eyes showed a linear relation between the evoked potential amplitude and the logarithm of the stimulus contrast; however, the slope of the line was flatter for the amblyopic eyes. Possible neural mechanisms for the amblyopic eyes' reduced responses to spatial contrast are considered.

The discrimination of depth and pattern was investigated using random dot stereograms. For all four subjects, the psychometric function for depth judgments (probability of correct response) was clearly superior to pattern judgments over a range of ten crossed or uncrossed binocular disparity values. Degrading the visual images, either monocularly or binocularly, with various convex lens power (fogging lenses) reduced both depth and pattern discrimination across all disparity values, rather than simply increasing the thresholds. The results are discussed with reference to current theories of stereopsis, and the question of the hierarchial processing of depth and pattern information is considered.

A simple reaction time paradigm was used to study binocular interactions for suprathreshold grating patterns before and after 8 days of constant monocular occlusion. Prior to patching, subjects demonstrated binocular summation for near threshold contrast stimuli and binocular summation and facilitation for high contrast stimuli. After the period of occlusion, the subjects showed binocular inhibition for high contrast stimuli but still demonstrated binocular summation for low contrast stimuli. The effects of occlusion were reversible and appear to be related to an oculomotor imbalance.