Maxwellian View Research Articles

Contributions of neural pathways to age-related losses in chromatic discrimination.

Chromatic-discrimination thresholds were measured for light mixtures lying along individually determined tritan axes and an axis of constant short-wavelength-sensitive- (S-) cone stimulation for 30 color-normal observers (age range 22-77 years). The stimulus was a foveally viewed 2 degrees, circular bipartite field consisting of a standard and a test light. Heterochromatic flicker photometry was used to equate the retinal illuminance of the stimuli at 120 Td for all observers. All stimuli were presented in Maxwellian view. Age-related losses in chromatic discrimination depended on the level of cone stimulation. At relatively lower levels of S- and long-wavelength-sensitive (L-) cone stimulation, discrimination thresholds were elevated for older relative to younger observers. As the level of simulation increased for these two cone types, thresholds converged, on average, for all observers. Application of a model of chromatic discrimination mediated by an S-cone pathway suggests that there is no significant age-related change in Weber fractions and that age-related losses in chromatic discrimination are due, at least in part, to spontaneous neural noise arising in the pathway and/or neural changes that multiplicatively scale all incident light.

Refraction, aliasing, and the absence of motion reversals in peripheral vision

Reversals in perceived direction of motion of a grating when its spatial frequency exceeds half that of the sampling mosaic provide a potential tool for estimating sampling frequency in peripheral retina. We used two-alternative forced-choice tasks to measure performance of three observers detecting or discriminating direction of motion of high contrast horizontal or vertical sinusoidal luminance gratings presented either 20 or 40 deg from the fovea along the horizontal meridian. A foveal target at a comfortable viewing distance aided fixation and accommodation. A Maxwellian view optometer with 3 mm artificial pupil was used to correct the refraction of the peripheral grating, which was presented in a circular patch, 1.8 deg in diameter, in a surround of similar colour and mean luminance (47.5 cd · m −2). The refractive correction at each eccentricity was measured by recording the aerial image of a point after a double pass through the eye. The highest frequency which can reliably be detected (7–14 c/deg at 20 deg, 5.5–7.5 c/deg at 40 deg) depends critically on refraction. Refraction differs by up to 5 D from the fovea to periphery, and by up to 6 D from horizontal to vertical. Direction discrimination performance shows no consistent reversals, and depends less on refraction. It falls to chance at frequencies as low as one-third of the highest that can be detected. Gratings which can be detected but whose direction of motion cannot be discriminated appear as irregular speckle patterns whose direction of motion varies from trial to trial. The absence of motion reversals may reflect irregularity of sampling, and suggests that reversals are not a simple tool for studying sampling in peripheral vision.

Spectral efficiency across the life span: flicker photometry and brightness matching.

Spectral efficiency functions were measured for 50 color-normal observers (aged 19-85 years) by means of heterochromatic flicker photometry (HFP) and heterochromatic brightness matching (HBM). Foveally viewed, circular, 1.2 degrees-diameter stimuli were presented as 3-s flashes (50% duty cycle) in Maxwellian view. Monochromatic lights (420-700 nm; 16 wavelengths) were equated to a 100-Td (trolands), broadband white standard in both procedures. In both HFP and HBM, average sensitivity (specified at the cornea) decreased at short wavelengths with increasing age, consistent with age-related increases in the density of the ocular media. The short-wavelength HBM decline was of lower magnitude than the HFP decline. HFP data were modeled by a weighted additive combination of long- and middle-wavelength-cone inputs and density spectra of the ocular media and macular pigment. HBM data were analyzed with an upper envelope of additive and subtractive combinations of log-transformed cone absorption functions. These analyses indicated that HBM sensitivity, specified at the retina by correction for estimated lens and macular pigment density, increased with age in an approximately wavelength-independent manner over a broad range from 420 to 560 nm.

Foveal Flicker Sensitivity Abnormalities in Early Glaucoma

This study tested the hypothesis that (a) evidence of foveal visual dysfunction could be elicited in glaucoma subjects by measuring flicker sensitivity as a function of time after onset of an adapting field for a suitably chosen set of test and adaptation parameters and that (b) such dysfunction would be related to high blood pressure. Three groups of subjects were tested: (a) subjects with primary open-angle glaucoma but only minimal field loss, (b) normal control subjects, and (c) control subjects found to be suspect for glaucoma. The protocol included measurement of pulse rate and blood pressure, administration of Humphrey 30-2 visual fields and optic nerve head photography, and administration of a battery of psychophysical tests in Maxwellian view. This battery included a test of flicker sensitivity measured at middle wavelengths as a dynamic function of time after onset of a long-wavelength adapting field. The dynamic light-adaptation functions of subjects with glaucoma were much more likely to be unstable than were the corresponding functions of normal subjects. In addition, the dynamic light-adaptation functions of subjects with high blood pressure for their pulse rate were significantly less stable than the correspond ing functions of subjects without high blood pressure for their pulse rate. Moreover, the ratio of mean arterial pressure to pulse rate was significantly less for normal subjects than for either glaucoma subjects or for glaucoma-suspect subjects. We infer that among people with primary open-angle glau coma but with only minimal field loss, there often is foveal dysfunction associated with cardiovascular disease. Evidence of such dysfunction appears to require the use of stimulus conditions that tax the ability of the visual system to respond appropriately.

Impairment of retinal increment thresholds in Huntington's disease.

We have investigated detection thresholds for a foveal blue test light using a Maxwellian view system in 61 normal subjects, 19 patients with Huntington's chorea, 14 patients with Tourette's syndrome, and 20 patients with schizophrenia. Ten measurements were made: The blue test light (1 degree diameter, 500 msec duration) was presented either superimposed on a yellow adaptation field (5 degree diameter) or 500 msec after switching off this field (transient tritanopia effect). In both cases five different background intensities were presented. The only abnormality found was in patients with Huntington's chorea. During adaptation these patients' thresholds are significantly higher than normal (p < 0.005). No change was found in the transient tritanopia effect. Huntington's disease causes degeneration of several different transmitter systems in the brain. Increment threshold testing allows for noninvasive investigation of patients and confirms the involvement of the retina in the degenerative process in Huntington's chorea.

Chromatic discrimination with variation in chromaticity and luminance: Data and theory

Boynton and Kambe developed a model of chromatic discrimination in which thresholds are mediated by two independent mechanisms: the short-wavelength sensitive (S-) cones (S-cone axis), and the middle-wavelength sensitive (M-) and long-wavelength sensitive (L-) cones (M/L-cone axis). In this study, we used a Maxwellian view optical system to investigate fundamental properties of the model as a function of chromaticity and luminance. We confirmed that discriminations along the S-cone axis were dependent on S-cone excitation level. However, changes in chromaticity and changes in mean luminance were not described by a single threshold-vs-radiance (TVR) template. We developed a model to account for the different effects of changing S-cone excitation by varying mean chromaticity and by varying mean luminance. M/L-cone discriminations showed a minimum at the L-cone excitation to white, indicating strong opponency. The thresholds increased with luminance approaching a Weber region and showing parallel functions for differing chromaticities. These data are fit by a model allowing retinal gain controls and spectral opponency.

Loci of achromatic points throughout the life span

Mixtures of monochromatic lights that appear achromatic were measured for 50 normal, trichromatic observers ranging in age from 11 to 78 years. Stimuli were presented to one eye as a 1 degree-diameter, 1-s flash (10-s interstimulus interval) in Maxwellian view. We found the achromatic locus by varying the intensity ratio of each observer's spectral unique blue and unique yellow while maintaining constant overall retinal illuminance. Measurements were made for three levels of retinal illuminance (10, 100, 1000 trolands). Additional verification of the position of the achromatic locus in color space was obtained for 23 subjects with the use of a mixture composed of 600-nm light and its spectral complement. There were no significant changes in the achromatic loci as a function of age. The mean achromatic locus in CIE chromaticity space was x, y = 0.31, 0.31 or u',v' = 0.21, 0.46. These results suggest that partial compensation for age-related changes in visual mechanisms occurs in a way that preserves constancy of the achromatic locus across the life span.

Subjective and Objective Assessment of Soft Bifocal Contact Lens Performance

Subjective and objective techniques were used to assess the on-eye performance of soft bifocal contact lenses. In the subjective technique a young observer whose accommodation had been paralyzed with a cycloplegic agent was fitted with the contact lens type under investigation and aligned with a Maxwellian view Badal optometer using a bite-bar. Visual acuity was measured as a function of both target vergence (0.00 to -4.00 D) and pupil size (1 to 5 mm). Aspheric, concentric, and diffractive soft bifocal contact lens designs were investigated. Diffractive and concentric bifocal designs with +2.00 D near additions showed "twin peaks" of visual acuity with one peak at 0.00 D target vergence (equivalent to distance viewing) and the second at -2.00 D target vergence (equivalent to a target at 50 cm). Some aspheric designs produced a relatively constant visual acuity across this vergence range, whereas others gave poor acuity at near. Visual acuity with the Echelon diffractive lens was relatively unaffected by pupil size, confirming theoretical predictions. Visual acuity with concentric designs was also relatively free of pupil size effects. In the objective technique, the variation in surface power across the lens was assessed using video-keratography. This technique provided an elegant means of visualizing the power profile of the lens. The location of the zones of increased power and the magnitude of power variations allowed an accurate prediction of the visual performance measured subjectively. Temporal displacement of lenses may explain the occasions where visual performance did not vary with pupil size.

Artificial pupils and Maxwellian view

When control of the pupil size is required, the simplest method is to use a physical artificial pupil or aperture that is placed in the spectacle plane. In some clinical applications (e.g., the potential acuity meter) an optical artificial pupil is imaged in the plane of the natural pupil by a Maxwellian view optical system. We compared visual performance with physical and Maxwellian artificial pupils by measuring the effects of the pupil diameter (0.5-5 mm in range) and defocus (5-D myopia to 4-D hyperopia) on minimum angles of resolution (MAR's) and on angular blur disk diameters. For pupil diameters down to ~ 2.0 mm there were no meaningful differences between the visual resolution that is obtained with the physical and the Maxwellian pupils. At the smallest diameter (0.5 mm) the physical artificial pupils caused the MAR to increase because of the diffraction limitation on resolution, and defocus no longer affected MAR. With the small Maxwellian pupils vision did not become diffraction-limited so that maximum resolution could still be obtained. MAR was still affected by defocus. The angular blur disk diameters measured with the smaller Maxwellian pupils were slightly but significantly larger than those found with physical artificial pupils. For physical artificial pupils, field-of-view restrictions may result from vignetting with the eye pupil. Thus small physical artificial pupils can act as pinholes causing resolution to become impaired but insensitive to defocus. Also vignetting by the eye pupil can restrict the field of view. Small optical artificial pupils from Maxwellian viewing do not impair resolution, and the resolution may remain sensitive to defocus. The eye pupil does not cause any field restriction, although, if small, it may filter higher spatial frequencies out of the retinal image.

The phase of PVEP in maxwellian view: Influence of contrast, spatial and temporal frequency

The temporal phase of the pattern reversal VEP has been investigated using stimulation with laser interference fringes in Maxwellian view. VEP phase was almost constant as function of spatial frequency (2–30 c/deg, 6.5 r/sec, 51 subjects). The phase function however shows a small phase increase at low spatial frequencies consistent with the existence of multiple temporal mechanisms. Contrast variation yields a smaller phase increase with decreasing contrast than conventional stimulation. The phase is linear as function of reversal rate (2–31 r/sec) for low and high spatial frequencies. The indication of more than one temporal mechanism has also been found when the test field diameter was varied. With decreasing test field size the phase increases at 6.5 r/sec, but decreases at 18 r/sec (for 12 c/deg).

The Julius F. Neumueller Award in Optics, 1989: change of pupil centration with change of illumination and pupil size.

Pupil centration is important to the optical blur on the retina. Using a dual Maxwellian view system we measured the centration of the pupil with respect to the achromatic axis of the eye as a function of pupil size. Significant shifts of the pupil center (up to 0.6 mm) with pupil dilation were measured in both nasal and temporal directions. The effect was usually symmetrical between the two eyes and the shift was linear with pupil size in one-half of the subjects. From their initial positions the linear pupil center shifts with dilation were in the direction of the achromatic axis.

Psychophysical measurement of the size and shape of the human foveal avascular zone

We have examined two psychophysical procedures for assessing the size and shape of the human foveal avascular zone (FAZ). Both procedures used a Maxwellian view system with a rotating beam to create a high contrast entoptic view of the retinal vasculature. Most subjects readily report a clear avascular zone surrounding their fixation point. The size of this FAZ was measured by (a) reducing the circular Maxwellian view field stop until it appeared to coincide with the edge of the FAZ, and (b) by tracing the boundary of the FAZ with a point source. Consistent with earlier angiographic and anatomical studies, the first method (N = 34 eyes) showed a mean FAZ diameter of 0.736 mm (area 0.42 mm2). FAZ diameter ranged from 0.46 to 1.13 mm (area 0.166-1.00 mm2) and the FAZ from the right and left eyes of each individual subject were very similar (r2 = 0.863). The tracing technique (n = 24 eyes) showed that FAZ's were generally not spherical but well fit by ellipses with, on average, a major axis 17% longer than the minor axis. The maximum diameters of the FAZ tracings were very similar and significantly correlated with the circle diameters matched to the FAZ in experiment 1.

Video monitoring of the principal ray of a Maxwellian view for the measurement of optical aberrations, the Stiles-Crawford effect, retinal resolution, and for investigating color vision

A video frame grabbing technique with a computer-generated overlay is used to obtain a direct visualization of the position of the image of a point source, producing a Maxwellian view, in the natural entrance pupil of the human eye. With this simple technique, the image point can be located with an accuracy of at least 0.07 mm. Applications are demonstrated: (a) in instruments using a single Maxwellian view for measuring retinal resolution in white light or superimposed Maxwellian views for color vision experiments, (b) in a dual Maxwellian view apparatus used for measuring the Stiles-Crawford effect of the first kind or for measuring chromatic aberration, and (c) in a new semi-Maxwellian view apparatus for the evaluation of monochromatic and chromatic aberrations.

Optical limitations of the Maxwellian view interferometer

Le Grand's technique for producing interference fringes directly on the retina was analyzed to determine the effect of ocular chromatic aberration on fringe parameters. For monochromatic light, fringe period is largely independent of refractive index of the ocular medium and of the axial location of the interferometer relative to the eye. Lateral displacement of the interferometer, however, shifts the retinal fringes by an amount which depends on wavelength. To close approximation, the relative shift between fringes of different colors is directly proportional to the amount of lateral displacement of the instrument, with the constant of proportionality equal to the longitudinal chromatic aberration of the eye. The net effect is a significant loss of retinal contrast for polychromatic fringes.

Evaluation of pupil constriction and dilation from cycling measurements

Pupil cycling was produced using an electronic circuit so that the retina was illuminated in Maxwellian view only when pupil area exceeded an adjustable area threshold, A ref. The maximum ( A max) and minimum ( A min) amplitude of the oscillations varied linearly with A ref. These observations are described by a delay-differential equation. The A ref-dependent changes in A max, A min were used, respectively, to quantitate dilation and constriction. A comparison of the predicted and observed period of pupil cycling suggests that the latency times for light onset and offset are the same. Measurements of A max, A min provide a method for determining the average pupil light response.

The effects of 0.025% hyoscine hydrobromide eyedrops on visual function in man.

Instillation of three drops of 0.025% hyoscine hydrobromide into one eye at 5 min intervals caused a mydriasis and cycloplegia of rapid onset and of 4-6 days' duration; this was similar to the previously reported actions of concentrations of up to 0.5%. After refraction for the working distance of the experiments, contrast sensitivity to stationary oscilloscope-generated grating patterns of 10 and 20 c/deg was unaffected despite the mydriasis. Also, contrast sensitivity to laser interference fringes of 10 and 20 c/deg observed in the Maxwellian view, in which the effects of the ocular media are bypassed, was unaffected. Contrast sensitivity to a 3 c/deg phase-reversed grating pattern was, however, transiently reduced by 40% after hyoscine. A second series of experiments showed a deleterious effect of hyoscine on contrast sensitivity to both stationary and phase-reversed grating patterns of 2, 3 and 5 c/deg while contrast sensitivity at 10 c/deg was not consistently affected. These results were explained by a direct deleterious action of hyoscine at the level of the retina, specific to channels that detect low spatial frequencies.

The effects of physostigmine sulphate eyedrops on human visual function.

Instillation of 0.25% physostigmine sulphate eyedrops in twelve subjects caused a sustained miosis, a transient reduction in near-point and a transient increase in the amplitude of accommodation. The latter had a peak at 30 min and had recovered by 90 min, though its amplitude varied widely between subjects. Contrast sensitivity to stationary grating patterns of 3-30 cycles/deg and phase-reversed grating patterns of 0.5-3 cycles/deg was reduced transiently with a time course similar to that of the increase in accommodation. The peak reduction in contrast sensitivity was correlated significantly with the peak amplitude of accommodation. Contrast sensitivity to laser interference fringes, observed in Maxwellian view where the effects of defocus are bypassed, was also reduced, indicating that physostigmine also had a direct deleterious action on the visual system.

Perception of electronic display colours as a function of retinal illuminance

Colour-naming data were objected from four colour-normal observers to determine the nature of the changes in colour appearance that can occur with changes in illuminance for spectral and nonspectral stimuli comparable to those used in electronic displays. Test stimuli were presented as one degree, foveal flashes of one second duration in Maxwellian view. The retinal illuminance of the nine test stimuli varied from −1.0 to 4.0 log trolands (0.01-1 147fL). Shifts in perceived colour were observed with changes in stimulus intensity. At low illuminance levels, the stimuli appeared desaturated, with black and white responses dominating chromatic responses. At higher retinal illuminances, the stimuli were more saturated, although perceived colour was not constant with changing light levels. In general, blue and yellow responses increased at a faster rate relative to red and green responses with increasing illuminance. The data have important implications for the use of colour on electronic displays.

Retinal illuminance using a wide-angle model of the eye.

A computer model of relative retinal illuminance, based on our optical wide-angle model of the eye, is proposed for the cases of Ganzfeld illumination and the Maxwellian view through a range of visual field angles from 0 degrees to 80 degrees. The proposed model is designed to be functionally correct and to represent closely the anatomical parameters of the eye. Unlike earlier proposed models, this model is based on our previously reported measurements of spherical aberration in 100 eyes in vivo and is designed to be correct in the peripheral field and with large pupils. Data are reported for pupil diameters of 2, 4, and 8 mm. The effect of crystalline lens extinction on retinal illumination is also estimated for average eyes of young (age 19 years) and old (age 63 years) individuals at two wavelengths (410 and 532 nm). In the Ganzfeld case, illumination of the retina decreases with increasing visual angle. In the Maxwellian view, retinal illuminance increases with increasing visual angle.

The pattern ERG in response to colored stimuli.

The pattern-evoked electroretinogram (PERG) was studied in response to square-wave stripe patterns contrasting either between dark and colored stripes ("red-black" or "green-black" luminance contrast pattern) or between the two colors (red-green chromatic contrast pattern). All lights were matched in photopic luminance. A two-channel Maxwellian view system was used to present the stimuli in the onset-offset mode. When no pattern was present a mixture of both colors was seen. Different spatial frequencies were studied and the amplitudes of the onset response were evaluated. When the two luminance contrast patterns were presented, the responses showed a spatial selectivity. However the combination of the two colors (red-green contrast) resulted in a monotonic decrease of the response with spatial frequency. The spatially selective behavior of the response to luminance contrast patterns could be associated with the on-off center-surround organization of retinal receptive fields. The behavior of the response to chromatic-contrast patterns, on the other hand, could be explained by the action of color-opponent center-surround receptive fields as described in the primate.