Biomedical subjects
Publications and source records attributed to W S Baron.
The paper provides: (1) a review of the ocular cyclorotation literature; (2) an overview of the environmental factors such as posture and viewing conditions, including biocularity and distance, that can produce ocular cyclorotation; and (3) a discussion of how ocular cyclorotation can affect toric contact lens fitting and performance.
We have used signal-averaging and vector voltmeter techniques to measure the rod-cone response transition of the primate corneal electroretinogram (ERG). Lights of 580 nm or shorter wavelength, flickering sinusoidally at 5 Hz, produced nearly sinusoidal ERG's with a rod-type spectral sensitivity. For stimuli of 600 nm or longer wavelength, the ERG exhibited cone as well as rod response components; the ERG response became increasingly nonlinear and measurements of sensitivity and phase grew more variable. Our results show how phase information may reveal changes in response nonlinearities. Response shape and vector voltmeter phase readings are more accurate indicators of the boundary between rod-only vs. rod-cone mixed responses than are vector voltmeter magnitude readings. Shifts in the rod-cone response boundary could perhaps be exploited clinically to test for some anomalous visual processes.
Using the local ERG in response to a long-wavelength stimulus as an indicator, field sensitivity functions have been obtained from cynomolgus macaque monkeys (Macaca fascicularis) with flashed and sinusoidally flickering test stimuli. These functions show the reciprocal of the relative radiance, for various adapting wavelengths, required to reduce a 667-nm test response to a criterion level. The resulting functions resemble both Stiles's pi 5 and the SR function of Smith and Pokorny, provided that pi 5 and SR are displaced about 7 nm toward longer wavelengths, in agreement with microspectrophotometric evidence. When field sensitivity functions are obtained with a 20 Hz sinusoidal test stimulus, using a continuous change of field wavelength, the direction of a slow spectral traverse has a large effect upon the shape of the sensitivity and phase functions--a hysteresis effect These effects do not occur at 5 Hz. The test light is proven to be ineffective upon G cones, and it appears unlikely that measurably significant signals are significantly induced into G cones as an indirect result of the modulation of R cones. Therefore, the steady background light absorbed in the G cones seems to be influencing the response of the R cones.
A procedure is proposed for computing the visual effectiveness of light incident on the retina considering the directional sensitivity of the underlying photoreceptor population. A half-sensitivity half-width measurement of the retinal directional sensitivity provides the basis for integrating a parabolic approximation to the directional sensitivity over the pupillary area penetrated by the incident light.
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Psychophysical and electrophysiological studies have shown that the perception of color is in part dependent upon an opponent signal between the long (R) and middle (G) wavelength--sensitive cone systems. Models of human color vision hypothesize that this signal is derived from the difference between sensitivities of the R and G classes of cones. We report here a slow potential in the foveal local electroretinogram (LERG) of primate that correlates well with the absolute logarithmic difference between psychophysically deduced R and G primaries. The foveal LERG is recorded from cynomolgus macaque monkeys with the use of low-frequency sinusoidally flickering stimuli. Responses obtained at the neural wavelength, typically in the region between 540 and 570 nm, or less like log-saturated sinusoids, whereas responses obtained to other wavelength stimuli have a negative component. The amplitude of the negative-going component is deduced by fitting waveforms obtained at the neutral wavelength to responses obtained to the other wavelengths. The validity of this nonlinear analysis is supported by fitting the deduced hyperpolarizing response vs. intensity (RvI) functions with the relationship, V/Vmax = I/(I r sigma), as previously found for single retinal units. The negative component RvI function does not follow this relationship--as expected for an R-G difference signal; a decrease in amplitude at high illuminances could account for perceptual luminance dependent hue shifts.
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A quantitative comparison is made between the psychophysical flicker response of man and similar data obtained electrophysiologically from the cones of macaque monkeys. When the psychophysical data are obtained from an eye that is strongly light-adapted, there is excellent agreement between the two sets of data at high frequencies. Under this condition, both kinds of data fit a distributed-parameter model, whose time constant also agrees with that derived from studies of the phosphenes elicited by electrical stimulation of the human eye. On the other hand, psychophysical data obtained with fully modulated stimuli (which minimally adapt the eye) yield a longer time constant for the same model. These results imply that the psychophysical flicker thresholds are normally controlled by a distributed filtering process that is proximal to the receptor stage. This slower, psychophysical process is evidently desensitized by intense adapting lights, so that the faster one that governs the electrophysiological responses can be detected.
1. The response of the primate cone photoreceptors to sinusoidally flickering stimuli has been obtained by monitoring the late receptor potential (LRP). 2. By comparing the response characteristics of the foveal local electroretinogram (LERG) before and after the intraocular infusion of sodium aspartate, it was found that the b-wave in the foveal LERG does not affect the monitoring of the LRP to steady-state flicker. 3. Functions describing the supra-threshold frequency response characteristics of the photoreceptors were obtained. 4. Linearity was found to hold for low amplitude responses, and temporal modulation transfer functions (MTFs) were obtained for the photoreceptors at various adaptation levels. 5. The cone photoreceptors were found to act approximately as passive low pass filters compounded with some low frequency attenuation. 6. The high frequency response of the photoreceptors at various adaptation levels tends toward a common high frequency asymptote, much like human psychophysical findings, and can be described by a diffusion model. 7. Non-linearities (convexity-upwards) suggest modest positive feedback at the level of the photoreceptors. 8. Mechanisms limiting the magnitude of the receptor response at low frequencies have little effect on the phase lag of the response.
Electrophysiological recordings of primate photorecptors have been obtained and frequency response characteristics of the red, green and blue cones have been determined and compared to previous psychophysical findings. Using cynomolgus monkeys, we recorded the foveal local electroretinogram, which is dominated by the late receptor potential, and obtained criterion-response threshold data for sinusoidally flickered test stimuli complementary chromatic adapting backgrounds. Our results support the hypotheses that (a) the shapes of the MTFs of the red and green cone systems are identical and are determined solely by the photoreceptors at high frequencies, and (b) the blue cones have an MTF with a lower corner frequency than the red- and green-cone systems.
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BACKGROUND: A duplex optical image is created when the ablation zone formed by excimer photorefractive keratectomy is smaller than the entrance pupil. Visual performance and secondary effects are analyzed using a theoretical model of the optical image. METHODS: A point-spread function having a centered in-focus component surrounded by an annular out-of-focus component is calculated from pupil size, ablation size, refractive error, and photoreceptor directional sensitivity. The line-spread, edge-spread, and optical transfer functions are derived. RESULTS: In the line- and edge-spread functions, secondary maxima and curvilinear ramps are most evident with low refractive errors. The half-height widths of the point- and line-spread functions change little. The optical transfer function is reduced in proportion to the distribution of light between the image components. CONCLUSIONS: Stable point and line half-height widths explain why Snellen visual acuity is insensitive to annular blur. Contrast sensitivity correlates with symptoms of haze and fog. Halos and ghost images are associated with secondary optical maxima and curvilinear ramps. Neither visual acuity nor contrast sensitivity can predict halos or ghost images. Halos and ghost images will be most prevalent in low illumination and for low refractive corrections. High refractive errors will produce fewer visual side effects than low refractive errors.