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Biomedical subjects

D G Green

Publications and source records attributed to D G Green.

At least 19 recordsLinked to original sources
PubMed1992

Effect of contact lens correction of sine wave contrast sensitivity in keratoconus patients after penetrating keratoplasty.

Contrast sensitivity testing reveals visual deficits not detected by standard acuity tests, providing a more sensitive measure of visual performance. Sine wave contrast sensitivity functions were examined for spectacle and contract lens correction of keratoconic eyes after penetrating keratoplasty (PKP). Contrast sensitivity was significantly higher for middle and high spatial frequencies with a rigid gas permeable contact lens than with glasses, even when Snellen acuity was identical for both forms of correction. Contrast performance correlated with subjective assessments of vision quality. This may be related to the quality of the keratometry mire images and suggests that mild irregularities in graft contour may be affecting contrast sensitivity. When considering the options for optical correction of grafted eyes, an evaluation of contrast sensitivity may help determine whether contact lens correction would optimize visual performance.

Adult↗
PubMed1992

Effect on grating identification of sampling with degenerate arrays.

We have measured observer performance in identifying gratings that were sampled by degenerate-sampling arrays. Grating identification for high-spatial-frequency stimuli fell to threshold levels only after more than 88% of the sampling elements were subtracted. Although several studies with normal observers have shown that foveal visual acuity corresponds to estimates of interphotoreceptor spacing derived from anatomical photoreceptor density, a simple transformation of sampling-element density to mean spacing does not describe the results in our simulation of degenerate-sampling arrays. To the extent that our model simulates the anatomical changes of retinal pathology, our results suggest that grating acuity reflects the normal high-spatial-sampling rate of small regions of preserved anatomy in the degenerate retina rather than the mean density or spacing of foveal receptors after massive degeneration.

Adult↗
PubMed1992

Contrast sensitivity measurements with the Echelon diffractive bifocal contact lens as compared to bifocal spectacles.

We examined the contrast sensitivity function of presbyopic subjects wearing the Hydron Echelon diffractive bifocal contact lens. Contrast sensitivity with the Echelon lens was compared to contrast sensitivity with bifocal spectacle correction. Each eye was tested for near and distance vision with the Echelon contact lens and with spectacle correction. Most subjects achieved a visual acuity of 20/20 with the Echelon lens; however, on average, contrast sensitivity was found to be significantly decreased with the contact lens as compared to spectacles for both distance and near vision. The largest decrease in contrast sensitivity was seen for mid-range spatial frequencies (4 to 16 cycles/degree). Although most subjects experienced a significant decrease in contrast sensitivity, two experienced only small decreases with the Echelon lens. Only one subject who met entry criteria reported that vision was as good with the Echelon lens as with spectacles. The decreased contrast sensitivity may result from contact lens decentration as well as inherent optical characteristics of the Echelon lens.

Adult↗
PubMed1991

Are albino rats night blind?

Based on single-unit recordings from the superior colliculus and optic nerve, albino rats recently were reported to have dark-adapted thresholds that are 2 log units higher than those of pigmented rats. To confirm this result, electroretinograms (ERG) were recorded with pupillary light reflex thresholds from the same strains of albino (CD) and pigmented (Long-Evans hooded) rats. Neither ERG nor pupil measurements showed higher dark-adapted thresholds for albino relative to pigmented animals. Both groups had dark-adapted thresholds close to the thresholds found for hooded animals in the reported study. These experiments measuring ERGs and pupillary light reflexes do not verify the report of night blindness in albino rats.

Albinism↗
PubMed1990

Evidence for a power law intensity code in the coupled cones of the turtle.

The hyperpolarizing responses to light were recorded intracellularly from red cones of the turtle, Pseudemys scripta elegans. Pairs of slit stimuli were flashed alone or together at various intensities, one slit positioned on the receptive field center and the other displaced 30 microns. The peak amplitude of the response was measured, and the results analyzed to quantify the relationship between the light intensity and the size of the neural signal evoked prior to the spatial interactions occurring in the network of coupled cones. This signal, E, was found to be described by a compressive power law, E = k.I0.5, where I is slit intensity. Evidence that the inferred excitation function describes a local mechanism independent of the slit position was obtained by measuring the response and the sensitivity receptive field profiles. The response and the sensitivity fields both decreased exponentially, but with space constants that differed by a factor of two, indicating in still another way the existence of an early square-root transformation.

Animals↗
PubMed1990

Square root intensity coding in turtle cones: physiological mechanisms.

The effect of coupling on the intensity-response functions for full field and for slit stimuli was studied by comparing the shapes of the curves obtained from several strongly coupled red cones of turtle with that of one very weakly coupled cone. The full field V-log I curves could be fitted by a Michaelis-Menten relationship, regardless of the strength of the coupling. For the weakly coupled cone the slit V-log I could also be fitted by a Michaelis-Menten curve. For strongly coupled cones a major portion of the curve (1.2-2 log units of intensity) was better fitted by V oc Im. For centered slits "m" was 0.5. With increased distance between the slit and the center of the receptive field "m" was found to increase slightly. The results were analyzed in terms of a theory in which the shape of the slit intensity-response curves arises from scattered light progressively recruiting neighboring cone responses. An analytical formulation of this idea is presented and a plausible light distribution function which supports the recruitment hypothesis is derived. A numerical model of the cone network, which includes the effect of scattering and transduction saturation, accounts well for all of the experimental data obtained with single and paired slits.

Animals↗
PubMed1987

Spatial spread of adaptation within the cone network of turtle retina.

1. The spatial characteristics of adaptation were studied in the red-sensitive cones of the snapping turtle retina using intracellular microelectrodes. Light responses elicited with slit-shaped test and adapting stimuli revealed that test response amplitudes and adaptation decline similarly with distance from the impaled cone. The spatial spread of adaptation and the light response cannot be accounted for by scattered light and must therefore result from electrical coupling between cones. 2. The reduction in the amplitude of the test response correlated strongly with the magnitude of the sustained hyperpolarization induced by the adapting fields. This dependence of adaptation on membrane potential was independent of the spatial configuration of the adapting field. 3. The time courses of flash responses were monotonically related to the membrane potential induced by adapting stimuli and were also independent of adapting field configuration. 4. Adapting slits imaged on the cone receptive field centres uniformly depressed sensitivity without altering the shape of the field or its exponential fall-off. Since the membrane potential evoked by the adapting slit falls off exponentially, the invariance of receptive field shape implies that the spread of adaptation cannot be attributed solely to voltage-dependent desensitization of the transduction apparatus in the cones. Therefore a substantial part of the membrane potential dependency of adaptation probably results from a shunting of signals across the plasma membrane of the cone. 5. Full field backgrounds depressed sensitivity but did not alter the receptive field profiles. On the model of electrical coupling proposed by Lamb & Simon (1976), this suggests that to the extent that the voltage-dependent desensitization results from an increased conductance and hence an increased shunt of the signals at the plasma membrane, there must be a concomitant increase in the conductance of the electrical pathways linking cones to one another.

Action Potentials↗
PubMed1985

The absence of spread of adaptation between rod photoreceptors in turtle retina.

Adaptation by weak backgrounds and the spatial spread of desensitization between rods was studied in the snapping turtle retina, Chelydra serpentina. Intracellular membrane potentials were recorded from these photoreceptors in an eyecup preparation. The kinetics and sensitivity of rod responses were changed significantly by large, very dim backgrounds. For the twenty-five most sensitive rods where the dark-adapted flash sensitivity, SDF, was greater than 1.0 mV/Rh*, Rh* being the number of effective photo-isomerizations per rod, the background intensity required to halve the amplitude of the linear range response averaged 0.21 Rh* s-1. The time-to-peak of the test responses was reduced up to 50% by these dim backgrounds. The desensitizing effects of full field backgrounds of various intensities on the responses to large test spots were measured. The dependence of incremental flash sensitivity, SF, on background intensity, IB, followed the form (FORMULA: SEE TEXT) where I0 is the background intensity which halved SDF. The same intensity dependence held for slit-shaped background fields that desensitized responses to small test spots. The desensitizing effects of large, very dim flashed and continuous backgrounds took several seconds to appear and decay to dark levels. This in conjunction with the sparsity of photons suggests, that the desensitization from a single photoisomerization can persist for several seconds. A comparison of the desensitizing effects of spot and annular backgrounds revealed that small spot backgrounds superimposed on the centered test spots desensitized rods more effectively than annular fields. This finding held true even when annular patterns produced a greater maintained hyperpolarization in the rods. Thus, there was no unique relationship between desensitization and the steady maintained hyperpolarization evoked by a background field. The dependence of adaptation on distance from the impaled rod was determined with slit-shaped background fields placed at different positions across the rod's receptive field. The desensitizing effect of displaced slit stimuli was found to decline much more rapidly with distance than excitation. Displacing the slit by 20 micron from the centre reduced its desensitizing effect by more than 1 log unit. In contrast, excitation fell to about 80% at the same distance (lambda ranging from 50 to 70 micron). The fall off of desensitization with distance matched the calculated fall off with distance of light scatter from a slit. No difference was noted in the kinetics of test responses in the presence of equally desensitizing, superimposed and displaced slits.(ABSTRACT TRUNCATED AT 400 WORDS)

Adaptation, Ocular↗
PubMed1985

Light exposure can reduce selectively or abolish the C-wave of the albino rat electroretinogram.

The ERG (electroretinogram) of the albino rat is reported to lack a c-wave. Observations of our own suggested that the conditions of light-rearing are important. Consequently, the authors recorded c-waves in two groups of albino rats. One group was reared from birth in dim illumination (dark-reared) and the other in 12/12 cyclic light (light-reared). Rats were tested after birth from 22 days to about 1 year. All dark-reared animals had a c-wave. Rats reared in cyclic light typically had no detectable c-wave. Physiologic and anatomic evidence suggests this consistent difference, c-waves present in dark-reared animals but absent or diminished in light-reared animals, is probably not due to extensive light induced retinal damage. No consistent differences between the two groups were seen in a- or b-wave thresholds, a- or b-wave intensity-response functions, and in the time-course of b-wave dark adaptation.

Animals↗
PubMed1979

Cone inputs to ganglion cells in hereditary retinal degeneration.

The photoreceptor layer degenerated, but cone nuclei apparently devoid of outer segments were retained in retinas of aged rats of the Royal College of Surgeons strain from which optic tract activity was recorded. Measures of sensitivity showed these single axons of retinal ganglion cells to have photopic spectral responses. Cone remnants containing a cone pigment may be the photoreceptive elements in these retinas.

Action Potentials↗
PubMed1978

Myocardial susceptibility to ischemic damage: a comparative study of disease models in the rat.

Using experimental models of various disease states, the ability of the isolated perfused working rat heart to withstand and recover from a period of severe ischemia was investigated. The results revealed that the coexistence of a diabetic state, obesity, or left ventricular hypertrophy increased the susceptibility of the hearts to ischemic damage and reduced the rate or the extent of postischemic recovery. In contrast, hearts obtained from moderately hypertensive rats exhibited a greater resistance to, and a superior recovery from, ischemia than did hearts obtained from normotensive controls.

Adenosine Triphosphate↗
PubMed1978

Visual acuity: the influence of refraction and diffraction and the use of interference fringes.

In summary, interference acuity provides an exceedingly useful technique for separating optical from retinal causes for poor vision. Opacities can cause gross disturbances of the fringe pattern, so failure in this test may or may not be caused by poor macular function. For patients who would be operated on only if indications of existing potential for vision could be obtained, this test is invaluable. We had several patients who would have been excluded from consideration for a cataract extraction but who were not because we were able to show good potential for acuity; postoperatively, they achieved acuities as predicted. The test succeeded in evaluating macular function in instances when the usual methods of testing for vision potential, such as two-point discrimination, electroretinography, and ultrasound, provided only crude estimates of retinal function. Although the apparatus required is simple-a small laser, a few optical components, and a patient chair-I am not sure how long it will take before ophthalmologists will have it available for office use. But I am optimistic about the possibilities offered by the widespread use of what I consider to be a promising new technique for evaluating macular function behind opacities of the ocular media.

Cataract↗