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D C Hood

Publications and source records attributed to D C Hood.

At least 73 records · Page 4Linked to original sources
PubMed1990

Models of the normal and abnormal rod system.

A framework is presented for using threshold data to test hypotheses about the action of a disease, a chemical agent, or a developmental process. A model of the normal rod system, based on models from the physiological and psychophysical literature, is presented. Hypotheses about the alteration of the rod system are specified in this model. The approach is illustrated with a class of hypotheses that places the decrease in sensitivity with retinal disease at the rod receptors and with data from patients with retinitis pigmentosa and congenital stationary night blindness. The implications for models of the normal rod system are considered.

Adaptation, Ocular↗
PubMed1990

Sites of sensitivity control within a long-wavelength cone pathway.

A flashed-field increment threshold paradigm was used to examine sites of sensitivity control within a long-wavelength cone pathway. The data were fit with a model containing two static nonlinearities, one at the receptors and the second at a red/green opponent stage. The nonlinearities are modified by multiplicative and subtractive processes of adaptation. Comparisons of the model's parameters with physiological measures of the long-wavelength cones suggest that, in the dark, sensitivity is controlled by the opponent site. At high adapting intensities, receptor nonlinearities may limit sensitivity under some conditions. The data also suggest that the spectral tuning of the opponent site varies with adapting intensity.

Color Perception↗
PubMed1990

A quantitative measure of the electrical activity of human rod photoreceptors using electroretinography.

An electrical potential recorded from the cornea, the a-wave of the ERG, is evaluated as a measure of human photoreceptor activity by comparing its behavior to a model derived from in vitro recordings from rod photoreceptors. The leading edge of the ERG exhibits both the linear and nonlinear behavior predicted by this model. The capability for recording the electrical activity of human photoreceptors in vivo opens new avenues for assessing normal and abnormal receptor activity in humans. Furthermore, the quantitative model of the receptor response can be used to isolate the inner retinal contribution, Granit's PII, to the gross ERG. Based on this analysis, the practice of using the trough-to-peak amplitude of the b-wave as a proxy for the amplitude of the inner nuclear layer activity is evaluated.

Adult↗
PubMed1990

The A-wave of the human electroretinogram and rod receptor function.

The amplitude of the leading edge of the a-wave of the human electroretinogram (ERG) was compared with predictions from a computational model of the light-induced responses of rod mammalian receptors. According to this model, a linear process describes the amplitude and time course of the response to relatively low flash intensities and at brief times after the onset of the flash. At higher flash intensities, a nonlinear process, described by the Naka-Rushton function or a saturating exponential, is involved. The primary focus here is on intensity-response data recorded with a clinical ganzfeld apparatus. The leading edge of the rod a-wave recorded from normal observers and patients with congenital stationary night blindness (CSNB) was described by a linear process for flash intensities up to the maximum available flash intensity, 2.0 log scot td-sec. This finding is consistent with the model of the rod's response. It suggests, however, that when ERGs are recorded with clinical systems limited to 2.0 log scot td-sec, these data cannot be used to distinguish between changes in the parameters (eg, semisaturation intensity versus maximum response) of the human rod receptors. Responses to flash intensities up to 3.4 log scot td-sec were recorded using a custom, high-intensity ganzfeld system. Both the linear and nonlinear components of the model were needed to fit the ERGs recorded with this system. This suggests that changes in different receptor parameters can be distinguished with higher intensity flashes.

Electroretinography↗
PubMed1989

S (blue) cone pathway vulnerability in retinitis pigmentosa, diabetes and glaucoma.

A variety of retinal disease lead to a decrease in the sensitivity of the S (blue) cone pathways. To determine the possible sites and mechanisms of this loss we compared the sensitivities of an S (blue/pi-1) and an M (green/pi-4) cone pathway in patients with retinal diseases that differ as to their primary locus of sensitivity loss. The sensitivities of an S and an M cone pathway were assessed in patients with retinitis pigmentosa, insulin-dependent diabetes mellitus and open-angle glaucoma using Stiles two-color increment threshold technique. A greater loss in sensitivity of an S than an M cone pathway was found for all three disease groups; however, the diabetic patients showed a more selective loss. The results suggest that multiple sites are involved and that the combined effects of metabolic abnormalities and hypoxia contribute to the selective loss.

Adaptation, Ocular↗
PubMed1988

Testing hypotheses about development with electroretinographic and incremental-threshold data.

Electroretinographic (ERG) and incremental-threshold [threshold-versus-intensity (tvi)] data have been used to infer mechanisms of development. For a hypothesis about development to be tested, the hypothesis must be specified in the context of a model of the adult visual system. Here, published ERG and tvi data obtained from infants are analyzed in the context of models with two sites. The first site in each of these models has the properties of the rod receptors. By a combination of hypotheses about development and these models, other hypotheses are considered. Taken together, the ERG and tvi data are consistent with a scheme in which developmental changes during the first 18 weeks occur largely, if not entirely, in the retina. Some of these changes may be receptoral in origin. The need for explicit adult models and explicit developmental hypotheses is emphasized.

Aging↗
PubMed1987

The time-course of multiplicative and subtractive adaptation process.

This paper examines, for foveal cone vision, the processes which mediate the transition to a steady state of adaptation following a change of illumination. In the steady state, the signal from an adapting field is attenuated not only by a multiplicative factor (reduction in gain) but also by a subtractive signal. We show that the multiplicative change is accomplished very rapidly following the onset of an adapting field (within about 50 msec). Much of the subtractive change is also accomplished rapidly, but it takes several sec to complete. At the offset of the field, the multiplicative process takes over 200 msec to recover. This slower time-course at offset may be a consequence of receptoral persistence.

Adaptation, Ocular↗
PubMed1986

Test of the decreased responsiveness hypothesis in retinitis pigmentosa.

Retinitis pigmentosa (RP) frequently leads to a decrease in cone system sensitivity. A number of alternative explanations have been proposed for this decrease. Based on the results of a psychophysical technique, the probe-flash paradigm, the authors suggest that a decrease in responsiveness of retinal elements can account for much of this loss. In this paper the decreased responsiveness hypothesis is tested by obtaining data at two levels of steady adaptation. The results of the study indicate that sensitivity loss is greater for the dark adapted than the light adapted state. The data rule out the decreased responsiveness hypothesis coupled with a simple model of adaptation. More complicated adaptation models cannot be excluded. The importance of considering models of adaptation when testing models of disease-related sensitivity loss is underscored.

Adult↗
PubMed1984

Detection and discrimination of small, brief lights: variable tuning of opponent channels.

There is disagreement as to how closely the properties of the pathways that detect small spots resemble the properties of the achromatic system. Spectral sensitivities for small, brief lights show a single, relatively broadband peak similar to the achromatic spectral sensitivity. The lights also appear relatively desaturated near threshold. However, recent field sensitivity data (Finkelstein and Hood, 1981, 1982) have shown that opponent mechanisms are either directly involved in detecting small spots, or they interact with, and desensitize mechanisms are either directly involved in detecting small spots, or they interact with, and desensitize, a strictly achromatic detection pathway. The test sensitivity, test mixture, and wavelength discrimination data of the present study support the first alternative; they implicate opponent (chromatic) mechanisms in the detection of small, brief lights. However, the data cannot be fit by the same opponent systems used to fit large-test data. They suggest instead an hypothesis in which large and small lights are detected by opponent mechanisms that change their spectral tuning with changes in stimulus parameters.

Color Perception↗
PubMed1984

The response range of the blue-cone pathways: a source of vulnerability to disease.

Retinal disease preferentially affects the sensitivity of the "blue"-cone pathways. This vulnerability to disease may be due, in part, to a more limited response range. A psychophysical technique, the probe-flash paradigm, was used to test this hypothesis. The data suggest that the S-cone pathways have a more limited response range than the L-cone pathways. Explanations for blue-cone vulnerability are discessed in the context of this finding.

Differential Threshold↗
PubMed1983

Improvements in visual performance following a pulsed field of light: a test of the equivalent-background principle.

The offset of a pulsed conditioning field of light has recently been shown to produce enhancements of temporal resolution and brightness discrimination. These enhancements are similar to those that are due to imposing a high level of light adaptation on the visual system. Here we analyze whether there is a true equivalence of adaptive state between some level of steady light adaptation and the offset of a conditioning field. The enhancements of visual function at field offset were replicated by using a suprathreshold two-pulse discrimination task and a task requiring detection of an incremental probe stimulus superimposed upon a suprathreshold pulse. These effects are shown to be qualitatively but not quantitatively similar to those produced by an equivalent background selected on the basis of its ability to raise threshold to the same degree as field offset. We conclude that the equivalent-background principle cannot be supported for our measures of foveal visual function.

Adaptation, Ocular↗
PubMed1982

Interactions between rod and cone channels above threshold: a test of various models.

Brightness-matching and magnitude estimation data were obtained to test alternative explanations of the rod-cone interaction above threshold. Steady adapting fields were selected such that 500 and 640 nm test flashes presented in the parafovea stimulated largely the rod and cone systems, respectively. In the brightness-matching experiment stimuli of combined 500 and 640 nm light were matched to a white foveal standard flash set at various levels above threshold. Similarly, in the magnitude estimation experiment, brightness estimates were obtained for single 500 and 640 nm flashes as well as for super-imposed combination flashes. Combination stimuli appeared brighter than their individual components. Overall, the results suggest that rod and cone signals elicited by a single flash combine in an excitatory fashion; there is no evidence for rod-cone inhibition in this study. Further, the results for the suprathreshold and threshold conditions are qualitatively similar. The signals from the rod and cone systems combine less well than signals within a system but to a greater degree than predicted by probability summations or vector addition models.

Adaptation, Ocular↗
PubMed1982

Opponent-color cells can influence detection of small, brief lights.

Spectral sensitivities for small, brief lights presented on a neutral adapting field resemble the photopic luminosity function. The lights often appear achromatic near threshold. These findings are consistent with a hypothesis linking sensitivity to an independent class of nonopponent cells. To test this hypothesis, a variation of the Stiles two-color threshold procedure was used. Field sensitivities were measured with a small, brief 580 nm target presented on flashed fields of monochromatic light; a 633/541 nm flash mixture was also used. All stimuli were delivered on a 4.0 log td steady neutral adapting field. The field sensitivity and field mixture data refute the hypothesis. They argue instead that reg/green opponent cells can substantially influence the detectability of small, brief lights.

Color Perception↗
PubMed1982

Effect of barium and tetraethylammonium on membrane circulation in frog retinal photoreceptors.

We studied the influence of altered ionic conditions on the recycling of synaptic vesicle membrane in frog retinal photoreceptors using horseradish peroxidase to monitor synaptic activity and trace the fate of internalized membrane. The addition of 1.2 mM barium or 20 mM tetraethylammonium to isolated retinas maintained in Ringer's solution, changes the usual balance of membrane circulation in the rod cells; the cone cells are much less affected. Retrieval of synaptic vesicle membrane in the rods, which normally regenerates small vesicles, becomes mediated predominantly by large sacs and vacuoles ("cisternae"). Because these cisternae can be labeled with peroxidase, they appear to arise from endocytized membrane. Morphometric analysis suggests strongly that the cisternae are formed of circulating synaptic vesicle membrane. The effects of barium and tetraethylammonium can be inhibited by high extracellular potassium, by high intensity light, and by 5 mM cobalt. They seem likely to depend on potassium channels, though additional more complex mediation may also be involved. The alterations in membrane retrieval that we find are of interest in terms of the multiple pathways of membrane cycling now being uncovered. They open potential experimental approaches to the controls of this circulation. In addition, the findings extend our previous ones demonstrating that rod cells and cone cells differ in their responses to divalent cations in ways that seem likely to be of physiological importance.

Animals↗