PubMed Health⌕ Search

Biomedical subjects

D C Hood

Publications and source records attributed to D C Hood.

At least 19 recordsLinked to original sources
PubMed1992

Temporal frequency dependent adaptation at the level of the outer retina in humans.

The focal electroretinogram (FERG) was used to examine temporal frequency tuning at the outer retinal level in humans by measuring temporal modulation thresholds. Changes in FERG thresholds as a function of ambient light level were compared to temporal modulation thresholds obtained psychophysically using the same stimuli. At lower temporal frequencies, both FERG and psychophysical thresholds changed sensitivity proportional to the mean illuminance level. At higher illuminance levels, both threshold measures were relatively independent of illuminance. The comparison of the FERG to the behavioral data suggest that most of the adaptation-dependent changes in temporal sensitivity in humans occur at the level of the photoreceptor complex.

Adaptation, Ocular↗
PubMed1992

Quantal noise and decision rules in dynamic models of light adaptation.

To evaluate some of the consequences of including probabilistic processes (e.g. quantal noise) in a computable model of light-adaptation dynamics, we considered the behavior of a general class of models. These models contain four stages: (1) early noise; (2) a deterministic filtering and gain-changing stage; (3) late noise; (4) a decision rule that is either an ideal (signal-known-exactly) detector or a peak-trough detector. With the ideal detector and without late noise, the observer's sensitivity as a function of mean luminance and temporal frequency is not affected by the filtering and gain-changing stage. Consequently, if the early noise is entirely quantal fluctuations, sensitivity will always be a square-root function of mean luminance and a uniform (flat) function of temporal frequency. This latter prediction is contradicted by all known data; either the ideal-detector is the wrong decision rule or sensitivity is almost always limited by sources of noise other than quantal fluctuations. With the peak-trough detector, however, with or without late noise, the observer's sensitivity as a function of temporal frequency does reflect the sensitivity of the low-level filtering and gain-changing stage. Late noise is needed, however, if the observer's sensitivity as a function of mean luminance is to go through both a square-root and a Weber region. Comparing these conclusions to similar work on the spatial frequency dimension highlights differences between the spatial and temporal frequency domains. Finally, on the basis of these analyses and evidence from the literature, we question whether quantal fluctuations limit visual sensitivity under any condition.

Adaptation, Ocular↗
PubMed1992

Modeling the dynamics of light adaptation: the merging of two traditions.

Light adaptation has been studied using both aperiodic and periodic stimuli. Two well-documented phenomena are described: the background-onset effect (from an aperiodic-stimulus tradition) and high-temporal-frequency linearity (from the periodic-stimulus tradition). These phenomena have been explained within two different theoretical frameworks. Here we briefly review those frameworks. We then show that the models developed to predict the phenomenon from one tradition cannot predict the phenomenon from the other tradition, but that the models from the two traditions can be merged into a class of models that predicts both phenomena.

Adaptation, Ocular↗
PubMed1992

A computational model of the amplitude and implicit time of the b-wave of the human ERG.

To improve the usefulness of the ERG in identifying the sites and mechanisms of adaptation, development, and disease processes, a theoretical framework based upon Granit's analysis of the ERG was evaluated. The framework assumes that the ERG is the sum of two potentials, one, P3, generated by the receptors and the other, P2, generated by the cells of the INL. Hood and Birch (1990a, b) demonstrated that the leading edge of the a-wave can be quantitatively described by a model used to describe the response from single rod receptors. This model provides P3(t), a theoretical receptor response as a function of time, for any given flash intensity. The ERGs from normal observers and patients with retinal diseases were analyzed in this framework, first by deriving P2 by computer subtracting the predicted P3(t) responses. This analysis was successful and a computational model of the ERG was then derived. The model of P2(t) was constructed with linear filters and a static nonlinearity and using P3(t) as the input. The ERG for any given flash intensity is then P3(t) + P2(t). The model describes (1) the change both in implicit times and in trough-to-peak b-wave amplitudes with flash intensity for the normal, dark-adapted observers; and (2) the changes in b-wave implicit times and amplitudes for three patients with retinal diseases. Among the implications drawn from these analyses were as follows: (1) The fits of the Naka-Rushton equation to trough-to-peak b-wave amplitudes must be interpreted with great care. (2) When the INL is affected by retinal disease, the b-wave may be a very poor reflection of INL activity. (3) The implicit time of the b-wave can provide a measure of receptor sensitivity.

Adult↗
PubMed1992

Psychophysical evidence for post-receptoral sensitivity loss in diabetics.

Although numerous reports show that the sensitivity of the S cone system is decreased in diabetic patients, few studies have been directed toward identifying the possible sites of the sensitivity loss. In this study, a psychophysical technique was used to test hypotheses about sites of S cone system sensitivity loss in a group of patients with early diabetic retinopathy. A model of the S cone system was assumed and the experimental conditions were chosen to distinguish between explanations for S cone sensitivity loss at the receptor level from explanations for loss at a post-receptoral level. Within the context of the model, the data were consistent with S cone system sensitivity loss occurring at a post-receptoral level.

Adult↗
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↗
PubMed1979

Comparison of changes in sensitivity and sensation: implications for the response-intensity function of the human photopic system.

Brightness magnitude estimations of foveally presented flashes deviate from a power function. Increment threshold functions for test lights presented upon these same flashes deviate drastically from a constant Weber fraction. These departures from the classic laws of sensation and sensitivity are shown to be in qualitative agreement and to be affected in similar ways by changes in steady adapting fields. Both sets of data can be fit over the lower range of flash intensities by models based on a saturating response function; both sets deviate from these models at high flash intensities in ways consistent with a nonsaturating response function.

Adaptation, Ocular↗