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At least 1,027 records · Page 57Linked to original sources
PubMed2001

Flashed stimuli and the suppression of flicker response from long-wavelength-sensitive cones: integrating two separate approaches.

The selective suppression of flicker response from LWS cones has been investigated with two approaches. One approach has emphasized the use of light-adaptation conditions, and the other has emphasized the use of dark-adaptation conditions. In both cases, stimuli are arranged to restrict or exceed the ability of adaptation processes to maintain an afferent flicker response, and long-wavelength stimuli are used to overload spectrally opponent processes. By integrating these two approaches, this study shows that diverse manifestations of flicker response suppression can be closely related mechanistically. For instance, the steep flicker TVI slopes that resulted from superimposing temporally modulated (100% contrast) test stimuli on flashed backgrounds corresponded to the disappearance of flicker that resulted from increasing the time-averaged illuminance of temporally modulated stimuli (contrast x < 100%) that were flashed alone in an otherwise dark field. For the stimulus parameters of this study, flicker response suppression was more evident for small (19' diameter) than for large (1 degree diameter) stimuli. However, flicker response suppression was elicited reliably for both sizes by adding a spatially coincident short-wavelength stimulus to the interstimulus interval between presentations of the long-wavelength stimuli. By showing that temporal contrast can be treated as an independent variable for an important set of test/background stimulus combinations, the results of this study make it possible to investigate the means by which changes of contrast gain help to maintain flicker response as assessed in a conventional flicker TVI paradigm. The reduced degree of suppression for relatively large stimuli probably is related to the increased action of spatially extensive contrast gain-control processes. These contrast gain-control processes might not act independently of spectrally opponent processes.

Adaptation, Ocular↗
PubMed2001

Preliminary evidence that pharmacologic melatonin treatment decreases rat ghrelin levels.

Ghrelin is a signal peptide isolated from rat stomach antagonistic to actions of leptin. Ghrelin stimulates the secretion of growth hormone (GH) and increases food intake, body mass, and adiposity in rodents. Photoperiod and melatonin regulate leptin secretion of mammals. The aim of the study was to investigate possible melatonin-ghrelin interactions in weight regulation by studying the effects of continuous pharmacologic melatonin treatment and constant light on plasma ghrelin, leptin, and GH levels in rats. Plasma ghrelin concentrations were significantly reduced by exogenous melatonin. Ghrelin levels correlated negatively with plasma leptin levels in control rats kept in 12 h of light/12 h of dark but not in the melatonin-treated animals. The inverse ghrelin-leptin relationship was also disrupted by constant illumination. The circulating ghrelin and GH levels may not be interrelated in all metabolic situations. The results suggest new interplay between the pineal gland and energy metabolism as well as reenforce the hypothesis that ghrelin is antagonistic to leptin.

Adaptation, Ocular↗
PubMed2001

The crustacean eye: dark/light adaptation, polarization sensitivity, flicker fusion frequency, and photoreceptor damage.

Compound eyes, nauplius eyes, frontal organs, intracerebral ocelli, and caudal photoreceptors are the main light and darkness detectors in crustaceans, but they need not be present all at once in an individual and in some crustaceans no photoreceptors whatsoever are known. Compound eye designs reflect on their functions and have evolved to allow the eye to operate optimally under a variety of environmental conditions. Dark-light-adaptational changes manifest themselves in pigment granule translocations, cell movements, and optical adjustments which fine-tune an eye's performance to rapid and unpredictable fluctuations in ambient light intensities as well as to the slower and predictable light level changes associated with day and night oscillations. Recycling of photoreceptive membrane and light-induced membrane collapse are superficially similar events that involve the transduction cascade, intracellular calcium, and membrane fatty acid composition, but which differ in aetiology and longterm consequence. Responses to intermittant illumination and linearly polarized light evoke in the eye of many crustaceans characteristic responses that appear to be attuned to each species' special needs. How the visual responses are processed more centrally and to what extent a crustacean makes behavioural use of e-vector discrimination and flickering lights are questions, however, that still have not been satisfactorily answered for the vast majority of all crustacean species. The degree of light-induced photoreceptor damage depends on a large number of variables, but once manifest, it tends to be progressive and irreversible. Concomittant temperature stress aggravates the situation and there is evidence that free radicals and lipid hydroperoxides are involved.

Adaptation, Ocular↗
PubMed1990

[Adaptational changes in cone electroretinograms in man].

Changes of amplitude and implicit time of human cone electroretinogram (ERG) were studied during dark adaptation and succeeding light adaptation. Dark-adapted cone ERG was isolated by subtracting scotopic blue response from matched scotopic red response. The former represented the rod-mediated b-wave, while the latter consisted of both rod-mediated b-wave and cone-mediated b-wave or x-wave. The b-wave amplitude of dark-adapted cone ERG remained unchanged during dark adaptation, while the implicit time increased systematically, reaching a plateau. Light-adapted cone ERG was obtained by red stimulus lights under a bright background light. The amplitude of light-adapted cone ERG was markedly suppressed through dark adaptation but it recovered gradually during light adaptation, reaching the base line level. The implicit time was unchanged during light adaptation.

Adaptation, Ocular↗
PubMed1988

Dark and light adaptation of pineal photoreceptors.

Dark and light adaptation of pineal photoreceptors was studied in the isolated pineal organ of the rainbow trout, Salmo gairdneri. After intracellular recording, the photoreceptors were iontophoretically injected with Lucifer yellow CH or with horseradish peroxidase for morphological characterization. Pineal photoreceptor cells responded to light with a hyperpolarization whose amplitude was graded with intensity. Following a 30-60 s bleach, receptor responsiveness was greatly reduced with a gradual recovery in the dark. Recovery of membrane potential was complete within 2-4 min in the dark. In response to flashes the hyperpolarizing response increased in darkness in amplitude and duration over a period of more than 30 min and the voltage-intensity curves continuously shifted to lower intensities. After exposure to strong light the time-course of dark adaptation, determined with a threshold criterion, was monophasic and receptor sensitivity increased by at least 5-6 log units. The results show that pineal photoreceptors exhibited the full characteristics of dark adaptation processes previously ascribed to cells proximal to the receptors, i.e. to ganglion cells. Exposure to steady illumination of different intensities induced graded and sustained hyperpolarizations of the receptor membrane potential. The incremental voltage range of responses to test flashes superimposed on the backgrounds was reduced. Voltage-intensity curves were shifted to higher intensities with increasing background illumination indicating that adaptation occurred over a range of about 2.5 log units before the receptors saturated.

Adaptation, Ocular↗