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At least 919 records · Page 51Linked to original sources
PubMed1986

Adaptation of tonic accommodation reduces accommodative lag and is masked in darkness.

Tonic aftereffects of accommodation were investigated under three open loop conditions; Maxwellian view (pin hole pupil), empty field, and darkness. Duration of accommodative aftereffects, following 5 sec of adaptation, ranged within several seconds for all three open loop conditions. After adapting for 1 min to a 2-D stimulus, the duration of accommodative aftereffects increased by approximately an order of magnitude for the Maxwellian view and empty field conditions; however, they increased modestly for the dark condition. Tonic aftereffects of accommodation were also stimulated by convergence with base out prism or divergence with base in prism while the accommodative loop was opened. The ratio of open-loop aftereffect amplitude/closed-loop accommodative response to lenses ranged from 0.95 to 0.48, and it became smaller when the accommodative stimulus approached the amplitude of accommodation. The rapid decay of tonic aftereffects of accommodation in darkness was reversable when a visible stimulus was presented while the accommodative loop was opened by Maxwellian view or empty field. Hence, darkness only masked aftereffects that were manifest primarily when there was light stimulation of the retina. The amplitudes of accommodative aftereffects were reciprocally related to the amplitude of the lag of accommodation. During a 2-min adaptation period, accommodative response increased gradually for 30-60 sec as it approached the amplitude of the accommodative stimulus. This resulted in a reduction of accommodative error or lag. Opening the accommodative loop before or after the lag was reduced resulted in a small or large aftereffect of accommodation respectively. Accommodative aftereffects appear to be related to accommodative lag as prism adaptation is related to fixation disparity.

Accommodation, Ocular↗
PubMed1986

Scotopic sensitivity loss in glaucoma.

Photopic and scotopic increment thresholds were measured at selected visual field positions for 39 patients suspected of having glaucoma, 39 patients with diagnosed glaucoma, and 31 age-matched normal control subjects. Stimuli were presented at equal-eccentricity pairs of positions just above and below the nasal horizontal meridian. Both photopic and scotopic thresholds were elevated significantly for both the suspect and glaucoma groups. The average photopic and scotopic threshold elevations were the same for the suspect group, but scotopic threshold elevations were substantially greater than photopic threshold elevations for the glaucoma group. In a more detailed analysis of the glaucoma group data, local and diffuse components of the photopic and scotopic threshold elevations were operationally defined. The analysis revealed that localized photopic and scotopic scotomas were of equal depth, but diffuse scotopic defects exceeded diffuse photopic defects by a factor of 2:1 log units. These findings suggest that not all ganglion cell types are equally susceptible to glaucomatous damage and also point to scotopic perimetry as a potentially sensitive test for early glaucoma detection.

Adaptation, Ocular↗
PubMed1998

Fine structure of the retinal pigment epithelium of Oreochromis niloticus L. (Cichlidae; Teleostei) in light- and-dark adaptation.

The fine structure of the retinal pigment epithelium (RPE) of the cichlid Oreochromis niloticus was investigated in both light- and dark-adaptation. The eyes of four light-adapted and from four dark-adapted O. niloticus were fixed routinely for light and transmission electron microscopy. The RPE consisted of a single layer of columnar cells showing minimal basal infolding but plentiful apical processes that in light-adaptation interdigitated with the photoreceptor outer segments. The epithelial cells were joined by a series of basally-located tight junctions. These cells showed a large vesicular nucleus, plentiful smooth endoplasmic reticulum and polysomes, but only small amounts of rough endoplasmic reticulum. Phagosomes, lysosome-like bodies, lipid droplets, and myeloid bodies were observed. The choriocapillaris was a single layer of large-caliber capillaries, and Bruch's membrane (complexus basalis) was a trilaminate structure typical of teleosts. The RPE melanosomes moved basally (sclerally) in dark-adaptation and apically (vitreally) during light-adaptation. Other morphological features which changed at least to some degree during retinomotor responses were: the location of the RPE nucleus; the location and electron density of the mitochondria; and the location, number, and size of the myeloid bodies. A number of unique morphological changes take place within the RPE cells of this species during the circadian cycle in addition to the movement of melanosomes characterized in other vertebrates.

Adaptation, Ocular↗
PubMed1998

Photoreceptor fine structure in Oreochromis niloticus L. (Cichlidae; Teleostei) in light- and dark-adaptation.

The structure and arrangement of both light- and dark-adapted retinal photoreceptors of Oreochromis niloticus L. were studied. Eyes of four light-adapted and four dark-adapted O. niloticus were fixed routinely for light and transmission electron microscopy. Rods, single cones, and double (twin) cones were present in a ratio of 30:1:2, respectively. Light-adapted rods were tall, extending into the retinal epithelial layer. Rod inner segments showed a distal ellipsoid of mitochondria that narrowed dramatically in the myoid region. Dark-adapted rod inner segments were much shorter with a thicker myoid region, indicating photomechanical movement. Rod synaptic spherules were small, with both superficial synapses and invaginated sites. Single cones were similar to individual members of a double cone. Cone outer segments consisted of uniform discs with a single incisure. All cones displayed a short, tapering outer segment, a large ellipsoid of mitochondria, and a myoid region rich in organelles. Both members of double cones had extensive subsurface cisternae along their contiguous surfaces. Cone inner segments changed little throughout the circadian cycle, suggesting an absence of significant retinomotor movements. Large, vesicular cone nuclei were located adjacent to or through the external limiting membrane. The cones' synaptic pedicles had larger synapses than rod spherules, with more of both invaginated (ribbon) and conventional (superficial) synaptic sites. Cone photoreceptors were arranged in a repeating square mosaic pattern with a single cone surrounded by four double (twin) cones. The photoreceptors of the Nile tilapia presented basic piscine characteristics, and also some more species-specific features.

Adaptation, Ocular↗
PubMed1976

Scotopic and photopic vision in the California ground squirrel: physiological and anatomical evidence.

The California ground squirrel is a highly diurnal species previously thought to have an all-cone retina. This issue was re-examined in physiological and anatomical experiments. The electroretinogram (ERG) was used to measure the spectral sensitivity of the eye under different conditions of adaptation. The occurrence of a Purkinje shift could be demonstrated, although there was some indication that not all members of this species show such a shift. Spectral sensitivity of the dark-adapted eye of this squirrel is close to that predicted by a typical mammalian rhodopsin. Light adaptation produces a shift in spectral sensitivity to a peak location of about 525 nm. It was shown that two mechanisms having different spectral sensitivities contribute to the photopically recorded ERG. The degree to which these two mechanisms contribute to the ERG was found to be strikingly different from the degree to which the two contribute to visual behavior. Our anatomical results indicate that the retina of the California ground squirrel has two structurally distinct photoreceptors which, on the basis of various criteria, can be classified as cone and rod-like. The rod-like receptors comprise about 6-7% of the total. The two photoreceptor types differ in placement of their inner segments, size of their outer segments, outer segment ultrastructure, and terminal structure and organization.

Adaptation, Ocular↗
PubMed1995

Retinoid cycling proteins redistribute in light-/dark-adapted octopus retinas.

In cephalopods, the complex rhodopsin-retinochrome system serves to regenerate metarhodopsin and metaretinochrome after illumination. In the dark, a soluble protein, retinal-binding protein (RALBP), shuttles 11-cis retinal released from metaretinochrome located in the photoreceptor inner segments to metarhodopsin present in the rhabdoms. While in the rhabdoms, RALBP delivers 11-cis retinal to regenerate rhodopsin and in turn binds the all-trans isomer released by metarhodopsin. RALBP then returns all-trans retinal to the inner segments to restore retinochrome. The conventional interpretation of retinoid cycling is contradicted by immunocytochemical studies showing that, in addition to rhodopsin, retinochrome is present in the rhabdomal compartment, making possible the direct exchange of chromophores between the metapigments with the potential exclusion of RALBP. By using immunofluorescence and laser scanning confocal microscopy, we have precisely located opsin, aporetinochrome, and RALBP in light-/dark-adapted octopus retinas. We found differences in the distribution of all three proteins throughout the retina. Most significantly, comparison of cross sections though light- and dark-adapted rhabdoms showed a dramatic shift in position of the proteins. In the dark, opsin and retinochrome colocalized at the base of the rhabdomal microvilli. In the light, opsin redistributed along the length of the microvillar membranes, and retinochrome retreated to a location that is perhaps extracellular. RALBP was present in the core cytoplasm of the photoreceptor outer segments in the dark, and RALBP moved to the periphery in the light. Because of the colocalization of opsin and retinochrome in the dark, we believe that the two metapigments participate directly in chromophore exchange. RALBP may serve to transport additional chromophore from the inner segments to the rhabdoms and may not be immediately involved in the exchange process.

Adaptation, Ocular↗
PubMed1990

On the duplex nature of the skate retina.

The skate retina contains only one type of photoreceptor which has rod-like properties in the dark-adapted state. In the presence of background illumination, the receptors take on cone-like properties, i.e., their photoresponses become much faster, are less sensitive to light, and can be elicited in the presence of very bright backgrounds. Although the transformation is an extremely slow process, the skate retina performs like that of animals with mixed, rod-cone retinae. In this report we examine some postreceptoral features which may relate to this remarkable behavior. We show, for example, that the skate photoreceptor terminals make two kinds of junctions (ribbon synapses and basal junctions) that appear analogous to the two kinds of synaptic contacts made by cone terminals in other species. Furthermore, two types of horizontal cells are seen in the skate retina. Although the light-evoked responses of these horizontal cells are similar, there are differences in their current profiles recorded under voltage clamp, and in the nature of their dendritic processes. We have also observed several unusual postreceptoral structural features that may have some bearing on the response properties of the skate retina. In addition, a comparison of the adaptive properties of the receptor potential with those of intraretinal responses (b-wave, PNR) reveals differences that suggest strongly the presence of a network adaptive mechanism originating within the proximal retina. The network mechanism appears to be controlled to a large extent by the extracellular concentration of potassium [K+]0; i.e., changes in [K+]0 affect significantly the sensitivities of the b-wave and PBR, but have little effect on responses arising in the distal retina.

Adaptation, Ocular↗
PubMed1976

Effects of light and dark upon photoreceptor synapses in the retina of Xenopus laevis.

Photoreceptor synapses in Xenopus retina were studied after exposure to day/night cycles and continuous light or dark. In the rods, dense-core vesicles appear alongside the synaptic ribbons in animals exposed to light. In dark-adapted rods, electron-dense material is present in the synaptic clefts, but no dense-core vesicles are found associated with the synaptic ribbons. Cone photoreceptors do not show these ultrastructural changes in response to light and dark. Prolonged exposure to light (21 days) causes flattening of the synaptic vesicles associated with the synaptic ribbons in both rods and cones. The results are discussed in the light of what is known about transmitter release from photoreceptors.

Adaptation, Ocular↗
PubMed1995

Light adaptation of the human photopic oscillatory potentials: influence of the length of the dark adaptation period.

Photopic electroretinograms recorded immediately after a period of dark adaptation show a regular increase in amplitude with time spent in light. The retinal mechanisms at the origin of this light adaptation effect remain obscure. The purpose of this study was to investigate the duration of the dark adaptation period needed to produce an optimal light adaptation effect as demonstrated by photopic oscillatory potential recordings. Our results indicate that the light adaptation effect can be separated into two distinct processes. The first one, activated early in the dark adaptation process, reduces the amplitude of the fourth oscillatory potential to 32% of control after less than 5 min of dark adaptation, while the second process, activated after more than 10 min of dark adaptation, appears to impact solely the amplitude of the earlier oscillatory potentials 2 and 3. Our results suggest that the light adaptation effect is mediated by two distinct retinal pathways or mechanisms.

Adaptation, Ocular↗
PubMed1995

Clinical electroretinography in the dog with ganzfeld stimulation: a practical method of examining rod and cone function.

We used a simple and reproducible technique with full-field electroretinography and a special-purpose computer system to test and evaluate outer retinal function in dogs. The standardized protocol included the following five basic responses: (1) a stable initial light-adapted, mainly cone derived response, (2) a dark-adapted rod response, (3) chromatically separated rod and cone responses, (4) a maximal rod and cone response and (5) an isolated cone flicker response. For evaluating the electroretinographic responses, a graphic-presentation was used that included data from the tested animal as well as normative data from dogs of the specific breed and age group, presented as the percentage of the median in which limits of normality were depicted in percentiles.

Adaptation, Ocular↗
PubMedDate not supplied

Reproducibility of electroretinograms recorded with DTL electrodes.

The purpose of this study was to examine whether the use of the DTL fiber electrode yields stable and reproducible electroretinographic recordings. To do so, luminance response function, derived from dark-adapted electroretinograms, was obtained from both eyes of 10 normal subjects at two recording sessions spaced by 7-14 days. The data thus generated was used to calculate Naka-Rushton Vmax and k parameters and values obtained at the two recording sessions were compared. Our results showed that there was no significant difference in the values of Vmax and k calculated from the data generated at the two recording sessions. The above clearly demonstrate that the use of the DTL fiber electrode does not jeopardize, in any way, the stability and reproducibility of ERG responses.

Adaptation, Ocular↗