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PubMed2004

Effect of visual stimulation on blood oxygenation in the optic nerve head of miniature pigs: a pilot study.

BACKGROUND: Visual stimulation is increasingly used to investigate the coupling between neuronal activity, blood flow and metabolism in the neural tissue of the ocular fundus. In an attempt to clarify whether the oxygen metabolism is involved in this coupling, we investigated the changes in the partial pressure of oxygen of venous blood (pO (2,blood)) in the optic nerve head of pigs in response to two different visual stimuli. MATERIALS AND METHODS: In 3 miniature pigs, the pO (2,blood) was measured in the optic disk rim using the technique of phosphorescence quenching by oxygen. This parameter was recorded every 8 seconds during a dark-to-light transition and during diffuse luminance flicker (field of 30 degrees centered at the optic disk, temporal frequencies of 2 to 80 Hz). RESULTS: The venous pO (2,blood) level (mean +/- standard deviation) did not change between dark- and light-adapted conditions (26.2 +/- 5.3 and 26.0 +/- 6.2 mm Hg, respectively), nor did we observe any transient change of pO (2,blood) during the light adaptation phase. On the other hand, the venous pO (2,blood) increased, on average, relative to its level during continuous light conditions (24.5 +/- 1.9 mm Hg) by at least 6 % for all flickering frequencies, with a maximum response of 14 % at 15 Hz. CONCLUSIONS: The phosphorescence quenching technique can reveal changes in venous pO (2,blood) induced by visual stimulation. Our results show that the pO (2,blood) in the optic nerve head of miniature pigs does not change with the light adaptation state of the retina, but increases during flicker stimulation with a band-pass type response. The previously reported increase of the ONH blood flow in response to flicker stimulation could lead to this increase of pO (2,blood).

Adaptation, Ocular↗
PubMedDate not supplied

Effects of 10 min tilt and visual directional information on ocular counterrolling.

The effects of 10 min of static tilt on ocular counterrolling (OCR) were investigated. Seven healthy volunteers were tilted 45 degrees around the naso-occipital axis, fixed at that position for 10 min, and the pre- and post-tilt OCRs were recorded. OCR was video-recorded using an infrared CCD camera and subjected to computer image analysis. The subjects were directed to fixate on an imagined target in the dark (experiment 1), or viewed an earth vertical, striped pattern placed in front of his/her eyes to enhance the recognition of extent of body tilt (experiment 2). In experiment 1, the average OCRs recorded before and after 10 min of tilt showed no significant changes. However, in experiment 2, OCR decreased significantly after 10 min of tilt. These results suggest that visual perception of spatial verticality can produce time-dependent adaptation of OCR.

Adaptation, Ocular↗
PubMed1995

An adaptable association between vertical and horizontal vergence.

Vertical phoria (vergence error under monocular viewing conditions) can be trained to vary with conjugate eye position. The adaptive response controls the vertical alignment of the two eyes in the absence of binocular disparity and is used to compensate for binocular changes of the oculomotor system induced by developmental and environmental factors. Vertical phoria was associated with horizontal disparity vergence by adapting vertical vergence to two vertically disparate targets separated along the depth axis. This association was primarily dependent on the horizontal vergence as opposed to monocular eye position or binocular conjugate eye position. Following this adapted association with horizontal disparity vergence, vertical phoria aftereffects were also evoked by accommodative vergence. Previous reports have demonstrated an adapted association between vertical phoria and conjugate eye position. The current report examines the difference in the vertical phoria resulting from adaptation to vertically disparate targets separated along either the vertical axis or depth axis. The amplitude of the vertical vergence aftereffect was approximately 4 times greater for targets separated along the depth axis than in the vertical meridian. The association between vertical phoria with conjugate eye position and horizontal vergence is proposed to result from a cross-coupling of vertical vergence with supranuclear regions that control conjugate and horizontal vergence eye movements. A selective interaction would enable the oculomotor system to correct disturbances in specific supranuclear regions as they interface with vertical vergence.

Accommodation, Ocular↗
PubMed2000

[Flexibility in the adaptation of the vestibulo-ocular reflex to modified visual inputs in humans].

The vestibulo-ocular reflex (VOR) serves to stabilize images on the retina. To maintain appropriate performance and minimize image slippage throughout life, the VOR is subject to long-term adaptive regulation in response to visual input. Adaptive changes in VOR gain (eye velocity/head velocity) can be evoked either by fitting subjects with magnifying, miniaturizing, or reversing spectacles during normal behavior or by moving a large visual field in or out of phase relative to the subject's head movement. These changes exhibit frequency-selectivity. Here, we examine the flexibility of VOR gains by causing VOR in similar directions to undergo different behavioral gain changes. Nine healthy adults, ranging in age from 24 to 38 (mean 28.5) with no history of neurotological symptoms participated in the study. All subjects demonstrated clinically normal functioning on a screening battery of tests that included combined neurologic and otologic physical examinations. Horizontal and vertical eye positions were recorded by bitemporal DC coupled electrooculography (EOG). The subject sat in a rotating chair. The axis of rotation of the body was always earth-vertical, the interaural axis crossing the axis of rotation of the chair. The head was positioned at 20 degrees down in all experiments and was stabilized in this position using a chin rest. The chair was 78 cm in diameter and was shielded by a half-cylindrical optokinetic screen positioned in front of the subjects. Random dot patterns were projected onto this screen. During per- and post-adaptation periods, goggles were fitted to ensure that the subject was in complete darkness and the chair was rotated sinusoidally. The amplitude of the rotating chair was 30 degrees and 60 degrees. Frequencies of rotation were 0.1 Hz, 0.2 Hz, 0.3 Hz and 0.4 Hz for amplitudes of 30 degrees and 0.1 Hz, 0.2 Hz, and 0.3 Hz for amplitudes of 60 degrees. To induce VOR adaptation, the retinal slippage velocity caused by the visual input of a large field was changed for short-term; the change was produced by a combination of sinusoidal head rotation and random dot patterns. During each adaptation session, the frequencies of sinusoidal head rotation were either 0.1 Hz or 0.3 Hz and the amplitude was 30 degrees. The random dot patterns were synchronized with the sinusoidal head rotation in the same direction to make the retinal slippage zero (x0 experiment) and in the opposite direction to double the retinal slippage (x2 experiment). Therefore, a total of four adaptation protocols were tested. The subjects were asked to fix their eyes on a single dot by looking straight ahead in the x0 experiment and to follow the dot in the random dot pattern in the x2 experiment. Each adaptation session lasted for 30 minutes. Each subject participated in couple of adaptation experiments everyday. The average VOR gain and phase lag were calculated using a Fourier transformation. Out of all the subjects who participated in the x2 adaptation experiment at 0.3 Hz with an amplitude of 30 degrees, seven subjects showed a steady increase in VOR gain during a couple of the trials. One out of the remaining two subjects showed a decrease in VOR gain in all three trials. Another subject showed an increase in VOR gain during three trials and a decrease in two trials. In the x2 adaptation experiment with a range of 30 degrees at 0.3 Hz (peak velocity: 28 degrees/s), the percent change in gain (post-pre/pre) was 133% at the same stimulation and 100% at 0.4 Hz (peak velocity: 37 degrees/s). The percent change in gain was 65% for amplitudes of 60 degrees at 0.1 Hz (peak velocity: 18 degrees/s) and 64% for amplitudes 60 degrees at 0.2 Hz (peak velocity: 37 degrees/s). In the x1 adaptation experiment (30 degrees at 0.3 Hz), the percent change in gain was -62% for the same conditions and -50% for amplitude of 60 degrees at 0.1 Hz and -30% for amplitudes of 60 degrees at 0.2 Hz. No change, in VOR gain was observed at the other frequencies. The

Adaptation, Physiological↗
PubMed1999

Identification and immunolocalization of actin cytoskeletal components in light- and dark-adapted octopus retinas.

Photoreceptors in the octopus retina are of the rhabdomeric type, with rhabdomeres arising from the plasma membrane on opposite sides of the cylindrical outer segment. Each rhabdomere microvillus has an actin filament core, but other actin-binding proteins have not been identified. We used immunoblotting techniques to identify actin-binding proteins in octopus retinal extracts and immunofluorescence microscopy to localize the same proteins in fixed tissue. Antibodies directed against alpha-actinin and vinculin recognized single protein bands on immunoblots of octopus retinal extract with molecular weights comparable to the same proteins in other tissues. Anti-filamin identified two closely spaced bands similar in molecular weight to filamin in other species. Antibodies to the larger of the Drosophila ninaC gene products, p174, identified two bands lower in molecular weight than p174. Anti-villin localized a band that was significantly less in molecular weight than villin found in other cells. Epifluorescence and confocal microscopy were used to map the location of the same actin-binding proteins in dark- and light-adapted octopus photoreceptors and other retinal cells. Antibodies to most of the actin-binding proteins showed heavy staining of the photoreceptor proximal/supportive cell region accompanied by rhabdom membrane and rhabdom tip staining, although subtle differences were detected with individual antibodies. In dark-adapted retinas anti-alpha-actinin stained the photoreceptor proximal/supportive cell region where an extensive junctional complex joins these two cell types, but in the light, immunoreactivity extended above the junctional complex into the rhabdom bases. Most antibodies densely stained the rhabdom tips but anti-villin exhibited a striated pattern of localization at the tips. We believe that the actin-binding proteins identified in the octopus retina may play a significant role in the formation of new rhabdomere microvilli in the dark. We speculate that these proteins and actin remain associated with an avillar membrane that connects opposing sets of rhabdomeres in light-adapted retinas. Association of these cytoskeletal proteins with the avillar membrane would constitute a pool of proteins that could be recruited for rapid microvillus formation from the previously avillar region.

Actinin↗
PubMed1997

Autonomic involvement in multiple sclerosis: a pupillometric study.

To study pupillary autonomic function in multiple sclerosis (MS), we examined 36 subjects with low disability, preserved visual acuity and no recent history (2 years) of optic neuritis or actual visual complaints. Compared to controls, MS patients showed a greater dilatator reaction with darkness and, for the light reflex, a lower amplitude and contraction rate and a greater recovery of pupillary diameter 5 s after the stimulus. Within the MS group, no difference was found comparing patients with or without the following characteristics: nuclear magnetic resonance imaging evidence of midbrain lesions; increased visual evoked potential P100 latency; and a previous history of optic neuritis. No correlation was found between P100 latency, duration of disease and pupillometric parameters. Our results indicate that in MS patients there is autonomic dysfunction with a reduction of parasympathetic tone and a relative increase in sympathetic dilatator tone to the pupils. We suggest that pupillary abnormalities could be due to non-specific impairment of the central pathways subserving pupil functions.

Adaptation, Ocular↗
PubMed2004

S-crystallin and arginine kinase bind F-actin in light- and dark-adapted octopus retinas.

PURPOSE: Rhabdomere microvilli dramatically reorganize in conditions of light and dark. This reorganization involves remodeling of the microvillus actin cytoskeleton. We are using the rhabdomeric retina of Octopus bimaculoides to identify actin-binding proteins that may be involved in this remodeling. METHODS: Octopus were light-/dark-adapted, retinas separated into dorsal and ventral halves, and homogenized. Actin-binding proteins were recognized using F-actin overlay blot assays and selected proteins from the overlays were identified using N-terminal sequencing methods or mass spectroscopy. Protein concentrations were quantified and compared by statistical analysis. RESULTS: Total protein gels of light-/dark-adapted, ventral/dorsal halves were almost identical except for a protein band at 26 kD. The relative amount of this protein in the dark was almost double that found in the light. The levels of other proteins did not vary significantly between the light and dark. F-actin overlays also showed matching patterns of actin-binding proteins except for the 26 kD protein. Although the 26 kD protein from light-adapted retinas transferred to the blotting membranes, it did not bind F-actin while the 26 kD protein on overlays from dark-adapted retinas always demonstrated F-actin binding. Besides the 26 kD protein, other proteins at 200 kD, 80 kD, 40 kD appeared on the overlays. These proteins and the 26 kD protein were sequenced and identified as hemocynanin, transitional ER ATPase, arginine kinase and S-crystallin, respectively. CONCLUSIONS: The amount of S-crystallin present in the octopus retina is significantly greater in dark-adapted retinas and it binds to F-actin. In the light, the level of S-crystallin is greatly reduced and there is no apparent F-actin binding. No other studies, to our knowledge, show that S-crystallin binds to the actin cytoskeleton or that its expression is regulated by light. Arginine kinase may provide energy for cytoskeletal remodeling as it may in other neural tissues.

Actins↗
PubMed1998

Preservation of visual sensitivity of older subjects: association with macular pigment density.

PURPOSE: The authors investigated how individual differences in macular pigment (MP) density are related to loss of visual sensitivity with age. METHODS: Macular pigment and visual sensitivity of 27 healthy older subjects (aged 60-84 years) were compared with data from 10 younger subjects (aged 24-36 years). Macular pigment density was measured psychophysically with a centrally fixated stimulus 1 degree in diameter. Photopic increment sensitivity in the fovea was measured as thresholds for centrally fixated 1 degree stimuli on a bright yellow background. Foveal sensitivity of the pi-1 mechanism driven by the S cones was measured with 440-nm light. Photopic increment thresholds not determined by the pi-1 mechanism were measured with 550-nm light. Sensitivity was specified at the photoreceptor outer segments by individually correcting for psychophysically determined lens density and MP density. Dark-adapted (scotopic) sensitivity of rod-dominated visual mechanisms was measured in the parafovea with 550-nm light at 8 degrees eccentricity. RESULTS: Consistent with past reports, photopic sensitivity declined significantly with age for both 440-nm (P < 0.025) and 550-nm (P < 0.0003) light. For older subjects, photopic sensitivity was positively related to MP density, although more strongly for 440-nm (P < 0.001) than for 550-nm (P < 0.01) light. Parafoveal scotopic sensitivity of the older subjects was also positively related to MP density (P < 0.02). Visual sensitivity of the young subjects was not significantly related to MP density. CONCLUSIONS: For subjects older than 60 years, visual sensitivity of those with high MP density was not significantly different from that of young subjects. Conversely, older subjects with low MP density had lower sensitivity than young subjects. Although this study cannot prove causality, the results show that high MP density was associated with the retention of youthful visual sensitivity, which suggested that MP may retard age-related declines in visual function.

Adaptation, Ocular↗
PubMed1999

Light- and focus-dependent expression of the transcription factor ZENK in the chick retina.

Ocular growth and refraction are regulated by visual processing in the retina. We identified candidate regulatory neurons by immunocytochemistry for immediate-early gene products, ZENK (zif268, Egr-1) and Fos, after appropriate visual stimulation. ZENK synthesis was enhanced by conditions that suppress ocular elongation (plus defocus, termination of form deprivation) and suppressed by conditions that enhance ocular elongation (minus defocus, form deprivation), particularly in glucagon-containing amacrine cells. Fos synthesis was enhanced by termination of visual deprivation, but not by defocus and not in glucagon-containing amacrine cells. We conclude that glucagon-containing amacrine cells respond differentially to the sign of defocus and may mediate lens-induced changes in ocular growth and refraction.

Adaptation, Ocular↗
PubMed2004

Short-term adaptation of the cervico-ocular reflex.

The cervico-ocular reflex (COR) works in conjunction with the vestibulo-ocular reflex (VOR) and the optokinetic reflex (OKR) in order to prevent visual slip over the retina during head movement. The COR induces eye movements in response to proprioceptive signals from the neck. We investigated whether the COR gain can be adapted by inducing a mismatch between vision and neck proprioception, in analogy to VOR adaptation. Thirteen healthy subjects were rotated in the dark in a trunk-to-head manner (the head fixed in space while the body passively rotated sinusoidally with a peak velocity of 1.25 degrees /s). Eye movements were recorded with infrared video-oculography under various adaptive conditions. Analysis showed a small but significant reduction in COR gain in the suppression conditions. This means that the cervico-ocular reflex can be modified after only 10 min of concurrent visual and cervical stimulation.

Adaptation, Physiological↗
PubMed1995

Influence of eye motion on adaptive modifications of the vestibulo-ocular reflex in the rat.

While sustained retinal slip is assumed to be the basic conditioning stimulus in adaptive modifications of the vestibulo-ocular reflex (VOR) gain, several observations suggest that eye motion-related signals might also be involved. We oscillated pigmented rats over periods of 20 min around the vertical axis, at 0.3 Hz and 20 degrees/s peak velocity, in different retinal slip and/or eye motion conditions in order to modify their VOR gain. The positions of both eyes were recorded by means of a phase-detection coil system with the head restrained. The main findings came from the comparison of two basic conditions--including their respective controls--in which one or both eyes were reversibly immobilised by threads sutured to the eyes. In the first condition the animals were rotated in the light with one eye immobilised and the other eye free to move but covered. Rotation in the light in this open-loop condition immediately elicited high-gain compensatory eye movements of the non-impeded, covered eye. At the end of this training procedure, the VOR gain increased by 43.2%. In the second condition, both eyes were immobilised and one eye was covered. The result was an increase in the VOR gain of 26.3%. These two conditions were similar as to the visuo-vestibular drive during the exposure, but different as to the resulting--and allowed--eye motion, showing that the condition where the larger eye movements occurred yielded the larger VOR gain change. Our data support the idea proposed by Collewijn and Grootendorst (1979, p. 779) and Collewijn (1981, p. 146) that "[retinal] slip and eye movements seem to be relevant signals for the adaptation of the rabbit's visuo-vestibular oculomotor reflexes".(ABSTRACT TRUNCATED AT 250 WORDS)

Adaptation, Physiological↗