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At least 1,405 records · Page 78Linked to original sources
PubMed1978

Excitation and adaptation in the vertebrate rod photoreceptor.

The excitatory processes and the reactions of the rhodopsin photoproducts in vertebrate rods are summarized, and an adaptation model of the rod is outlined. The transmitter factor Q describes the ability of the discs to release a transmitter carrying the signal from the site of light adaptation to the plasma membrane of the receptor. If Q decreases appropriately when the background light intensity increases, the response range of the rod is preserved and the rod obeys Weber's law. This kind of receptor adaptation is experimentally observed in several species. A photoisomerization in a disc apparently affects the release of a transmitter also in neighbouring discs. The background light also affects the neuronal network of the retina, preserving the response range of the ganglion cell. After an exposure bleaching an appreciable fraction of rhodopsin the sensitivity of the rod returns in two phases: a fast intermediate adaptation and a slow opsin adaptation. After a rather small bleach the photoresponse is fairly well preserved (mainly Q-adaptation), but large bleaches cause a strong reduction in the response range. Different experimental methods utilized when correlating rhodopsin photoproducts and intermediate adaptation are discussed.

Adaptation, Ocular↗
PubMed1998

[Oscillatory potentials in human electroretinography during adaptation to light and dark].

In 20 healthy subjects no changes of the peak-latencies of the four wavelets of electroretinographic oscillatory potentials (OP) was found in the course of dark- and light-adaptation. On the other hand, their peak-to-peak amplitudes and dominant frequencies diminish significantly with rising levels of light adaptation. No single OP wavelet was affected by these changes selectively. The OP peak latencies are well suited for clinical diagnostic testing owing to their stability and low variability. On the other hand, because of the great interindividual variability of the peak-to-peak amplitude values of the OP a better clinical tool appears to be a sum of their amplitudes.

Adaptation, Ocular↗
PubMed1976

The effect of light deprivation on the flickering B-wave.

The effect of light deprivation on the flickering beta-wave was investagated. Results indicate that the flickering beta-wave is extremely reduced in the eye light-deprived for 14 to 20 days relative to the normal eye. The ratio of the second beta-wave (in a series of flickering beta-waves) to the ratio of the first beta-wave appears relatively unaffected by light deprivation, but the ratio of the fourth beta-wave to the first is significantly affected by light deprivation. The effect of light deprivation on the beta-waves evoked by a 13.4 milliambert flickering light is greater for beta-waves evoked later in the series than for beta-waves evoked earlier in the series.

Adaptation, Ocular↗
PubMedDate not supplied

The effect of space missions on gravity-responsive torsional eye movements.

Three astronauts underwent preflight, inflight, and postflight testing of spontaneous ocular torsion and of ocular counterrolling (OCR), reflexes governed by the gravity-responsive otolith organs in the inner ear. One astronaut, A, had a 30-day space mission on Euromir '94 and was examined monocularly with SensoMotoric Instruments video-oculography (VOG). The other two astronauts, B and C, were studied with a binocular VOG and flew an 180-day mission on Euromir '95. In space, spontaneous eye torsion in the upright position was found to be substantially offset from baseline Earth-based recordings in all three subjects for the duration of the flights. In addition, the binocular studies showed a marked torsional disconjugacy. On return to Earth, offset and torsional disconjugacy persisted for many days. OCR in response to 30 degrees right and left tilt was examined preflight and postflight. Compared to preflight, Astronaut A showed reduced OCR immediately postflight, which increased over the next few days. Both Astronauts B and C had increased OCR postflight, which gradually approached but did not achieve the preflight values over 13 days postflight. The adaptation of ocular torsion in space in one astronaut and not in the other two, and slow adaptation postflight, may reflect the lack of visual feed-back and the open loop nature of the otolith-ocular torsion reflex.

Adaptation, Ocular↗
PubMed2002

Cerebellar LTD facilitates but is not essential for long-term adaptation of the vestibulo-ocular reflex.

The induction of cerebellar long-term depression (LTD) at the parallel fibre-Purkinje cell synapse is selectively blocked in L7-PKCi transgenic mice, rendering these mice unable to adaptively modify their vestibulo-ocular reflex (VOR) during visuo-vestibular training for a few hours. Despite this deficit, their eye movement performance as well as their general motor behaviour appears unaffected. This combination suggests that, in the long term, residual forms of plasticity in the vestibulo-cerebellar circuitry can compensate for the absence of cerebellar LTD. To investigate whether LTD-deficient mice exhibit motor learning in the long run, we subjected L7-PKCi transgenic mice to visuo-vestibular training paradigms that were aimed at either increasing or decreasing the VOR response in the course of eight consecutive days. During the increasing paradigm, the VOR gain of transgenic mice increased significantly, while VOR gain decreased and VOR phase-lead increased during the decreasing paradigm. The impact of these long training periods on the VOR was significantly smaller in LTD-deficient mice than in wild type littermates. Thus, while LTD may be necessary for short-term VOR adaptation, it facilitates but is not required for long-term adaptation of the VOR.

Adaptation, Physiological↗
PubMed1996

Changes in sensitivity of vestibular nucleus neurons induced by cross-axis adaptation of the vestibulo-ocular reflex in the cat.

In alert, chronically-prepared cats, we studied response characteristics of well-isolated vestibular nucleus neurons (n = 9) while pairing yaw rotation with a pitch optokinetic stimulus, resulting in cross-axis adaptation of the horizontal vestibulo-ocular reflex. Each neuron's sensitivity to whole body rotation in a variety of axes in three-dimensional space was determined. When tested in darkness following adaptation, neurons showed statistically significant increases in sensitivity to yaw but not vertical plane rotations, suggesting participation in reflex plasticity.

Animals↗
PubMed1987

Adaptation of the resting states of accommodation.

Without an adequate stimulus to focus, the eyes accommodate for a "resting state" that averages about 1.6 diopter (D) (62 cm). The resting state can be made more "myopic" by adaptation to a near accommodative or vergence stimulus. In the present experiment, 21 subjects were tested for adaptation to both types of stimuli. Resting state was measured before and after adaptation in the dark (dark focus) and in the presence of an illuminated empty field. The authors found that preadaptation resting states were more myopic in the light field than in the dark test conditions; accommodative and vergence stimuli produce myopic shifts in dark and light field resting states; and a subset of the subjects show much larger aftereffects in the light field conditions. These subjects also show the largest difference between preadaptation dark- and light-field measures. Differences between dark- and light-field measures of resting state in these and other experiments may require a re-examination of the hypothesis that there is a single resting state for each subject.

Accommodation, Ocular↗
PubMed1986

Short-latency ocular following responses of monkey. III. Plasticity.

The adaptability of the ocular following responses elicited by brief movements of the visual scene was studied in five rhesus monkeys. Adapting stimuli were double-ramp sequences, designed to initiate ocular following and then induce consistent visual errors that would challenge any adaptive mechanism regulating the performance of such tracking. Both ramps lasted 150 ms and could differ from one another in speed (speed steps) or direction (direction steps), depending on the experiment. Monkeys were repeatedly exposed to these adapting stimuli over a period of 3 days with overnight rest periods in darkness. Ocular following responses were calibrated with 100-ms test ramps that covered a range of speeds (10-100 degrees/s) and directions (right, left, up, down) and were applied 50 ms after spontaneous saccades. Repeated exposure to speed steps involving abrupt increases or decreases in speed resulted in clear increases or decreases, respectively, in the ocular following responses elicited by the standard test ramps. Changes were greatest during the first part of the day, with partial recovery during the overnight periods. Changes were specific to the direction and, to a lesser extent, the speed of the adapting stimuli. Repeated exposure to direction steps involving abrupt 90 degrees counterclockwise changes in the direction of movement resulted in the emergence of a counterclockwise orthogonal component of ocular following. Orthogonal responses differed from isogonal ones in having smoother response profiles and in showing no overnight recovery. All of the recorded changes were adaptive insofar as they would result in improved tracking of the second ramp in the double-ramp sequence. It is concluded that the ocular following responses of the monkey are subject to extensive visually mediated adaptive regulation that influences both the magnitude and direction of tracking.

Adaptation, Ocular↗
PubMed2000

Calcium transients in the rhabdomeres of dark- and light-adapted fly photoreceptor cells.

The light response of fly photoreceptor cells is modulated by changes in free Ca(2+) concentration. Fly phototransduction and most processes regulating it take place in or very close to the rhabdomere. We therefore measured the kinetics and the absolute values of the free Ca(2+) concentration in the rhabdomere of fly photoreceptor cells in vivo by making use of the natural optics of the fly's eye. We show that Ca(2+) flowing into the rhabdomere after light stimulation of dark-adapted cells causes fast Ca(2+) transients that reach peak values higher than 200 microM in <20 msec. Approximately 500 msec later, the free Ca(2+) concentration has declined again to approximately 20 microM. The duration of the Ca(2+) transients becomes still shorter, and their size reduced, when the photoreceptor cell is light-adapted. This reduction in duration and size of the Ca(2+) transients is graded with the intensity of the adapting light. The kinetics and absolute values of the free calcium concentration found to occur in the rhabdomere are suitable to mediate the fast feedback signals known to act on the fly phototransduction cascade.

Adaptation, Ocular↗