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PubMed2003

Effects of reversible shutdown of the monkey flocculus on the retention of adaptation of the horizontal vestibulo-ocular reflex.

There are two different proposals regarding the role of the cerebellar flocculus in the adaptation of the vestibulo-ocular reflex: that the flocculus is the site for both the induction and retention of the adaptation, or that the flocculus plays an important role in the induction, but the vestibular nuclei to which the flocculus issues its efferents are the site of retention. To locate the memory trace for the adaptation of the vestibulo-ocular reflex, we determined effects of acute pharmacological shutdown of the bilateral cerebellar flocculi in four Macaca fuscata. The gain of the horizontal vestibulo-ocular reflex was measured by sinusoidal oscillation of the turntable by 10 degrees (peak-to-peak) at 0.33 Hz in the dark. Two hours of 0.33 Hz-10 degrees sinusoidal oscillation of the turntable while viewing the stationary checked-pattern screen through a x2.2 magnifying lens adaptively increased the horizontal vestibulo-ocular reflex gain by 0.2 on the average. The gain increase lasted for 1 h when the monkeys were left with their heads fixed in darkness, but it disappeared within 24 h after the monkeys were returned to their cages where they had free movements. The effects of injections of 5 or 10% lidocaine chloride into the bilateral floccular areas (8 microl for each side) immediately after adaptation occurred were determined in nine sessions. The gain of the horizontal vestibulo-ocular reflex immediately decreased to the level before the adaptation. These effects of lidocaine lasted for at least 1 h. On the contrary, injections of the same amount of Ringer's solution, tested in eight sessions, hardly affected gain of the horizontal vestibulo-ocular reflex over 1-h period. These depressant effects of lidocaine injections were specific to the gains increased by adaptation. These results suggest that the memory trace for the short-term adaptation of the horizontal vestibulo-ocular reflex gain induced by 2 h of sustained visual-vestibular interactions resides within the flocculus.

Adaptation, Physiological↗
PubMed1971

Dynamics of excitation and inhibition in the light-adapted Limulus eye in situ.

The dynamics of spike discharge in eccentric cell axons from the in situ lateral eye of Limulus, under small sinusoidal modulation of light to which the eye is adapted, are described over two decades of light intensity and nearly three decades of frequency. Steady-state lateral inhibition coefficients, derived from the very low-frequency response, average 0.04 at three interommatidial spacings. The gain vs. frequency of a singly illuminated ommatidium is described closely from 0.004 to 0.4 cps by the linear transfer function s(0.25); this function also accounts approximately for the measured phase leads, the small signal adaptation following small step inputs, and for Pinter's (1966) earlier low-frequency generator potential data. We suggest that such dynamics could arise from a summation in the generator potential of distributed intensity-dependent relaxation processes along the dendrite and rhabdome. Analysis of the dynamic responses of an eccentric cell with and without simultaneously modulated illumination of particular neighbors indicates an effect equivalent to self-inhibition acting via a first-order low-pass filter with time constant 0.42 sec, and steady-state gain near 4.0. The corresponding filters for lateral inhibition required time constants from 0.35 to 1 sec and effective finite delay of 50-90 msec.

Action Potentials↗
PubMed1991

Optokinetic response and adaptation of the vestibulo-ocular reflex (VOR) in a patient with chronic cortical blindness.

Optokinetic response and adaptation of the VOR were investigated in a patient with chronic cortical blindness. Our results suggest that: 1) optokinetic response was present in a patient with cortical blindness. This optokinetic response may relate to the extrastriate pathways: 2) the occurrence of VOR adaptation was shown in a patient with cortical blindness, which may indicate preservation of the pathways of the visual-vestibular interaction in the brain stem and cerebellum. The result is in agreement with previous reports in experimental animals; 3) less adaptation of the VOR in this patient than in normal adults may relate to a chronic loss of visual feed back. To the best of our knowledge, this is the first report of adaptation of the VOR in a patient with cortical blindness.

Adaptation, Physiological↗
PubMed1969

The Purkinje shift.

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Adaptation, Ocular↗
PubMed1971

Spectral sensitivity of the barnacle, Balanus amphitrite.

The extracellular ocellar potential was used to evaluate the spectral sensitivity of the ocellus of the barnacle, Balanus amphitrite. Maximum relative sensitivity was at 530-540 nm. Studies with chromatic adapting lights suggest that the receptors contain a single photopigment. The spectra were relatively broader in the dark as compared to the light-adapted state. This effect was shown to be due to an increase in the slope of the amplitude-intensity function, caused by light adaptation. Studies of tapetal fluorescence and corneal transmission indicate little effect of the ocellar media on the determination of sensitivity.

Action Potentials↗
PubMed2005

Two realms of dark adaptation.

The recovery of rod responsiveness after saturating flashes is greatly retarded above a certain critical level of rhodopsin bleaching (approximately 0.1%). A mathematical description of the process of turn-off of the phototransduction cascade allows attributing different phases of the recovery to specific products of rhodopsin photolysis. The fast phase is determined by quenching of metarhodopsin II and activated transducin. The slow phase is controlled by decay of partially inactivated (phosphorylated and arrestin-bound) metarhodopsins, and by regeneration of rhodopsin. The transition between the two regimes of adaptation is rather abrupt, occurring within a few-fold range of stimulus intensity. This marks the border between reversal of light adaptation and dark adaptation, as it is commonly defined.

Adaptation, Ocular↗
PubMed1994

The simultaneous interocular brightness sense test. A test of optic nerve function.

OBJECTIVE: To describe and establish control data from controls for a simultaneous interocular brightness sense test in which the apparent brightness of two independently viewed light patches was compared. DESIGN: Dichoptic viewing was achieved by dissociation of right and left eyes using a septum and cross-polarization. Brightness matches were obtained by using adjustments to the illumination of the left retina while the illumination of the right retina was kept constant. OBSERVERS: Ninety-one control observers (age range, 20 to 91 years) participated in the study to provide normal brightness-matching data. RESULTS: Effects from ocular dominance, age, and long- and short-term adaptation were negligible, but the test was sensitive to anisocoria. Just noticeable differences in dichoptic brightness matching were proportional to retinal illuminance; this finding was consistent with the Weber-Fechner law. CONCLUSION: The test is quick and simple to complete and appears to offer considerable scope for the assessment of visual function when differences in the functional integrity of the optic nerves of the right and left eyes are suspected.

Adaptation, Ocular↗
PubMed2000

The role of steady phosphodiesterase activity in the kinetics and sensitivity of the light-adapted salamander rod photoresponse.

We investigated the kinetics and sensitivity of photocurrent responses of salamander rods, both in darkness and during adaptation to steady backgrounds producing 20-3,000 photoisomerizations per second, using suction pipet recordings. The most intense backgrounds suppressed 80% of the circulating dark current and decreased the flash sensitivity approximately 30-fold. To investigate the underlying transduction mechanism, we expressed the responses as a fraction of the steady level of cGMP-activated current recorded in the background. The fractional responses to flashes of any fixed intensity began rising along a common trajectory, regardless of background intensity. We interpret these invariant initial trajectories to indicate that, at these background intensities, light adaptation does not alter the gain of any of the amplifying steps of phototransduction. For subsaturating flashes of fixed intensity, the fractional responses obtained on backgrounds of different intensity were found to "peel off" from their common initial trajectory in a background-dependent manner: the more intense the background, the earlier the time of peeling off. This behavior is consistent with a background-induced reduction in the effective lifetime of at least one of the three major integrating steps in phototransduction; i.e., an acceleration of one or more of the following: (1) the inactivation of activated rhodopsin (R*); (2) the inactivation of activated phosphodiesterase (E*, representing the complex G(alpha)-PDE of phosphodiesterase with the transducin alpha-subunit); or (3) the hydrolysis of cGMP, with rate constant beta. Our measurements show that, over the range of background intensities we used, beta increased on average to approximately 20 times its dark-adapted value; and our theoretical analysis indicates that this increase in beta is the primary mechanism underlying the measured shortening of time-to-peak of the dim-flash response and the decrease in sensitivity of the fractional response.

Adaptation, Ocular↗
PubMedDate not supplied

Characteristics and variability of vertical phoria adaptation in normal adults.

We evaluated the characteristics of phoria adaptation for vertically induced retinal disparity. An adaptive change in the fusion-free ocular alignment, phoria adaptation, was measured with a computer-aided mirror haploscope at 10, 30, and 60 minutes after the start of wearing of a 3-prism-diopter base up prism by 35 normal subjects ranging in age from 21 to 67 years (mean: 37 years). The relationships between phoria adaptation and the subjects' age, the vertical fusional amplitude, the amount of heterophoria, and the starting time of the examination were evaluated. All subjects showed phoria adaptation, with the mean (+/- SD) degree of 0.78 +/- 0.28 degree, 0.96 +/- 0.26 degree and 1.02 +/- 0.30 degrees at 10, 30, and 60 minutes, respectively, after wearing the prism. The repeatability (95% confidence interval) for the measurements was less than +/- 0.24 degree. There was a significant correlation between the vertical fusional amplitude and the gain of phoria adaptation (at 10 and 60 minutes, P < 0.05). The gain of phoria adaptation measured at 60 minutes showed a significant decrease with age (P < 0.01). The results indicate that the time course of phoria adaptation in the vertical direction is similar to that in the horizontal direction and its gain differs considerably among subjects.

Adaptation, Ocular↗
PubMed1975

Normal values in clinical electrooculography. 1. Material, method, methodological investigations and distribution of the potential and time parameters.

For the evaluation of normal EOG potential and time parameters a case series (142 eyes) is presented. An EOG technique aiming at the largest possible amplitude between the dark trough and the light peak is described. It is shown that the light-induced response following a 20 min period of adaptation to a very low degree of illumination (about 1/10 lx, allowing a rough orientation on the displayed trace) does not differ from that produced after 20 min of total darkness. Also, the independence of the pupillary area for the light-induced response is demonstrated, which means that the light stimulus employed (2500-4000 lx) is supramaximal. Furthermore, the good quality of the gaze fixation in the actual test situation with respect to the recording of equidistant saccades is proven. A DC amplification of the signal is used, which secures independency of inter- and intra-individual differences in the saccadic velocity and makes possible accurate measurements of deflections disfigured by correcting movements. The following EOG parameters are recorded: A base value after 10 min of preadaptation with the light stimulus, the dark trough and the light peak potentials. Also the periods between the beginning of the dark adaptation and the dark trough and between the dark trough and the light peak are noted. The frequency distribution and the general level of the various parameters are discussed in the light of comparable figures of previous publications.

Adaptation, Ocular↗
PubMed1987

Optically-induced changes in tonic vergence and AC/A ratio in normal monkeys and monkeys with lesions of the flocculus and ventral paraflocculus.

Monkeys wore either base-out prisms to promote changes in tonic vergence or periscopic spectacles to promote changes in the coupling between accommodation and vergence. Eye movements were recorded using the magnetic search coil technique and the monkeys were rewarded for accurate fixation. Two normal monkeys and two monkeys which had previously received lesions of the flocculus and ventral paraflocculus were studied. After 30 min of prism viewing the two normal monkeys had elevated phoria--increased by approximately 50% of the prism stimulus. The two lesioned monkeys also had phoria increases of a similar size after prism viewing. The effect of duration of prism viewing on the magnitude and time course of phoria elevation was studied in one normal monkey. The initial magnitude of phoria elevation and the time constant of relaxation of phoria both increased as the duration of prism exposure varied from 5 s to 30 min. Initial phoria increased approximately in proportion to the logarithm of duration of prism exposure whereas the time constant of phoria relaxation increased linearly with duration. In the same normal monkey it was shown that increasing vergence by means of accommodative-vergence did not induce phoria changes. The effect of periscopic spectacle viewing was studied in all four monkeys. After 30 min of periscopic spectacle viewing all four monkeys had a higher AC/A ratio. The magnitude of the changes was (paradoxically) greater in the two lesioned monkeys than in the one normal monkey studied fully.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗