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PubMed1976

Ontogenetic shift of spectral phototactic preferences in anuran tadpoles.

Tadpoles of three species of anurans initially had a midspectrum ("green") preference in laboratory phototactic tests, which was shown experimentally to involve a form of true color vision in one species and probably in the other two as well. During development, the preference shifted to shorter wave-lengths (higher frequencies) until a short-wavelength ("blue") preference predominated in the pre- and postmetamorphic stages and in the adults of six species tested; color vision was involved in all of these stages. The green preference of young tadpoles is ecologically adaptive, in that it directs larvae to green plants that provide food or shelter. Tadpoles observed in a pond congregated heavily in vegetated areas rather than in open water. Spectroradiometric field measurements showed that pond illumination in vegetated areas had a more highly saturated yellow-green spectral dominance compared with a desaturated white illumination in open water. During all ontogenetic stages and as adults, the animals had a preference for high illuminance of white light, which correlates with the high illumination of their habitat. Microspectrophotometric data from Liebman and Entine suggested that the green rods are active receptors in tadpoles, making unlikely Muntz's hypothesis that the ontogenetic shift in spectral preferences is due to premetamorphic maturation of these receptors. However, the visual pigments of all five types of photoreceptors shifted from vitamin A2- to vitamin A1-based chromophores during ontogeny, and the resulting shift in spectral response of the receptors might be related to the spectral shift in phototactic preferences.

Adaptation, Ocular↗
PubMed1998

Prism adaptation to a rightward optical deviation rehabilitates left hemispatial neglect.

A large proportion of right-hemisphere stroke patients show hemispatial neglect-a neurological deficit of perception, attention, representation, and/or performing actions within their left-sided space, inducing many functional debilitating effects on everyday life, and responsible for poor functional recovery and ability to benefit from treatment. The frequent parietal locus of the lesion producing neglect reflects the impairment of coordinate transformation used by the nervous system to represent extrapersonal space. Given that adaptation to a visual distortion can provide an efficient way to stimulate neural structures responsible for the transformation of sensorimotor coordinates, the aim of our study was to investigate the effect of prism adaptation on various neglect symptoms, including the pathological shift of the subjective midline to the right. All patients exposed to the optical shift of the visual field to the right were improved on their manual body-midline demonstration and on classical neuropsychological tests. Unlike other physiological manipulations used to improve neglect, this improvement lasted for at least two hours after prism removal and thus could be useful in rehabilitation programmes. The positive effect found for both sensorimotor and more cognitive spatial functions suggests that they share or depend on a common level of space representation linked to multisensory integration.

Adaptation, Ocular↗
PubMedDate not supplied

Does cortical motion adaptation exhibit functional properties analogous to light adaptation in the retina?

PURPOSE: The retina codes variations in luminance by adapting to and hence discounting, the mean luminance. During adaptation to a moving pattern, perceived speed decreases. Thus we know that the adapted visual system does not simply code the absolute speed of a stimulus. We hypothesize that adaptation to a moving stimulus serves to optimize coding of changes in speed at the expense of maintaining an accurate representation of absolute speed. In this case we would expect discrimination of speeds around the adapted level to be preserved or enhanced by motion adaptation. METHODS AND RESULTS: After adaptation to motion in the same direction as a subsequent test stimulus, seven of eight subjects showed a reduction of perceived speed in the adapted region and seven showed enhanced discrimination. CONCLUSIONS: We conclude that motion adaptation preserves or enhances differential speed sensitivity at the expense of an accurate representation of absolute speed in a manner analogous to retinal light adaptation.

Adaptation, Ocular↗
PubMed2000

Neuroanatomical correlates of the near response: voluntary modulation of accommodation/vergence in the human visual system.

This study identifies brain regions participating in the execution of eye movements for voluntary positive accommodation (VPA) during open-loop vergence conditions. Neuronal activity was estimated by measurement of changes in regional cerebral blood flow (rCBF) with positron emission tomography and 15O-water. Thirteen naive volunteers viewed a checkerboard pattern with their dominant right eye, while a lens interrupted the line of gaze during alternate 1.5 s intervals. Three counterbalanced tasks required central fixation and viewing of a stationary checkerboard pattern: (i) through a 0.0 diopter (D) lens; (ii) through a -5.0-D lens while avoiding volitional accommodation and permitting blur; and (iii) through a -5. 0-D lens while maintaining maximal focus. The latter required large-amplitude, high-frequency VPA. As an additional control, seven of the subjects viewed passively a digitally blurred checkerboard through a 0.0-D lens as above. Optometric measurements confirmed normal visual acuity and ability to perform the focusing task (VPA). Large-amplitude saccadic eye movements, verified absent by electro-oculography, were inhibited by central fixation. Image averaging across subjects demonstrated multifocal changes in rCBF during VPA: striate and extrastriate visual cortices; superior temporal cortices; and cerebellar cortex and vermis. Decreases in rCBF occurred in the lateral intraparietal area, prefrontal and frontal and/or supplementary eye fields. Analysis of regions of interest in the visual cortex showed systematic and appropriate task dependence of rCBF. Activations may reflect sensorimotor processing along the reflex arc of the accommodation system, while deactivations may indicate inhibition of systems participating in visual search.

Accommodation, Ocular↗
PubMed2003

Refractive error, cognitive demand and nearwork-induced transient myopia.

PURPOSE: Whereas many previous studies have identified the association between sustained near work and myopia, few have assessed the influence of concomitant levels of cognitive effort. This study investigates the effect of cognitive effort on near-work induced transient myopia (NITM). METHODS: Subjects comprised of six early onset myopes (EOM; mean age 23.7 yrs; mean onset 10.8 yrs), six late-onset myopes (LOM; mean age 23.2 yrs; mean onset 20.0 yrs) and six emmetropes (EMM; mean age 23.8 yrs). Dynamic, monocular, ocular accommodation was measured with the Shin-Nippon SRW-5000 autorefractor. Subjects engaged passively or actively in a 5 minute arithmetic sum checking task presented monocularly on an LCD monitor via a Badal optical system. In all conditions the task was initially located at near (4.50 D) and immediately following the task instantaneously changed to far (0.00 D) for a further 5 minutes. The combinations of active (A) and passive (P) cognition were randomly allocated as P:P; A:P; A:A; P:A. RESULTS: For the initial near task, LOMs were shown to have a significantly less accurate accommodative response than either EOMs or EMMs (p < 0.001). For the far task, post hoc analyses for refraction identified EOMs as demonstrating significant NITM compared to LOMs (p < 0.05), who in turn showed greater NITM than EMMs (p < 0.001). The data show that for EOMs the level of cognitive activity operating during the near and far tasks determines the persistence of NITM; persistence being maximal when active cognition at near is followed by passive cognition at far. CONCLUSIONS: Compared with EMMs, EOMs and LOMs are particularly susceptible to NITM such that sustained near vision reduces subsequent accommodative accuracy for far vision. It is speculated that the marked NITM found in EOM may be a consequence of the crystalline lens thinning shown to be a developmental feature of EOM. Whereas the role of small amounts of retinal defocus in myopigenesis remains equivocal, the results show that account needs to be taken of cognitive demand in assessing phenomena such as NITM.

Accommodation, Ocular↗
PubMedDate not supplied

A review of age changes in perceptual information processing ability with regard to driving.

Age related changes in the sensory modalities of hearing and vision, along with changes in the information processing abilities of selective attention, perceptual style, and perceptual-motor reaction time, were reviewed in the context of driving behavior. Literature reported, indicated age related changes in these abilities have relevance for the understanding of the driving behavior of the older adult.

Accommodation, Ocular↗
PubMed2004

Developmental regulation of calcium-dependent feedback in Xenopus rods.

The kinetics of activation and inactivation in the phototransduction pathway of developing Xenopus rods were studied. The gain of the activation steps in transduction (amplification) increased and photoresponses became more rapid as the rods matured from the larval to the adult stage. The time to peak was significantly shorter in adults (1.3 s) than tadpoles (2 s). Moreover, adult rods recovered twice as fast from saturating flashes than did larval rods without changes of the dominant time constant (2.5 s). Guanylate cyclase (GC) activity, determined using IBMX steps, increased in adult rods from approximately 1.1 s(-1) to 3.7 s(-1) 5 s after a saturating flash delivering 6,000 photoisomerizations. In larval rods, it increased from 1.8 s(-1) to 4.0 s(-1) 9 s after an equivalent flash. However, the ratio of amplification to the measured dark phosphodiesterase activity was constant. Guanylate cyclase-activating protein (GCAP1) levels and normalized Na+/Ca2+, K+ exchanger currents were increased in adults compared with tadpoles. Together, these results are consistent with the acceleration of the recovery phase in adult rods via developmental regulation of calcium homeostasis. Despite these large changes, the single photon response amplitude was approximately 0.6 pA throughout development. Reduction of calcium feedback with BAPTA increased adult single photon response amplitudes threefold and reduced its cutoff frequency to that observed with tadpole rods. Linear mathematical modeling suggests that calcium-dependent feedback can account for the observed differences in the power spectra of larval and adult rods. We conclude that larval Xenopus maximize sensitivity at the expense of slower response kinetics while adults maximize response kinetics at the expense of sensitivity.

Adaptation, Ocular↗
PubMed1975

Marked accommodation, retinal stretch, monocular space perception and retinal receptor orientation.

In recent years we have come to recognize that retinal receptors are probably aligned with the center of the exit pupil of the eye. It becomes crucial to define those factors influencing and determining photoreceptor alignment. Here, the effect of retinal stretch associated with marked accommodation has been considered. Assuming that a sense of direction is associated with a given retinal locus, we have used a classical monocular bisection test technique in order to see if marked accommodation altered bisection settings. Substantial changes in bisection settings were found when testing in the region of the posterior pole. These could be localized most probably in the retina and choroid. In separate studies, the effect of anterior retinal stretch upon cone receptor orientation in the region of the fovea was evaluated. Marked accommodation caused a transient nasal displacement of the peak of the Stiles-Crawford directional sensitivity function. These studies suggest that the posterior pole of the eye is routinely subject to stresses and strains. This point is of clinical significance. Our studies suggest that the presence of the blind spot influences the magnitude of change of measured parameters. We have only begun the important task of evaluating the nature of recovery processes following marked accommodation. Lastly, in order to obtain the necessary responses, we have had to train our young observers to voluntarily utilize nearly their entire accommodative capability.

Accommodation, Ocular↗
PubMed1987

Analysis of binocular visual function using tests made under binocular conditions.

Accommodation and vergence have a complex relation which occasionally breaks down, resulting in a loss of visual efficiency along with symptoms of discomfort associated with use of the eyes. Studies of accommodation/vergence interactions and tonic vergence disorders indicate that, using classical analysis techniques, separate methods are frequently necessary to determine whether existing binocular deficiencies are causing reported symptoms. The problem with current systems of binocular visual function analysis is that the vergence error which exists under binocular conditions is often not the same as that which is measured under monocular conditions. A rationale for, and technique of, analyzing binocular function using results of tests made under binocular conditions is described. This analysis incorporates the concepts of CA/C, proximal vergence (PV), and fixation disparity along with several accommodative measures (facility, lag, sustaining ability, and accuracy). By identifying relevant binocular components and the interrelations, the clinician should be better able to assess the contribution of each and examine which may be modified most easily by vision therapy, lenses, and/or prism intervention.

Accommodation, Ocular↗
PubMed2000

Contrast is enhanced by yellow lenses because of selective reduction of short-wavelength light.

PURPOSE: Although many studies have shown a subjective preference for yellow lenses, there has been little success in determining the clinical nature of this benefit. METHOD: Contrast sensitivity, color vision, accommodative-convergence, and visual acuity were measured in a group of 20 young subjects along with subjective rating of their perception through clear control lenses (380-nm cut-off), yellow lenses (450-nm cut-off), dark yellow lenses (511-nm cut-off), and orange lenses (527-nm cut-off). RESULTS: A systematic detriment to color vision was found to occur with increasing cut-off wavelength of the yellow lenses (p < 0.001) and this was significantly correlated to subjective ratings of color (r = -0.66) and brightness (r = -0.34). Perceived brightness significantly improved for the yellow (450-nm cut-off) lens only (p < 0.001). Although tinted lenses reduced contrast sensitivity to a white on black grating, there was a significant improvement in low to midrange spatial frequencies when measured using a white-on-blue grating. CONCLUSIONS: The detriment in color vision caused by yellow-colored lenses enhances contrast when viewing bright objects against a blue-based background, such as the sky. Contrast of overlying objects is enhanced is due to the selective reduction of short-wavelength light by the yellow lenses.

Accommodation, Ocular↗
PubMed2004

Step response of mouse rod photoreceptors modeled in terms of elemental photic signals.

The process of light adaptation in rod photoreceptors enables these sensory cells of the retina to remain responsive to photic stimuli over a broad range of light intensity. Recent studies have employed the technique of paired-flash electroretinography to determine properties of phototransduction, and of light and dark adaptation, in rod photoreceptors in the living eye. Building on these studies, we have developed a theoretical model aimed at explaining the rod electrical response to a step of light based on known physiology. The central feature of the model is its description of the macroscopic (i.e., measured) response in terms of a time-evolving, weighted sum of elemental responses determined under dark-adapted and near fully light-adapted conditions. The model yields a time-dependent function that describes the course of desensitization and putatively represents the cumulative dynamics of underlying biochemical processes involved in light adaptation of the rod.

Action Potentials↗
PubMed2004

Functional neuroanatomy of the human near/far response to blur cues: eye-lens accommodation/vergence to point targets varying in depth.

The purpose of this study was to identify the networks involved in the regulation of visual accommodation/vergence by contrasting the cortical functions subservient to eye-lens accommodation with those evoked by foveal fixation. Neural activity was assessed in normal volunteers by changes in rCBF measured with PET. Thirteen right-handed subjects participated in three monocular tasks: (i) resting with eyes closed; (ii) sustained foveal fixation upon a LED at 1.2 m (0.83 D); and (iii) accommodating alternately on a near (24 cm, 4.16 D) vs. a far (3.0 m, 0.33 D) LED alternately illuminated in sequential 2 s epochs. The contrast between the conditions of near/far accommodation and of constant foveal fixation revealed activation in cerebellar hemispheres and vermis; middle and inferior temporal cortex (BA 20, 21, 37); striate cortex and associative visual areas (BA 17/18). Comparison of the condition of constant fixation with the condition of resting with closed eyes indicated activation of cerebellar hemispheres and vermis; visual cortices (BA 17/18); a right hemisphere dominant network encompassing prefrontal (BA 6, 9, 47), superior parietal (BA 7), and superior temporal (BA 40) cortices; and bilateral thalamus. The contrast between the conditions of near/far accommodation with closed-eye rest reflected an incremental summation of the activations found in the previous comparisons (i.e. activations associated with constant fixation). Neural circuits activated selectively during the near/far response to blur cues over those during constant visual fixation, occupy posterior structures that include occipital visual regions, cerebellar hemispheres and vermis, and temporal cortex.

Accommodation, Ocular↗
PubMed1993

Temporal characteristics of proximally-induced accommodation.

This study sought to determine whether proximally-induced accommodation (PIA) is capable of sustaining an accommodative response over time. Accordingly, the steady-state PIA response was measured during the course of a continuous 5 min stimulus period for targets located at distances of 0.2 or 6 m. The vergence and accommodation loops were opened by subjects (n = 8) monocularly viewing the targets through a 0.5 mm pinhole, while accommodation was assessed using an objective, open-field, infrared optometer. The mean PIA response did not change significantly during the fixation period. This finding suggests that PIA is indeed capable of maintaining a sustained response. This observation is consistent with the constancy of apparent target distance perception during extended fixation of a stationary object of regard under these conditions.

Accommodation, Ocular↗
PubMed1993

Effect of target luminance on microfluctuations of accommodation.

It is feasible that small temporal variations in steady-state accommodation may provide feedback to the accommodation control system through changes in retinal image contrast and that this feedback may be used to maintain an optimal accommodation response. The complex waveform of microfluctuations is dominated by two distinct regions of activity; a low frequency component (LFC < 0.6 Hz) and a high frequency component (1.0 < or = HFC < or = 2.3 Hz). Whereas the HFCs appear to be correlated with some intraocular manifestation of arterial pulse the contribution of the LFCs to the control of steady-state accommodation is unclear. The present study investigates the effect of target luminance on the waveform of accommodative microfluctuations. Three young emmetropic observers viewed monocularly a high contrast (90%) Maltese cross target placed at a vergence equal to their dark-focus level of accommodation in a Badal stimulus system. The luminance of the target was varied from 0.002 to 11.63 cd m-2 in nine equal logarithmic steps. Five continuous accommodation signals were collected for each viewing condition at a sampling rate of 102.4 Hz, and average power spectra subsequently calculated with a frequency resolution of 0.1 Hz. One-way ANOVA revealed a significant variation in the root-mean-square (r.m.s.) value of the microfluctuations (F = 19.795, d.f. 124, P = 0.0001) which could be attributed mainly to increases in the r.m.s. value for the two lowest luminances (0.002 and 0.004 cd m-2). Power spectrum analysis revealed that these changes in the microfluctuations could be attributed to increases of power in the LFC.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗