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PubMed1990

The size of the pool for bleaching adaptation in human rod vision.

We present new psychophysical estimates of the size of the rod pool for bleaching adaptation in the human retina. We estimate that at 5 deg nasal eccentricity in the human retina the size of the adaptation pool for rods is between 5 and 7.5 min arc. This estimate is compatible with the extent of the dendritic spread of rod bipolars located in this region of the primate retina and with the area occupied by roughly 50 rods in this parafoveal region of the human retina. Thus a candidate for the site of adaptation is the bipolar cell whose receptive field is comprised of approx. 50 rods. These estimates represents the lowest measurements to date of the size of the adaptation pool for rods.

Accommodation, Ocular↗
PubMed1990

The refractive structure and optical properties of the isolated crystalline lens of the cat.

Direct measurements of the shape and internal refractive index distribution of the isolated cat lens were used to construct individual optical models of the lenses from the left eyes of five cats. The right eyes were used in a companion study of the optics of the cat eye. The lens refractive index spatial distribution and dispersion were measured with a specially designed Pulfrich areal refractometer. Agreement of calculated ray paths through these models with the observed paths of laser beam fans incident parallel to, and at an oblique angle to the lens axis indicates that the models, which contain no freely adjustable parameters, are good physical and optical representations of the isolated (accommodated) crystalline lenses.

Accommodation, Ocular↗
PubMed1990

The frequency characteristics of accommodative microfluctuations for central and peripheral zones of the human crystalline lens.

Temporal variations in the nominally steady-state accommodation response can be characterized approximately by two principal regions of dominant activity: a low frequency region (LFR less than 0.6 Hz) and a high frequency region (HFR greater than 1.0 Hz). The observed changes of activity in the LFR induced by varying stimulus conditions has led to the proposal that this region may be utilized as an error detector in the accommodation feed-back loop. The role of fluctuations in the HFR is, however, equivocal particularly as several reports have shown their magnitude to be positively correlated with pupil size. The implication is that HFRs are a consequence of the mechanical and elastic properties of peripheral rather than central zones of the crystalline lens and hence do not contribute to the accommodation control process. The aim of the study was to compare the incidence and magnitude of microfluctuations in LFRs and HFRs for central and peripheral lens zones when viewing a near target. Accommodation was measured continuously with a modified Canon Autoref R1 infra-red optometer on five young emmetropic subjects. We show that although the total power of the microfluctuations is reduced in the periphery, the form of the power spectra is similar for central and peripheral zones. Thus rather than being spurious effects caused by the periphery of the lens HFRs are present for both central and peripheral lens zones.

Accommodation, Ocular↗
PubMed1990

The subjective sensitivity to small changes in the contrast of a suprathreshold grating.

Small periodic (0.25-40 Hz) changes in the contrast of suprathreshold, spatially sinusoidal (0.8-10.5 c/deg) grating test objects were generated on an oscilloscope screen. It was found that the minimum temporal modulation in spatial contrast which could be detected subjectively was approximately proportional to the mean contrast of the grating during the change, provided that the minimal spatial modulation of the grating was above threshold. The contrast increment threshold was dependent on both the grating spatial frequency and the temporal frequency of the modulation: for any fixed temporal frequency in the range 2-8 Hz it increased approximately linearly with spatial frequency; for fixed spatial frequency the threshold was minimal at temporal frequencies of a few Hz. The relevance of these results to the possible role of fast fluctuations in the control of accommodation response is considered.

Accommodation, Ocular↗
PubMed1991

Steady-state and dynamic response properties of the Mandelbaum effect.

The Mandelbaum effect refers to an inadvertent accommodation response to an intervening surface (e.g. a window screen) while attempting to focus a distant object of interest. Previous research has attributed the Mandelbaum effect to a tendency for the accommodation mechanism to preferentially focus the stimulus nearest the dark (resting) focus position. The present study extends this work by examining dynamic accommodation response properties for different distances and stimulus separations. Accommodation measurements of nine young, emmetropic subjects were obtained with an infrared optometer while they viewed superimposed horizontal and vertical square-wave gratings at various dioptric separations. Three subjects demonstrated a strong focussing preference for the target located nearest their dark focus position for all stimulus conditions, while two other subjects showed a similar, but weaker, preference. Conversely, two subjects primarily focussed the nearer of the two targets, and two subjects tended to focus the vertical target (regardless of whether it was near or far, or was closet to the dark focus). For all subjects, accommodation rarely fluctuated from one target to the other, and was seldom focussed midway between the two targets. Our findings indicate that the influence of the dark focus on the Mandelbaum effect varies among individuals, and plays little or no role in some individuals. Thus, accommodation responses to competing stimuli are more complex than previous findings would suggest.

Accommodation, Ocular↗
PubMed1991

Elasticity of tissues involved in accommodation.

Uniaxial loading of human lens, zonules, ciliary muscle, and choroid shows a nonlinear relationship between stress and deformation of the specimen, and hysteresis on unloading. Spring constants, at 10% elongation, have been calculated for all specimens and for several combinations. Except for zonules they are significantly correlated with age. They will provide a basis for a model of accommodation. The mean force required for 10% elongation of the lens, at mean age 43, is about 22 x greater than for the zonules; and the mean elongation of the zonules is nearly 7 x that of the lens at a load of 0.01 N (approximately 1 g). Ranges of forces and pressures in the accommodated and nonaccommodated eye are estimated.

Accommodation, Ocular↗
PubMed1991

Accommodation without pupillary constriction.

In this paper it will be shown that the association of pupillary contraction with even substantial accommodation breaks down under rigorous control of alignment. Conditions exclude other than "accommodative" (blur induced) pupillary responses by eliminating light reflexes due to temporal and spatial luminance differences, colour and polarization differences. The visual solid angles of the targets were also precisely equalized. A subtle procedure is described to obtain accurate alignment along a single line of sight of the stimulated eye. The pupillary system appears to be quite sensitive to alignment errors, but the "accommodative pupillary reflex" is totally absent under rigorous alignment. Pupillometry used an infrared television camera to monitor the non-stimulated eye in total darkness and silence. Occurrence of correct accommodation for every stimulus event was confirmed by observing accommodative vergence. Judging by the size of the accommodative vergence response and, occasionally, the size of Purkinje-Sanson images, the accommodative response was found to be essentially equal to the stimulation.

Accommodation, Ocular↗
PubMed1992

Barn owls have symmetrical accommodation in both eyes, but independent pupillary responses to light.

We have studied accommodation behaviour in the barn owl (Tyto alba). By defocussing one eye with various spectacle lenses and recording the refractive state in both eyes continuously during pecking, we found that the owls' accommodation was symmetrical in both eyes, with no regard to the power of the lens used. Even with no visual input to one eye, the amount of accommodation was always identical in both eyes. On the other hand, pupillary responses to light were independent. This finding differs from an earlier observation in the chicken, where both accommodation and pupillary responses were found to be independent. The result is discussed with regard to current knowledge on the central pathways for control of accommodation and pupillary responses in birds.

Accommodation, Ocular↗
PubMed1992

The influence of chromatic aberration on the static accommodative response.

Previous measurements of static accommodation have consistently shown steady state errors over most of the range; the response lags below the stimulus and, at low levels, the response leads the stimulus. A series of experiments is presented in which the longitudinal and, for the first time, transverse chromatic aberrations of the eye were varied and the resultant stimulus-response functions of accommodation were measured. The results show that the steady state error of accommodation is not influenced by manipulations of the magnitude or the direction of either longitudinal or transverse chromatic aberration. This indicates that a particular wavelength is not preferentially focussed on the retina as a function of stimulus level and supports the negative feedback theory of accommodation.

Accommodation, Ocular↗
PubMed1992

Accommodative hysteresis as a function of target--dark focus separation.

The concept of dark focus (DF) as fulcrum for accommodative effort was examined by inducing adaptation of tonic accommodation with the target at varying dioptric distances from the DF. Mean pre- to post-changes in DF at the initial post fixation test essentially were linear (r2 = 0.975), with zero intercept, with respect to the target-DF separation, as were the data of most individual subjects. Target-DF distance thus is a major determinant of adaptation of tonic accommodation. These results are consistent with the interpretation of DF as a zero reference point or fulcrum about which accommodative effort varies.

Accommodation, Ocular↗
PubMed1992

Absence of pupil response to blur-driven accommodation.

The drive to the pupil constriction associated with near fixation has generally been attributed to accommodation with convergence and fusional convergence having secondary roles. However, our previous investigations have shown that significant changes in accommodation can take place without concomitant pupil response. To investigate further, the present study recorded pupil and accommodation responses to a blur-only accommodative stimulus using a target moved sinusoidally at a range of temporal frequencies. Care was taken to minimise target size change and apparent lateral or vertical target displacement. Results show that pupil response could be very much reduced or absent irrespective of stimulus temporal frequency and despite maintained accommodation response. The results suggest that blur-driven accommodation alone is not sufficient to drive pupil near response and that the presence of cues such as size change and lateral or vertical displacement of an approaching object may be necessary to elicit a response.

Accommodation, Ocular↗
PubMed1992

Detection of eyeshine by flashlight fishes of the family Anomalopidae.

Flashlight fishes of the family Anomalopidae live in clear tropical waters and are nocturnally active. They have luminescent organs located just below the pupils of their eyes. The relation of the luminescent organ to the pupil of the anomalopid eye is similar to that of the illumination and pupil system of the ophthalmoscope and identical to that of some photoretinoscopes. Indeed, one species of flashlight fish, Anomalops katoptron, actually moves its luminous organ away from its pupils in the process of occluding its light organ, making a retinoscopic-like movement. By photographing the eyeshine of a number of fishes with a photoretinoscope and by analyzing the optics of light organs of fish of the family Anomalopidae as well as the optics of reflecting eyes, we show under what light conditions and ranges flashlight fishes may reasonably be able to detect eyeshine from other fishes in the environment. Further, we suggest that flashlight fishes may be able to communicate with each other by altering the accommodation of their eyes. In such a communication system, the sender radiates no energy and communicates only with the interrogating receiver of the information. To our knowledge, this utilization of eyeshine, both for detection and for communication, is unique in the animal kingdom.

Accommodation, Ocular↗
PubMed1993

Accommodative microfluctuations and pupil diameter.

The nominally steady-state accommodation response exhibits temporal variations which can be characterized by two dominant regions of activity; a low frequency component (LFC < 0.6 Hz) and a high frequency component (1.0 < or = HFC < or = 2.1 Hz). There is no consensus as to the relative contribution made by each of the frequency components of the microfluctuations to the control of steady-state accommodation. We investigate the effect of variations in artificial pupil diameter (0.5, 1, 2, 3, 4 and 5 mm pupils) on the microfluctuations of accommodation, while three young emmetropic subjects view, monocularly, a photopic high contrast Maltese cross target placed at a dioptric distance equal to their open-loop accommodation level. Average power spectra were calculated for five accommodation signals, each of 10 sec duration, collected for each viewing condition at a sampling rate of 102.4 Hz using a continuously recording infrared objective optometer. For artificial pupil diameters < or = 2 mm the power of the LFC was found to increase as a function of reducing pupil diameter, while for artificial pupil diameters > 2 mm the LFC was found to be relatively constant. No systematic change in the HFC with varying artificial pupil diameter was observed. Changes in the root-mean-square (r.m.s.) value of the fluctuations with varying pupil diameter were significant (one-way ANOVA, F = 8.507, P = 0.0001, d.f. = 89) and showed a similar form to the changes in the LFC.(ABSTRACT TRUNCATED AT 250 WORDS)

Accommodation, Ocular↗
PubMed1993

Chromatic aberration and accommodation: their role in emmetropization in the chick.

The roles of chromatic aberration and accommodation as cues to emmetropization in the chick were investigated. Myopia was induced monocularly by lid suture for a period of 1-2 weeks from hatching, after which eyes were reopened and the recovery process followed. Monochromatic light (ML) rearing conditions and ciliary nerve section surgery were used to eliminate chromatic aberration and accommodative activity respectively. Control animals were reared in white light (WL). When accommodation was left intact, chickens reared under monochromatic light were able to recover normally. However, ciliary nerve section produced hyperopia, deepening of the anterior chamber and a tendency towards axial lens thinning, irrespective of the light conditions used. Hyperopic refractive errors peaked at 4 weeks (mean refractive errors: +5.7 D, +4.21 D for ML, WL groups respectively, 4 weeks), with the ML group still exhibiting significant hyperopia at 7 weeks. Ciliary nerve section did not prevent the myopic response to lid suture (mean refractive errors: -22.65 D; -25 D for ML, WL groups respectively, 1 week) nor the elimination of myopia when eyes were reopened. These data indicate that neither accommodation nor chromatic aberration are fundamental to the gross operation of the emmetropization process although they may be essential for the fine tuning of refraction.

Accommodation, Ocular↗
PubMedDate not supplied

Application of a simple mechanical model of accommodation to the aging eye.

A simple model for the mechanics of accommodation in the human eye, previously proposed, is applied to the eye at different ages. Data from work by R. F. Fisher is used to derive values for the elasticity of lens-plus-zonules and data from work by G. W. H. M. van Alphen and W. Graebel is used to derive values for the elasticity of the choroid, each at a series of ages. The model is applied in two ways: (i) the elasticity of one component is allowed to change with age, while the other (and also ciliary muscle force) is held constant and the effect on accommodative amplitude is estimated; (ii) both elasticities are allowed to change with age, and muscle force is assumed to take on values producing a constant change in the radius of the "ciliary ring". It is shown that, in principle, age-related changes in choroid could account for a substantial component of the observed loss of accommodative amplitude, and it is suggested that this may be the case for monkeys, but not for humans. For the case of "all components changing", which is probably the case for humans, the ciliary muscle is likely to become stronger with age.

Accommodation, Ocular↗
PubMed1995

Fourth Purkinje image signals reveal eye-lens deviations and retinal image distortions during saccades.

Saccadic eye movements of various sizes and directions were registered simultaneously with the scleral search coil and a fifth-generation Dual Purkinje image eyetracker. Comparison of the search coil and the Purkinje image tracker records reveal considerable dynamic deviations during and immediately after the saccade, which we ascribe to the movements of the eye lens relative to the optical axis of the eye. Due to the increased stiffness of the tissues these deviations are smaller in older subjects. Also, they are larger at near accommodation. The size of the retinal image displacement which results from the lens movement proper can be as large as 0.5 deg, which may not be negligible in a number of visual tasks.

Accommodation, Ocular↗
PubMed1995

The effect of perceived distance on accommodation under binocular steady-state conditions.

Spatiotopic cues, such as perceived distance, have little effect on accommodation unless blur has been reduced or eliminated. We investigated the effect of perceived distance on accommodation under binocular steady-state conditions, about which little is known. Blur was reduced but not eliminated by using a stimulus with a moderately low luminance. Accommodation was measured under two conditions: (1) when cues from perceived distance, blur, and convergence were aligned; and (2) when perceived distance was opposed by both blur and convergence. We found a significant difference in accommodation between the two conditions, which we attribute to perceived distance.

Accommodation, Ocular↗
PubMed1995

Accommodation in humans with juvenile macular degeneration.

Accommodation, or the change in refractive power of the eye to focus objects at different distances, is driven by many stimuli including defocus blur, the awareness of target distance or proximal cues, and through the vergence crosslink (convergence accommodation). The effectiveness of defocus blur as an accommodative stimulus is decreased in normally-sighted subjects as visual acuity is experimentally reduced and as the target is imaged at increasing eccentricities from the fovea. Since subjects with central retinal abnormalities have reduced visual acuity and typically fixate eccentrically, one would predict that defocus blur would not be an effective accommodative stimulus for them. Using an infrared optometer, steady-state accommodative responses of six subjects with juvenile macular degeneration (JMD) and of three normally-sighted controls were measured. The effectiveness of defocus blur in stimulating accommodation varied across the subjects and was related to visual acuity, with those subjects having worse acuity showing less accurate accommodative responses. When provided with additional cues to accommodative demand (i.e. proximal and/or binocular cues), subjects with JMD showed more accurate accommodative responses. In general, those subjects who did not modulate accommodative response with changing defocus blur cues showed the most accurate accommodation under binocular viewing. In contrast, those subjects who did change accommodative response with changing defocus blur cues showed the most accurate accommodation under monocular viewing.

Accommodation, Ocular↗