Shift in the peak of the photopic Stiles-Crawford function with marked accomodation

Subjective measurement of the Stiles-Crawford effect of the first kind with variation in accommodation

PURPOSE

To measure the Stiles-Crawford effect of the first kind (SCE-I), corresponding to central vision, with innovative technology to evaluate changes in the directionality and photoreceptor alignment with accommodation.

METHODS

A uniaxial Maxwellian system (spot size in pupil 0.5 mm diameter) was employed, incorporating a spatial light modulator to flicker at 2 Hz between two 2.3° fields corresponding to test (peripheral pupil) and reference (pupil centre) positions. Participants determined thresholds at 13 positions along the horizontal pupil meridian by indicating if the test field was brighter or dimmer than the reference field. Thresholds were determined by a staircase procedure after four reversals at each pupil location. After pupil dilation, seven emmetropes were tested at 0 D to 6 D accommodation stimulus levels in 2 D intervals. Data were fit by the Gaussian function, both when the fits were unforced or forced to pass through the sensitivity expected for the reference point. Directionality (ρ) and peak location values ( x max ) were determined for unforced and forced fits.

RESULTS

Regression slopes for ρ as a function of accommodation stimulus were not significant. There was a tendency for x max to shift temporally with increasing accommodation across the 6 D stimulus range. This was not significant for regression fitting (-0.059 mm/D, R²  = 0.06, p = 0.20), but a paired t-test for 0 and 6 D stimuli showed a weakly significant change of 0.62 mm (p = 0.05). The differences between the two fitting approaches were small and non-significant.

CONCLUSIONS

Directionality did not change with accommodation, but the pupil peak location showed a significant temporal shift of approximately 0.62 mm with 6 D accommodation stimulus. It is possible that substantial changes in the directionality and a shift in the direction of peak location might occur at very high levels of accommodation.

Does transient increase in axial length during accommodation attenuate with age?

Background

The aim was to profile transient accommodative axial length changes from early adulthood to advanced presbyopia and to determine whether any differences exist between the responses of myopic and emmetropic individuals.

Methods

Ocular biometry was measured by the LenStar biometer (Haag‐Streit, Switzerland) in response to zero, 3.00 and 4.50 D accommodative stimuli in 35 emmetropes and 37 myopes, aged 18 to 60 years. All results were corrected to reduce errors arising from the increase in crystalline lens thickness with accommodation. Accommodative responses were measured sequentially by the WAM 5500 Auto Ref/Keratometer (Grand Seiko, Hiroshima, Japan).

Results

Axial length increased significantly with accommodation (p < 0.001), with a mean corrected increase in axial length of 2 ± 18 µm and 8 ± 16 µm observed at 3.00 and 4.50 D, respectively. The magnitude of accommodative change in axial length was not dependent on refractive error classification (p = 0.959); however, a significant reduction in the magnitude and variance of axial length change was evident after 43 to 44 years of age (p < 0.002).

Conclusion

The negative association between transient increase in axial length and age, in combination with reduced variance of data after age 43 to 44 years, is consistent with a significant increase in posterior ocular rigidity, which may be influential in the development of presbyopia.

Accommodation-induced variations in retinal thickness measured by spectral domain optical coherence tomography

To research retinal stretching or distortion with accommodation, accommodation-induced changes in retinal thickness (RT) in the macular area were investigated in a population of young adults (n = 23) by using a dual-channel spectral domain optical coherence tomography (SD-OCT) system manufactured in-house for this study. This dual-channel SD-OCT is capable of imaging the cornea and retina simultaneously with an imaging speed of 24 kHz A-line scan rate, which can provide the anatomical dimensions of the eye, including the RT and axial length. Thus, the modification of the RT with accommodation can be calculated. A significant decrease in the RT (13.50 ± 1.25 μm) was observed during maximum accommodation. In the 4 mm × 4 mm macular area centered at the fovea, we did not find a significant quadrant-dependent difference in retinal volume change, which indicates that neither retinal stretching nor distortion was quadrant-dependent during accommodation. We speculate that the changes in RT with maximum accommodation resulted from accommodation-induced ciliary muscle contractions.

Axial length changes during accommodation in myopes and emmetropes

PURPOSE

To investigate the influence of accommodation on axial length (AXL) and a comprehensive range of ocular biometric parameters) in populations of young adult myopic and emmetropic subjects.

METHODS

Forty young adult subjects had ocular biometry measured using a noncontact optical biometer (Lenstar LS 900) based on the principle of optical low coherence reflectometry under 3 different accommodation demands (0 D, 3 D, and 6 D). Subjects were classified as emmetropes (n = 19) or myopes (n = 21) based on their spherical equivalent refraction (mean emmetropic refraction -0.05 +/- 0.27 D and mean myopic refraction -1.82 +/- 0.84 D).

RESULTS

AXL changed significantly with accommodation, with a mean increase of 11.9 +/- 12.3 mum and 24.1 +/- 22.7 mum for the 3 D and 6 D accommodation stimuli, respectively. A significant axial elongation associated with accommodation was still evident even after correction of the AXL data for potential error because of lens thickness change. The mean "corrected" increase in AXL was 5.2 +/- 11.2 mum and 7.4 +/- 18.9 mum for the 3 D and 6 D stimuli, respectively. There was no significant difference between the myopic and emmetropic populations in terms of the magnitude of change in AXL with accommodation, regardless of whether the data were corrected or not. A number of other ocular biometric parameters, such as anterior chamber depth, lens thickness, and vitreous chamber depth also exhibited significant change with accommodation. The myopic and emmetropic populations also exhibited no significant difference in the magnitude of change in these parameters with accommodation.

CONCLUSIONS

The eye undergoes a significant axial elongation associated with a brief period of accommodation, and the magnitude of this change in eye length increases for larger accommodation demands, however, there is no significant difference in the magnitude of eye elongation in myopic and emmetropic subjects.

Bifocal lens control of myopic progression in children

Bifocal spectacle lenses have been used as a strategy to slow myopic progression in children since the 1950s and perhaps earlier. The reported success of this strategy varies greatly, as does the design of studies reporting the outcomes of their use-from earlier retrospective analysis of records to later prospective clinical trials. Collectively, published data support the suggestion that bifocal lenses inhibit myopic development in children but only by a small amount and only in a subset of children. Possible reasons for the greatly varying outcomes include a lack of individualism of the treatment and failure to take the vergence system into account. This review summarises the results of bifocal and multifocal studies, describes how accommodation, convergence and their interaction are linked to myopic development and details how a bifocal treatment that takes this into account may be devised. Also discussed is whether alterations to peripheral retinal blur contribute to bifocal lens effects.

Directional sensitivity of the retina: 75 years of Stiles-Crawford effect

The reduction of the brightness when a light beam's entry into the eye is shifted from the centre to the edge of the pupil has from the outset been shown to be due to a change in luminous efficiency of radiation when it is incident obliquely on the retina. The phenomenon is most prominent in photopic vision and this has concentrated attention on the properties of retinal cones, where responsibility has yet to be assigned to factors such as differences in shape, fine structure and configuration, and membrane anchoring of photopigment molecules. Geometrical optics and waveguide formulations have been applied to the question of how light is guided in receptors, but details of their geometry and optical parameters even if they become available will make calculations complex and of only moderate generality. In practice, the diminution of oblique light helps visual performance by reducing deleterious influence of ocular aberrations and of glare caused by light scattering when the pupil is wide. Receptor orientation can come into play in ocular conditions due to mechanical disturbance and has been shown to have potentiality as a tool for clinical diagnosis. Currently, open questions include microanatomical and molecular differences between rods and cones, the coupling of the optical image of the eye with the transducing apparatus in the photoreceptors, possible phototropism and more convincing methods of estimating the actual spatial distribution of photon events as it affects visual resolution.

Small foveal targets for studies of accommodation and the Stiles–Crawford effect

The properties of small monochromatic targets as accommodative stimuli are not well understood. We used a dynamic optometer to record accommodation responses to monochromatic disc targets (1.0-27.3 min arc) and to a Maltese cross. Accommodation responded adequately to points as small as 13.6 min arc. The response to these small targets is relevant to the question of whether the Stiles-Crawford (SC) effect could provide a stimulus to accommodation. Previous studies have used pupil apodizing filters to neutralise the natural SC function and so determine how visual performance or accommodation is influenced by the SC effect. However, these filters cannot correct for known inhomogeneities in the SC function across the retina for extended targets. Therefore, we calculated the SC function inhomogeneities across the retinal image of a smaller 13.6-min arc target. Unfortunately, even this small target is too large to permit a homogenous SC function across its extent. Alternatives to the apodizing filter approach are discussed.

Retinal image motion during deliberate fixation: implications to laser safety for long duration viewing

With the easy attainability of hand-held laser devices and the burgeoning light emitting diode (LED) technology, safety standards for long-term viewing of continuous light sources are being scrutinized. One concern is with quantifying the effect of head and eye movements on the distribution of energy over the retina. This experiment describes target motion over the retina as a result of head and eye movements during a deliberate fixation task. Volunteers deliberately fixated, with (fettered) and without (unfettered) head and chin rest support, on LEDs that subtended a 0.1 min of arc visual angle. A Dual Purkinje Image Eyetracker measured eye position during each 100-s fixation trial. The data showed an elliptical retinal energy distribution, oriented on the temporal/ nasal retinal axis, with a major axis 1.5 times greater than the minor. The average half-maximum diameter for the major axis was 40 microm for the fettered and 107 microm for the unfettered condition. The retinal area illuminated from head and eye movements showed a positive linear relationship with time. Peak retinal radiant exposure at 100 s was reduced by a factor of 10 as a result of eye movements and by a factor of 30 as a result of head and eye movements. Although the net result was a reduction in radiant exposure from a "no-movement" baseline, the distribution of energy over the retina was well within the 200 microm foveal boundary. The data suggest that the laser permissible exposure limits for long term viewing of a small continuous wave source include a factor that adjusts for this reduction in retinal radiant exposure with time.

Parametric representation of Stiles-Crawford functions: normal variation of peak location and directionality

Evidence suggests that the psychophysically determined Stiles-Crawford effect of the first kind (SCE) reflects waveguide properties of human photoreceptors. The peak of the SCE data set is assumed to reflect the principal alignment tendencies, and the spread (e.g., rho value, the curvature or width at half-height) is assumed to reflect the directionality (i.e., interreceptor differences in alignment) of the population of photoreceptors being tested. As such, disruption of the normal SCE can be used and/or has been used (1) to document early natural history of retinal pathology involving the photoreceptors, (2) to provide a firm rationale for therapeutic intervention, and (3) to provide a method for monitoring therapies designed to alter the natural course of retinal-disease processes. We report large-sample norms for foveal SCE peak location and spread (horizontal peak location, nasal 0.51 +/- 0.72, horizontal rho value 0.047 +/- 0.013, vertical peak location, superior 0.20 +/- 0.64, vertical rho value 0.053 +/- 0.012), compare these norms with values determined in other laboratories, and discuss the various mathematical forms used for the empirical description of SCE data sets.

Video monitoring of the principal ray of a Maxwellian view for the measurement of optical aberrations, the Stiles-Crawford effect, retinal resolution, and for investigating color vision

A video frame grabbing technique with a computer-generated overlay is used to obtain a direct visualization of the position of the image of a point source, producing a Maxwellian view, in the natural entrance pupil of the human eye. With this simple technique, the image point can be located with an accuracy of at least 0.07 mm. Applications are demonstrated: (a) in instruments using a single Maxwellian view for measuring retinal resolution in white light or superimposed Maxwellian views for color vision experiments, (b) in a dual Maxwellian view apparatus used for measuring the Stiles-Crawford effect of the first kind or for measuring chromatic aberration, and (c) in a new semi-Maxwellian view apparatus for the evaluation of monochromatic and chromatic aberrations.

The Stiles--Crawford function of an albino observer

Stiles--Crawford functions (SCF's) were measured for both eyes of an albino observer with a marked pendular nystagmus using an eye position monitoring system. Both eyes demonstrated directional sensitivity as revealed by their SCF's. The directional sensitivity of both eyes was less than that of either the foveal or parafoveal region of normal observers. In addition, there was a significant difference between the SCF's for the albino observer's two eyes. Our results suggest that the psychophysical SCF is predominantly the result of the orientational distribution of groups of receptors on the retina.

Inferred positive phototropic activity in human photoreceptors

The Stiles-Crawford (S.-C.) function, a measure of the directional sensitivity of the retina, was used to infer the alignment characteristics of the sampled retinal elements. One assumes that the peak of the photopic S.-C. function reflects the central alignment tendency of renal elements sampled, and that the shape of the function reflects, among other factors, distributive qualities. Here two tests were performed to determine whether the function sampled reflected positive phototropic activity. The natural eye pupil was dilated and artificial pupils were substituted having specified eccentricity from the centre of the natural pupil. This was achieved with a displaced iris contact lens. After a series of complex experiments, it was finally shown that the peaks of the S.-C. function shifted towards the displaced aperture of the contact lens. As a second test, individuals were occluded uniocularly with a black patch for periods of time up to 10 days. This caused remarkable flattening of the measured S.-C. function. That flattening occurred in determinations of both photopic and scotopic S.-C. functions. Comparable effects were not seen in the second eye or if a diffuser was substituted for the black patch. Change and recovery in both experiments occurred within 3-5 days. On the basis of these experiments it is inferred that there is an active mechanism behaving in a positive phototropic manner present in the human retina.

Evidence for alteration in photoreceptor orientation

The effect of incident light, its point of origin, and its magnitude on receptor orientation was determined. A non-invasive test of vision, the Stiles-Crawford Effect (SCE), was used to investigate retinal directional sensitivity and alignment. The simple act of monocular black patching for a period of three to five days caused marked reduction in directional sensitivity and/or apparent dispersal of alignments. Recovery occurred in a comparable time period in normal adults 20 to 50 years of age. Special painted iris contact lenses with displaced pupillary apertures were worn to investigate the factors influencing alignment. Both light induced effects which caused alignment to shift toward a displaced aperture, and mechanical factors resulting in other alignment shifts were recorded. Thus, retinal receptors and/or associated anatomic components are apparently phototropic and subject to multiple forces influencing their directionality and/or alignment, and the milieu of the retinal receptor is active.

Monocular light exclusion for a period of days reduces directional sensitivity of the human retina

Single eyes of young adult observers were occluded for as long as 10 days. Directional sensitivity of the retina (the Stiles-Crawford effect of the first kind) under photopic conditions was dramatically reduced at every retinal location tested in all subjects. The maximum effect was observed within 3 to 5 days, and recovery took place at approximately the same rate after termination of patching.

A study of the Stiles-Crawford (S-C) function at 35 degrees in the temporal field and the stability of the foveal S-C function peak over time

Directional sensitivity of the retina (the Stiles-Crawford function of the first kind) was measured at a point 35 degrees from fixation and at the point of fixation. Three normal observers were used. The peripheral test point intercepted the retina between the optic nerve head and the ora serrata. At both test points, photopic and scotopic (one subject) Stiles-Crawford function peaks were contained within the pupillary bound and approximated the center of the pupil. Directionality at the two points was rather similar. These findings add strength to the argument that retinal receptors align approximately with the center of the exit pupil of the eye. Evidence for stability in Stiles-Crawford peak location in time is also presented.

Retinal receptive field-like properties and Stiles-Crawford effect in a patient with a traumatic choroidal rupture

Simple psychophysical techniques were used in order to assess layer-by-layer retinal functions in a patient with sub-retinal fluid due to a choroidal rupture following ocular trauma. A substained-like and a transient-like function believed to reflect retinal receptive-field-like properties, and an indicator of retinal receptor orientation (the Stiles-Crawford effect) have been followed in time. Central visual acuity was also measured. Initially all measured functions provided anomalous responses in affected retinal areas. Of interest here is the nature and order of recovery of the measured response functions. Most notable was the rapid rate of recovery of receptor orientation and the slower rate of recovery of the transient-like function.

Experimentally induced variations in the dark adaptation functions of a severe strabismic amblyope

Dark adaptation curves were determined for an amblyopic and a normal control subject following a variety of preadaptation conditions. Sizable between-eye differences were found in the dark adaptation functions of only the amblyope; the magnitude of such differences varied with this subject's distance from the light preadapting screen and with the effective size of his pupils during the light preadaptation period. Marked differences were also found between the dark adaptation functions of both eyes of the amblyopic subject and those of the normal subject. The preadaptation-dependent changes in the dark adaptation function of the amblyopic subject are hypothesized to result from anomalies of amblyopic eye light adaptation which are referable to disturbances of retinal receptor alignment.