bluuebunny/crossref_metadata_2025 huggingface dataset:
Tinted lenses and related therapies for learning disabilities – a review
Authors: Evans Bruce J. W. , Drasdo Neville
Published: December 2007

Research relating to the use of Irlen tinted lenses and coloured overlays for underachieves is reviewed Many of the studies were not published in refereed journals and were methodologically poor. The weaknesses of the Irlen argument arc discussed, including the absence of evidence to support the claims that these tints need to he uniquely prescribed and manufactured. Syntonics is another form of visual colour therapy that has been applied to those with a learning disability. Research on this is reviewed, and is also shown lo have procedural irregularities which preclude firm conclusions. Owing to the poor quality of much of this research the claims of the protagonists of these therapies cannot be proved or disproved. A proposed new therapy is normally preceded by a valid theoretical hypothesis: this has been lacking in the present topic. Recently, a feasible explanation has been proposed in terms of ‘pattern glare’ resulting from mild hypersensitivity to epileptogenic patterns. This, together with potential alternative theories, is discussed, In the conclusion of this review, advice is given for eye‐care practitioners who may be consulted on these therapies.
The Optical Services Audit Committee (OSAC) and UK optometry
Authors: Taylor S. P.
Published: December 2007

In 1989, following a series of legislative moves the UK Government to remove the ‘monopoly’ within optical services, LI proposal was made ID introduce a two‐tier system of eye care. Within the two tiers would be a cheap, rapid spectacle prescription assessment that took no account of binocularity or ocular health. The initial response to this with in professional groups involved was split, dispensing opticians keen to see the arrival of the system, the optometric profession concerned over the possible health consequences. In order to provide an overview of the situation the General Optical Council proposed the establishment of the Optical Services Audit Committee to investigate the whole of UK eye care services and recommend direction for future developments. The main conclusions of the report produced by this committee are discussed here.
Variations in hyperacuity performance with age
Authors: Whitaker David , Elliott David B. , MacVeigh David
Published: December 2007

The term hyperacuity has been applied in a group of stimuli which produce spatial thresholds smaller than those expected given the relatively large receptor spacing and the retinal image quality of the human eye. It is not yet firmly established whether hyperacuity performance declines with increasing age in the same way as most other measures of visual ability. This is perhaps, due to the use of varying task configurations and criterion‐dependent psychophysical techniques. The present study examines age‐related performance in three different hyperacuity tasks using a criterion independent forced‐choice method. Both displacement and bisection thresholds were found in increase with age, but there was no significant chance in vernier acuity. This indicates that age has a differential effect on thresholds for various hyperacuities depending upon the task requirements. No significant age‐related trend was observed in hyperacuity bias. which represents the difference between subjective and true physical alignment.
Calculation and least‐squares estimation of surface curvature and dioptric power from meridional measurements
Authors: Harris W. F.
Published: December 2007

The tabulation or estimation of dioptric power or surface curvature from meridional measurements is an important problem with applications in a number of areas, By phrasing the problem in terms of matrices and applying new results in the mathematics of matrices one is able lo reformulate the problem into a standard form that is well known in mathematics and statistics. As a result the solution can be written down directly. The method copes with any number of measurements along any number of meridians, including repeated measurements along the same meridian, and is more general than methods previously proposed. It gives least‐squares and best estimates of the true surface curvature or dioptric power. Being phrased in standard statistical terms the method lends itself readily to extension to related types of problems such as least‐squares estimation under certain types of constraints. Matrix methods employed in this paper are likely to find wider application in optometry and the vision sciences.
Motion reveals spatial visual defects
Authors: Campbell F. W. , Andrews P. R.
Published: December 2007

The question of how we can be unaware of the deficit in our (monocular) visual field, equivalent in size to 76 Full moons, is examined. A new method of investigating the response to moving images on and near the Wind spot has been found. The image of a computer‐generated line, which is made to lengthen with time and pass over the blind spot, is seen as shorter than that of a similar, parallel line which passes outside the blind spot. This perceived difference in length corresponds to the actual width of the blind spot. Our unawareness of blind spots and seotomata is often described as involving some form of ‘filling in’ process. The rapid variation in cortical receptive field site. recently found lo occur in response to stabilized images, may provide a general filling‐in mechanism for the removal from perception of otherwise‐distracting, stabilized image regions (such as the shadows of blood vessels and clinical scotomata).
Counterphase dichoptic flicker is seen as its own second harmonic
Authors: Cavonius C. R. , Estévez O. , van der Tweel L. H.
Published: December 2007

If a paifernless field is modulated sinusoidally in time so that the luminance change in one eye is in coumerphase to that in the other, the resulting flicker appears faster ihan if the modulation to both eyes has the me phase. If observers set the frequency and the amplitude of a comparison in‐phase field so that it matches a neighbouring Counterphase field, modulated at, say, 2.5 * its threshold, they set the frequency to twice the eounterphase frequency, and the amplitude to a value that is. for a given frequency, a constant ratio of the modulation of the counterphase field. Counterphase stimulation thus appears to cause an internal second‐harmonic signal. However, it is not possible to cancel this by adding a second harmonic component to the stimulus.
Interactions between colour and luminance contrast in the perception of motion
Authors: Mullen K. T. , Boulton J. C.
Published: December 2007

It has been demonstrated widely that at isoluminance moving chromatic stimuli arc seen to be stationary or moving more slowly than their luminance counterparts. We have examined the effect on perceived velocity of adding luminance contrast to an isoluminant chromatic stimulus. We show that mount; luminance contrast ‘captures’ colour so that a combined colour and luminance stimulus is seen moving as a unified percept. However, in the presence of colour contrast, significantly higher levels of luminance contrast are required to achieve a veridical velocity than for monochromatic stimuli with only luminance contrast. We show that this interactive effect between colour and luminance contrast cannot be fully explained by a threshold masking of luminance by colour contrast. The effect suggests that a breakdown in the veridical perception of velocity should be expected for colours with a wide range of associated luminance contrasts and not just for those at the point of isoluminance.
Use of interferometric visual stimulators in optometry
Authors: Thibos Larry N. , Bradley Arthur
Published: December 2007

Professor Fergus Campbell and colleagues pioneered the use of the interferometric visual stimulator as a tool for separating optical from neural Factors which limit spatial vision. The device is now commercially available as a clinical instrument which ‘bypasses the optics of the eye’ to measure the potential visual acuity of the neural portion of the visual system. However, the clinician needs to be aware of limitations of current instrument designs and recent advances in our understanding of peripheral vision, to correctly interpret patient data gathered with these devices.
New acuity test for toddlers
Authors: Woodhouse J. Margaret , Adoh Tonnie O. , Oduwaiye Kola A. , Batchelor Bruce G. , Megji Shiraz , Unwin Nick , Jones Nick
Published: December 2007

Preferential looking is the technique of choice for measuring visual acuity in infants and young children. Most workers agree that the toddler age group, 1 to 3 years, is the most difficult to lest. This is because of their short attention span and restlessness but mostly because they find the grating target used in the test, frankly boring. The concept of the vanishing optotype chart offers alternative test targets, while utilizing the technique of preferential looking. We have designed a lest which comprises a familiar shape (house, car etc.) on the upper or lower part of a neutral grey card. The shape is computer generated, and designed to fade completely when beyond the resolution limit. Acuity is determined by the width of the while lines making up the shape. As with conventional preferential looking, the observer notes the child's eye movements to determine the position of the target shape. Although picture naming is not required, the shapes help to maintain the child's interest in the test. The test is quick, and is successful with the toddler age group and older patients with intellectual impairment.
Current perspective on microfluctuations of accommodation
Authors: Wirm Barry , Gilmartin Bernard
Published: December 2007

The collaboration of Fergus Campbell. Gerald Westheimer and John Robson in the 1950s produced insight into the nature of accommodation mierofluctuations and instigated work which has led to the current view thai 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 (HFC ≥ 1.0 ≤ 2.3 Hz). A functional role has been attributed to these microfluctuations as they offer a means by which a directional cue could be derived from an even‐error stimulus. However, there is no consensus regarding the respective contribution made by each of the dominant components in the accommodation control process. Using a newly‐designed measurement and recording system we have conducted a series of experiments to investigate the nature and aetiology of the microfluctuations. The incidence and magnitude of microrluctuations in LFCs and MFCs for central and peripheral lens zones were investigated while live young cmmetropic subjects viewed a near target. The form of the power spectra of the fluctuations was found to be similar for central and peripheral zones although an overall reduction in magnitude was observed in the periphery. The HFC's arc thus a consistent feature of mierofluctuations in central /ones and not. as previously suggested, merely a spurious feature of peripheral zones. A significant between‐subject variation in the location of HKC's was found and led us to consider the relationship between HFCs and other physiological systems which provide intraocular rhythmic variation. Simultaneous measurements of ocular accommodation and systemic arterial pulse made on 20 subjects demonstrate that the location of the HFC is significantly correlated with arterial pulse frequency. The intraocular mechanisms by which arterial pulse modulates the HFC are likely to involve rhythmic changes in both choroidal blood flow and intraocular pressure (IOP). Consequently, examination of the effect of 0.5% timolol maleate. a drug which exerts an ocular hypertensive effect and also induces a reduction in IOP pulse, on the accommodation microfluctualions of lOemmetropic subjects when fixating a near target was undertaken. A double‐blind protocol against saline showed timolol to reduce significantly the root‐mean‐square value of the fluctuations. The results thus indicate that factors relating to IOP and ocular vaseulature will affect the magnitude and frequency composition of accommodative fluctuations. The studies to date suggest that the complex waveform of the accommodative microfhiciuations is a consequence of the combination of neurological control and physiological rhythmic variation: the former is probably attributable to the LFCs; the latter to arterial pulse.
Retrospective study of optometric referrals
Authors: Hobley A. J. , Woodward E. G. , Port M. J. A.
Published: December 2007

We undertook a retrospective study of the referral patterns of optometrists. During the study period 13 107 patients had been seen, of whom 2.86% had been referred. Of the referred patients. 66.67% were referred to an ophthalmologist via the general practitioner. 25.87% to their general practitioner alone, and 7.47% directly to an ophthalmologist. The conditions for which patients were referred are given along with information on any supplementary tests used. The effect of practitioner age and seasonal variations on referrals was also determined. The role of the optometrist in screening for ocular disease is discussed.
Rapid and slow‐velocity vergence eye movements *
Authors: Wick Bruce , Bedell Harold E.
Published: December 2007

Previously we have measured rapid‐velocity vergence responses to targets at different distances that provided no disparity or accommodative stimulation. To evaluate the possibility that this rapid‐velocity vergence occurs during succades. the latencies or eye movement between two long dim luminous rods were compared under two conditions. Rapid‐velocity vergence with an average latency of ≈300ms was elicited when subjects alternately viewed horizontal rods at distances of 38 and 78 cm, and with a vertical separation of 5.2°. Horizontal saccades with a comparable latency were measured when the rods were equidistant and oriented vertically. The correlation between the mean latencies of vergence and saccadic movements was 0.97, sugesting that the two movements occurred together. In a second experiment vegence responses were measured when the subject looked between u bright vertical line on a screen at 76cm and ii second pair of lines (vertically displaced between 0.15 and 3°) with crossed disparity to simulate a target al 38cm. Slow‐velocity vergence often occurred alone when the vertical separation between targets was small: rapid‐velocity vergence intruded when the separation between the targets was larger. The results can be accounted for if proximity and disparity stimulation act through a single vergence controller, the output of which produces slow‐ or rapid‐velocity vergence depending upon whether the saccadic system is concurrently active.
Contribution of accommodation and disparity‐vergence to transient nearwork‐induced myopic shifts
Authors: Rosenfield Mark , Ciuffreda Kenneth J. , Novogrodsky Lisa
Published: December 2007

Many investigations have attempted to demonstrate a relationship between sustained near‐vision and the aetiology of environmentally‐induced myopia, but it remains unclear whether myopic development relates to the actions of either accommodation or vergence. The present study investigated the effect of varying the disparity‐vergence demand during a near‐vision task while maintaining the accommodative stimulus constant. We required 20 young subjects to perform a near task (viewing distance = 20cm) for 20 min through base‐in, zero power and base‐out prisms. The magnitude of the prisms for each individual corresponded to one‐third their near vergence range. Pre‐ and post‐task measurements of refractive state were determined objectively using an infrared optometer, with post‐task readings being recorded over the initial 50s following task completion. The results indicated no significant difference between the transient far‐point shift for the three disparity‐vergence conditions. However, combining data across conditions, a significant, mean shift in the myopic direction of 0.14 D was observed during the 10–20 s period immediately following task completion which dissipated within 20–50 s post‐task. These findings suggest that the transient post‐task myopic shift was not related lo the output of disparity‐vergence.
Relationship between axial length and chromatic refraction of the eye
Authors: Dick Graham L.
Published: December 2007

A new mathematical expression is derived to provide an estimate, ax, of the axial length of the human schematic eye. Approximations in the expression are made after considering effects in the Gullstrand‐Emsley schematic eye. Parameters required to solve the expression are, for two wavelengths, the refractive indices of the humours, the focal power of the cornea and refractive errors in two states of accommodation. Unlike recognized optical methods, additional intraocular parameters are not required. For cmmetropic variants of the Gullstrand‐Emsley schematic eye with axial lengths between 19 and 27 mm, ax is within 0.06 to 0.26 mm of the actual value. Furthermore, ax is essentially independent of variations in the refractive indices of the crystalline lens. However, the estimate of ax is critically dependent on the accuracy of values assigned to the parameters. For that reason, the model is considered impractical for clinical application.
Spatial adaptation to text on a video display terminal
Authors: Greenhouse Daniel S. , Bailey Ian L. , Howarth Peter A. , Berman Samuel M.
Published: December 2007

We employed vertical sinusoidal test gratings to search for spatial adaptation to lower‐case text presented on a standard video display terminal. The parameters of the contrast sensitivity test were selected on the basis of waveform analysis of horizontal spatial luminance profiles of the text. We found that subjects exhibited a small (4–5dB), but significant, frequency‐specific spatial adaptation consistent with the frequency spectrum of the stimulus. The theoretical and practical significance of this finding is discussed.
Time‐averaged accommodation response to flickering stimuli
Authors: Chauhan K. , Charman W. N. , Halnan A. M. , Kelly C. M. , Loughlin A. , Neilson K. J. , Walsh G.
Published: December 2007

Experiments are described in which the steady‐state accommodation response versus stimulus curve was measured with an infrared autorefractor for high‐contrast stimuli having a 100% square‐wave temporal modulation of luminance in the frequency hand 1–200 Hz. Slightly more accurate responses were found at frequencies ‐50 100 Hz. i.e. above flicker fusion. The relevance of the results to practical situations in which flicker may be experienced is discussed.
Effects of monochromatic and chromatic oblique aberrations on visual performance during spectacle lens wear
Authors: Tang C. Y. , Charman W. Neil
Published: December 2007

An optical system is described which allows overall visual performance to be studied when the wearer of a spectacle lens views objects through the lens periphery. Performance with lenses of any refractive power can be explored without any need for subjects to have the appropriate complementary refractive errors. In comparison with the effects of dioptric defocus. transverse chromatic aberration (TCA) is found to cause a relatively greater degradation in contrast sensitivity al high spatial frequencies when tangentially‐oriented gratings are observed. The relative importance of TCA appears lo have been underestimated in some previous studies. As vision is found to be affected by both TCA and defocus, a careful compromise between chromatic and monochromatic aberrations is required.
Preliminary assessment of the Dex Frame for assisting children with specific learning difficulties
Authors: Taylor Steve , Francis Marie , Sawyer Chris
Published: December 2007

A device called the Des Frame has recently been marketed as an aid for children with specific learning difficulties. Using a test of reading speed and a test of reading accuracy we have attempted to evaluate the device. Two groups of children with learning difficulties were used, one having sole use of the device and being allowed to carry it with them and a second group who only used the device when attending for remedial tuition and assessment. The results show that no significant improvement occurred in reading performance of the groups with learning difficulties using the Dex Frame as compared with a control group. The teachers of the groups did, however, feel that on an individual basis some children (< 10%) expressed that they felt more comfortable using the device.
Survey of the accuracy of new pseudoisochromatic plates
Authors: Birch Jennifer , McKeever Lisa M.
Published: December 2007

The accuracy of three new pseudoisochrometic tests for detecting red–green colour deficiency was assessed. These were the Ishihara plates, the Ishihara test for ‘unlettered persons’ and Ohkuma's lest cards. We examined 500 subjects; 471 normal trichromats and 29 colour‐deficient people, Results obtained for the 1989 edition of the Ishiliara places were compared with the 9th edition and the most efficient plates identified. Although normal trichromats may be expected to make several interpretive misreadings, the Ishihara plates were found to be superior to the 9th edition and to the Ohkuma test (1986) for colour vision screening. The new symbol designs of the Ishihara plates for ‘unlettered persons’ (1990) were found to be very effective for colour vision screening, and a further study with young children is proposed. The 38 plate I9S9 edition of the Ishihara les; is recommended for use in clinical practice. The designs included in the concise 24 plate edition and the new abbreviated 14 plate edition are not selected from the point of view of accuracy and more reliable results are obtained if the full lest is given or if the practitioner shows only the most efficient designs.
Vision of the famous: the artist's eye
Authors: Elliott David B. , Skaff Amanda
Published: December 2007

This article reviews the possible effects of various acquired or inherited eye disorders on the paintings and sculptures of some of the most famous artists. Were Leonardo da Vinci, Rembrandt, Michelangelo and Monet affected by age‐related eye disease which influenced their later works? Was El Greco's oblique elongation of his characters due to astigmatism? Was Constable a colour‐defective? Was the Impressionist style of painting due to the myopia and cataract of its leading components? Arc van Gogh's yellow and haloed paintings due to the ocular side effects of digitalis toxicity? These questions are discussed and some of the artists’ works are shown to illustrate the points.
Clinical evaluation of a range of autorefractors
Authors: McCaghrey G. E. , Matthews F. E.
Published: December 2007

A practice‐based study of nine autorefractors was undertaken lo assess reliability and validity. Various statistical methods were used to test aspects of validity of the machines compared with one another. In routine optometric practice the autorefractor is shown lo be a useful complement to existing techniques, but there is a measurable difference in the performance of machines from different manufacturers, and even between two models from the same manufacturer.
Reliability of visual performance measurement under optical degradation
Authors: Woods Russell L
Published: December 2007

Small groups of experienced subjects wore three types of bifocal contact tens which were shown to reduce visual performance at distance and near conditions. Monitor‐based contrast sensitivity. Pelli Rubson contrast threshold, and high and low‐contrast visual acuity were each measured on two separate occasions. For each bifocal contact lens type the lest retest difference did not increase significantly with reduction in visual performance. Despite this the reliability of the visual performance measures was shown 10 be reduced compared lo previous reports with normal well‐corrected subjects and the test‐retest repeatability coefficients (95% confidence limits) increased as the average visual performance declined. The poor reliability is of interest to the practitioner fitting bifocal contact lenses, and confirms previous suggestions that reliability may be related to visual degradation (e.g. ocular disease). In addition Pelli‐Robson contrast thresholds were shown to be correlated with the spatial frequency represented by the presumed fundamental frequency of the component letters.
Temporal characteristics of proximally‐induced accommodation
Authors: Rosenfield Mark , D'Amico Jennifer L. , Nowbotsing Sujata , Kapoor Neera , Ciuffreda Kenneth J.
Published: December 2007

This study sought to determine whether proximally‐induced accommodation (PIA) is capable of sustaining an accommodative response over time. Accordingly, the steady‐stale 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 und 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.
Diffractive properties of the Diffrax® bifocal contact lens
Authors: Atchison David A. , Thibos Larry N.
Published: December 2007

We have verified that the Pilkington Diffrax® lens behaves optically as a diffractive bifocal contact lens. The longitudinal chromatic aberration of the lens is similar to that predicted by theory, both on and off the eye. The over‐refraction with the lens on the eye is similar to the distance subjective refraction, which is also as predicted.
Effect of target luminance on microfluctuations of accommodation
Authors: Gray Lyle S. , Winn Barry , Gilmartin Bernard
Published: December 2007

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 ≤ HFC ≤ 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 microfiuctuations. 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 11.63cd 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 O.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 lo increases of power in the LFC. No systematic variation of the HFC could be observed with changing luminance level. The increase in power of the LFC as retinal‐image quality is degraded suggests that it is this component which is used in the control of blur‐driven accommodation responses.
Estimation of the thickness of the crystalline lens from on‐axis and off‐axis Scheimpflug photographs
Authors: Sparrow John M. , Brown Nicholas A. Phelps , Bron Anthony J.
Published: December 2007

Measurement of human lens biometry directly from Scheimpflug images of the anterior ocular segment may be impossible when pupil size precludes an adequate Scheimpflug view of the posterior lens surface. The authors describe a simple and accurate geometric method of overcoming this problem by estimating the true lens thickness from pairs of on‐axis and off‐axis Scheimpflug images. The method is validated and the variability of the estimate is quantified. This new method has utility in the study of lens biometry in subjects with large lenses whose pupils cannot be adequately dilated.
Ophthalmic contact lens with internal fixation light for examination of the optic nerve head*
Authors: Richards David W. , Murphy Kent A. , Cartwright Mont J.
Published: December 2007

Stereoscopic slitlamp examination of the optic nerve head in one‐eyed patients is difficult because there is currently no way to provide the patient with a fixation target. This situation is often aggravated in glaucoma patients by a miotic and fixed pupil. We have developed a funduscopic contact lens which incorporates a fixation target, allowing the patient lo view the target with the eye being examined. The device has been successfully tested on normal volunteers.
Effect of uncorrected refractive errors upon central visual field testing
Authors: Henson David B. , Morris Ellen J.
Published: December 2007

An investigation of the effects of uncorrected refractive errors upon a central visual field examination has been made with a view to establishing what the likely effects would be of omitting a refractive correction when screening the visual field with a threshold related, supra‐threshold strategy on the Henson CFA3000. The results indicate that, while a linear relationship exists between the extent of threshold elevation and the product of residual refractive error and pupil size, the scatter in the results means that accurate predictions cannot he made in individual cases. The threshold elevation was found to be independent of eccentricity (within the central 21° from fixation) and did not increase the variability of results. These last two findings indicate that uncorrected refractive errors are unlikely to affect the sensitivity of the threshold related, supra‐threshold strategy to localized visual field defects.
Binocular vision in a virtual world: visual deficits following the wearing of a head‐mounted display
Authors: Mon‐Williams Mark , Warm John P. , Rushton Simon
Published: December 2007

The short‐term effects on binocular stability of wearing a conventional head‐mounted display (HMD) to explore a virtual reality environment were examined. Twenty adult subjects (aged 19–29 years) wore a commercially available HMD for 10 min while cycling around a computer generated 3‐D world. The twin screen presentations were set lo suit the average interpupillary distance of our subject population, to mimic the conditions of public access virtual reality systems. Subjects were examined before and after exposure to the HMD and there were clear signs of induced binocular stress for a number of the subjects. The implications of introducing such HMDs into the workplace and entertainment environments are discussed.
Form and behaviour of Haidinger's brushes
Authors: Misson Gary P.
Published: December 2007

A computer simulation of Haidinger's brushes that is based on the optics of a radially symmetrical array of dichroic elements is presented. The simulation describes the perceived distribution of intensity and contrast of Haidinger's brushes when observed with monochromatic plane polarized light at different orientations. In addition, the behaviour of Haidinger's brushes when the plane of polarization is rotated, and their modification by optical retarders, is reproduced. The similarity between the simulation and the reported behaviour of Haidinger's brushes is consistent with the theory that the phenomenon results from a symmetrical distribution of dichroic pigment molecules within the macula.
Model for co‐ordination of corneal and crystalline lens power in emmetropic human eyes
Authors: Dunne Mark C. M.
Published: December 2007

A mathematical model is used to investigate the co‐ordination of corneal and crystalline lens power in emmetroptia. Modelled depth changes of the anterior chamber, crystalline lens and vitreous chamber were based upon empirical data. Modelled power changes of the corneal and crystalline lens were then purely governed by eyeball shape. Power changes generated by the model closely agreed with empirical data indicating that the co‐ordination of both, to maintain emmetropia, can be accounted for by relative axial and equatorial growth of the eyeball.
Sensitivity of off‐axis performance of aspheric spectacle lenses to tilt and decentration
Authors: Atchison David A. , Tame Simon A.
Published: December 2007

Apparatus was modified lo measure, and theoretical raytracing was used to predict, off‐axis powers of spectacle lenses in the presence of till or decentration. In response lo poor fitting in the form of lilt or decentration, lenses with aspheric from surfaces were found to have greater off‐axis power errors than best‐form lenses with spherical surfaces. This is attributable to the aspheric lenses having flatter surfaces than the spherical lenses. The errors are up to twice those occurring for the spherical lenses, and can be quite high. e.g. 0.9 D astigmatism for + 6D power with 10° tilt in 20° upgaze. Negative lenses are more sensitive to poor fitting than are positive lenses of the same power. The errors for straight ahead vision associated with tilt are approximately proportional to the square of the angle of tilt, and the errors for straight ahead vision associated with decentration are approximately proportional to the square of decentration. It is most important that aspheric lenses be correctly fitted, which means that each 2° of pantascopic tilt should be accompanied by approximately 1 mm decentration.
Isovergence surfaces: the conjugacy of vertical eye movements in tertiary positions of gaze
Authors: Schor Clifton M. , Maxwell James S. , Stevenson Scott B.
Published: December 2007

Conjugate gaze is often defined as the equal angle rotation of the two eyes. For fixation at far distances, the optical axes are parallel and conjugacy is defined irrespective of the coordinate system. For nearby or finite fixation distances, the evaluation of conjugacy for many gaze postures depends on the coordinate system used to measure it. For example, if the eye is elevated or depressed and the eye is rotated about a vertical axis, the intersections of lines of sight with a tangent screen will describe either straight lines for arcs depending on whether the vertical axis is fixed with respect to the head or to the eye. Because of the horizontal separation of the two eyes, the binocular fixation of near targets at tertiary positions of gaze will require a vertical vergence component for head‐referenced but not eye‐referenced measurements. The vertical gaze alignment of three human subjects was measured as they viewed targets placed at secondary and tertiary eye positions at two different distances. Vertical vergence was either held open or closed‐loop. The lines of sight were found lo intersect (i.e. vertical gaze was aligned) regardless of target position or viewing condition.
Assessment of the ocular response to topical β‐adrenoceptor antagonists using accommodative microfluctuations
Authors: Strang Niall C. , Wirm Barry , Gilmartin Bernard , Brosnahan Donal
Published: December 2007

Power spectrum analysis of accommodative microfluctuations has identified two dominant frequency components: a low frequency component (LFC < 0.6 Hz) and a high frequency component (1.3 Hz < HFC < 2.5 Hz). Computer‐driven models of accommodation and experimental manipulation of accommodative feedback loops indicate that LFCs are likely to have a functional role in monitoring retinal image contrast during sustained accommodation. In contrast HFCs have been shown lo he correlated with arterial pulse frequency and consequently their characteristics can be modified by the extra‐ and intra‐ocular vascular (and possibly CNS) effects. For example, topical instillation of the non‐selective β‐antagonist timolol maleate has shown previously the ability lo modify the HFC. In an attempt to clarify proposed differences between β‐adrenoceptor antagonist agents with regard to their effects on systemic and ocular vasculature. we extend the potential offered by HFCs as a non‐invasive method of assessing the ocular response to β‐antagonists to the cardioselecttve β‐antagonist betaxolol HCI. Accommodative microfluctuations were measured using a continuously recording infrared optometer on 10 emmetropic subjects (mean age 23.9 × 2.3 years) who viewed a high contrast target located at a vergence of ‐4 D. A double‐blind protocol was employed between saline and hetaxolol (0.5%, 2 x 30 μl) following corneal anaesthesia. Local and systemic effects were separated by examining the treated and untreated eyes of three subjects. Power spectrum analysis indicated that the root mean square (r.m.s.) value and power of LFCs and HFCs were equivalent for the saline and betaxolol trials.) This demonstrates the utility of microfluctuations as a technique for differentiating ocular responses to β‐adrenoreceptor antagonists that are likely to emanate from a combination of ocular and systemic vascular effects
Reduction in stereoacuity with age and reduced retinal illuminance
Authors: Yap M. , Brown B. , Clarke J.
Published: December 2007

Stereoacuity has been reported to decrease significantly in the seventh decade of life. Could this decrease be due to a reduction of retinal illuminance in the ageing eye? In this study, we used a more rigorous psychophysical procedure than that used previously, on 35 subjects, aged 21–67 years, to determine if the reported effect of age on stereoacuity could be replicated. We also compared the stereoacuity of 15 healthy young adults under normal and reduced retinal illuminance. A forced‐choice algorithm was used to improve the accuracy of measurements. Results suggest that the reduced retinal illuminance resulting from normal ageing is not the cause of the decreased stereoacuity found with ageing.
Extent and duration of practice effects on performance with the Farnsworth–Munsell 100–Hue test
Authors: Hardy Kevin J. , Craven Ben , Foster David H. , Scarpello John H. B.
Published: December 2007

The extent and persistence of practice effects on serial performance in the Farnsworth‐Munsell 100‐Hue test (100‐Hue test) were evaluated in an experiment in which six subjects performed the 100‐Hue lest up to 17 times over six weeks, and then once more after 7 months. A practice effect occurred which was highly statistically significant for the group as a whole (P < 0.001) and for three individual subjects (P < 0.05). A practice effect was still evident 7 months after the last test performance. Error scores were reduced almost to zero after 5–10 retests so that it was unclear whether the effects of practice had disappeared or whether failure to improve further was a ‘floor effect’. As a control against a floor effect, a second experiment was performed in which subjects’performance was impaired by placing neutral density filters in from of their eyes (thus artificially raising their 100‐Hue error score) Under these conditions, error scores continued to fall and were halved after 17 tests (P < 0.03). It is concluded that practice has a large effect on 100‐Hue test performance which continues over many retests and for many months after testing.
Precision and reliability study of a modified keratometric technique for measuring the radius of curvature of soft contact lenses
Authors: Quesnel Nadia‐Marie , Simonet Pierre
Published: December 2007

The back optic zone radius of soft contact lenses cannot be measured with conventional methods used for rigid lenses. Although a variety of techniques has been proposed, none has received wide clinical acceptance. A study was conducted to determine the precision and limitations of a method using a Zeiss keratometer. A total of 192 lenses from categories 1 (Optima 38) and 4 (Hydrocurve II 55%) of the FDA classification were used. These lenses were divided into six groups according to their water content and refractive power. The difference versus the mean of two different measurements (repeatability) has been plotted to determine the bias (mean of the differences relative to zero), the 95% limits of agreement and the repeatability coefficient of the technique. Variation between measurements was also assessed by calculating the reproducibility coefficient R for each soft lens. The precision expressed by the SD of a series of ten measurements on one lens in each group, was computed for comparison with previous studies. The repeatability of the technique expressed in term of 95% limits of agreement (range ± 0.058 to ± 0.198 mm) meets the American Standards Tolerances for each group of soft lenses studied. The coefficient of reproducibility R for each soft lens studied seems to be related to lens flexibility and dioptric power. The technique used gives the same degree of precision as other keratometric methods described in previous studies. The clinical use of this method is discussed with regard to practical factors.
Effects of posterior segment disorders on oscillatory displacement thresholds, and on acuities as measured using the Potential Acuity Meter and laser interferometer
Authors: Barrett Brendan T. , Davison Peter A. , Eustace Peter E.
Published: December 2007

Oscillatory displacement thresholds (ODTs) were measured in 67 subjects with clear ocular media, but known posterior segment disorders. The ability of these thresholds to assess reduced visual function, as quantified by visual acuity deficits, was compared with that of the Potential Acuity Meter (PAM) and the Rodenstock Retinometer (RR). ODTs were found to be sensitive to acuity deficits which were retinal/neural in origin. Despite the fact that interferometric acuities are, unlike ODTs, a measure of resolution, the RR proved no more accurate in assessing the level of letter acuity which existed than did ODT measurement. As would be expected, the PAM provided for the most accurate means of assessing non‐optical visual acuity deficits. However, the PAM proved lo be less useful in eyes with: very poor levels of acuity, and certain anomalous conditions. The results have implications for the relative ability of each of the three tests to identify reduced visual function behind cataract.
Resolving power to narrow and extended light sources and its influence on legibility of the optotypes
Authors: Koczorowski Pawel , Kędzia Boleslaw
Published: December 2007

The resolving power of the human visual system has been found to depend on the sizes of the light sources. The sources used in this experiment were two rectangular white fields separated by a vertical black bar, The minimal angle of resolution measured on five subjects decreased by about 10‐20 times while the light sources were extended up to the effective range of spatial summation. For larger sources no significant changes of spatial resolution were observed. The experimental data revealed biphasic dependence of the minimal angle of resolution on the sizes of the light sources and permitted estimation of the effective range of spatial summation for 3‐9 min arc. This range encompasses the angular diameters of typical optotypes recognized by normal observers (visual acuity greater than 1, optotypes smaller than 5 min arc), therefore we show how the spatial arrangement of the lines and strokes of optotypes determines their legibility.
Minimal angle of resolution in monocular single‐colour vision and the TAIS model
Authors: Koczorowski Paweł
Published: December 2007

The mathematical model TAIS simulates visual perception of single‐colour or black‐and‐white, non‐moving objects. It makes an analysis of the luminance spatial distribution in an optional direction and describes it in linguistic terms. The model comprises three modules that successively convolve the luminance distribution with a spatial litter (a normalized Gaussian function), calculate the second derivative of the convolution and, with use of context‐free and context‐sensitive grammars, generate a string of symbols. The string of symbols contains the information, simplified in form and rectified from visual noise, about perceived spatial changes of brightness. The model simulates various visual phenomena, e.g. edge detection and spatial resolution, and it predicts the resolving power dependence on the light source size. The mode I‐gene rated dependence of minimal angle of resolution on the light source width fits the experimental data with linear correlation coefficients in the range 0.70–0.94. The diagram consists of decreasing and constant pans, its shape adjusted to experimental data through selection of the Gaussian filter parameter and second derivative threshold.
Optical method for estimating the equivalent refractive index of the crystalline lens in vivo
Authors: Dick Graham L.
Published: December 2007

In numerous studies it has been necessary to represent refractive indices of the crystalline lens by an equivalent refractive index (here denoted nL).Its traditional value of 1.416. based on in vitro measurements, may be in error by more than 0.025 in some cases. In viva estimates are subject to recognized deficiencies of ultrasonography. This study describes and evaluates a new and exclusively optical technique. Using exact ray tracing, nL, is found by combining routine optical section data with optical depths that may be measured by a ranging system termed OCDR. Gullstand's no. 1 eye is used to calculate the effects of errors in eye data on the accuracy of the technique. From pooled errors estimated for 22 parameters, a standard deviation of 0.0045 is anticipated for the estimate of nL, although allowance for additional uncertainty associated with the refractive index profile of the lens must await further research.
VDT screen reflections and accommodation response
Authors: Collins Michael , Davis Brett , Atchison David
Published: December 2007

Reflections in computer screens are imaged behind the screen and therefore potentially create a conflicting cue to appropriate accommodation response to the plane of screen. We investigated the effect of screen reflections on accommodation response by producing reflections (1.33 D accommodation demand) surrounding and overlying the fixation point (2 D accommodation demand) in the screen. We have found little evidence that reflections in visual display terminal screens can influence the accuracy of the users' accommodation responses under normal binocular viewing conditions. The accuracy of the accommodation response was not significantly influenced by the position of the reflections, the presence of surrounding text (peripheral cues), or the subjects' dark focus level. The variability of accommodation response did not significantly change under the various test conditions. However, accommodation errors were apparent to a small degree (<0.25 D) under some monocular viewing conditions in the presence of screen reflections.
Prismatic effect in bifocal lenses and the location of the near optical centre*
Authors: Harris W. F.
Published: December 2007

A matrix expression is derived for the prismatic effect in the reading portion of a bifocal lens. Using this expression the problem of finding the location of the optical centre of the reading portion is examined. Matrix methods lead to a complete solution that holds in all circumstances within the usual limitations of first‐order optics and thin lenses Both the main and segment lenses may be astigmatic. In the usual case there is a unique near optical centre but situations can occur in which there is no near optical centre or an infinite number of them. These situations are examined systematically. Numerical examples illustrate the method.