Low spatial frequencies are suppressively masked across spatial scale, orientation, field position, and eye of origin

Masking is said to occur when a mask stimulus interferes with the visibility of a target (test) stimulus. One widely held view of this process supposes interactions between mask and test mechanisms (cross-channel masking), and explicit models (e.g., J. M. Foley, 1994) have proposed that the interactions are inhibitory. Unlike a within-channel model, where masking involves the combination of mask and test stimulus within a single mechanism, this cross-channel inhibitory model predicts that the mask should attenuate the perceived contrast of a test stimulus. Another possibility is that masking is due to an increase in noise, in which case, perception of contrast should be unaffected once the signal exceeds detection threshold. We use circular patches and annuli of sine-wave grating in contrast detection and contrast matching experiments to test these hypotheses and investigate interactions across spatial frequency, orientation, field position, and eye of origin. In both types of experiments we found substantial effects of masking that can occur over a factor of 3 in spatial frequency, 45° in orientation, across different field positions and between different eyes. We found the effects to be greatest at the lowest test spatial frequency we used (0.46 c/deg), and when the mask and test differed in all four dimensions simultaneously. This is surprising in light of previous work where it was concluded that suppression from the surround was strictly monocular (C. Chubb, G. Sperling, & J. A. Solomon, 1989). The results confirm that above detection threshold, cross-channel masking involves contrast suppression and not (purely) mask-induced noise. We conclude that cross-channel masking can be a powerful phenomenon, particularly at low test spatial frequencies and when mask and test are presented to different eyes. © 2004 ARVO.

The investigation of the sensitivity gradient of the visual field, as a function of stimulus dynamic range, in the normal and abnormal eye

The study utilized the advanced technology provided by automated perimeters to investigate the hypothesis that patients with retinitis pigmentosa behave atypically over the dynamic range and to concurrently determine the influence of extraneous factors on the format of the normal perimetric sensitivity profile. The perimetric processing of some patients with retinitis pigmentosa was considered to be abnormal in either the temporal and/or the spatial domain. The standard size III stimulus saturated the central regions and was thus ineffective in detecting early depressions in sensitivity in these areas. When stimulus size was scaled in inverse proportion to the square root of ganglion cell receptive field density (M-scaled), isosensitive profiles did not result, although cortical representation was theoretically equivalent across the visual field. It was conjectured that this was due to variations in the ganglion cell characteristics with increasing peripheral angle, most notably spatial summation. It was concluded that the development of perimetric routines incorporating stimulus sizes adjusted in proportion to the coverage factor of retinal ganglion cells would enhance the diagnostic capacity of perimetry. Good general and local correspondence was found between perimetric sensitivity and the available retinal cell counts. Intraocular light scatter arising both from simulations and media opacities depressed perimetric sensitivity. Attenuation was greater centrally for the smaller LED stimuli, whereas the reverse was true for the larger projected stimuli. Prior perimetric experience and pupil size also demonstrated eccentricity-dependent effect on sensitivity. Practice improved perimetric sensitivity for projected stimuli at eccentricities greater than or equal to 30o; particularly in the superior region. Increase in pupil size for LED stimuli enhanced sensitivity at eccentricities greater than 10o. Conversely, microfluctuation in the accommodative response during perimetric examination and the correction of peripheral refractive error had no significant influence on perimetric sensitivity.

Modelling frontal discontinuities in wind fields

A Bayesian procedure for the retrieval of wind vectors over the ocean using satellite borne scatterometers requires realistic prior near-surface wind field models over the oceans. We have implemented carefully chosen vector Gaussian Process models; however in some cases these models are too smooth to reproduce real atmospheric features, such as fronts. At the scale of the scatterometer observations, fronts appear as discontinuities in wind direction. Due to the nature of the retrieval problem a simple discontinuity model is not feasible, and hence we have developed a constrained discontinuity vector Gaussian Process model which ensures realistic fronts. We describe the generative model and show how to compute the data likelihood given the model. We show the results of inference using the model with Markov Chain Monte Carlo methods on both synthetic and real data.

Evaluation of an open-field autorefractor's ability to measure refraction and hence potential to assess objective accommodation in pseudophakes

Background: To evaluate the accuracy of an open-field autorefractor compared with subjective refraction in pseudophakes and hence its ability to assess objective eye focus with intraocular lenses (IOLs). Methods: Objective refraction was measured at 6 m using the Shin-Nippon NVision-K 5001/Grand Seiko WR-5100K open-field autorefractor (five repeats) and by subjective refraction on 141 eyes implanted with a spherical (Softec1 n=53), aspherical (SoftecHD n=37) or accommodating (1CU n=22; Tetraflex n=29) IOL. Autorefraction was repeated 2 months later. Results: The autorefractor prescription was similar (average difference: 0.09±0.53 D; p=0.19) to that found by subjective refraction, with ~71% within ±0.50 D. The horizontal cylindrical components were similar (difference: 0.00±0.39 D; p=0.96), although the oblique (J45) autorefractor cylindrical vector was slightly more negative (by -0.06±0.25 D; p=0.06) than the subjective refraction. The results were similar for each of the IOL designs except for the spherical IOL, where the mean spherical equivalent difference between autorefraction and subjective was more hypermetropic than the Tetraflex accommodating IOL (F=2.77, p=0.04). The intrasession repeatability was

The influence of age on the pattern and flash visual evoked field

We have investigated the effect of ageing on the visual system using the relatively new technique of magentoencephalography (MEG). This technique measures the magnetic signals produced by the visual system using a SQUID magnetometer. The magnetic visual evoked field (VEF) was measured over the occipital cortex to pattern and flash stimuli in 86 normal subjects aged 15 - 86 years. Factors that influenced subject defocussing or defixating the stimulus or selective attention were controlled as far as possible. The latency of the major positive component to the pattern reversal stimulus (P100M) increased with age particularly after the age of 55 years while the amplitude of the P100M decreased over the life span. The latency of the major flash component (P2M) increased much more slowly with age, while its amplitude decreased in only a proportion of elderly subjects. Changes in the P100M with age may reflect senile changes in the eye and optic nerve, e.g. senile miosis or degenerative changes in the retina. The P2M may be more susceptible to senile changes in the retina. The data suggest that the spatial frequency channels deteriorate more rapidly with age than the luminance channels and that MEG may be an effective method of studying ageing in the visual system.

The correlation between tests of visual function and perceived visual ability in central field loss patients

Purpose: To investigate the correlation between tests of visual function and perceived visual ability recorded with a 'quality-of-life' questionnaire for patients with central field loss. Method: 12 females and 7 males (mean age = 53.1 years; Range = 23 - 80 years) with subfoveal neovascular membranes underwent a comprehensive assessment of visual function. Tests included unaided distance vision, high and low contrast distance logMAR visual acuity (VA), Pelli-Robson contrast senstivity (at 1m), near logMAR word VA and text reading speed. All tests were done both monocularly and binocularly. The patients also completed a 28 point questionnaire separated into a 'core' section consisting of general questions about perceived visual function and a 'module' section with specific questions on reading function. Results: Step-wise multiple regression analysis was used to determine which visual function tests were correlated with the patients's perceived visual function and to rank them in order of importance. The visual function test that explains most of the variance in both 'core' score (66%0 and the 'module' score (68%) of the questionnaire is low contrast VA in the better eye (P<0.001 in both cases). Further, the module score also accounts for a significant proportion of the variance (P<0.01) of the distance logMAR VA in both the better and worse eye, and the near logMAR in both the better eye and binocularly. Conclusions: The best predictor of both perceived reading ability and of general perceived visual ability in this study is low contrast logMAR VA. The results highlight that distance VA is not the only relevant measure of visual fucntion in relation to a patients's perceived visual performance and should not be considered a determinant of surgical or management success.

Investigation of chromatic function in the normal and abnormal eye

This study examined the use of non-standard parameters to investigate the visual field, with particular reference to the detection of glaucomatous visual field loss. Evaluation of the new perimetric strategy for threshold estimation - FASTPAC, demonstrated a reduction in the examination time of normals compared to the standard strategy. Despite an increased within-test variability the FASTPAC strategy produced a similar mean sensitivity to the standard strategy, reducing the effects of patient fatigue. The new technique of Blue-Yellow perimetry was compared to White-White perimetry for the detection of glaucomatous field loss in OHT and POAG. Using a database of normal subjects, confidence limits for normality were constructed to account for the increased between-subject variability with increase in age and eccentricity and for the greater variability of the Blue-Yellow field compared to the White-White field. Effects of individual ocular media absorption had little effect on Blue-Yellow field variability. Total and pattern probability analysis revealed five of 27 OHTs to exhibit Blue-Yellow focal abnormalities; two of these patients subsequently developed White-White loss. Twelve of the 24 POAGs revealed wider and/or deeper Blue-Yellow loss compared with the White-White field. Blue-Yellow perimetry showed good sensitivity and specificity characteristics, however, lack of perimetric experience and the presence of cataract influenced the Blue-Yellow visual field and may confound the interpretation of Blue-Yellow visual field loss. Visual field indices demonstrated a moderate relationship to the structural parameters of the optic nerve head using scanning laser tomography. No abnormalities in Blue-Yellow or Red-Green colour CS was apparent for the OHT patients. A greater vulnerability of the SWS pathway in glaucoma was demonstrated using Blue-Yellow perimetry however predicting which patients may benefit from B-Y perimetric examination is difficult. Furthermore, cataract and the extent of the field loss may limit the extent to which the integrity of the SWS channels can be selectively examined.

Non-standard techniques for the investigation of the visual field

The study investigated the potential applications and the limitations of non-standard techniques of visual field investigation utilizing automated perimetry. Normal subjects exhibited a greater sensitivity to kinetic stimuli than to static stimuli of identical size. The magnitude of physiological SKD was found to be largely independent of age, stimulus size, meridian and eccentricity. The absence of a dependency on stimulus size indicated that successive lateral spatial summation could not totally account for the underlying mechanism of physiological SKD. The visual field indices MD and LV exhibited a progressive deterioration during the time course of a conventional central visual field examination both for normal subjects and for ocular hypertensive patients. The fatigue effect was more pronounced in the latter stages and for the second eye tested. The confidence limits for the definition of abnormality should reflect the greater effect of fatigue on the second eye. A 330 cdm-2 yellow background was employed for blue-on-yellow perimetry. Instrument measurement range was preserved by positioning a concave mirror behind the stimulus bulb to increase the light output by 60% . The mean magnitude of SWS pathway isolation was approximately 1.4 log units relative to a 460nm stimulus filter. The absorption spectra of the ocular media exhibited an exponential increase with increase in age, whilst that of the macular pigment showed no systematic trend. The magnitude of ocular media absorption was demonstrated to reduce with increase in wavelength. Ocular media absorption was significantly greater in diabetic patients than in normal subjects. Five diabetic patients with either normal or borderline achromatic sensitivity exhibited an abnormal blue-on-yellow sensitivity; two of these patients showed no signs of retinopathy. A greater vulnerability of the SWS pathway to the diabetic disease process was hypothesized.

Visual field and structural correlates of progression in age-related macular degeneration

Presentation Purpose:To examine the correlation of central visual field loss and progression of structural changes in the macular area in age-related macular degeneration (AMD). Methods:Central 10° standard and short-wavelength automated perimetry (SWAP) visual fields were acquired in 39 eyes of 24 patients with AMD using a Humphrey Field Analyzer. Stereoscopic fundus photographs were graded1 by two independent observers and the stage of disease determined2. Custom software mapped perimetric data onto fundus images in order to relate structural changes to functional loss. Results:Mean deviation (MD) in standard perimetry changed from 0.04 dB at stage 1 to -12.39 dB at stage 4 (r2=0.48, p<0.001). The group mean SWAP MD was -5.26 dB at stage 1 and increased to -17.08 dB at stage 4 (r2=0.53, p<0.001). Pattern standard deviation (PSD) also increased with advancing stage in standard perimetry; 1.32 dB to 8.67 dB at stage 1 and 4, respectively (r2=0.54, p<0.001). In SWAP, PSD increased from 2.86 dB to 5.63 dB at stage 1 and stage 4 (r2=0.43, p<0.001). Defect frequency was greater in SWAP than standard perimetry. Early stage defects occurred with the greatest frequency at eccentricities of 3.2° and 5.1° in standard perimetry and at 4.2° in SWAP. Late stage defects were most frequent at 1° eccentricity in standard perimetry and at 1° and 9° in SWAP. MD declined with increasing affected retinal area over the central 3000µm; by 0.20 dB (r2=0.67, p<0.001) and 0.18 dB (r2=0.49, p<0.001) per % increase in defect area for standard perimetry and SWAP respectively. 41% of defects were associated with structural changes on the retina in standard perimetry and 43% in SWAP. Conclusions:Sensitivity decreased with advancing stage of AMD, with a greater effect demonstrated in SWAP compared to standard perimetry. The central field became less uniform as stage increased. SWAP defects occurred at similar locations but were deeper and wider than corresponding defects in standard perimetry. Central loss in SWAP is a sensitive marker of functional progression in AMD.1. Bird et al. (1995) Surv Ophthalmol 39:367-3742. van Leeuwen et al. (2003) Arch Ophthalmol 121:519-526

Design and validity of a miniaturized open-field aberrometer

PURPOSE: To validate a new miniaturised, open-field wavefront device which has been developed with the capacity to be attached to an ophthalmic surgical microscope or slit-lamp. SETTING: Solihull Hospital and Aston University, Birmingham, UK DESIGN: Comparative non-interventional study. METHODS: The dynamic range of the Aston Aberrometer was assessed using a calibrated model eye. The validity of the Aston Aberrometer was compared to a conventional desk mounted Shack-Hartmann aberrometer (Topcon KR1W) by measuring the refractive error and higher order aberrations of 75 dilated eyes with both instruments in random order. The Aston Aberrometer measurements were repeated five times to assess intra-session repeatability. Data was converted to vector form for analysis. RESULTS: The Aston Aberrometer had a large dynamic range of at least +21.0 D to -25.0 D. It gave similar measurements to a conventional aberrometer for mean spherical equivalent (mean difference ± 95% confidence interval: 0.02 ± 0.49D; correlation: r=0.995, p<0.001), astigmatic components (J0: 0.02 ± 0.15D; r=0.977, p<0.001; J45: 0.03 ± 0.28; r=0.666, p<0.001) and higher order aberrations RMS (0.02 ± 0.20D; r=0.620, p<0.001). Intraclass correlation coefficient assessments of intra-sessional repeatability for the Aston Aberrometer were excellent (spherical equivalent =1.000, p<0.001; astigmatic components J0 =0.998, p<0.001, J45=0.980, p<0.01; higher order aberrations RMS =0.961, p<0.001). CONCLUSIONS: The Aston Aberrometer gives valid and repeatable measures of refractive error and higher order aberrations over a large range. As it is able to measure continuously, it can provide direct feedback to surgeons during intraocular lens implantations and corneal surgery as to the optical status of the visual system.

Eye essentials:visual fields

This book covers the essentials of visual field measurement from the anatomical concepts of the visual pathway and how they relate to visual field loss to the measurement and analysis of visual fields. Eye Essentials is a major new series which provides authoritative and accessible information for all eye care professionals, whether in training or in practice. Each book is a rapid revision aid for students taking higher professional qualifications and a handy clinical reference guide for practitioners in busy clinics. Highly designed with synoptic text, handy tables, key bullet points, summaries, icons and stunning full colour illustrations, the books have rapidly established themselves as the essential eye clinic pocket books.

Visual field severity indices demonstrate dose-dependent visual loss from Vigabatrin therapy

Purpose: The aims of this study were to develop an algorithm to accurately quantify Vigabatrin (VGB)-induced central visual field loss and to investigate the relationship between visual field loss and maximum daily dose, cumulative dose and duration of dose. Methods: The sample comprised 31 patients (mean age 37.9 years; SD 14.4 years) diagnosed with epilepsy and exposed to VGB. Each participant underwent standard automated static visual field examination of the central visual field. Central visual field loss was determined using continuous scales quantifying severity in terms of area and depth of defect and additionally by symmetry of defect between the two eyes. A simultaneous multiple regression model was used to explore the relationship between these visual field parameters and the drug predictor variables. Results: The regression model indicated that maximum VGB dose was the only factor to be significantly correlated with individual eye severity (right eye: p = 0.020; left eye: p = 0.012) and symmetry of visual field defect (p = 0.024). Conclusions: Maximum daily dose was the single most reliable indicator of those patients likely to exhibit visual field defects due to VGB. These findings suggest that high maximum dose is more likely to result in visual field defects than high cumulative doses or those of long duration.

Regional variability in visual field sensitivity during hypercapnia

PURPOSE: Previous investigations have demonstrated a relative vascular autoregulatory inefficiency of the inferior compared to the superior retina in healthy subjects breathing increased CO2. The purpose of this study was to determine whether the superior and inferior visual field sensitivities of healthy eyes are similarly affected during mild hypercapnia. DESIGN: Experimental study. METHODS: Visual field analysis (Humphrey Field Analyser; SITA standard 24-2 program) was carried out on one randomly selected eye of 22 subjects (mean age, 27.7 ± 5 years) during normal room air breathing and isoxic hypercapnia. The Student paired t-tests were used to compare the visual field indices mean deviation (MD) and pattern standard deviation (PSD) for each breathing condition. A secondary, sectoral analysis of mean pointwise sensitivity was performed for each condition. In each case a P value of <.01 was considered statistically significant (Bonferroni corrected). RESULTS: Visual field MD was -0.23 ± 0.95dB during room air breathing and -0.49 ± 1.04dB during hypercapnia (P = .034). Sectoral pointwise mean sensitivity deteriorated by 0.46dB (P = .006) in the upper visual hemifield during hypercapnia, whereas no significant difference was observed for the lower hemifield (P = .331). CONCLUSIONS: The upper visual hemifield exhibited a significantly greater degree of deterioration in pointwise visual field mean sensitivity compared to the lower hemifield during hypercapnic conditions. This suggests that the upper visual hemifield and hence inferior retina is more susceptible to insult during hypercapnia than the superior retina in healthy individuals. A regional susceptibility of inferior retinal function to altered vascular or metabolic effects may account for the earlier and more frequent inferior nerve fibre damage associated with glaucomatous optic neuropathy. © 2003 by Elsevier Science Inc. All rights reserved.

Impact in contact lenses and the anterior eye:challenges prevailing in 2015

As we settle into a new year, this second issue of Contact Lens and Anterior Eye allows us to reflect on how new research in this field impacts our understanding, but more importantly, how we use this evidence basis to enhance our day to day practice, to educate the next generation of students and to construct the research studies to deepen our knowledge still further. The end of 2014 saw the publication of the UK governments Research Exercise Framework (REF) which ranks Universities in terms of their outputs (which includes their paper, publications and research income), environment (infrastructure and staff support) and for the first time impact (defined as “any effect on, change or benefit to the economy, society, culture, public policy or services, health, the environment or quality of life, beyond academia” [8]). The REF is a process of expert review, carried out in 36 subject-based units of assessment, of which our field is typically submitted to the Allied Health, Dentistry, Nursing and Pharmacy panel. Universities that offer Optometry did very well with Cardiff, Manchester and Aston in the top 10% out of the 94 Universities that submitted to this panel (Grade point Average ranked order). While the format of the new exercise (probably in 2010) to allocate the more than £2 billion of UK government research funds is yet to be determined, it is already rumoured that impact will contribute an even larger proportion to the weighting. Hence it is even more important to reflect on the impact of our research. In this issue, Elisseef and colleagues [5] examine the intriguing potential of modifying a lens surface to allow it to bind to known wetting agents (in this case hyaluronic acid) to enhance water retention. Such a technique has the capacity to reduced friction between the lens surface and the eyelids/ocular surface, presumably leading to higher comfort and less reason for patients to discontinue with lens wear. Several papers in this issue report on the validity of new high precision, fast scanning imaging and quantification equipment, utilising techniques such as Scheimpflug, partial coherence interferometry, aberrometry and video allowing detailed assessment of anterior chamber biometry, corneal topography, corneal biomechanics, peripheral refraction, ocular aberrations and lens fit. The challenge is how to use this advanced instrumentation which is becoming increasingly available to create real impact. Many challenges in contact lenses and the anterior eye still prevail in 2015 such as: -While contact lens and refractive surgery complications are relatively rare, they are still too often devastating to the individual and their quality of life (such as the impact and prognosis of patients with Acanthmoeba Keratitis reported by Jhanji and colleagues in this issue [7]). How can we detect those patients who are going to be affected and what modifications do we need to make to contact lenses and patient management prevent this occurring? -Drop out from contact lenses still occurs at a rapid rate and symptoms of dry eye seem to be the leading cause driving this discontinuation of wear [1] and [2]. What design, coating, material and lubricant release mechanism will make a step change in end of day comfort in particular? -Presbyopia is a major challenge to hassle free quality vision and is one of the first signs of ageing noticed by many people. As an emmetrope approaching presbyopia, I have a vested interest in new medical devices that will give me high quality vision at all distances when my arms won’t stretch any further. Perhaps a new definition of presbyopia could be when you start to orientate your smartphone in the landscape direction to gain the small increase in print size needed to read! Effective accommodating intraocular lenses that truly mimic the pre-presbyopic crystalline lenses are still a way off [3] and hence simultaneous images achieved through contact lenses, intraocular lenses or refractive surgery still have a secure future. However, splitting light reaching the retina and requiring the brain to supress blurred images will always be a compromise on contrast sensitivity and is liable to cause dysphotopsia; so how will new designs account for differences in a patient's task demands and own optical aberrations to allow focused patient selection, optimising satisfaction? -Drug delivery from contact lenses offers much in terms of compliance and quality of life for patients with chronic ocular conditions such as glaucoma, dry eye and perhaps in the future, dry age-related macular degeneration; but scientific proof-of-concept publications (see EIShaer et al. [6]) have not yet led to commercial products. Part of this is presumably the regulatory complexity of combining a medical device (the contact lens) and a pharmaceutical agent. Will 2015 be the year when this innovation finally becomes a reality for patients, bringing them an enhanced quality of life through their eye care practitioners and allowing researchers to further validate the use of pharmaceutical contact lenses and propose enhancements as the technology matures? -Last, but no means least is the field of myopia control, the topic of the first day of the BCLA's Conference in Liverpool, June 6–9th 2015. The epidemic of myopia is a blight, particularly in Asia, with significant concerns over sight threatening pathology resulting from the elongated eye. This is a field where real impact is already being realised through new soft contact lens optics, orthokeratology and low dose pharmaceuticals [4], but we still need to be able to better predict which technique will work best for an individual and to develop new techniques to retard myopia progression in those who don’t respond to current treatments, without increasing their risk of complications or the treatment impacting their quality of life So what will your New Year's resolution be to make 2015 a year of real impact, whether by advancing science or applying the findings published in journals such as Contact Lens and Anterior Eye to make a real difference to your patients’ lives?

Does the Swedish Interactive Threshold Algorithm (SITA) accurately map visual field loss attributed to vigabatrin?

Background: Vigabatrin (VGB) is an anti-epileptic medication which has been linked to peripheral constriction of the visual field. Documenting the natural history associated with continued VGB exposure is important when making decisions about the risk and benefits associated with the treatment. Due to its speed the Swedish Interactive Threshold Algorithm (SITA) has become the algorithm of choice when carrying out Full Threshold automated static perimetry. SITA uses prior distributions of normal and glaucomatous visual field behaviour to estimate threshold sensitivity. As the abnormal model is based on glaucomatous behaviour this algorithm has not been validated for VGB recipients. We aim to assess the clinical utility of the SITA algorithm for accurately mapping VGB attributed field loss. Methods: The sample comprised one randomly selected eye of 16 patients diagnosed with epilepsy, exposed to VGB therapy. A clinical diagnosis of VGB attributed visual field loss was documented in 44% of the group. The mean age was 39.3 years∈±∈14.5 years and the mean deviation was -4.76 dB ±4.34 dB. Each patient was examined with the Full Threshold, SITA Standard and SITA Fast algorithm. Results: SITA Standard was on average approximately twice as fast (7.6 minutes) and SITA Fast approximately 3 times as fast (4.7 minutes) as examinations completed using the Full Threshold algorithm (15.8 minutes). In the clinical environment, the visual field outcome with both SITA algorithms was equivalent to visual field examination using the Full Threshold algorithm in terms of visual inspection of the grey scale plots, defect area and defect severity. Conclusions: Our research shows that both SITA algorithms are able to accurately map visual field loss attributed to VGB. As patients diagnosed with epilepsy are often vulnerable to fatigue, the time saving offered by SITA Fast means that this algorithm has a significant advantage for use with VGB recipients.