Comparing paediatric optometric vision care in Canada over a 14-year period

PURPOSE

In Canada, teaching in paediatric eye care has increased in the past decade both within the optometry curriculum and as continuing education to optometrists. Paediatric vision care guidelines have also been established by North American optometric associations. This study examined whether this exposure was associated with changes in paediatric eye care in Canada over a 14-year period.

METHODS

Canadian optometrists were invited to participate in an anonymous 35-item survey in 2007 and 2021. The surveys sought to investigate optometrist's recommendations for first eye examinations, the number of paediatric patients seen in a typical week and preparedness to provide eye examinations to children. Response frequencies were determined for each survey item.

RESULTS

Across Canada, 133/1000 (13.3%) and 261/~6419 (~4.1%) optometrists responded to the survey in 2007 and 2021, respectively. No significant difference was found in the number of years practicing, days per week in practice and total number of patients seen per week. The modal age optometrists recommended children be seen for their first eye examination changed from 3-4 years in 2007 (53%) to 6-12 months in 2021 (61%). In 2007, 87% of respondents provided eye examinations to children <2 years, increasing to 94% in 2021 (p = 0.02). Despite a reduction in the recommended age between the two survey years, the most frequent age children were seen for their first eye examination was 3-4 years (30% in both surveys) and the most common age seen in a typical week remained unchanged (4-6 years-56% 2007; 66% 2021).

CONCLUSION

Although optometrists' willingness to provide paediatric eye care increased over the past 14 years, the number of children seen in a typical week did not change. Barriers to determine why more children are not being seen at an earlier age need to be investigated.

An Urgent Call for Concussion Incidence Measures in Para Sport for Athletes with Vision Impairment: A Narrative Review

Concussion in para athletes with vision impairment (VI) is poorly understood. Recently published studies have suggested that athletes with VI may be more likely to sustain sport-related concussions compared to non-disabled athletes and athletes with other impairment types. There is a critical need for objective concussion incidence measures to determine concussion injury rates and risks more accurately. The aim of this review was to examine the limited available evidence of concussion incidence rates across six different para sports for athletes with VI and encourage the future collection of concussion incidence data and the adoption of injury prevention strategies in VI para sport. A literature search was conducted using four unique databases, which formed the basis of this narrative review. Injury prevention strategies such as modifying sport rules, introducing protective equipment, and incorporating additional safety measures into the field of play have been introduced sporadically, but the effectiveness of most strategies remains unknown. More prospective, sport-specific research examining mechanisms of injury and risk factors for concussion injuries in athletes with VI in both training and competition is needed. This research will help inform the development of targeted injury prevention strategies to reduce the likelihood of concussion for athletes with VI.

Measuring recognition visual acuity in young children - testability with the Waterloo Differential Acuity Test (WatDAT)

CLINICAL RELEVANCE

Visual acuity measurement is important for the detection and monitoring of eye disorders. Developing accurate and sensitive visual acuity tests suitable for young children is therefore desirable.

BACKGROUND

Recognition or form visual acuity (VA), which is measured with matching in children aged 3 years and up, is more sensitive for detecting visual deficits compared to resolution VA. The Waterloo Differential Acuity Test (WatDAT) is a proposed recognition VA test using the concept of identifying the "odd one out" among distractors. The WatDAT is expected to be cognitively easier than matching tests and therefore may be used in younger children. The purpose of this study is to investigate the testability of the WatDAT paradigm in children aged 12-36 months, and to determine the optimum format and number of distractors.

METHODS

Fifty-one typically-developing children aged 12-36 months participated in the study. Data for Patti Pics (PP) and Face targets (FT) were collected for formats with 3, 4 and 5 distractors. The targets were presented binocularly on a computer touch screen at 30 cm. The task was to touch the face among identical non-faces or a house among circles. Following initial training, there were 5 presentations for each distractor format. Testability was defined as correctly identifying at least 4/5 presentations and was also determined for uncrowded PP symbols using matching.

RESULTS

Of participants aged 18-36 months, 87% could perform the WatDAT PP targets with 3 distractors compared to 68% for the FT, while 48% could perform matching with PP. The testability for FT increased to 85% for children ≥22 months. Younger children showed lower testability. For the 3 distractor format, PP targets gave 9% testability in children 12 to <18 months, and FT gave a testability of 16% in children 12 to <22 months.

CONCLUSION

WatDAT testability is higher than matching VA tests. This indicates that the newly developed WatDAT has potential for measuring recognition VA in children 18 months and older.

Validation of the Waterloo Differential Visual Acuity Test (WatDAT) and Comparison With Existing Pediatric Tests of Visual Acuity

Purpose

The new Waterloo Differential Acuity Test (WatDAT) is designed to allow recognition visual acuity (VA) measurement in children before they can typically undertake matching tests. The study purpose was to validate WatDAT in adults with normal and reduced VA.

Methods

Eighty adults (18 to <40 years of age) participated (32 normal VA, 12 reduced VA, and 36 simulated reduced VA). Monocular VA was measured on two occasions in random order for WatDAT (versions with 3 and 5 distractors for Faces and Patti Pics house among circles), Lea Symbols, Kay Pictures and Patti Pics matching tests, Teller Acuity Cards, Cardiff Acuity Test, and Early Treatment Diabetic Retinopathy Study (ETDRS) letter chart. Pediatric tests were validated against ETDRS using limits of agreement (LoA), sensitivity, and specificity. The LoA for repeatability were also determined.

Results

WatDAT showed minimal bias compared with ETDRS, and LoAs, which were similar to pediatric matching tests (0.241-0.250). Both preferential looking tests showed higher bias and LoAs than ETDRS. Matching tests showed good agreement with ETDRS, except for Kay Pictures and Lea Uncrowded test, which overestimated VA. WatDAT showed high sensitivity (>0.96) and specificity (>0.79), which improved with criterion adjustment and were significantly higher than for the preferential looking tests. LoA for repeatability for WatDAT 3 Faces and WatDAT 5 Faces were comparable with the ETDRS.

Conclusions

WatDAT demonstrates good agreement and repeatability compared with the gold-standard ETDRS letter chart, and performed better than preferential looking tests, the alternative until a child can undertake a matching VA test.

Translational Relevance

Good validity of the Waterloo Differential Acuity Test was demonstrated in adults as a first step to showing its potential for detecting childhood visual disorders.

Trust and Access: Eye Information Seeking Practices and Preferences among Canadians

SIGNIFICANCE

This study highlights the value that the public places on obtaining trusted and accessible health-related information and their preference for obtaining it from their healthcare practitioners. Previous research has not been specific to Canadians or vision. Findings can be used to increase eye health literacy and eye care utilization.

PURPOSE

Canadians underutilize eye care and underestimate the occurrence of asymptomatic eye disease. This study explored eye information seeking practices and preferences among a group of Canadians.

METHODS

Using snowball sampling, a 28-item online survey collected respondent perceptions about their eye and health information seeking practices and preferences. Questions examined electronic device access, information source use, and demographics. Two open-ended questions examined information seeking practices and preferences. Respondents were at least 18 years old and living in Canada. Individuals working in eye care were excluded. Response frequencies and Z scores were computed. Written comments were assessed using content analysis.

RESULTS

Respondents searched for less eye than health information (Z scores ≥ 2.25, P < .05). For eye and health information, primary care providers were the used and preferred source, and reliance on internet searches was greater than desired. Trust and access drove information seeking practices. Respondent comments suggested a hierarchy of trust operates across My Health Team, My Network, and My External Sources, with a persistent threat posed by Discredited Sources. Access to information sources appeared mediated by enablers (Convenience and Accessible Features) and barriers (Unreachable Health Team and Absent Systems). Eye information was seen as more specialized and harder to find. There was a high regard for healthcare practitioners who provide their patients with curated trusted information.

CONCLUSIONS

These Canadians value trusted and accessible health-related information. They prefer eye and health information from their healthcare practitioners and value when their health team provides online curated information, particularly regarding eyes.

What the Canadian public (mis)understands about eyes and eye care

CLINICAL RELEVANCE

Inadequate public knowledge about eyes and eye care poses avoidable risks to vision-related quality of life.

BACKGROUND

This study of eye care knowledge among Canadians extends earlier findings from focus groups.

METHODS

Perceptions about eyes and eye care were sought using a 21-item online survey and snowball sampling. Inclusion criteria were living in Canada and being at least 18-years old; eye care professionals and staff were excluded. Response frequencies were converted to percentages, with eye condition items analysed according to 'expected' or 'unexpected' eye impacts. Proportions selecting these impacts or 'unsure' were determined.

RESULTS

There were 424 respondents: 83.0% aged 20-65 years and 69.6% female. Mismatches existed between perceived recommendations and behaviours for booking eye exams: within two years (86.7% vs. 68.4%) and symptom-driven (3.3% vs. 13.0%). First eye exams after age one year were deemed appropriate by 43.6%. Few respondents associated glaucoma with no symptoms (6.0%) or amblyopia with blurred vision (13.5%). A notable proportion incorrectly related tunnel vision with age-related macular degeneration (AMD, 36.8%) and cataract (21.9%). Identifying all 'expected' responses was unlikely for glaucoma (1.9%), amblyopia (6.7%), and cataract (12.0%). Most respondents identified no 'expected' effects for glaucoma (63.8%) and AMD (46.2%) and some 'expected' effects for cataract (59.5%) and amblyopia (72.6%). Selecting 'unsure' was 9-10 times more common among respondents choosing no 'expected' impacts than those choosing some. Awareness of thyroid-associated eye disease was lowest (32.4%) of seven conditions. Respondents were most likely to consult optometrists for routine eye exams, eye disease, diabetes eye checks and blurred vision but family physicians for red eyes and sore eyes. Respondents typically paid for their eye exams and eyewear but wanted government to pay.

CONCLUSION

Vision-threatening knowledge gaps and misinformation about eyes and eye care among Canadian respondents highlight the need for accessible, targeted public education.

Shape judgments in natural scenes: Convexity biases versus stereopsis

Determining the relief of upcoming terrain is critical to locomotion over rough or uneven ground. Given the significant contribution of stereopsis to perceived surface shape, it should play a crucial role in determining the shape of ground surfaces. The aim of this series of experiments was to evaluate the relative contribution of monocular and binocular depth cues to judgments of ground relief. To accomplish this goal, we simulated a depth discrimination task using naturalistic imagery. Stimuli consisted of a stereoscopically rendered grassy terrain with a central mound or a dip with varying height. We measured thresholds for discrimination of the direction of the depth offset. To determine the relationship between relief discrimination and measures of stereopsis, we used two stereoacuity tasks performed under the same viewing conditions. To assess the impact of ambiguous two-dimensional shading cues on depth judgments in our terrain task, we manipulated the intensity of the shading (low and high). Our results show that observers reliably discriminated ground reliefs as small as 20 cm at a viewing distance of 9.1 m. As the shading was intensified, a large proportion of observers (30%) exhibited a strong convexity bias, even when stereopsis indicated a concave depression. This finding suggests that there are significant individual differences in the reliance on assumptions of surface curvature that must be considered in experimental conditions. In impoverished viewing environments with limiting depth cues, these convexity biases could persist in judgments of ground relief, especially when shading cues are highly salient.

Contributions of Stereopsis and Aviation Experience to Simulated Rotary Wing Altitude Estimation

OBJECTIVE

We examined the contribution of binocular vision and experience to performance on a simulated helicopter flight task.

BACKGROUND

Although there is a long history of research on the role of binocular vision and stereopsis in aviation, there is no consensus on its operational relevance. This work addresses this using a naturalistic task in a virtual environment.

METHOD

Four high-resolution stereoscopic terrain types were viewed monocularly and binocularly. In separate experiments, we evaluated performance of undergraduate students and military aircrew on a simulated low hover altitude judgment task. Observers were asked to judge the distance between a virtual helicopter skid and the ground plane.

RESULTS

Our results show that for both groups, altitude judgments are more accurate in the binocular viewing condition than in the monocular condition. However, in the monocular condition, aircrew were more accurate than undergraduate observers in estimating height of the skid above the ground.

CONCLUSION

At simulated altitudes of 5 ft (1.5 m) or less, binocular vision provides a significant advantage for estimation of the depth separation between the landing skid and the ground, regardless of relevant operational experience. However, when binocular cues are unavailable aircrew outperform undergraduate observers, a result that likely reflects the impact of training on the ability to interpret monocular depth cues.

Differential visual acuity - A new approach to measuring visual acuity

BACKGROUND

A novel type of acuity measurement, which we refer to as 'differential acuity', requires the observer to identify one unique target among three others which are identical. This is a proof of concept study aimed to determine if differential acuity is equivalent to standard measures of recognition acuity.

METHODS

To create a range of visual acuity, vision was optically blurred in sixteen adults with normal visual acuity. Visual acuity was then measured with the differential acuity targets in both crowded and uncrowded format, and compared with standard ETDRS acuity or with singly presented letters and uncrowded letters were analysed separately.

RESULTS

The visual acuity results for crowded and uncrowded letters were analysed separately. Repeated measures analysis of variance showed that when a crowded Sloan C had to be differentiated from three crowded Os (CvsO), the results were not significantly different from ETDRS acuity or from naming one of four letters presented centrally (Name4) (p<0.05). Similar results were found for uncrowded letters - the C versus O and Name4 gave similar visual acuity. The 95% limits of agreement between the naming and C versus O differential acuity measures were between 0.17 and 0.27 logMAR.

CONCLUSION

From this proof of concept study we conclude that differential acuity gives similar results to the ETDRS chart in adults. We infer that the comparable but cognitively simpler differential visual acuity task could be applied in clinical settings for young children or patients with developmental delay who cannot respond by naming or matching.

Motivators and deterrents for seeking eye care in a Canadian region

BACKGROUND

Motivators and deterrents for seeking eye care in a Canadian setting were sought using a qualitative study. Provincial deregulation of eyewear dispensing in 2010 allows consumers to order eyewear without an optical prescription, thus eliminating a potential motivator for obtaining an eye examination.

METHODS

Convenience sampling was used to obtain 25 members of the public who contributed to one of seven focus groups that were facilitated, audiotaped, anonymised and transcribed. Participants completed a demographic questionnaire. Focus group data analysis employed grounded theory and theme saturation determined the number of focus groups.

RESULTS

Nine men and 16 women participated, ranging in age from 18 to 71 years (mean 41.5; median 40.0). Three main themes were identified as influencers for seeking eye care: priority; advice; and capacity. Priority served as a motivator ('lived experience', 'symptoms', and 'habit') and deterrent ('test distress', 'asymptomatic', 'don't know' and 'other priorities'). Advice was a motivator ('professional' and 'family/friends'), while capacity was a motivator ('insurance') and deterrent ('cost').

CONCLUSION

The motivators and deterrents of seeking eye care in these focus groups were framed by three themes. Key findings not reported previously included the motivators of 'habit', 'advice' and 'insurance' and the potential deterrent of 'test distress'. These factors should be added to other previously reported motivators and deterrents in further exploration of Canadian eye-care seeking behaviours. Such knowledge is needed to develop strategies for improving eye-care literacy in Canada. This is particularly important because eyewear deregulation and/or online eye examinations may encourage members of the public to bypass comprehensive eye care without fully understanding the implications of this decision for their health and wellness.

Refractive error magnitude and variability: Relation to age

PURPOSE

To investigate mean ocular refraction (MOR) and astigmatism, over the human age range and compare severity of refractive error to earlier studies from clinical populations having large age ranges.

METHODS

For this descriptive study patient age, refractive error and history of surgery affecting refraction were abstracted from the Waterloo Eye Study database (WatES). Average MOR, standard deviation of MOR and astigmatism were assessed in relation to age. Refractive distributions for developmental age groups were determined. MOR standard deviation relative to average MOR was evaluated. Data from earlier clinically based studies with similar age ranges were compared to WatES.

RESULTS

Right eye refractive errors were available for 5933 patients with no history of surgery affecting refraction. Average MOR varied with age. Children <1 yr of age were the most hyperopic (+1.79D) and the highest magnitude of myopia was found at 27yrs (-2.86D). MOR distributions were leptokurtic, and negatively skewed. The mode varied with age group. MOR variability increased with increasing myopia. Average astigmatism increased gradually to age 60 after which it increased at a faster rate. By 85+ years it was 1.25D. J₀ power vector became increasingly negative with age. J₄₅ power vector values remained close to zero but variability increased at approximately 70 years. In relation to comparable earlier studies, WatES data were most myopic.

CONCLUSIONS

Mean ocular refraction and refractive error distribution vary with age. The highest magnitude of myopia is found in young adults. Similar to prevalence, the severity of myopia also appears to have increased since 1931.

Visual and binocular status in elementary school children with a reading problem

PURPOSE

This descriptive study provides a summary of the binocular anomalies seen in elementary school children identified with reading problems.

METHODS

A retrospective chart review of all children identified with reading problems and seen by the University of Waterloo, Optometry Clinic, from September 2012 to June 2013.

RESULTS

Files of 121 children (mean age 8.6 years, range 6-14 years) were reviewed. No significant refractive error was found in 81% of children. Five and 8 children were identified as strabismic at distance and near respectively. Phoria test revealed 90% and 65% of patients had normal distance and near phoria. Near point of convergencia (NPC) was <5cm in 68% of children, and 77% had stereoacuity of ≤40seconds of arc. More than 50% of the children had normal fusional vergence ranges except for near positive fusional vergencce (base out) break (46%). Tests for accommodation showed 91% of children were normal for binocular facility, and approximately 70% of children had an expected accuracy of accommodation.

CONCLUSION

Findings indicate that some children with an identified reading problem also present with abnormal binocular test results compared to published normal values. Further investigation should be performed to investigate the relationship between binocular vision function and reading performance.

Orthoptic Treatment of Convergence Insufficiency in Parkinson's Disease: A Case Series

Introduction: This study reports a case series of orthoptic treatment (OT) for convergence insufficiency (CI) in individuals with Parkinson's disease (PD). Method: We are reporting two cases of individuals with PD who completed OT for CI. Both had a confirmed diagnosis of CI, accompanied by CI-type symptomatology. They each underwent an OT program consisting of three office-based visits and 8 weeks of home-based exercises. Treatment outcome was based on the changes measured pre- versus post-OT on the near point of convergence, positive fusional vergences, and symptomatology score. Results: The two participants successfully completed therapy, gained ability to converge, had fewer symptoms, and were satisfied with the OT-induced changes they felt in their day-to-day lives. Conclusion: This case series show that OT for CI in PD is possible. Further research is required as these results demonstrate that OT has the potential to improve symptomatic CI in these patients. In the meantime, the positive results obtained in these two cases should encourage clinicians to consider OT (a therapy with no/minimal risk) for CI in patients with PD whose quality of life is affected by this binocular dysfunction.

Prevalence of Convergence Insufficiency in Parkinson's Disease

Background

We recently reported that convergence insufficiency (CI)-type visual symptomatology was more prevalent in participants with Parkinson's disease (PD), compared to controls. The objective of this work was to determine the prevalence of a confirmed clinical diagnosis of CI in PD, compared to controls.

Methods

Participants with (n = 80) and without (n = 80) PD were recruited and received an eye exam. Published criteria were used to arrive at a clinical diagnosis of CI. The Convergence Insufficiency Symptom Survey (CISS-15) questionnaire was administered to each participant, with a score of ≥21 being considered positive for CI symptomatology. Student t test, chi-square, or nonparametric tests at the 0.05 level were used for statistical significance.

Results

A total of 43.8% of participants with versus 16.3% without PD had a clinical diagnosis of CI (P ≤ 0.001). A total of 53.8% of participants with versus 18.8% without PD had scores on the CISS-15 of ≥21 (P ≤ 0.001).

Conclusions

These results indicate that individuals with PD have a higher prevalence of CI and CI symptomatology than controls. These data provide evidence supporting the notion that treatment for symptomatic CI should be investigated in individuals with PD.

Refractive plasticity of the developing chick eye: a summary and update

PURPOSE

To summarize the OPO 1992 Classic Paper: Refractive plasticity of the developing chick eye (12: 448-452) and discuss recent findings in refractive development.

SUMMARY AND RECENT FINDINGS

The classic paper shows that when lightweight plastic goggles with rigid contact lens inserts are applied to the eyes of newly hatched chicks, the eye responds accurately to defocus between -10 and +20 D, although hyperopia develops more rapidly. While the changes largely are due to change in axial length, high levels of hyperopia are associated with corneal flattening. Also, newly hatched chicks are better able to compensate for the induced defocus than chicks that are 9 days old. In addition, astigmatism of 2-6 D can be produced by applying 9 D toric inducing lenses on the day of hatching, and the most myopic meridian coincides with the power meridian of the inducing lens. This astigmatism appears to be primarily due to corneal toricity. Furthermore, the greatest magnitude was produced when the plano meridian of the inducing lens was placed 45° from the line of the palpebral fissure. Since our publication in 1992, it has been shown that similar results can be produced in a variety of species, including; tree shrews, marmosets, monkeys and fish. Considerable effort has been spent in trying to determine what the eye uses, if not the brain, as the signal to the sign of the defocus. Accommodation, chromatic aberration, diurnal variation, astigmatism and higher order monochromatic aberrations have all been considered. Choroidal thinning and thickening play a role in myopia and hyperopia development, respectively, in chicks. High light levels (15,000 lux) increase the rate at which chicks compensate for positive lenses and decrease the compensation rate for negative lenses. However these light levels do not prevent the eye from fully compensating for either type of lens. It has also been shown that brief periods of normal vision prevent the development of form deprivation myopia. Finally, the importance of the peripheral retina in refractive development has been explored and lenses designed to reduce relative peripheral hyperopia have resulted in variable effects as far as myopia control is concerned.

CONCLUSIONS

A growing body of evidence, from both animal models and human clinical trials indicates that the development of myopia is related both to genetics and environment / lifestyle. Nevertheless, we are far from understanding how this interaction takes place.

Effect of age and pop out distracter on attended field of view

PURPOSE

To investigate the functional field of view (FFOV) of younger and older individuals using the attended field of view (AFOV), a method which allows for eye and head movement. The impact of a pop out distracter and a dual task on the FFOV measure was also investigated.

METHODS

Nine young adult (25±6 years) and 9 older participants (72±4 years) took part in the experiment. The AFOV test involved the binocular detection and localization of a white target (Landolt-C) in a field of 24 white rings (distracters). The further AFOV tests were modified to include the presence of a pop out distracter, a dual task condition, and a combination of the two.

RESULTS

Older observers had lower viewing efficiency (log [1/presentation time]) in all conditions (pooled mean across conditions: older: 0.05±0.02; younger: 0.48±0.04) than the younger group. The addition of dual or a pop out distracter did not affect the older group (mean difference ∼104±150ms and ∼124±122ms respectively) but the additional pop out distracter reduced the efficiency of the younger group for targets near fixation (mean difference ∼68±35ms).

CONCLUSION

Better viewing efficiency was observed in younger individuals compared to older individuals. Difficulty in disregarding irrelevant stimuli and thereby resorting to inefficient search strategy is proposed as the reason for the differences. The finding that both older and younger individuals are not affected significantly by the presence of the irrelevant pop out distracter has implications in situations such as driving or hazard avoidance. In such scenarios, search performance is likely not impaired beyond what is found with distracters (visual clutter) in the environment.

Increase in myopia prevalence in clinic-based populations across a century

PURPOSE

The study's aim was to report prevalence estimates and age-related trends in refractive error in a clinic-based series and compare them to results from studies of a similar nature conducted since 1892.

METHODS

Refractive error, patient age, and sex were collected from the files of all patients seen at the University of Waterloo School of Optometry and Vision Science clinic between January 2007 and January 2008. Patients were categorized as having myopia (<-0.5D mean ocular refraction [MOR]), hyperopia (>0.5D MOR), emmetropia, astigmatism (<-0.5D), and/or anisometropia (>1.0D difference between eyes). The prevalence of all refractive components was determined overall and in 1- and 5-year age groups, and then compared to data from older clinic-based studies. Refractive trends over time were noted. The prevalence of myopia and hyperopia were compared to older studies.

RESULTS

The lowest prevalence of myopia was 5% at 0 to 5 years of age, after which it increased to 72% at 20 to 30 years of age and then decreased to 22% in patients older than 70 years. A myopic peak occurred at 24 years of age. The prevalence of hyperopia followed opposite trends with a minimum prevalence of 6% at 25 to 30 years of age. Peaks in emmetropia prevalence were 55% at 5 to 10 years of age and 37% at 45 to 50 years of age. The prevalence of astigmatism and anisometropia increased with age. The Waterloo Eye Study showed a higher prevalence of myopia across all ages compared to the older studies with a peak prevalence of 72% compared to 21% in the oldest (Herrnheiser) study from 1892.

CONCLUSIONS

In the last 100 years, there appears to have been a myopic shift in clinic-based populations and myopia prevalence appears to follow a predictable pattern with age.

Eye examinations improve visual acuity across ages

OBJECTIVE

To investigate changes in distance best corrected visual acuity (BCVA) relative to presenting visual acuity (PVA) as a function of age and the number of years between eye examinations.

DESIGN

Retrospective cross-sectional study.

PARTICIPANTS

6397 patient files at a large Canadian Optometric Clinic.

METHODS

A retrospective file review of patients aged 4 to 93 years from the University of Waterloo, Optometry Clinic was conducted. Mean decimal PVA values were calculated for all patients grouped by year of age, and a function was fit to these data using nonlinear regression. The same was done for BCVA and the functions were compared. In addition, the mean logMAR difference between PVA and BCVA was determined for patients grouped by the length of time between the study visual assessment and their previous visual assessment independent of patient age.

RESULTS

Right and left eye functions were not significantly different from each other for either PVA or BCVA. BCVA functions were significantly greater than PVA functions for both right (F[3174] = 194.7, p < 0.0001) and left (F[3174] = 206.0, p < 0.0001) eyes. PVA differed (≥1 line) from BCVA in at least 1 eye in 70% (n = 4054) of patients. The difference between BCVA and PVA increased as the length of time between assessments increased from 0.075 logMAR for <1 year to 0.107 logMAR for ≥5 years.

CONCLUSIONS

Refractive testing resulted in an improvement in BCVA over PVA in the majority of patients, over a wide range of ages.

Diurnal rhythms of spherical refractive error, optical axial length, and power in the chick

PURPOSE

To measure the diurnal variation of spherical equivalent refractive error (mean ocular refraction or MOR) and to investigate factors contributing to it in chick, an important animal myopia model.

METHODS

Nine chicks developed naturally on a 14-hour light/10-hour dark cycle. Optical axial length (OAL) and Hartmann-Shack wavefront error (HSWE) measurements, including pupil size, were taken starting on day 7, at eight times during the following 32 hours. MOR was calculated for a constant pupil size from HSWE measurements.

RESULTS

MOR, OAL, and pupil size showed significant diurnal variation (P < 0.0001). Most eyes showed significant sinusoidal variations in MOR and in pupil size with periods close to 24 hours. On average, MOR oscillated ±0.84 diopters. OAL varied with a period not different from 12 hours. Diurnally varying MOR and OAL were correlated (P = 0.0003, R² = 0.62). However, as previously reported, the variation in OAL did not account for the variation in MOR. From these results, we derived the diurnal variation in ocular power necessary to give the measured MOR variation.

CONCLUSIONS

We confirmed a diurnal variation in OAL and found diurnal variations in pupil size and MOR. Although changes in OAL explain the MOR previously observed in response to lenses and diffusers, they do not completely account for the observed diurnal variation of MOR nor for the reduction in hyperopia during normal development. We infer that the diurnal variation in MOR and normal emmetropization both result from small differences in the relative changes of OAL and ocular power.

Effect of stimulus type on the eye movements of children

PURPOSE

The authors investigated whether pictures elicit superior response rates and eye movement dynamics on saccade and pursuit tasks than do dots or spots of light and whether the need for more interesting stimuli is age dependent.

METHODS

Using video eye tracking, horizontal eye movements were investigated in children and adults using dots and small colored pictures as stimuli. Saccade data were obtained from 61 people and pursuit data from 53 people, ages 3 to 30 years, with no known ocular, ocular motor, neurologic, or systemic disease. Saccadic stimuli were randomly presented in steps ranging in size from 5° to 30°. Pursuits at four velocities (5°/s, 10°/s, 20°/s, and 30°/s) were tested using step ramp stimuli.

RESULTS

Picture targets result in age-dependent improvements in ocular motor responses compared with dots. With the exception of saccadic accuracy, the youngest children are most affected by the type of target. Adults are affected very little. For pictures, saccadic response rates (t((60)) = 4.30, P < 0.001), saccadic peak velocities (t((60)) = 2.24, P = 0.03), saccadic accuracy (t((59)) = 2.34, P = 0.02), and closed-loop pursuit gains (F((3,50)) = 2.86, P = 0.046) are higher. Saccadic error rates (t((60)) = 3.91, P < 0.001) and saccadic latencies (t((59)) = 9.5, P < 0.001) are lower with pictures.

CONCLUSIONS

Stimulus characteristics can affect response rates and eye movement dynamics, particularly in young children. To avoid underestimation of eye movement performance in young children, it is important to use meaningful targets. Furthermore, when comparing the ocular motor performance of children across studies one must consider the type of stimuli used.

Automatic system for 3D reconstruction of the chick eye based on digital photographs

The geometry of anatomical specimens is very complex and accurate 3D reconstruction is important for morphological studies, finite element analysis (FEA) and rapid prototyping. Although magnetic resonance imaging, computed tomography and laser scanners can be used for reconstructing biological structures, the cost of the equipment is fairly high and specialised technicians are required to operate the equipment, making such approaches limiting in terms of accessibility. In this paper, a novel automatic system for 3D surface reconstruction of the chick eye from digital photographs of a serially sectioned specimen is presented as a potential cost-effective and practical alternative. The system is designed to allow for automatic detection of the external surface of the chick eye. Automatic alignment of the photographs is performed using a combination of coloured markers and an algorithm based on complex phase order likelihood that is robust to noise and illumination variations. Automatic segmentation of the external boundaries of the eye from the aligned photographs is performed using a novel level-set segmentation approach based on a complex phase order energy functional. The extracted boundaries are sampled to construct a 3D point cloud, and a combination of Delaunay triangulation and subdivision surfaces is employed to construct the final triangular mesh. Experimental results using digital photographs of the chick eye show that the proposed system is capable of producing accurate 3D reconstructions of the external surface of the eye. The 3D model geometry is similar to a real chick eye and could be used for morphological studies and FEA.

Threshold determination in sweep VEP and the effects of criterion

In order to develop criteria for the range of data points used for regression line fitting in sweep visually evoked potential (sVEP), which would be objective, clearly specified and give good repeatability and validity, and in order to investigate the effect of luminance on sVEP measurement, visual acuity (VA) and contrast sensitivity (CS) were measured with sVEP in adults aged 17-30 years and children aged 6-8 years. Six to ten participants took part in each experiment. Five criteria (C0-C4) for fitting the regression line were implemented. Test-retest repeatability and validity against psychophysical thresholds at three luminance levels were considered for thresholds and the number of acceptable readings. There were significant effects of criteria (repeated measures ANOVAs, P < 0.05). The criteria, C2 and C3 (based on the range over which the signal-to-noise ratio >or=1), consistently gave better VA and CS, more viable readings, better agreement with psychophysical thresholds in adults and better repeatability than the other criteria. In the case of adults, C2 gave thresholds that were not significantly different from the psychophysical thresholds (P > 0.05). There was little effect of luminance over the 25-100 cd/m(2) range used. Overall, C2 performed the best and would be the criterion of choice, giving better repeatability, better validity and more viable plots.

Development of Visual Acuity and Contrast Sensitivity in Children

Purpose

Most studies of visual development have concentrated on visual development of infants. Only a few studies have extended this to children and determined the point at which visual function becomes truly adult-like. Yet from a clinical and research perspective it is important to know this. This review paper is a discussion of the development of visual acuity and contrast sensitivity into childhood.

Methods

The literature on subjective (measured with preferential looking or psychophysical methods) and objective (visually-evoked potential) measures of visual acuity and contrast sensitivity was examined with particular emphasis on studies of children over the age of 5 years and those articles that compared different age groups and those that made a comparison with adults.

Results

Visual acuity was found to be fully mature between the ages of 5 and the mid teenage years, while contrast sensitivity was found to mature fully between the ages of 8 to 19 years. Thus, there is still no clear answer to the fundamental question of when these basic aspects of visual function mature, but it may be later than previously thought.

Conclusions

Further studies are needed to answer this basic question more precisely and objective measures, such as VEP, may be able to answer this question better than psychophysical methods.

Binocular adaptation to near addition lenses in emmetropic adults

Near addition lenses are prescribed to pre-presbyopic individuals for treatment of binocular motor problems such as convergence excess and to control the progression of myopia. To date, no investigation has looked at the complete sequence of binocular motor responses during a period of near work with +2D lenses. This investigation evaluated changes to accommodation and vergence responses when young adults sustained fixation at 33 cm with +2D addition lenses. In addition, the effect of the accommodative vergence cross-link (AV/A) on the magnitude and the completeness of binocular adaptation to these lenses were evaluated. The results showed that +2D lenses initiate an increase in exophoria and convergence driven accommodation. The degree of the initial induced phoria was dependant upon the magnitude of the AV/A ratio. Vergence adaptation occurred after 3 min of near fixation and reduced the exophoria and convergence driven accommodation. The magnitude of vergence adaptation was dependant upon the size of the induced phoria and hence the AV/A ratio. The completeness of adaptation was seen to vary inversely with induced exophoria and thus the AV/A ratio.

Eye and head movement alterations in naïve progressive addition lens wearers

AIMS

To examine adaptation to progressive addition lenses (PALs) by investigating eye and head movement strategies of individuals naïve to PAL wear; to determine whether adaptation is a short-term change in strategy or is maintained with continued PAL wear; to determine if adaptation strategies are different between PAL designs.

METHODS

Ten presbyopic individuals participated in a double-blind crossover study. Participants wore one PAL design for < or = 4 weeks, had approximately 1 week without PAL wear and then wore the second PAL for < or = 4 weeks. Eye and head movements were recorded at the beginning and end of each period of PAL wear in response to several visual tasks. Visual tasks were a flash discrimination task at 2 m and 40 cm, and reading text aloud. Quantitative analysis was undertaken for eye and head movement dynamics (latency, duration, peak velocity, etc.). Categorical analysis used eye and head movement metrics to classify participants as eye-movers, head-movers or mixed at each visit.

RESULTS

There was significant between-participant variability. A number of eye/head movement parameters exhibited carry-over effects. The number of vertical head movements for the distance task showed a significant increase between the first and second visit (F(1,9) = 5.578; p = 0.042), irrespective of lens design. Categorical analysis showed that participants employed task dependent strategies for all tasks. Participants that altered their strategy tended to include more head movements. Seven of the 10 participants preferred the second PAL worn.

CONCLUSIONS

Participants recruited more head movements for flash discrimination tasks and for reading text during adaptation to PAL wear. The many and various eye and head movement parameters analysed were unable to discriminate differences dependent on PAL design.

Vergence accommodation and monocular closed loop blur accommodation have similar dynamic characteristics

Retinal blur and disparity are two different sensory signals known to cause a change in accommodative response. These inputs have differing neurological correlates that feed into a final common pathway. The purpose of this study was to investigate the dynamic properties of monocular blur driven accommodation and binocular disparity driven vergence-accommodation (VA) in human subjects. The results show that when response amplitudes are matched, blur accommodation and VA share similar dynamic properties.

Application of video-based technology for the simultaneous measurement of accommodation and vergence

Accommodation and vergence are two ocular motor systems that interact during binocular vision. Independent measurement of the response dynamics of each system has been achieved by the application of optometers and eye trackers. However, relatively few devices, typically earlier model optometers, allow the simultaneous assessment of accommodation and vergence. In this study we describe the development and application of a custom designed high-speed digital photorefractor that allows for rapid measures of accommodation (up to 75Hz). In addition the photorefractor was also synchronized with a video-based stereo eye tracker to allow a simultaneous measurement of accommodation and vergence. Analysis of accommodation and vergence could then be conducted offline. The new instrumentation is suitable for investigation of young children and could be potentially used for clinical populations.

Dissociated hypotropia: clinical features and surgical management of two cases

BACKGROUND

Dissociated vertical deviation (DVD), which almost always shows a hypertropia of the affected eye, occasionally can manifest as a hypotropia, either unilaterally or bilaterally. Two patients are presented who had this rare variant of DVD, which has been termed "dissociated hypotropia," both of whom underwent surgery and had good outcomes.

METHODS

Two patients, ages 13 and 14, with unilateral dissociated hypotropia underwent large inferior rectus muscle recessions on their affected eyes. In one case, a video-based eye tracker was used to record the dissociated hypotropic drifts in the eye to plot the changes in the drift amplitudes in response to increasing density filters over the fixating eye (Bielschowsky phenomenon) and to confirm whether the deviation changed in the light versus the dark.

RESULTS

In case 1, the hypotropia drift measured up to 18(Delta), and it was reduced to zero with a 6 mm inferior rectus recession in the affected eye. In case 2, the drift measured up to 20(Delta). Eye movement recordings showed a time course of the downward drift that mirrored that of upward drift of hypertropic DVD and confirmed the presence of a Bielschowsky phenomenon. The drift amplitude did not change in bright versus dark conditions. An 8 mm inferior rectus recession on the affected eye reduced the hypotropia to 5(Delta).

CONCLUSIONS

Dissociated hypotropia is a rare form of the dissociated strabismus complex. The downward drift has a time course similar to the upward drift of hypertropic DVD and shows the Bielschowsky phenomenon. It responds to a large inferior rectus muscle recession.

Horizontal saccade dynamics across the human life span

PURPOSE

To investigate saccade dynamics as a function of age to determine whether they follow the pattern of development and decline predicted by Weale's model of aging.

METHODS

One hundred ninety-five participants between the ages of 3 and 86 years made visually guided horizontal prosaccades ranging in size from 1 degrees to 60 degrees in response to dot stimuli. Eye movements were recorded binocularly with a video-based eye tracker, sampling at 120 Hz. Saccadic latency, accuracy, and velocity were measured and analyzed as a function of age.

RESULTS

Mean saccadic latency decreased from 439 ms at 3 years to 172 ms at 14 years, followed by a period of relative stability to age 50 and finally, gradually increased to 264 ms at >or=80 years. For saccadic accuracy (amplitude gain), there was a statistically significant (P<0.05) interaction between saccade size and age. Participants made increasingly hypometric saccades as age and saccade size increased. Average age group saccadic asymptotic peak velocity (Vmax) increased during childhood from 446 deg/s at age 3, to a peak of 610 deg/s at 14 years and then gradually declined with age to approximately 345 deg/s for participants>or=80 years.

CONCLUSIONS

Age affected saccadic latency, accuracy, and velocity. For each parameter there was a different pattern of development and decline probably related to the way in which the portion of the brain that controls each function develops and ages.

Aberrations of chick eyes during normal growth and lens induction of myopia

Understanding the control of eye growth may lead to the prevention of nearsightedness (myopia). Chicks develop refractive errors in response to defocusing lenses by changing the rate of eye elongation. Changes in optical image quality and the optical signal in lens compensation are not understood. Monochromatic ocular aberrations were measured in 16 chicks that unilaterally developed myopia in response to unilateral goggles with -15D lenses and in 6 chicks developing naturally. There is no significant difference in higher-order root mean square aberrations (RMSA) between control eyes of the goggled birds and eyes of naturally developing chicks. Higher-order RMSA for a constant pupil size exponentially decreases in the chick eye with age more slowly than defocus. In the presence of a defocusing lens, the exponential decrease begins after day 2. In goggled eyes, asymmetric aberrations initially increase significantly, followed by an exponential decrease. Higher-order RMSA is significantly higher in goggled eyes than in controls. Equivalent blur, a new measure of image quality that accounts for increasing pupil size with age, exponentially decreases with age. In goggled eyes, this decrease also occurs after day 2. The fine optical structure, reflected in higher-order aberrations, may be important in understanding normal development and the development of myopia.

Dynamics of Saccadic Adaptation: Differences Between Athletes and Nonathletes

PURPOSE

The aim of the study was to delineate differences in saccadic adaptation characteristics between a population of racquet sports athletes and nonathletes.

METHODS

Eye movements were recorded at 120 Hz using a video-based eye tracker (ELMAR 2020) in a sample of 27 athletes (varsity badminton and squash players) and 14 nonathletes (<3 hours/week participation in recreational sports). Responses to negative positional error and positive positional error were studied in two sessions on separate days. Negative positional errors were induced by displacing the stimuli backwards by 3 degrees from the initial target step (12 degrees). Likewise, positive positional errors were induced by displacing the stimuli forward by 3 degrees . Amplitude gains were calculated for trials before, during, and after the adaptation phase. The magnitude and the rate of change of saccadic adaptation were determined from the amplitude gains. Differences between the groups were compared using regression analysis.

RESULTS

No significant differences were found between the two groups in the magnitude of saccadic adaptation, both for negative (athletes -60%, nonathletes -57%) and positive (athletes +26%, and nonathletes +27%) positional error. Racquet sports athletes showed a significantly faster rate of adaptation for the positive positional error. A significant difference was not observed in the rate of adaptation for the negative positional error.

CONCLUSIONS

Racquet sports athletes and nonathletes adapt to positional error signals by similar amounts. However, racquet sports athletes respond to positive positional errors at a faster rate, suggesting that a strategic component or environmental influences (such as practice) may play a role in saccadic adaptation.

Effect of retrobulbar injection of lidocaine on saccadic velocities

PURPOSE

To determine whether exposing the extraocular muscles (EOMs) to lidocaine via retrobulbar injection for cataract surgery has a demonstrable negative effect on subsequent function of the muscle.

SETTING

York Finch Eye Associates, Humber River Regional Hospital, and Toronto Western Hospital Research Institute, Toronto, Ontario, Canada.

METHODS

This study comprised 37 eyes that had phacoemulsification and posterior chamber intraocular lens implantation; 13 eyes had retrobulbar lidocaine with hyaluronidase and 24 eyes, topical anesthesia. The postoperative saccadic velocities were compared with the preoperative velocities using a sensitive recording device. The results were compared within and between the retrobulbar lidocaine and topical anesthesia groups.

RESULTS

No detectable decrement in postoperative saccadic velocities was detected in any patient, and no difference was found between the groups.

CONCLUSIONS

Exposing EOMs to lidocaine for cataract surgery had no detectable negative effect on saccadic velocities 1 week after surgery.

A case of different null zones for distance and near fixation

A compensatory head posture (CHP) is a common clinical feature of congenital motor nystagmus. It is usually caused by an eccentric null zone, which is the eye position where the nystagmus intensity (amplitude x frequency) is least. A patient adopts the posture to maximize the binocular visual acuity. Occasionally, a patient may have more than one null zone, leading to the adoption of different CHPs at various times. A 10-year-old boy with congenital motor nystagmus and orthophoria, and with good corrected vision in both eyes, presented with a face turn that had been noted since infancy. For distance fixation, he consistently adopted a left face turn due to a null zone in right gaze. For near fixation, he adopted a right face turn due to a null zone in left gaze. Eye movement recordings confirmed the different locations of the null zones for distance and near fixation. After a trial of base-out prisms to stimulate convergence, which eliminated his head posture at both positions, he underwent artificial divergence surgery. He has had a satisfactory result for 18 months after surgery with a satisfactory head posture and a well-controlled exophoria. Measures to induce convergence, with prisms and then surgery, can be an effective strategy to correct the head postures caused by two different null zones in a patient with congenital motor nystagmus.

Effects of scleral search coil wear on visual function

PURPOSE

The scleral search coil is widely regarded as the gold standard measurement technique for eye movements. The effect of wearing scleral search coils on human vision has not been systematically studied. However, there are anecdotal reports of degraded visual acuity, mild eye irritation, and an increase rise in intraocular pressure (IOP). The current study was conducted to investigate the effect of scleral search coil use on visual acuity and ocular integrity.

METHODS

Six subjects were examined; all had previously worn search coils. Two drops of topical anesthetic were administered before insertion of the coils. Coils were inserted by hand and secured by applying mild pressure. The coils were removed after 45 minutes or on request of either the subject or the clinician. Before, during (at 15-minutes intervals), and after the coil was worn, the following measurements were taken for both eyes: tonometry (noncontact), corneal topography, biomicroscopic examination, visual acuity (monocular Snellen), and an eye-discomfort rating.

RESULTS

Scleral coils produced a variety of effects, including ocular discomfort, hyperemia of the bulbar conjunctiva, increased IOP, buckling of the iris, grade 2 and 3 corneal staining, and reduction in visual acuity. Effects appeared as early as 15 minutes after insertion of the coils. All observed effects seemed to be transient and dissipated after coils were removed.

CONCLUSIONS

Scleral coils may not be appropriate for all subjects. The findings suggest that there is a need for thorough subject prescreening and that clinicians should consider the risk/benefit ratio. Acute reduction in visual acuity may confound search coil findings. More research is needed to determine the maximum wearing time for properly screened subjects.

Ultrasound biomicroscopy of the anterior segment of the enucleated chicken eye during accommodation

Ultrasound biomicroscopy produces real-time two-dimensional images of ocular structures measured non-invasively. Given recent work which shows that lenses from myopic eyes show shorter focal lengths and reduced accommodative amplitudes compared with controls, this study was undertaken to determine the structural characteristics of the anterior segment of chicken eyes during accommodation using the ultrasound biomicroscope (UBM). Form-deprivation myopia and hyperopia were induced in hatching chicks by the application of either translucent or +15 D defocus goggles. After 7 days, eyes were enucleated and ultrasound biomicrographs of the eye, at rest and during ciliary nerve-stimulated accommodation, were collected. For all eyes, accommodation was associated with a decrease in anterior chamber depth, an increase in lenticular thickness and a steepening of the front lenticular surface curvature. Changes related to refractive error were more difficult to detect. Myopic eyes showed deeper anterior chamber depths and differences in lenticular thicknesses just above the resolution limit for detection. In +15 D lens-treated eyes, anterior chamber differences were opposite but smaller, just at the limit of resolution, while differences in mean lenticular thickness were not resolvable at a pixel or above. The UBM is a good tool for measuring robust changes during accommodation, but is limited in its ability to detect subtle changes associated with experimentally induced ametropias.

Effects of experimentally induced ametropia on the morphology and optical quality of the avian crystalline lens

PURPOSE

To examine the effects of refractive error on avian lens morphology and optical quality.

METHODS

Hatchling white leghorn chicks were unilaterally goggled for 7 days with either a form-deprivation goggle (n = 12), a -10 D defocus goggle (n = 12), or a +10 D defocus goggle (n = 12) to induce myopia and hyperopia. Optical quality of lenses (focal length and focal length variability) from treated and contralateral control eyes was assessed using a scanning laser apparatus. Lens morphology was examined by light and electron microscopy.

RESULTS

Although the induction of refractive errors did not significantly alter lens size, shape, paraxial focal length, or average focal length, average focal length variability increased. Lenses from eyes goggled with form-deprivation and +10 D defocus goggles demonstrated a twofold increase in average focal length variability, when compared with their contralateral controls. The morphology of the lens is not altered by these experimental manipulations.

CONCLUSIONS

This study provides evidence that the refractive development of the chick lens is not independent of the refractive development of the ocular globe and that chick lenticular development is influenced by both genetics and visual experience.

Afocal magnification does not influence chick eye development

In defocus-induced ametropia experiments, retinal blur circles are a likely source of information as to the magnitude but not the sign of the defocus. However, magnification (and minification) produced by the lenses may be a cue. In this study, 1-day-old broiler chicks (N = 13) were treated monocularly for 7 days with special goggles containing approximately afocal iseikonic lenses which were designed to produce 10% retinal image magnification. This is a little less than the magnification produced by +10 D defocusing lenses used to produce about 10 D of hyperopia in earlier work. Intraocular dimensions of both eyes were measured by A-scan ultrasonography on the first and last day. Refractive states of both eyes were measured daily with a retinoscope and trial lenses. After the birds were sacrificed, the eyes were enucleated, weighed, and measured with calipers. Before the treatment there was no difference in the refractive state or dimensions of the right and left eyes. After 1 week of goggle wear there was still no significant difference between the eyes in spite of the magnification produced by the goggles. These data suggest that factors other than magnification are responsible for the ability of the eye to respond to the sign of defocus.

Vertical latent nystagmus component and vertical saccadic asymmetries in subjects with dissociated vertical deviation

PURPOSE

This study was conducted to quantify the vertical component of a latent nystagmus observed in subjects with dissociated vertical deviation (DVD), as well as to provide further evidence for vertical saccadic asymmetries in these individuals.

METHODS

Binocular eye movements of subjects with DVD were recorded in two dimensions using a noninvasive video-based eye tracker while cover/uncover tests, alternate cover tests, and vertical saccades were performed.

RESULTS

A small amplitude (1.5 degrees or less) vertical component of latent nystagmus can be observed in some subjects with DVD and is larger in the deviating eye than in the viewing eye. The frequency of the vertical nystagmus component is the same in each eye for any given fixation condition but may change depending on which eye is fixating. DVD in the presence of a vertical component of latent nystagmus can be adequately modeled by the algebraic sum of an exponentially decreasing velocity DVD and a nystagmus with an exponentially decreasing slow phase velocity. In general, the occluded eyes of DVD subjects make smaller downward saccades than the viewing eyes.

CONCLUSIONS

It is possible but not obligatory that DVD subjects will have a vertical component of latent nystagmus. Algebraic summation of an exponentially decreasing velocity DVD and a vertical component of latent nystagmus provides a more parsimonious explanation of the observed saccadic eye movements than modeling the DVD itself as a combination of vergence and saccadic movements. Subjects with DVD show a range of saccadic yoking from nearly complete saccadic conjugacy to nearly complete dissociation.

Scattering properties of Bagolini lenses and their effects on spatial vision

The effect of a Bagolini lens on spatial vision was investigated by studying its far-field diffraction pattern as produced by a coherent beam of laser light, and its effect on the contrast sensitivity function (CSF) in human subjects. For lenses of the main type studied, which were crossed by a series of slightly-irregular striated bands, each consisting of fine, parallel, etched lines of various widths and separations, the diffraction pattern consisted of undiffracted light giving a bright central spot and wide-angle, diffracted light giving a dim streak. The latter was due to the sum of the diffraction patterns associated by the irregular fine etched lines. The streak produced by a single striated band was modulated by a series of regular maxima and minima related to the width of the band. Analysis of this pattern gave the width of the band as 0.6 mm, in close agreement with direct microscopical measurements. When four bands were illuminated by a beam of about 3 mm diameter, similar to the diameter of the photopic pupil, the diffraction pattern showed no obvious maxima and minima, due to irregularity in the width and separation of the bands. The central spot contained more than 90% of the total light in the diffraction pattern. Thus the Bagolini lens, with its relatively weak far-field diffraction pattern lacking regular maxima and minima when areas > or = 3 mm in diameter were used, was expected to have only a small effect on the apparent contrast of the targets in CSF experiments. This was confirmed by the measurements: Bagolini lenses showed no significant effect on either the monocular or binocular CSF. Further similar measurements with lenses of slightly different design from another manufacturer confirmed these findings. Therefore Bagolini lenses do not disrupt vision when they are used to determine the presence of suppression and anomalous retinal correspondence.