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Biomedical subjects

E J Casson

Publications and source records attributed to E J Casson.

14 recordsLinked to original sources
PubMed2000

Vision standards for driving in Canada and the United States. A review for the Canadian Ophthalmological Society.

We review the vision requirements for driving with the goal of revising current Canadian Ophthalmological Society (COS) recommendations for vision standards for driving. The report comprises two sections. In the first section we report the results of a survey of transportation authorities in Canada and the United States conducted on behalf of the COS to determine the current standards and medical review procedures. The results suggest that although the standards in Canada are more consistent than those in the United States, few of the standards in either country are evidence-based. In the second section we review the recent literature on visual function and driving. We conclude from this review that adequate contrast sensitivity is as important as, if not more important than, good visual acuity for driving and that there is little evidence to support a monocular standard for acuity, contrast sensitivity or visual field. Although there is evidence that the extent of a visual field defect is related to the ability to perform driving tasks, there is little evidence to suggest a relation between the location of the visual field defect and fitness to drive.

Automobile Driving↗
PubMed1999

The temporal order judgment paradigm: subcortical attentional contribution under exogenous and endogenous cueing conditions.

The role of subcortical attentional processing was investigated under exogenous and endogenous cueing conditions. As retinotectal projections arise predominantly from the nasal retina i.e., temporal hemifield, subcortical attention should be distributed asymmetrically under monocular viewing conditions with a temporal hemifield advantage. We compared the results of monocular and binocular viewing conditions using a temporal order judgment (TOJ) paradigm. Subjects fixated a centrally located cross and two stimuli were presented with a variable onset asynchrony. Three experiments were conducted: no cue, exogenous cue and endogenous cue. Subjects reported which stimulus seemed to appear first. An effect consistent with subcortical processing was found under exogenous cueing conditions. No such effect was found under endogenous cueing conditions. We believe that subcortical attentional processing in response to an exogenous cue facilitates rapid shifts in attention towards environmental stimuli. We found no evidence for subcortical processing in voluntary directed attention and believe this process to be cortical in nature.

Adolescent↗
PubMed1997

Excimer laser photorefractive keratectomy for low hyperopia: safety and efficacy.

PURPOSE: To assess the safety and efficacy of photorefractive keratectomy (PRK) to correct low hyperopia. SETTING: University of Ottawa Eye Institute, Ottawa General Hospital, Ontario, Canada. METHODS: Twenty-five eyes with refractions of +1.00 to +4.00 diopters (D) and cylinder of 1.00 D or less were treated for hyperopia with the VISX Star excimer laser system using a refined ablation architecture. Thorough visual assessments were performed preoperatively (baseline) and 1, 3, and 6 months postoperatively. Complications were recorded and the level of patient satisfaction was noted. RESULTS: Mean spherical equivalent at 6 months was +0.27 D +/- 0.55 (SD), which was an 89% reduction over baseline. Eighty-four percent of patients gained two to seven lines of near uncorrected visual acuity (UCVA) and 1 patient (4%) lost more than one line. Eight percent achieved 20/25 or better UCVA. Approximately half realized their preoperative distance best corrected visual acuity (BCVA) by 1 month. By the end of the study, all patients had improved, achieved, or were within one line of their baseline distance BCVA. There were some slight reductions in lower contrast acuity at 6 months, although dim lighting conditions did not further reduce these acuities. Most patients had no clinically meaningful change in cylinder. The most common complications included early, transient corneal surface irregularities and visual symptoms and trace haze (grade < or = 0.5) in 14 of 23 patients at 6 months. All but 1 patient expressed a high degree of satisfaction. CONCLUSIONS: These results support the hypothesis that PRK shows great promise as a safe and effective treatment for low hyperopia. There were no significant complications and no decentered ablations. The slight regression occurred with or without the presence of trace haze. Overall, refractive stability was encouraging, although longer follow-up is needed.

Adult↗
PubMed1997

Distinguishing subcortical and cortical influences in visual attention. Subcortical attentional processing.

PURPOSE: The purpose of the study was to investigate the role of subcortical processing in human visual attention. The midbrain contribution to visual attention is unclear. Although evidence exists for a subcortical attentional advantage in ocular motor tasks, such an advantage has not been shown in perceptual tasks. Because retinotectal projections arise predominantly from nasal retina (i.e., temporal hemifield), subcortical attention should be distributed asymmetrically for monocular viewing conditions with an advantage to the temporal hemifield. METHODS: To test for a subcortical attentional effect, the authors compared the results of binocular and monocular viewing conditions using the split priming motion induction paradigm. In this perceptual attention paradigm, priming cues are presented to the left and right of fixation followed by an instantaneously presented horizontal bar. As a result of attention to the priming cues, motion is perceived within the bar as it appears to draw in from the two lateral cues toward a central collision point. Asymmetrically distributed attention results in an asymmetry in the perception of motion within the bar, and thus the perceived collision point will be shifted away from the center. RESULTS: In two separate studies, one with and one without control of eye movements, the authors found significant differences between the results for monocular and binocular presentation. When the stimulus configuration is presented to the left eye, the perceived collision point is shifted toward the center consistent with a subcortical attentional effect. However, presentation of the stimulus configuration to the right eye yields the same results as those of binocular presentation. CONCLUSIONS: This pattern of results can be explained by a separate and additive interaction between cortical and subcortical attentional effects in the visual field. Dominance of the left visual field for cortical attention and dominance of the temporal visual field for subcortical attention act together when the initial priming cue occurs in the temporal (left) visual field of the left eye. However, these influences compete when the same stimulus configuration is presented to the right eye, where cortical attention predominates in the left visual field and subcortical attention predominates in the temporal (right) visual field.

Adult↗
PubMed1997

To wear or not to wear: current contact lens use in the Royal Canadian Mounted Police.

OBJECTIVE: The Canadian Ophthalmological Society was asked by the Royal Canadian Mounted Police (RCMP) and the Canadian Human Rights Commission to render an opinion on the acceptability of contact lenses as a reasonable accommodation to the uncorrected visual acuity standard. DESIGN: Survey by mailed questionnaire. SETTING: Canada. SUBJECTS: All RCMP general duty constables with a visual acuity code of V3, V4, V5 or V6 (n = 348) and a random sample of approximately 25% of the constables with an acuity code of V2 (n = 809). Of the 1040 questionnaires returned, 1037 were usable (final response rate 89.6%). Of the 1037 respondents 316 were in the V3 to V6 group and 721 were in the V2 group. OUTCOME MEASURES: Reported frequency of problems with spectacles or contact lenses, weighted according to sampling fraction. RESULTS: A total of 934 respondents indicated that they used some form of visual acuity correction while on duty; of the 934, 360 reported that they wore contact lenses at least some of the time. Approximately 75% of the spectacle wearers reported having to remove their spectacles because of fogging or rain. Although contact lens dislogement or fogging (21.2%) was less frequent than spectacle dislogement (59.2%), 35.4% of the contact lens wearers reported that they were unable to wear their lenses because of irritation on at least one occasion in the previous 2 years; the median length of time was 3.14 days. When the additional amount of time due to other causes is factored in, it is clear that contact lens users wear spectacles for substantial periods while on duty. CONCLUSIONS: Not only are RCMP general duty constables who usually wear contact lenses likely to have to wear spectacles at some time, but it is also possible that they will have to remove their spectacles and function in an uncorrected state in critical situations. Thus, altering the current standard to allow the use of contact lenses as a reasonable accommodation would not ensure effective and safe job performance.

Canada↗
PubMed1995

Effects of luminance, contrast, and blur on visual acuity.

Although previous investigations have reported that changes in background luminance, stimulus contrast, and dioptric blur can each affect visual acuity independently, it has not been shown how these three variables interact to influence visual acuity. This is a particularly important issue if one is interested in predicting how individuals with different refractive characteristics will be able to perform acuity-based tasks in degraded visual environments with low background lighting and contrast levels. To investigate these relations, we conducted a series of experiments in which measurements of visual acuity were obtained for four subjects using Landolt C targets of varying contrast at several background luminances for levels of blur between 0 and 8 diopters (D). Our results show that visual acuity is significantly affected by all three factors, and that their effects are essentially additive. At all luminance and contrast levels, the reduction in visual acuity is greatest for dioptric blur up to 2.0 D, with a more gradual reduction in visual acuity for dioptric blur of greater than 2.0 D. At all blur and luminance levels, visual acuity decreases gradually for contrast levels down to 20%, and decreases sharply for lower contrast levels. Over the range of background luminances we tested (75.0 to 0.075 cd/m2), visual acuity decreases linearly with reductions in luminance. The additive effects of dioptric blur, contrast, and luminance provide a basis for predicting visual acuity-related task performance for individuals in different visual environments. For example, an individual with 6/6 (20/20) visual acuity under high luminance-high contrast conditions will fall to 6/18 (20/60) acuity for low luminance conditions and 6/30 (20/100) for low luminance-low contrast conditions. Similarly, an individual with an uncorrected visual acuity of 6/30 (20/100) under optimal conditions will fall to approximately 6/120 (20/400) under low luminance conditions and 6/240 (20/800) under low luminance-low contrast conditions.

Adult↗
PubMed1993

Blue-on-yellow perimetry can predict the development of glaucomatous visual field loss.

OBJECTIVE: The purpose of this investigation was to determine whether blue-on-yellow (B/Y) perimetry is capable of predicting the onset and location of impending glaucomatous visual field loss in patients with ocular hypertension. DESIGN: A Humphrey Field Analyzer (Humphrey Instruments, San Leandro, Calif) was modified to perform B/Y perimetry to isolate and measure the sensitivity of short-wavelength-sensitive mechanisms. Participants were tested annually with standard white-on-white (W/W) and B/Y automated perimetry for a period of 5 years. PATIENTS: The study population consisted of 38 patients with ocular hypertension and 62 age-matched normal control subjects. RESULTS: Initially, all 76 ocular hypertensive eyes had normal W/W automated perimetry results, with 67 eyes having normal and nine eyes having abnormal B/Y test results. Five years later, five of the nine ocular hypertensive eyes with initial B/Y abnormal results developed glaucomatous visual field loss measured by standard W/W automated perimetry, while none of the 67 ocular hypertensive eyes with initially normal B/Y results developed abnormal W/W perimetry results. CONCLUSIONS: Blue-on-yellow perimetry deficits are an early indicator of glaucomatous damage and are predictive of impending glaucomatous visual field loss for standard W/W automated perimetry. To our knowledge, this is the first prospective, long-term longitudinal study that demonstrates the ability to predict the onset of glaucomatous visual field loss in patients with ocular hypertension on the basis of psychophysical testing.

Adult↗
PubMed1993

Progression of early glaucomatous visual field loss as detected by blue-on-yellow and standard white-on-white automated perimetry.

OBJECTIVE: To determine whether blue-on-yellow perimetry reveals progression of glaucomatous damage before it is evident with standard white-on-white perimetry. DESIGN: A Humphrey field analyzer (Humphrey Instruments, San Leandro, Calif) was modified to perform blue-on-yellow perimetry to isolate and measure the sensitivity of short wavelength-sensitive mechanisms. Participants were tested annually with standard white-on-white automated perimetry and blue-on-yellow automated perimetry for 5 years. PATIENTS: Sixteen patients with early glaucomatous visual field loss in one or both eyes and 62 age-matched normal control subjects. RESULTS: At baseline, 25 (78.1%) of the 32 eyes exhibited larger deficits with blue-on-yellow perimetry, five (15.6%) had equivalent loss with both tests, and two (6.3%) had larger deficits with standard white-on-white perimetry. Seven (21.9%) of the 32 eyes demonstrated evidence of progressive visual field loss with standard white-on-white perimetry in 5 years, while the other 25 eyes (78.1%) were relatively stable. Deficits with blue-on-yellow perimetry were twice as large as deficits with white-on-white perimetry in the stable group and were three to four times as large in the group with progressive field loss. CONCLUSIONS: Blue-on-yellow perimetry is effective in predicting which patients with early glaucomatous visual field loss are most likely to have progressive loss. The rate of progressive loss is greater with blue-on-yellow perimetry than with standard white-on-white perimetry.

Adult↗
PubMed1993

Benzodiazepine effects on flicker sensitivity: role of stimulus frequency and size.

1. Benzodiazepines (BZDs) impair sensitivity to temporally modulated visual stimuli (flicker). Critical flicker-fusion frequency (CFF) is commonly used as a measure of this effect, but it only measures sensitivity to a narrow range of frequencies, usually above 25 Hz. Are other frequencies more sensitive to the effects of BZDs? 2. Flicker sensitivity at 1, 2, 4, 8, 16, and 32 Hz was measured for 1 degrees and 5 degrees stimuli before and 50 to 100 minutes after triazolam (0.25 mg), lorazepam (1.0 mg) and placebo. Drug effects on CFF were also measured. 3. Both BZDs significantly impaired overall flicker sensitivity. Triazolam produced 50% more impairment than lorazepam. CFF was significantly impaired by triazolam. BZD effects did not vary with stimulus size. 4. Significantly greater BZD-induced impairment of flicker sensitivity occurred at 16 Hz than at 1, 2, 4, or 32 Hz. 5. The magnitude of BZD effects on flicker sensitivity vary with the temporal frequency of the stimulus. BZD effects are greatest for 8-16 Hz stimuli.

Adult↗
PubMed1993

Longitudinal comparison of temporal-modulation perimetry with white-on-white and blue-on-yellow perimetry in ocular hypertension and early glaucoma.

We obtained data over 3 years on temporal-modulation perimetry (TMP), standard automated [white-on-white (W/W)] perimetry, and short-wavelength-sensitive [blue-on-yellow (B/Y)] perimetry in ocular hypertensive (OH) patients and patients with early glaucomatous visual-field loss (EG). Evidence of visual-field defects was obtained with the use of both B/Y perimetry and TMP in the majority of OH and EG eyes that demonstrated progression on W/W perimetry as well as in all stable EG eyes. Using the nerve-fiber-bundle pattern to compare testing procedures, we determined that these defects were generally as extensive or more extensive than the concurrent W/W abnormalities. In terms of location over the 3 years of testing, TMP and B/Y defects were reasonably consistent in the EG eyes, somewhat less consistent in the OH eyes demonstrating progression, and both inconsistent and infrequent in the stable OH eyes. The greatest degree of overlap occurred between the location of defects obtained by use of the higher TMP frequencies (8 and 16 Hz) and that of defects obtained by use of B/Y perimetry. Since these two methods are thought to isolate different visual mechanisms subserved by different visual pathways, these results suggest that early glaucomatous visual-field damage as revealed by TMP and B/Y perimetry may not be specific to a single visual pathway.

Color Perception Tests↗
PubMed1993

Temporal modulation perimetry: the effects of aging and eccentricity on sensitivity in normals.

PURPOSE: Temporal modulation perimetry (TMP) is a new test procedure designed to measure sensitivity to sinusoidal flickering stimuli throughout the central 27 degrees visual field. The purpose of this investigation was to determine the effect of age and visual field eccentricity on temporal modulation sensitivity. METHOD: In its present form, TMP is used to determine modulation sensitivity for three temporal frequencies (2, 8, and 16 Hz) at 45 visual field locations. Both eyes of 43 normal observers between 20 and 75 years of age were examined. RESULTS: Our results indicate that sensitivity to all temporal frequencies tested showed a decline with age, particularly in the peripheral visual field. Furthermore, the age-related sensitivity loss was more pronounced for 16 Hz than for 2 or 8 Hz. CONCLUSIONS: These data demonstrate an age-related loss in temporal modulation sensitivity in the peripheral visual field. They also provide a baseline comparison standard for evaluation of clinical patient populations.

Adult↗
PubMed1990

Short-term fluctuation as an estimate of variability in visual field data.

The short-term fluctuation index (SF) is one of several values that provide an indication of a patient's response reliability during an automated perimetry examination. The authors investigated the number of visual field locations used and the number of determinations per location as factors affecting the SF estimate. A computer simulation program for perimetry was used to measure the SF index for 350 normal visual fields with various levels of response fluctuation. As expected, the variability of the SF estimate decreased as the number of locations used to estimate SF increased. There was a more important finding that, for an equal number of threshold estimates, a larger number of determinations at a smaller number of locations produced greater consistency in the SF estimate (eg, ten determinations at two locations instead of two determinations at ten locations). However, it is also important to sample from a representative spatial distribution of visual field locations. These results suggest that five determinations at four locations in the visual field is optimal for most clinical perimetric testing situations.

Computer Simulation↗
PubMed1984

Duration discrimination between weak test lights.

Maloney and Wandell [Vision Res. 24, 633-640 (1984)] describe a model of the response of a single visual channel to weak test signals. In the model an initial continuous visual response is randomly sampled, and each sample gives rise--with a probability that increases with the magnitude of the sample--to a discrete detection event. The authors derive a parameter-free prediction for the upper bound on the discriminability of two lights of different durations. In this paper we describe an experimental test of that prediction. We find that the model accurately distinguishes between discrimination performance under conditions where both test lights are detected by a single channel and conditions where the test lights are detected by different channels.

Discrimination, Psychological↗