Insights for mfVEPs from perimetry using large spatial frequency-doubling and near frequency-doubling stimuli in glaucoma.

PURPOSE: To compare two forms of perimetry that use large contrast-modulated grating stimuli in terms of: their relative diagnostic power, their independent diagnostic information about glaucoma and their utility for mfVEPs. We evaluated a contrast-threshold mfVEP in normal controls using the same stimuli as one of the tests. METHODS: We measured psychophysical contrast thresholds in one eye of 16 control subjects and 19 patients aged 67.8 ± 5.65 and 71.9 ± 7.15, respectively, (mean ± SD). Patients ranged in disease severity from suspects to severe glaucoma. We used the 17-region FDT-perimeter C20-threshold program and a custom 9-region test (R9) with similar visual field coverage. The R9 stimuli scaled their spatial frequencies with eccentricity and were modulated at lower temporal frequencies than C20 and thus did not display a clear spatial frequency-doubling (FD) appearance. Based on the overlapping areas of the stimuli, we transformed the C20 results to 9 measures for direct comparison with R9. We also compared mfVEP-based and psychophysical contrast thresholds in 26 younger (26.6 ± 7.3 y, mean ± SD) and 20 older normal control subjects (66.5 ± 7.3 y) control subjects using the R9 stimuli. RESULTS: The best intraclass correlations between R9/C20 thresholds were for the central and outer regions: 0.82 ± 0.05 (mean ± SD, p ≤ 0.0001). The areas under receiver operator characteristic plots for C20 and R9 were as high as 0.99 ± 0.012 (mean ± SE). Canonical correlation analysis (CCA) showed significant correlation (r = 0.638, p = 0.029) with 1 dimension of the C20 and R9 data, suggesting that the lower and higher temporal frequency tests probed the same neural mechanism(s). Low signal quality made the contrast-threshold mfVEPs non-viable. The resulting mfVEP thresholds were limited by noise to artificially high contrasts, which unlike the psychophysical versions, were not correlated with age. CONCLUSION: The lower temporal frequency R9 stimuli had similar diagnostic power to the FDT-C20 stimuli. CCA indicated the both stimuli drove similar neural mechanisms, possibly suggesting no advantage of FD stimuli for mfVEPs. Given that the contrast-threshold mfVEPs were non-viable, we used the present and published results to make recommendations for future mfVEP tests.

Prismatic displacement effect of progressive multifocal glasses on reaction time and accuracy in elderly people.

BACKGROUND: Multifocal glasses (bifocals, trifocals, and progressives) increase the risk of falling in elderly people, but how they do so is unclear. To explain why glasses with progressive addition lenses increase the risk of falls and whether this can be attributed to false projection, this study aimed to 1) map the prismatic displacement of a progressive lens, and 2) test whether this displacement impaired reaction time and accuracy. METHODS: The reaction times of healthy ≥75-year-olds (31 participants) were measured when grasping for a bar and touching a black line. Participants performed each test twice, wearing their progressives and new, matched single vision (distance) glasses in random order. The line and bar targets were positioned according to the maximum and minimum prismatic displacement effect through the progressive lens, mapped using a focimeter. RESULTS: Progressive spectacle lenses have large areas of prismatic displacement in the central visual axis and edges. Reaction time was faster for progressives compared with single vision glasses with a centrally-placed horizontal grab bar (mean difference 101 ms, P=0.011 [repeated measures analysis]) and a horizontal black line placed 300 mm below center (mean difference 80 ms, P=0.007). There was no difference in accuracy between the two types of glasses. CONCLUSION: Older people appear to adapt to the false projection of progressives in the central visual axis. This adaptation means that swapping to new glasses or a large change in prescription may lead to a fall. Frequently updating glasses may be more beneficial.

Visual evoked potential and psychophysical contrast thresholds in glaucoma.

PURPOSE: We compared the diagnostic power of electrophysiologically and psychophysically measured contrast thresholds for the diagnosis of glaucoma. Additionally, we investigated whether combining results from the two methods improved diagnostic power. METHODS: Seven-eight subjects between 40 and 88 years formed the main study group: 21 normal controls (9 males) and 57 glaucoma patients (30 males) were tested. Twenty-two younger control subjects were also tested. Contrast thresholds were determined for a 1 cpd sinusoidal grating, subtending 41° × 52° modulated at 14.3 rps. The thresholds were based on the same staircase method applied to visual evoked potential (VEP) and psychophysical responses (Psyc). Diagnostic power was assessed by the percent area under the curve (%AUC) of receiver operating characteristic plots. RESULTS: Psyc showed significant age dependence, -0.10 ± 0.02 dB, while VEPs did not. Diagnostic performance for moderate and severe eyes combined was modest: Psyc 74 ± 9.0 % and VEP 72 ± 9.1 %, but improved significantly (p < 0.05) for a simple combined method, up to 90 ± 6.0 % for moderate disease. The combined method improved %AUC for all severities on average (p < 0.03). Canonical correlation analysis indicated that the four threshold measures contained independent information and that these independent dimensions were each correlated with glaucoma severity (p < 0.0015). CONCLUSION: Combining the VEP and Psyc thresholds appeared to improve diagnostic power. Canonical correlation analysis indicated that they measured statistically independent aspects of glaucoma possibly related to disease severity. Adding the 20-s psychophysical test to a VEP test produced a significant benefit for a small time cost.

Glaucoma affects steady state VEP contrast thresholds before psychophysics.

PURPOSE: Frequency doubling technology (FDT) is a recent psychophysical test for glaucoma. It measures the contrast threshold to low spatial frequency, high temporal frequency sinusoidal luminance profile bars. We wanted to confirm, with stricter controls, Vaegan and Hollow's report that contrast thresholds of steady state visual evoked potentials (ssVEPs) to a stimulus resembling the central field of the FDT test was more sensitive to glaucoma than the subjective threshold to the same stimulus and to start to optimize the technique. METHODS: A double masked trial using 57 eyes of 42 subjects. Both thresholds were estimated by modified binary search. In psychophysical testing, subjects were given a two alternative forced choice task. In ssVEP testing a significant signal in any one of eight channels was deemed to be a detection. In some subjects electrode positions were compared, both eyes were tested, tests were repeated to estimate reliability, stimulus frequencies were varied or full contrast functions were obtained. RESULTS: Thresholds and percent abnormal increased as a function of glaucoma severity for ssVEPs but not for psychophysics. Both threshold measures were reliable. Interocular correlations were low. SsVEP amplitude against contrast functions had similar thresholds to those found by modified binary search. The data was too irregular for individual thresholds to be estimated from a fitted exponential. Amplitudes were greatest at 7 to 10 Hz, < or =6 cm above inion and least laterally. Noise was similar across electrodes. CONCLUSIONS: SsVEP contrast thresholds to a stimulus like the central field of the FDT test detected glaucoma better than corresponding psychophysical thresholds at 18.29 Hz, when formal controls were used, as they had in a less controlled previous study at 7.14 Hz.

Randomised controlled trial of prevention of falls in people aged > or =75 with severe visual impairment: the VIP trial.

OBJECTIVES: To assess the efficacy and cost effectiveness of a home safety programme and a home exercise programme to reduce falls and injuries in older people with low vision. DESIGN: Randomised controlled trial. SETTING: Dunedin and Auckland, New Zealand. PARTICIPANTS: 391 women and men aged > or =75 with visual acuity of 6/24 or worse who were living in the community; 92% (361 of 391) completed one year of follow-up. INTERVENTIONS: Participants received a home safety assessment and modification programme delivered by an occupational therapist (n = 100), an exercise programme prescribed at home by a physiotherapist plus vitamin D supplementation (n = 97), both interventions (n = 98), or social visits (n = 96). MAIN OUTCOME MEASURES: Numbers of falls and injuries resulting from falls, costs of implementing the home safety programme. RESULTS: Fewer falls occurred in the group randomised to the home safety programme but not in the exercise programme (incidence rate ratios 0.59 (95% confidence interval 0.42 to 0.83) and 1.15 (0.82 to 1.61), respectively). However, within the exercise programme, stricter adherence was associated with fewer falls (P = 0.001). A conservative analysis showed neither intervention was effective in reducing injuries from falls. Delivering the home safety programme cost NZ650 dollars (234 pounds sterling, 344 euros, US432 dollars) (at 2004 prices) per fall prevented. CONCLUSION: The home safety programme reduced falls and was more cost effective than an exercise programme in this group of elderly people with poor vision. The Otago exercise programme with vitamin D supplementation was not effective in reducing falls or injuries in this group, possibly due to low levels of adherence. Trial registration number ISRCTN15342873.