Biomedical subjects

Andrew John Anderson

Publications and source records attributed to Andrew John Anderson.

6 recordsLinked to original sources

PubMed2005

Characteristics of the normative database for the Humphrey matrix perimeter.

PURPOSE: The Humphrey Matrix (Carl Zeiss Meditec, Dublin CA; Welch-Allyn, Skaneateles, NY) is a high-spatial-resolution perimeter that uses frequency-doubling stimuli. It incorporates an efficient test strategy that assumes that age, eccentricity, and test procedure type have only small effects on sensitivity. The results used to create the normative database for the perimeter were examined, to see whether these assumptions were met and to examine the form of the normative data. METHOD: Visual fields were measured (Matrix 30-2, 24-2, 10-2 and Macula patterns) in >275 subjects judged to be normal by a battery of clinical procedures. The right eye was always tested first. RESULTS: Sensitivity decreased by approximately 0.7 dB per age decade across all eccentricities; sensitivity decreased with eccentricity, typically by <5 dB at the most peripheral points tested. Although there was no systematic difference in sensitivity between the 30-2 and 24-2 tests, the Macula test sensitivities were typically 1 dB higher than for the 10-2 test. Sensitivity in the left eye was slightly lower than in the right, with the difference being significantly greater in the temporal visual field. In most test locations, the 95% confidence interval of normal sensitivity was approximately 6 dB below the median sensitivity. CONCLUSIONS: The performance of the test strategy in the Matrix perimeter is appropriately matched to the response characteristics of the normal population. The finding of a spatially nonuniform difference in sensitivity between left and right eyes is attributable to light-adaptation differences between the eyes. This effect is accounted for in the perimeter's normative database.

Adolescent↗

PubMed2003

Frequency-doubling technology perimetry and optical defocus.

PURPOSE: The frequency-doubling technology (FDT) perimeter has no provision for introducing corrective lenses, save for the patient's spectacles, and so patients are sometimes tested in the presence of moderate levels of defocus. The effect of defocus on frequency-doubling (FD) sensitivity was determined for both the commercially available instrument and for smaller targets that may be useful in spatially localizing visual field defects. In addition, whether stimulus artifacts may be detectable in the presence of defocus was assessed. METHODS: Detection and resolution thresholds for FD stimuli, along with detection thresholds for spatially uniform flickering stimuli, were measured in normal observers in the presence of up to +6 D of defocus. In addition, the effect of defocus on the summary indices (mean deviation [MD], and pattern standard deviation [PSD]) of both the commercial FDT perimeter and a customized perimeter with smaller, higher spatial frequency targets was determined. RESULTS: Thresholds for conventional FD targets (0.25 cyc/deg, 25 Hz) increased 0.1 log unit with +6 D defocus, whereas thresholds for smaller, higher spatial frequency (0.5 cyc/deg) targets increased 0.4 log unit. Despite the presence of a luminance artifact in the FD stimuli, detection, and resolution thresholds remained coincident at maximum defocus, suggesting that the artifact was not detectable. MD was significantly affected by defocus for the customized FD perimeter only, and PSD was not altered for either the conventional or customized test. CONCLUSIONS: In normal observers, optical defocus has little effect on FDT perimetry sensitivity and does not make low spatial frequency artifacts detectable. Improving the spatial resolution of FDT perimetry decreases its robustness to defocus.

Adult↗

PubMed2003

Anatomy of a supergroup: does a criterion of normal perimetric performance generate a supernormal population?

PURPOSE: To interpret individual results from automated perimeters, a normative database must be developed. Typically, a set of criteria determines those subjects that may be included in the database. This study examined whether a criterion of normal performance on an established perimeter generates a subgroup with supernormal perimetric performance. METHODS: The right-eye perimetric results of 100 subjects were analyzed. Subjects had visual acuities of 6/12 or better, no history of eye disease, and normal slit lamp biomicroscopic and ophthalmoscopic examinations. Subjects performed test-retest visual field examinations on a Humphrey Field Analyzer (HFA) 24-2 test (Zeiss Humphrey Systems, Dublin, CA), and on a custom frequency-doubling (FD) perimeter with targets spaced in the same 24-2 pattern. RESULTS: Test-retest correlation (Spearman rank correlation coefficients, rs) for mean defect (MD) and pattern SD (PSD) were 0.65 and 0.40 (HFA), and 0.82 and 0.39 (FD perimeter). Three subjects with HFA MDs in the lower 5% had similarly low MDs on retest, whereas no subject was common between the test and retest for the lower 5% of HFA PSD. Correlation between the HFA and FD test results were 0.41 (MD) and 0.05 (PSD). Based on these correlations, the bias introduced into perimetric probability limits were determined, by using Monte Carlo simulations. CONCLUSIONS: Although a criterion of a normal MD may produce a subpopulation with supernormal perimetric performance, a criterion of a normal PSD is less likely to do so. Also, a criterion on one test type is less likely to create a supernormal group on a different test type. The bias introduced into perimetric probability limits is small.

Adult↗

PubMed2002

Effect of eccentricity on luminance-pedestal flicker thresholds.

We investigated the effect that spatially coincident luminance increments (luminance pedestals) have on flicker thresholds at several eccentricities and target sizes. Luminance pedestals elevated flicker amplitude-thresholds more when stimuli were presented eccentrically, both at low (4 Hz) and high (20 Hz) temporal frequencies. Altering the size of the eccentric stimulus failed to equate central and eccentric thresholds at all pedestal amplitudes. Comparisons with flicker thresholds at various background luminances suggests that the increase in luminance-pedestal flicker thresholds peripherally is due to increased suppressive rod-cone interactions, increased effectiveness of luminous contrast on edge-sensitive flicker mechanisms, as well as increased gain in the light adaptation response.

Adaptation, Ocular↗

PubMed2002

Effect of dichoptic adaptation on frequency-doubling perimetry.

BACKGROUND: In frequency-doubling technology (FDT) perimetry, the second eye tested has reduced sensitivity. We investigated the cause of this sensitivity reduction. METHODS: Contrast sensitivity was measured for frequency doubling (0.25 cpd, 25 Hz) and nonflickering (4 cpd) gratings, arranged similarly to the targets in the FDT perimeter (Welch Allyn, Skaneateles Falls, NY, and Humphrey Instruments, San Leandro, CA). Various test orders were examined. RESULTS: Frequency-doubling (FD) sensitivity was reduced (0.15 log) in the second eye. A similar reduction occurred when first and second eye gratings were oriented orthogonally, suggesting that dichoptic contrast adaptation was not the cause. FD sensitivity was little affected after testing with nonflickering gratings, suggesting that fatigue effects were small. Sensitivity was reduced when testing was performed after 5 min of opaque occlusion. The use of a translucent occluder eliminated the reduction in sensitivity in the second eye. CONCLUSION: We confirm the presence of reduced sensitivity in the second eye tested with FD perimetry and find that it is caused by delayed light adaptation post-occlusion.

Adaptation, Ocular↗

PubMed2002

Mechanisms isolated by frequency-doubling technology perimetry.

PURPOSE: The frequency-doubling (FD) phenomenon describes the increase in apparent spatial frequency occurring when low-spatial-frequency sine wave gratings undergo rapid counterphase flicker. It is unclear whether the visual mechanisms isolated when pattern appearance is used as a threshold criterion are the same as when a simple detection criterion (as in FD perimetry) is used. It is also unclear whether the FD stimulus isolates mechanisms that differ from those isolated by spatially uniform flicker. In the current study, adaptation and spatiotemporal tuning functions were determined, using the FD stimulus, uniform flicker, and static (nonflickering) grating targets, to establish whether distinct mechanisms are isolated by the FD stimulus. METHODS: Spatiotemporal tuning functions were determined in six observers, using an FD stimulus under conditions of detection and resolution and using spatially uniform flickering stimuli and static grating stimuli. The effect of light adaptation on these stimulus classes was also assessed. All stimuli were 10 degrees-wide squares. RESULTS: Spatiotemporal tuning functions and adaptation characteristics were identical for both the FD detection and resolution paradigms. Spatially uniform flicker gave indistinguishable tuning functions and adaptation characteristics to the FD stimulus at 25 Hz and higher, but differed below this frequency. Static grating stimuli differed from FD stimuli in both tuning functions and adaptation characteristics. CONCLUSIONS: Absolute detection of the FD stimulus involves mechanisms that are indistinguishable from those involved when a criterion based on spatial form (i.e., resolution of a pattern) is used, indicating that a simple detection criterion can be used in FD perimetry. The FD stimulus isolates similar mechanisms to spatially uniform flickering stimuli at high temporal frequencies.

Adaptation, Ocular↗