Biomedical subjects

A M McKendrick

Publications and source records attributed to A M McKendrick.

10 recordsLinked to original sources

PubMed2003

Measurement error of visual field tests in glaucoma.

AIM: Psychophysical strategies designed for clinical visual field testing produce rapid estimates of threshold with relatively few stimulus presentations and so represent a trade-off between test quality and efficiency. The aim of this study was to determine the measurement error of a staircase algorithm similar to full threshold with standard automated perimetry (SAP) and frequency doubling perimetry (FDP) in glaucoma patients. METHODS: Seven patients with early open angle glaucoma (OAG) were prospectively recruited. All were experienced in laboratory based psychophysics. Three matched test locations were examined with SAP (externally driven Humphrey field analyser) and FDP (CRT) in a single arbitrarily selected eye of each subject. Each location was tested twice with a 4-2-2 dB staircase strategy, similar to full threshold, and then with the method of constant stimuli (MOCS). Accuracy (threshold estimation error) was quantified by determination of differences between "true" threshold measurements made by MOCS and single staircase threshold estimates. Precision (repeatability) was quantified by the differences between repeated staircase threshold estimates. RESULTS: Precision was relatively high for both tests, although higher for FDP than SAP at depressed sensitivity levels. The staircase strategy significantly underestimated threshold sensitivity for both test types, with the mean difference (95% CI) between staircase and MOCS thresholds being 4.48 dB (2.35 to 7.32) and 1.35 dB (0.56 to 1.73) for SAP and FDP respectively. Agreement levels (weighted kappa) between MOCS and staircase thresholds were found to be 0.48 for SAP and 0.85 for FDP. Although this "bias" appeared constant for FDP across all sensitivity levels, this was not the case for SAP where accuracy decreased at lower sensitivity levels. CONCLUSION: Estimations of threshold sensitivity made using staircase strategies common to clinical visual field test instrumentation are associated with varying degrees of measurement error according to visual field test type and sensitivity. In particular, SAP significantly overestimates the "true" level of sensitivity, particularly in damaged areas of the visual field, suggesting that clinical data of this type should be interpreted with caution.

Aged↗

PubMed2001

Visual dysfunction between migraine events.

PURPOSE: To evaluate interictal visual dysfunction in persons with migraine in terms of spatiotemporal selectivity and location within the visual pathways. METHODS: The vision of a group of 15 persons who had experienced migraine with aura was compared with that of 15 normal age-matched control subjects. A range of thresholds was measured to evaluate precortical (background modulation, contrast thresholds for static, and moving stimuli), area V1 (orientation discrimination and motion discrimination thresholds), and higher order (global dot motion thresholds) visual processes. Testing was performed centrally and at 10 degrees in the superior visual field. For each of the tests, the spatial and temporal parameters of the stimuli were selected to bias detection toward either parvocellular or magnocellular visual mechanisms. RESULTS: No defects were found for parvocellular processes. Significant (P: < 0.05) losses were apparent with the temporal background modulation method (16 Hz), orientation discrimination (0.5 cyc/deg), and global dot motion tasks. CONCLUSIONS: Both cortical and precortical visual dysfunction were identified in migraine group 7 days after the headache. This loss was selective for targets with temporal modulation of approximately 16 Hz.

Adult↗

PubMed2001

Variability components of standard automated perimetry and frequency-doubling technology perimetry.

PURPOSE: To evaluate and compare intra- and intertest variability components for both standard automated perimetry (SAP) and frequency-doubling technology (FDT) perimetry in a small group of normal individuals and patients with glaucoma. METHODS: The method of constant stimuli (MOCS) was used to examine matched test locations with both SAP and FDT perimetry stimuli in a group of eight normal individuals and seven patients with glaucoma. Subjects were tested weekly at three predetermined visual field loci for 5 consecutive weeks. Frequency-of-seeing (FOS) curves were generated and used to quantify threshold sensitivity (50% seen on FOS, in decibels), intratest variability (FOS interquartile range, in decibels), and intertest variability (interquartile range of weekly repeated threshold determinations, in decibels). RESULTS: In patients with glaucoma, SAP intra- and intertest variabilities were found to increase with sensitivity reductions, as previously reported. FDT perimetry revealed that both intra- and intertest variability components did not appreciably change with reductions in sensitivity. With the measurement scales used in this investigation, both intra- and intertest variability components were significantly greater for SAP than for FDT perimetry (P < 0.001 and P = 0.003, respectively). Intratest variability exceeded intertest variability for both SAP (P = 0.001) and FDT perimetry (P < 0.001). CONCLUSIONS: For both SAP and FDT perimetry, variability occurring within a single test session contributed more to total variability than between-session variability. When the measurement scales available on commercial instrumentation were used, FDT perimetry exhibited significantly less variability than SAP, especially within regions of visual field sensitivity loss. FDT perimetry therefore shows promise as an effective test for detecting progressive glaucomatous visual field loss, although prospective longitudinal validation is still required to determine sensitivity to change.

Adult↗

PubMed2000

Spatiotemporal filters in the detection of background modulation targets.

The background modulation method has been proposed as a useful test of early visual mechanisms [Biol. Cybern. 37, 77 (1980); Biol. Cybern. 47, 173 (1983)]. The task involves measuring detection thresholds for a luminous spot (increment) drifting over a spatially or temporally modulated background. The study explores the nature of the detecting mechanism in terms of spatial and temporal filters for both spatial and temporal background modulations. In both cases we find that thresholds can be explained by spatial contrast cues generated by the moving spot and that their spatiotemporal characteristics suggest detection by magnocellular processes.

Adult↗

PubMed2000

Aliasing for rapidly counterphasing stimuli: a failure to demonstrate an M-cell sampling limit to resolution.

We investigated whether resolution is sampling limited for stimuli optimized for detection by magnocellular mechanisms. We measured peripheral (15 degrees and 30 degrees) spatial detection and resolution thresholds using 50% and 90% contrast flicker-defined gratings (25 Hz) and 90% contrast counterphasing sinusoidal gratings (25 Hz). Direction-discrimination performance for 90% contrast counterphasing sinusoidal gratings (25 Hz) was measured foveally. Our results indicate that resolution of rapidly counterphasing stimuli is sampling limited in peripheral vision but is consistent with limiting of performance by parvocellular mechanisms. Also, undersampling may not be necessary to account for motion reversals observed with gratings that both drift and flicker.

Adult↗

PubMed2000

Visual field losses in subjects with migraine headaches.

PURPOSE: To characterize the visual fields of subjects with migraine headaches using static and temporal modulation perimetry. METHODS: Sixteen subjects with migraines (15 with aura, 1 without) and 15 nonheadache controls were tested. Perimetry was conducted 7 days after the offset of a headache with both static and temporally modulated targets using the Medmont M-600 automated perimeter (Medmont Pty Ltd., Camberwell, Victoria, Australia). Flicker thresholds were measured using the autoflicker test, which varies flicker rate with eccentricity. A subset of four subjects with migraines (3 with aura, 1 without) had the temporal tuning characteristics of their loss evaluated using fixed temporal frequencies (4, 6, 9, 12, and 16 Hz). RESULTS: Field losses were identified with temporal modulation perimetry in 11 of 16 migraine subjects. The majority of these losses occurred in the presence of normal static thresholds (8/11). The deficits displayed temporal tuning, being greatest for higher temporal frequencies (> or =9 Hz). None of the subjects revealed deficits typical of cortical lesions. The migraine-without-aura subject displayed a selective loss to temporally modulated stimuli, which was consistent with the aura group. This defect altered over time, decreasing for 30 to 40 days but remaining, to a smaller extent, for up to 75 days after the headache event. CONCLUSIONS: Visual dysfunction that is selective for temporally modulated targets occurs in migraine subjects. The migrainous pattern of dysfunction shares some features with that identified in early stages of glaucoma and raises the possibility for a common precortical vascular involvement in these two conditions.

Adult↗

PubMed1998

Effects of migraine on visual function.

BACKGROUND: Data is reported from an ongoing trial considering whether visual loss associated with migraine is consistent with a selective deficit in M- or P-cell functioning. METHODS: The psychophysical tests used include spatio-temporal contrast thresholds and luminance increment thresholds for a spot drifting across a spatially or temporally modulated background. A migraineur and 15 non-headache controls participated in the present study. Measurements were made foveally and at 10 degrees in the superior field. Static and flicker perimetry were conducted. RESULT/CONCLUSION: Regionalized losses occurred for stimuli with moderate temporal frequencies (16 Hz), which is consistent with M-cell dysfunction.

Adult↗

PubMed1997

The axis of astigmatism in right and left eye pairs.

This study was designed to investigate the relationship between the astigmatic axes of right and left eye pairs, with particular attention given to determining the degree to which either direct or mirror symmetry (enantiomorphism) of the astigmatic axes exists. A sample of 192 "nonvisually" selected adults participated, with refractive error and corneal curvature data being measured using autokerato-refractive equipment. Total, corneal, and residual astigmatism were investigated, with residual astigmatism being taken as the vector difference between total and corneal astigmatism. There was no evidence for a predominance of either mirror or direct symmetry of the astigmatic axes within this sample. The patterns of astigmatic axis distribution of right and left eyes were remarkably similar but, within this context, there was no definite evidence for a definable association between the axis of the left and right pairs of individuals. These findings remained unchanged when the effect of the modulus of astigmatism was incorporated, either through weighting the frequency distributions or through analyzing a subgroup of the overall population (those individuals with greater than 0.50 D of astigmatism). We believe these findings contradict commonly held clinical impressions regarding the symmetry of astigmatic axes, and should be considered when performing statistical analysis of astigmatic data.

Adolescent↗

PubMed1996

Distribution of astigmatism in the adult population.

We have quantified statistically the astigmatic frequency distribution. Vectorial analysis was used, as it enables formal multivariate statistical techniques to be applied to astigmatic data, allowing the simultaneous inclusion of both modulus and axis in the analytical procedure. These methods were applied to population data for each of total, corneal, and residual astigmatism from a sample of 198 adults. Right and left eyes were analyzed separately. All the distributions were found to depart significantly from a normal distribution. All the distributions were significantly leptokurtic (p < 0.005), and the distributions of total right eye, corneal right eye, and residual left eye astigmatism were also found to be significantly skewed (p < 0.05). Significant mild correlations were found between total and corneal astigmatism (p < 0.05). These findings add to the database of knowledge of astigmatic refractive error and may be of interest to those investigating refractive-error development.

Adolescent↗

PubMed1995

Clinical evaluation of refractive techniques.

BACKGROUND: The accuracy of two commercially available auto kerato-refractometers (Shin-Nippon Automatic Ref-Keratometer model RC380 and Topcon Auto Kerato-Refractometer model KR-3100) and one autokeratometer (Alcon Systems Auto-Keratometer) was compared to that of subjective refraction and conventional keratometry. METHODS: Refractive error and corneal curvature were measured on 20 subjects. Measurements were converted from the standard clinical format of sphere, cylinder and axis to a vector format to assess the contribution of spherical and cylindrical errors simultaneously. RESULTS: For measurements of refractive error taken in immediate succession, the Shin-Nippon instrument was found to be more repeatable than the Topcon instrument. This trend reversed when subjects were realigned between measurements. The 95 percent confidence limit for precision for subjective refraction was considerably greater: 93 percent of subjective refractions resulted in visual acuities better than or equal to 6/6, compared with 85 percent for the Topcon instrument and 45 percent for the Shin-Nippon instrument. Each of the methods for measuring corneal curvature showed minimal bias and comparable precision. CONCLUSIONS: Although subjective methods of determining refractive error generally achieved the same or better visual acuity as the automated methods, they displayed considerably poorer precision. Each of the methods of measurement of corneal curvature produced similar results.

Adult↗