Biomedical subjects

Allison M McKendrick

Publications and source records attributed to Allison M McKendrick.

17 recordsLinked to original sources

PubMed2006

Vernier acuity is normal in migraine, whereas global form and global motion perception are not.

PURPOSE: A recent study has demonstrated that some people with migraine display impairments of intermediate stages of motion and form processing. Deficits were identified by using tasks that required that local stimulus attributes be integrated into global percepts. Neurons capable of global processing of form and motion are known to be present in extrastriate cortical areas V4 and V5, respectively. It is not clear from the literature whether deficits of global processing in migraineurs are likely to arise from reduced input to extrastriate cortex from primary visual cortex (V1). The purpose of the study was to compare presumed measures of V1 performance (vernier acuity) to measures of global form and motion perception in migraineurs. METHODS: Thirty migraineurs (17 with aura, and 13 without) and 20 age-matched nonheadache control subjects participated. Intermediate level motion and form perceptions were measured using global dot motion stimuli and Glass patterns, respectively. Vernier stimuli were broad vertical bars composed of small dot elements. Both a static luminance stimulus and a motion defined form vernier stimulus were used. RESULTS: Mean migraine and control group performance were not significantly different for either vernier task (static: t(48)=0.39, P=0.70; motion: t(48)=0.29, P=0.77). Mean migraine group performance was significantly worse than in control subjects for both the global form (t(48)=2.06, P=0.04) and global motion (t(48)=2.87, P<0.01) tasks. CONCLUSIONS: On average, migraineurs demonstrate abnormalities of intermediate stages of both motion and form processing. These abnormalities do not appear to arise from dropout of performance at V1, as vernier acuity was normal in the same individuals.

Adolescent↗

PubMed2005

Processing of global form and motion in migraineurs.

Previous studies have identified anomalies of cortical visual processing in migraineurs that appear to extend beyond V1. Migraineurs respond differently than controls to transcranial magnetic stimulation of V5, and can demonstrate impairments of global motion processing. This study was designed to assess the integrity of intermediate stages of both motion and form processing in people with migraine. We measured the ability to integrate local orientation information into a global form percept, and to integrate local motion information into a global motion percept. Control subjects performed significantly better than migraineurs on both tasks, suggesting a diffuse visual cortical processing anomaly in migraine.

Adolescent↗

PubMed2005

Observer-based rather than population-based confidence limits for determining probability of change in visual fields.

Determining confidence intervals on psychophysical thresholds is straight forward if the psychometric function is known. In clinical settings, however, there is only partial information about the psychometric function, hence confidence limits are usually derived from test-retest data collected from many subjects. In this paper, we introduce a computational technique for deriving confidence limits for an individual's endpoint threshold using data typically obtained in a clinical setting, rather than a database of test-retest performance. The technique uses probabilistic analysis of all possible response sequences in a test procedure. We then extend this procedure to allow for levels of typical uncertainty in data measurement.

Algorithms↗

PubMed2005

Recent developments in perimetry: test stimuli and procedures.

Automated perimetry has evolved substantially in recent years, in part due to modern computer technology that enables more complex visual stimuli and test procedures to be realised than those incorporated in traditional white-on-white luminance increment perimetry. This paper reviews briefly a number of advances in automated perimetry. The review includes discussion of new test types: frequency doubling technology perimetry, short wavelength automated perimetry, flicker perimetry, high-pass resolution perimetry and rarebit perimetry. Test algorithms applied to perimetry such as zippy estimation of sequential thresholds (ZEST), Swedish interactive thresholding algorithm (SITA), tendency-oriented perimetry (TOP) and multi-sampling supra-threshold perimetry are also discussed.

Algorithms↗

PubMed2005

The detection of both global motion and global form is disrupted in glaucoma.

PURPOSE: It is well known that glaucoma results in performance impairments on tasks processed early in the visual pathways. Glaucoma should also impair cortical visual processing because of reduced input from retinal ganglion cells and also possibly because of abnormal cortical function. This study was undertaken to assess whether cortically processed global percepts are disrupted in glaucoma in areas of visual field classified as normal by standard automated perimetry (SAP). Performance on global tasks (motion and form) was compared to measures of presumed precortical magnocellular and parvocellular function in the same individuals. METHODS: Fifteen control subjects and 12 patients with primary open-angle glaucoma participated. Testing was performed foveally and midperipherally (12.5 degrees). Contrast-discrimination thresholds were measured by using the steady-pedestal (magnocellular) and pulsed-pedestal (parvocellular) contrast-discrimination tasks of Pokorny and Smith. Global motion coherence and global form coherence thresholds were measured at high and low contrast. RESULTS: Patients with glaucoma demonstrated higher global motion and form-coherence thresholds than did control subjects for targets presented in the midperiphery (P < 0.05), but not foveally. Different individuals performed poorly on the motion and form tasks. The subjects with the greatest presumed magnocellular and parvocellular loss were those with the largest deficits on the global motion and form tasks, respectively. CONCLUSIONS: Some subjects with glaucoma demonstrate profound impairments of global motion or global form integration in areas of visual field classified as normal by SAP. This finding implies that some people with glaucoma may have far greater difficulty with complex visual tasks (for example, navigation through the environment or face recognition) than is predicted by their visual field loss.

Aged↗

PubMed2005

Combining perimetric suprathreshold and threshold procedures to reduce measurement variability in areas of visual field loss.

PURPOSE: Several current perimetric test strategies use information from neighboring test locations to seed the procedure. When this initial estimate is close to the true threshold, the algorithms terminate quickly with low error. However, when neighboring thresholds are dissimilar, seeding procedures in this manner results in poor accuracy and repeatability. This study aims to develop a test procedure that avoids assumptions based on neighboring locations yet terminates with an acceptable number of presentations. We explore the utility of a combined screening and threshold approach. METHODS: Our approach [estimation minimizing uncertainty (EMU)] first applies suprathreshold screening and then in those locations that fail the screening, a ZEST procedure commencing with a uniform probability density function (pdf). EMU was compared with full threshold (FT) using computer simulation. Input to the simulation was empirical standard automated perimetry data (FT) from 265 normal subjects and 163 observers with glaucomatous visual field loss. Test performance was assessed using four patient error models designed to represent patients who respond with no errors, typical false-positive errors, typical false-negative errors, and extremely unreliable patients. RESULTS: When patients made typical false-positive errors, EMU required about 20% fewer presentations than FT averaged across the visual field. EMU required a greater number of presentations than FT when false-negative errors were made. The percentage of locations misclassified as either normal (defined as the thresholds greater than the lower 95% confidence limit of age-corrected norms) or abnormal by EMU was lower than for FT. Point-wise analysis demonstrated that in the presence of false-positive errors, the average error and SD of error of thresholds returned by EMU were reduced compared with FT. CONCLUSIONS: EMU enabled accurate estimates of threshold for situations in which neighboring locations are a poor predictor of true threshold. Combining suprathreshold and threshold strategies enables an acceptable number of total visual field presentations, while improving the accuracy and repeatability of threshold estimates in regions of abnormal sensitivity.

Computer Simulation↗

PubMed2005

Advantages of terminating Zippy Estimation by Sequential Testing (ZEST) with dynamic criteria for white-on-white perimetry.

PURPOSE: A number of automated perimeters use the Zippy Estimation by Sequential Testing (ZEST) algorithm, which is an adaptive Bayesian method, for determining sensitivity measures. There are two popular rules for deciding when to terminate Bayesian procedures: (1) after a fixed number of presentations; or (2) when the probability density function (pdf) over all thresholds modified by the procedure becomes sufficiently narrow (a dynamic termination criterion). It has recently been argued that fixed termination criteria perform equally as well as dynamic criteria when applied in a fashion typical of laboratory-based visual psychophysics. Perimetry, however, has specific requirements; the tests must be very short, there is a wide range of possible sensitivities, and erroneous responses from the patient must be tolerated. This study used computer simulation to compare fixed and dynamic termination criteria for the ZEST algorithm using conditions typical of white-on-white perimetry. METHODS: Eight ZEST procedures were compared using the following termination criteria: fixed termination after 4, 5, 6, 7, and 8 presentations; dynamic termination when the standard deviation of the pdf was 1 dB, 1.5 dB, and 2 dB. Four patient error models were used: ideal, typical false-positive, typical false-negative, and unreliable patients. We also ran a version of ZEST that set the likelihood function exactly equal to the patient's frequency of seeing curve. RESULTS: The mean absolute error and standard deviation of error in threshold measurement was higher for the fixed termination criteria than for dynamic termination criteria of the same average number of presentations. CONCLUSIONS: The results of our simulations indicate that dynamic procedures have some distinct benefits over fixed termination procedures when a minimum of presentations are required and response errors are made as in a white-on-white perimetric setting. Dynamic termination criteria are at least partially successful in expending more presentations when required to enhance test precision.

Algorithms↗

PubMed2004

Decreased visual field sensitivity measured 1 day, then 1 week, after migraine.

PURPOSE: To determine whether perimetric performance is worse the day after a migraine than prior interictal measurements, and if so, to determine whether differences have resolved by 1 week after migraine. METHODS: Twenty-two nonheadache control subjects (aged 18-45 years) and 22 migraineurs (aged 18-45 years: 10 migraine with visual aura, 12 migraine without aura) participated. Standard automated perimetry (SAP) and temporal modulation perimetry (TMP) were measured by perimeter (model M-700; Medmont, Pty Ltd., Camberwell, Victoria, Australia). Control subjects attended two test visits: baseline and retest. Migraineurs attended three times: baseline (>or=4 days after migraine), the day after the offset of the next migraine, and 7 days later. Groups were compared using the global indices of the perimeter: Average Defect (AD) and Pattern Defect (PD), in addition to point-wise comparisons. RESULTS: Group migraineur TMP performance was significantly worse the day after a migraine, showing decreased general sensitivity and increased localized loss. Performance measured 7 days later was not significantly different from that measured the day after a migraine. Group migraineur SAP performance was not significantly worse after migraine; however, a subgroup of six eyes from five patients had 10 or more visual field locations with decreases in sensitivity greater than control test-retest 95% confidence limits. CONCLUSIONS: Decreased visual field performance was present after migraine, as well as greater test-retest variability in the migraine group compared with control subjects. As migraineurs constitute 10% to 15% of the general population, the presence of this subgroup of patients with periodic prolonged decreased visual field sensitivity after migraine has implications for differential clinical diagnosis, and for clinical research using perimetry.

Adolescent↗

PubMed2004

Psychophysical measurement of neural adaptation abnormalities in magnocellular and parvocellular pathways in glaucoma.

PURPOSE: It is well established that contrast sensitivity is reduced in glaucoma. This study explored whether such contrast processing abnormalities consist of an absolute threshold level difference or a problem with contrast gain control. METHODS: Seventeen patients with primary open-angle glaucoma and 17 approximately age-matched control subjects participated. Subjects were tested foveally and midperipherally (12.5 degrees ). Subjects with glaucoma were tested in a peripheral region of relatively normal visual field (neighboring locations required to be within the normal 95% confidence limit on the total deviation plot of their most recent SITA/full threshold Humphrey Field Analyzer assessment; Carl Zeiss Meditec, Dublin, CA). Control subjects were tested in matching locations. Contrast discrimination was assessed using the steady-pedestal (magnocellular [M] pathway) and pulsed-pedestal (parvocellular [P] pathway) stimuli of Pokorny and Smith for seven pedestal luminances between 15 and 75 cd/m(2), presented on a background of 30 cd/m(2). RESULTS: Glaucoma group thresholds were significantly elevated compared with control subjects foveally and peripherally on both the pulsed-pedestal (P) and steady-pedestal (M) tasks (P < 0.01). Effect size statistics revealed slightly greater deficits on the P pathway task and greater deficits for pedestals that were decrements, rather than increments, from the surround luminance. Foveal deficits were of a magnitude to be explained by a reduction in contrast sensitivity; however, the peripheral deficits were greater than predicted by this factor alone. CONCLUSIONS: Foveal and midperipheral dysfunction of both M and P pathways was identified in people with glaucoma, in areas of relatively normal visual field performance. These findings are supportive of nonselective neural adaptation abnormalities in early glaucoma.

Adaptation, Physiological↗

PubMed2003

Pattern cues disambiguate perceived direction in simple moving stimuli.

Much of our knowledge about motion perception has been obtained by studying bars moving within apertures. When viewed within an ambiguously oriented aperture such as a circle, bars appear to move orthogonal to their orientation. We demonstrate that if the local orientation of the aperture edges is altered, a direction consistent with the edge orientation is seen. Indeed, the perceived direction can be strongly influenced by static lines separated from the edge of the moving stimulus. These results support recent suggestions that precise motion direction is likely to be determined by static orientation cues.

Adult↗

PubMed2003

Appearance of the frequency doubling stimulus in normal subjects and patients with glaucoma.

PURPOSE: To determine whether the spatial structure of the frequency doubling technology (FDT) perimetry stimulus is visible at detection-contrast threshold in normal observers and those with glaucoma and to assess its perceived spatial frequency at threshold and suprathreshold contrast. METHODS: Three subject groups were assessed: 10 young normal observers (aged <40 years), 10 older normal observers (aged >50 years), and 10 subjects with glaucoma. Detection thresholds for centrally and eccentrically presented 10 degrees squares, 0.25-cyc/deg, 25-Hz counterphase flicker sine-wave gratings were obtained by using a yes-no staircase procedure. Eccentric locations were in areas of loss of FDT sensitivity (< or =21 degrees ) in subjects with glaucoma, or at 7 degrees or 21 degrees inferonasally in normal observers. Resolution-contrast thresholds were determined by a two-alternative, forced-choice staircase procedure in which subjects selected the orientation of the grating stimulus tilted at +/-45 degrees. Perceived spatial frequency was determined by having subjects alter the spatial frequency of a temporally interleaved stationary sine-wave grating to match the FDT stimulus. RESULTS: No significant difference was found between detection- and resolution-contrast thresholds, implying that spatial structure was visible at detection threshold. In general, subjects perceived the spatial structure to have a spatial frequency closer to doubled than to veridical, although the young normal subjects reported a lower apparent spatial frequency than older individuals. CONCLUSIONS: When instructed as for clinical testing, subjects respond to the presence of the structure of the grating, and perceive the FDT stimulus to have a spatial frequency greater than its true spatial frequency. These findings were consistent across both normal observers and those with glaucoma, at both central and eccentric test locations.

Adult↗

PubMed2003

Contrast-processing dysfunction in both magnocellular and parvocellular pathways in migraineurs with or without aura.

PURPOSE: To assess contrast-discrimination thresholds in patients with migraine who have manifest visual field loss. This study was undertaken to determine whether contrast processing abnormalities in migraineurs are more readily identified by using stimuli that elicit a response from the subject that depends, at least in part, on adaptation mechanisms, and if so, whether deficits appear more pronounced in magnocellular (M) or parvocellular (P) visual pathways. METHODS: Ten patients with migraine who had abnormal visual fields measured with flicker perimetry but had normal standard automated perimetry (SAP) thresholds participated, along with 15 age-matched control subjects. Contrast-discrimination performance was assessed with the steady-pedestal (magnocellular) and pulsed-pedestal (parvocellular) stimuli of Pokorny and Smith for seven pedestal luminances between 15 and 60 cd/m(2) on a background of 30 cd/m(2). Subjects were tested foveally and midperipherally at 12.5 degrees. Migraineurs were tested in the quadrant of worst visual field performance. Control subjects were assessed in locations matched to those of the migraine group. RESULTS: Foveal performance was not significantly different between the migraine and control groups for either task. At 12.5 degrees the migraine group had significantly raised thresholds for both conditions. Effect size statistics revealed similar deficit magnitudes for each test (steady pedestal, -1.06; pulsed pedestal, -1.04). CONCLUSIONS: Dysfunction in both the M and P pathways was identified in the midperipheral visual field of the migraine group. The P pathway dysfunction was not identified by SAP. These findings support the possibility of nonselective neural adaptation abnormalities in some subjects with migraine.

Adult↗

PubMed2003

Properties of perimetric threshold estimates from full threshold, ZEST, and SITA-like strategies, as determined by computer simulation.

PURPOSE: To investigate the accuracy and precision of threshold estimates returned by two Bayesian perimetric strategies, staircase-QUEST or SQ (a Swedish interactive threshold algorithm [SITA]-like strategy) and ZEST (zippy estimation by sequential testing), and to compare these measures with those of the full-threshold (FT) algorithm. METHODS: A computerized visual field simulation model was developed to compare the performance (accuracy, precision, and number of presentations) of the three algorithms. SQ implemented aspects of the SITA algorithm that are in the public domain. The simulation was tested by using standard automated perimetry (SAP) visual field data from 265 normal subjects and 163 observers with glaucomatous visual field loss and by exploring the effect of response variability and response errors on algorithm performance. RESULTS: SQ was faster than FT or ZEST, with a comparable mean error when simulating field tests on patients. Point-wise analysis revealed similar error and standard deviation of error as a function of threshold for FT and SQ. If the initial estimate of threshold for either procedure was incorrect, the means and standard deviations of the error increased markedly. ZEST produced more accurate thresholds than did the other two strategies when the initial estimate was removed from the true threshold. CONCLUSIONS: When simulated patients made errors, the accuracy and precision of sensitivity estimates were poor when the initial estimate of threshold either overestimated or underestimated the true threshold. This was particularly so for FT and SQ. ZEST demonstrated more consistent error properties than the other two measures.

Algorithms↗

PubMed2002

Short-wavelength sensitivity deficits in patients with migraine.

OBJECTIVE: To examine short-wavelength sensitivity in patients with migraine using short-wavelength automated perimetry (SWAP) and Stiles 2-color increment threshold procedures. METHODS: Twenty-five subjects with migraine with (n = 11) and without (n = 14) aura and 20 age-matched headache-free subjects underwent testing. All subjects underwent standard automated perimetry (SAP) and SWAP (using a Humphrey field analyzer; 24-2 presentation pattern). In 2 migraine patients (one with and another without aura), the 2-color increment threshold procedure was used to determine whether sensitivity losses were specific to short-wavelength sensitivity pathways or a generalized loss to multiple pathways. RESULTS: No statistically significant differences between migraine patients and controls were found for mean deviation (MD) or pattern-standard deviation (PSD) for SAP. However, for SWAP, MD and PSD were worse for the migraine group (P =.04). Twelve migraine patients had more than 4 locations with sensitivity worse than the 5% probability level (reference value). Increment threshold determinations in the 2 selected migraine patients indicated a selectively greater loss for short-wavelength sensitivity mechanisms. CONCLUSIONS: Approximately 50% of subjects with migraine (with or without aura) demonstrate SWAP sensitivity losses, at times between migraine events. These findings, in conjunction with previous results for SAP and flicker perimetry, suggest that migraine patients should be excluded from normative databases of visual function, and warrant further investigations of the relationship between migraine and glaucoma.

Adolescent↗

PubMed2002

Development of efficient threshold strategies for frequency doubling technology perimetry using computer simulation.

PURPOSE: To develop new test procedures for frequency-doubling technology (FDT) perimetry that improve performance beyond those currently used. METHODS: Two novel threshold estimation procedures were evaluated: a rapid, efficient binary search technique (REBS) and a maximum-likelihood estimation (ZEST) procedure. A computerized visual field simulation model was developed to determine the accuracy and efficiency of these procedures. This model was constructed using previously derived characteristics of FDT perimetry from both normal observers (n = 506) and those with glaucomatous visual field loss (n = 352). The computer simulation program was used to determine the best parameters for the two new procedures and the effect of variability and response errors on algorithm performance. Comparisons were made to the performance of the modified binary search (MOBS) procedure used in the current commercial implementation of the FDT perimeter. RESULTS: Both the optimized REBS and ZEST procedures approximately halved the time required for FDT threshold testing without loss of accuracy or reproducibility. CONCLUSIONS: With suitable parameter choices, comparable performance was achieved using either ZEST or REBS. Simulation results indicate that accurate thresholds can be measured with an optimized ZEST or REBS procedure in approximately half the time required by traditional estimation methods.

Aged↗

PubMed2002

Performance of efficient test procedures for frequency-doubling technology perimetry in normal and glaucomatous eyes.

PURPOSE: To validate the clinical performance of two new efficient threshold-estimation procedures for frequency-doubling technology (FDT) perimetry in both visually normal individuals and patients with glaucomatous visual field loss. METHODS: Forty-one normal subjects (mean age, 48.3 +/- 11.6 years) and 50 patients with glaucomatous visual field loss (mean age, 72.7 +/- 10.0 years) were tested. Some of these participants were retested within a 3-month period. FDT perimetry was performed on a color monitor driven by a visual-stimulus-generating video board, with stimulus parameters designed to closely mimic those of the commercial FDT test. Visual field sensitivity was measured using three procedures: a modified binary search (MOBS) identical with the one used in the commercial FDT device, a rapid efficient binary search (REBS), and a procedure the uses Bayesian methods (zippy estimation of sequential testing; ZEST). The selection of optimum parameters for REBS and ZEST were based on results from previous simulations. RESULTS: Both ZEST and REBS were 40% to 50% faster than MOBS. All three methods produced similar visual field sensitivity measures, with 95% of the differences occurring between +/-2 dB for normal subjects and +/-3 dB for glaucoma patients. Test-retest performance was similar for all three procedures. CONCLUSIONS: The test time for full-threshold FDT perimetry can be approximately halved, by using either the ZEST or REBS procedure, without affecting the accuracy or reliability of the measurements. These findings in normal subjects and patients with glaucoma provide clinical confirmation of our previous investigations of these test strategies that use computer simulation.

Aged↗

PubMed2002

Elevated vernier acuity thresholds in glaucoma.

PURPOSE: In 1993, Piltz et al. observed that foveal vernier acuity thresholds for achromatic targets are elevated in patients with glaucoma. This study was undertaken to explore whether such elevated thresholds are present when subject groups are measured with targets of effectively equivalent contrast. Vernier acuity measures were also obtained with short-wavelength and frequency-doubled stimuli, to assess spatial hyperacuity performance in the short-wavelength-sensitive and magnocellular pathways, respectively. METHODS: Twenty patients with glaucoma and 19 subjects with normal vision participated. All subjects had visual acuity of 20/25 or better. Achromatic two-dot vernier thresholds were obtained for 90% contrast dots. In addition, individual contrast thresholds to the achromatic dots were measured for each subject, and vernier thresholds were measured at 4, 8, 12, and 16 times contrast threshold. Short-wavelength vernier acuity thresholds were measured for blue dots presented on a bright yellow background. The stimulus for the frequency-doubling grating vernier acuity task was a 90% contrast, 1-cyc/deg, 25-Hz sinusoidal grating. RESULTS: The glaucoma group demonstrated significantly higher foveal vernier acuity thresholds than control subjects for the blue-on-yellow stimulus (P = 0.002) and frequency-doubling grating stimulus (P < 0.001). No significant difference in vernier acuity between groups was found for the 90% contrast achromatic dots (P = 0.09), however a significant difference was found for the normalized contrast targets (P = 0.04). CONCLUSIONS: Vernier acuity tasks can be used to demonstrate abnormal foveal function in glaucoma. Testing with visual-function-specific stimuli may be effective in identifying such dysfunction. Vernier acuity, or other similar hyperacuity tasks that assess spatial sampling, may be useful in the detection of early glaucomatous damage, before it is detected with traditional perimetric tests.

Aged↗