Biomedical subjects
Publications and source records attributed to Andrew J Anderson.
When searching with our eyes, parallel programming of successive eye movements ensures that visual information arriving too late to alter the direction of one eye movement can still influence the direction of the next. Paradoxically, we can use random noise to probe the time period over which visual information influences where next to direct our gaze.
Bayesian adaptive threshold procedures may be run for a fixed number of trials, or may be stopped when the calculated confidence interval for the threshold reaches a selected limit (a dynamic termination criterion). This study used Monte-Carlo simulations to determine whether the confidence interval is a useful predictor of errors in the estimated threshold. No difference was found between the distribution of errors in a fixed trial procedure versus a dynamically terminated procedure of the same average number of trials. In addition, the width of the confidence interval failed to usefully predict observer variability arising from a shallow psychometric function slope or increased false positive response probabilities. This study suggests that dynamic termination criteria are of little use in Bayesian adaptive threshold procedures.
The FDT perimeter is a compact and relatively inexpensive perimeter whose transportability, tolerance to refractive errors, and rapid test times (less than 1 minute per eye) make it a suitable candidate for visual field screening. It is a uniform finding that the C-20-1 screening protocol of the FDT perimeter provides good sensitivity and specificity for the detection of moderate and severe losses in glaucoma. Sensitivity can be increased by use of the C-20-5 screening protocol. In addition, the FDT perimeter demonstrates good sensitivity and specificity for detecting the presence of neuro-ophthalmic disorders, though it may have a limited ability to determine whether a field defect is hemianopic. There is only limited evidence that the FDT can appropriately detect retinal disease. Despite some evidence that the current FDT perimeter may be suitable for staging and monitoring the progression of visual field damage, the large targets used in the test make this of limited practicability. The development of a frequency-doubling test with smaller targets spaced over narrower intervals would improve the ability of FDT perimetry to determine the spatial extent of visual field defects.
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.
PURPOSE: Tendency-oriented perimetry (TOP) is a new strategy designed to estimate the sensitivity of the visual field quickly, by using linear interpolation between test locations. This study determined the spatial resolution characteristics of TOP. METHODS: A Monte-Carlo technique was used to simulate visual fields, and incorporated realistic amounts of subject response variability as well as variability in the average sensitivity of the field. Visual field defects of various depths, ranging from a single point through to 18 contiguous points, were added to the simulated fields. An estimate of the visual field was made using the TOP algorithm. Global indices (mean deviation [MD] and loss variance [LV]) were calculated for both the true visual field and the TOP estimate. RESULTS: For small defects of one or two points, the TOP algorithm typically overestimated sensitivity. Sensitivity estimates tended to stratify into one of two possible values, with the lower value being dependent on the absolute position of the defect within the visual field. Although MD was satisfactorily predicted by TOP, LV was underestimated and reached a plateau when defects were deep, especially with smaller defects. For relatively large defects of nine contiguous points, both defect depth and LV was predicted with reasonable accuracy by TOP. The TOP sensitivity estimate for normal locations surrounding a defect was systematically reduced. CONCLUSIONS: The TOP procedure has a number of unusual spatial characteristics that prevent it from accurately estimating the spatial extent and absolute sensitivity of visual field defects.
We examined the effect of spatial waveform on the perceived spatial frequency of a grating target. The luminance profile of 0.5 c/ degrees sinusoidal gratings was modified by either compressive or expansive power functions, and was presented alternately with a true sinusoidal grating. Subjects matched the apparent spatial frequency of the two gratings using a method of adjustment. Both compressive and expansive power functions lowered the perceived spatial frequency of the grating, irrespective of the stimulus contrast. Rectified sine wave gratings were also found to reduce apparent spatial frequency. The magnitude of the spatial frequency shifts with spatial waveform diminished with successive matches, which may represent a change in matching strategy employed by observers. Calculations and a further experiment suggest that judgements of spatial frequency may in part be determined by the separation between edges in a grating.
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.
Background: The Medmont AT-20 has incorporated a contrast threshold test using a predetermined letter size that can be applied in clinical settings. This paper describes a pilot study that evaluates this technology and the effects of certain parameters on test outcomes. Methods: A photometric calibration of the test was performed to define the relationship between the AT-20 scale and Weber contrast (W%). We determined the effects of repeated measures (precision), target size (6/6 to 6/96), viewing duration (50 to 1,000 msec), defocus (+0.50 to +1.50 DS) and a macula scotoma on thresholds. The accuracy of the staircase (PEST) procedure was evaluated with and without false-negative responses. Results: The AT-20 scale has an almost linear relationship to a logarithmic transformation of W% and provides a suitable measure of contrast threshold. In the absence of monitor calibration, threshold uncertainty could be as great as 0.22 log units (W%) compared with published norms. We found that threshold variability averaged +/- 7.1 AT-20 scale units (95 per cent limits of agreement) and was proportional to threshold magnitude. One dioptre of defocus decreased thresholds by about one log unit (W%) for a 6/24 target. We propose that a 6/24 letter shown for 500 msec should provide a useful target for most clinical settings. The PEST procedure can yield endpoints in 47 (+/-12) seconds, is robust to false negative (FN) responses and gives abnormal thresholds in the presence of a macula scotoma. Conclusions: The Medmont AT-20 contrast test provides a useful clinical measure of contrast threshold. With calibration, the test could also be applied to research projects.