Biomedical subjects

C Bowd

Publications and source records attributed to C Bowd.

At least 19 recordsLinked to original sources

PubMed2001

Discriminating between normal and glaucomatous eyes using the Heidelberg Retina Tomograph, GDx Nerve Fiber Analyzer, and Optical Coherence Tomograph.

OBJECTIVE: To compare the ability of 3 instruments, the Heidelberg Retina Tomograph (HRT), the GDx Nerve Fiber Analyzer (GDx), and the Optical Coherence Tomograph (OCT), to discriminate between healthy eyes and eyes with early to moderate glaucomatous visual field loss. SUBJECTS AND METHODS: Forty-one patients with early to moderate glaucomatous visual field loss and 50 healthy subjects were included in the study. The HRT, GDx, and OCT imaging and visual field testing were completed on 1 eye from each subject within a 6-month interval. Statistical differences in sensitivity at fixed specificities of 85%, 90%, and 95% were evaluated. In addition, areas under the receiver operating characteristic (ROC) curve were compared. RESULTS: No significant differences were found between the area under the ROC curve and the best parameter from each instrument: OCT thickness at the 5-o'clock inferior temporal position (mean +/- SE, 0.87 +/- 0.04), HRT mean height contour in the nasal inferior region (mean +/- SE, 0.86 +/- 0.04), and GDx linear discriminant function (mean +/- SE, 0.84 +/- 0.04). Twelve HRT, 2 GDx, and 9 OCT parameters had an area under the ROC curve of at least 0.81. At a fixed specificity of 90%, significant differences were found between the sensitivity of OCT thickness at the 5-o'clock inferior temporal position (71%) and parameters with sensitivities less than 52%. Qualitative assessment of stereophotographs resulted in a sensitivity of 80%. CONCLUSION: Although the area under the ROC curves was similar among the best parameters from each instrument, qualitative assessment of stereophotographs and measurements from the OCT and HRT generally had higher sensitivities than measurements from the GDx.

Adult↗

PubMed2001

Relationship between visual field testing and scanning laser polarimetry in patients with a large cup-to-disk ratio.

PURPOSE: To determine the relationship between quantitative nerve fiber layer measurements and visual field testing in patients with large cup-to-disk ratios. METHODS: Seventy-six patients with vertical cup-to-disk ratios by contour of at least 0.8 on stereoscopic photographs and 50 normal subjects were included. One eye was randomly selected for study. All patients underwent standard achromatic automated perimetry, short-wavelength automated perimetry, and retinal nerve fiber layer measurements with scanning laser polarimetry. Analysis of variance was used to evaluate differences between the subject groups. Significance of pairwise comparisons was determined using the Tukey-Kramer multiple comparison test. RESULTS: Statistically significant differences in nerve fiber layer measurements between patients with large cup-to-disk ratios and both abnormal standard achromatic automated perimetry and short-wavelength automated perimetry (n = 22) and patients with large cup-to-disk ratios and both normal standard achromatic automated perimetry and short-wavelength automated perimetry (n = 42) were found for superior nasal ratio, maximum modulation, ellipse modulation, and the linear discriminant function (Tukey-Kramer less than.05). There was no significant difference in patients with abnormal short-wavelength automated perimetry only (n = 9) as compared with patients with both normal standard achromatic automated perimetry and short-wavelength automated perimetry and patients with both abnormal standard achromatic automated perimetry and short-wavelength automated perimetry. Statistically significant differences between the normal subjects and patients with large cup-to-disk ratios and both abnormal standard achromatic automated perimetry and short-wavelength automated perimetry were found for all retinal nerve fiber layer parameters, with the exception of symmetry, superior ratio, and inferior ratio. CONCLUSION: Our results show considerable overlap in nerve fiber layer measurements in eyes with large cup-to-disk ratio and abnormal visual fields as compared with eyes with large cup-to-disk ratios and normal visual fields. This may limit the clinical usefulness of scanning laser polarimetry for detection of early glaucoma in patients with large cup-to-disk ratios. Longitudinal studies are needed to determine if patients with large cup-to-disk ratios with normal standard achromatic automated perimetry and abnormal short-wavelength automated perimetry subsequently develop standard achromatic automated perimetry defects and if scanning laser polarimetry can concurrently detect progression of nerve fiber layer damage.

Aged↗

PubMed2001

Using optical imaging summary data to detect glaucoma.

PURPOSE: To evaluate the sensitivity and specificity for discriminating between early to moderate glaucomatous and normal eyes using summary data reports from the Heidelberg Retina Tomograph (HRT), the GDx Nerve Fiber Analyzer (GDx), and the Optical Coherence Tomograph (OCT). DESIGN: Comparative cross-sectional study PARTICIPANTS: One eye each of 50 normal subjects and 39 glaucoma patients with early to moderate visual field damage (mean deviation, -5.04 +/- 3.32 dB; range, -0.85 to -13.2 dB). METHODS: Three experienced graders masked to patient identity and diagnosis evaluated each summary data report from the HRT, GDx, and OCT independently. MAIN OUTCOME MEASURES: Each summary report was classified as either normal or glaucomatous. Sensitivity and specificity are reported for each grader, and agreement between graders is reported. RESULTS: For the HRT, sensitivity and specificity ranged from 64% to 75% and 68% to 80%, respectively. Agreement (kappa +/- standard error [SE]) between observers one and two, two and three, and one and three was 0.73 +/- 0.07, 0.77 +/- 0.07, and 0.67 +/- 0.08, respectively. For the GDx, sensitivity and specificity ranged from 72% to 82% and 56% to 82%, respectively. Agreement (kappa +/- SE) between observers one and two, two and three, and one and three was 0.66 +/- 0.08, 0.66 +/- 0.08, and 0.50 +/- 0.09, respectively. For the OCT, sensitivity and specificity ranged from 76% to 79% and 68% to 81%, respectively. Agreement (kappa +/- SE) between observers one and two, two and three, and one and three was 0.73 +/- 0.07, 0.58 +/- 0.08, and 0.51 +/- 0.09, respectively. CONCLUSIONS: When used alone, HRT, GDx, and OCT summary data reports can differentiate between normal and glaucomatous eyes with mild to moderate visual field loss. However, none of the instruments provided sensitivity and specificity that justify summary data reports being used as a screening tool for early to moderate glaucoma.

Adult↗

PubMed2001

Detecting early glaucoma by assessment of retinal nerve fiber layer thickness and visual function.

PURPOSE: To compare the abilities of scanning laser polarimetry (SLP), optical coherence tomography (OCT), short-wavelength automated perimetry (SWAP), and frequency-doubling technology (FDT) perimetry to discriminate between healthy eyes and those with early glaucoma, classified based on standard automated perimetry (SAP) and optic disc appearance. To determine the agreement among instruments for classifying eyes as glaucomatous. METHODS: One eye of each of 94 subjects was included. Healthy eyes (n = 38) had both normal-appearing optic discs and normal SAP results. Glaucoma by SAP (n = 42) required a repeatable abnormal result (glaucoma hemifield test [GHT] or corrected pattern standard deviation [CPSD] outside normal limits). Glaucoma by disc appearance (n = 51) was based on masked stereoscopic photograph evaluation. Receiver operating characteristic (ROC) curve areas, sensitivities, and specificities were calculated for each instrument separately for each diagnosis. RESULTS: The largest area under the ROC curve was found for OCT inferior quadrant thickness (0.91 for diagnosis based on SAP, 0.89 for diagnosis based on disc appearance), followed by the FDT number of total deviation plot points of < or =5% (0.88 and 0.87, respectively), SLP linear discriminant function (0.79 and 0.81, respectively), and SWAP PSD (0.78 and 0.76, respectively). For diagnosis based on SAP, the ROC curve area was significantly larger for OCT than for SLP and SWAP. For diagnosis based on disc appearance, the ROC curve area was significantly larger for OCT than for SWAP. For both diagnostic criteria, at specificities of > or =90% and > or =70%, the most sensitive OCT parameter was more sensitive than the most sensitive SWAP and SLP parameters. For diagnosis based on SAP, the most sensitive FDT parameter was more sensitive than the most sensitive SLP parameter at specificities of > or =90% and > or =70% and was more sensitive than the most sensitive SWAP parameter at specificity of > or =70%. For diagnosis based on disc appearance at specificity of > or =90%, the most sensitive FDT parameter was more sensitive than the most sensitive SWAP and SLP parameters. At specificity > or = 90%, agreement among instruments for classifying eyes as glaucomatous was poor. CONCLUSIONS: In general, areas under the ROC curve were largest (although not always significantly so) for OCT parameters, followed by FDT, SLP, and SWAP, regardless of the definition of glaucoma used. The most sensitive OCT and FDT parameters tended to be more sensitive than the most sensitive SWAP and SLP parameters at the specificities investigated, regardless of diagnostic criteria.

Diagnostic Techniques, Ophthalmological↗

PubMed2000

The retinal nerve fiber layer thickness in ocular hypertensive, normal, and glaucomatous eyes with optical coherence tomography.

OBJECTIVE: To quantitatively assess and compare the thickness of the retinal nerve fiber layer (RNFL) in ocular hypertensive eyes with normal and glaucomatous eyes using the Optical Coherence Tomograph (OCT 2000, software version A4X1; Humphrey Instruments, San Leandro, Calif). METHODS: The mean RNFL thickness of ocular hypertensive (n = 28) eyes was compared with age-matched normal (n = 30) and glaucomatous (n = 29) eyes. Subject eyes were classified into diagnostic groups based on intraocular pressure, stereoscopic disc photographs, and standard automated perimetry. Three circular scans were obtained for each eye at a diameter of 3.4 mm around the optic disc. In each eye, average RNFL thickness measurements were obtained in temporal, superior, nasal, and inferior quadrants. A single index of average RNFL thickness throughout 360 degrees also was obtained. RESULTS: Mean (95% confidence interval) RNFL was significantly thinner in ocular hypertensive eyes than in normal eyes, 72.8 microm (66.4-78.1 microm) and 85.8 microm (80.2-91.7 microm), respectively. More specifically, RNFL was significantly thinner in ocular hypertensive eyes than in normal eyes in the inferior quadrant, 84.8 microm (75.6-94.0 microm) vs 107.6 microm (99.3-115.9 microm); and in the nasal quadrant, 44.1 microm (37.5-51.7 microm) vs 61.8 microm (53.0-65.6 microm). Retinal nerve fiber layer was significantly thinner in glaucomatous eyes than in ocular hypertensive and normal eyes throughout 360 degrees and in all quadrants. CONCLUSION: These findings suggest that quantitative differences in RNFL thickness exist between age-matched ocular hypertensive, normal, and glaucomatous eyes.

Adult↗

PubMed2000

Optic disk topography after medical treatment to reduce intraocular pressure.

PURPOSE: We examined changes in optic disk topography using confocal scanning laser ophthalmoscopy after reducing intraocular pressure with administration of latanoprost. METHODS: Twenty-nine patients with glaucoma or ocular hypertension were imaged using the Heidelberg Retina Tomograph before and after the administration of latanoprost to decrease intraocular pressure. Average time between pretreatment and posttreatment imaging was 2.7 +/- 1.8 weeks. Heidelberg Retina Tomograph software-measured parameters were mean height of contour, cup area, cup volume, mean cup depth, maximum cup depth, cup shape, rim area, rim volume, cup-to-disk ratio, and retinal nerve fiber thickness. RESULTS: Average intraocular pressure decreased significantly (mean +/- SD) by 7.2 +/- 5.4 mm Hg (25 +/- 16% decrease). No statistically significant changes in measured topographic parameters were found. When data from patients with decreases in intraocular pressure of 7 mm Hg or greater were analyzed separately (mean intraocular pressure decrease = 10.79 +/- 4.32 mm Hg, 36 +/- 8% decrease), cup area (P =.005), cup volume (P =. 002), and cup-to-disk ratio (P =.005) decreased significantly, and rim area (P =.005) increased significantly. Linear regression analysis of the data from all subjects showed that a change in intraocular pressure after latanoprost administration accounted for 12% or more of the variance in two measured topographic parameters (mean cup depth and cup shape). CONCLUSIONS: These results suggest that, in some patients, moderate decreases in intraocular pressure may affect disk topography, as measured by Heidelberg Retina Tomograph. Intraocular pressure should be considered when analyzing consecutive confocal scanning laser ophthalmoscopy images for glaucomatous progression.

Antihypertensive Agents↗

PubMed2000

Exposure duration affects the perceived direction of cyclopean type II plaids.

This study investigated the effect of exposure duration on the perceived direction of cyclopean Type I and Type II plaids moving in the X/Y plane. The cyclopean plaids were created from grating components defined by binocular disparity embedded in a dynamic random-dot stereogram. The results showed that the cyclopean Type I plaid appeared to move in the intersection-of-constraints (IOC) direction across the range of exposures tested. However, the cyclopean Type II plaids appeared to move in a direction different from the IOC with short exposures but near the IOC with long exposures. This perceived directional shift was also obtained with luminance-defined Type II plaids. A common pattern-motion mechanism that processes cyclopean and luminance motion signals appears responsible for the perceived directional shift of the Type II plaids.

Depth Perception↗

PubMed2000

Cross-domain adaptation reveals that a common mechanism computes stereoscopic (cyclopean) and luminance plaid motion.

Across three experiments, this study investigated the visual processing of moving stereoscopic plaid patterns (plaids created with cyclopean components defined by moving binocular disparity embedded in a dynamic random-dot stereogram). Results showed that adaptation to a moving stereoscopic plaid or its components affected the perceived coherence of a luminance test plaid, and vice versa. Cross-domain adaptation suggests that stereoscopic and luminance motion signals feed into a common pattern-motion mechanism, consistent with the idea that stereoscopic motion signals are computed early in the motion processing stream.

Adaptation, Ocular↗

PubMed2000

Interference from multi-dimensional objects during feature and conjunction discriminations.

Feature discrimination performance within an attended object and interference from irrelevant, multi-dimensional objects (distractors) were examined in a two-choice, response compatibility paradigm. Results showed that the amount of interference by multi-dimensional distractors was dependent on three factors: (1) the discriminability of the incompatible, task-relevant distractor features; (2) the number of incompatible, task-relevant distractor features; and (3) whether the task-relevant, incompatible features matched the task goals. The most interesting finding was that additive priming effects were found for multiple, task-relevant features that matched the task goals, whether these features were present in the attended object or in the ignored object. Models that assume that each task-relevant feature primes its corresponding decision/response asynchronously and that this priming is combined to meet a decision/response criterion (at least when attended) can account for distractor interference during conjunction discriminations. Implications of these findings for feature integration models, template models, and a response selection model are discussed.

Adult↗

PubMed1999

Occlusion contributes to temporal processing differences between crossed and uncrossed stereopsis in random-dot displays.

Stereoscopic depth discrimination was investigated in crossed and uncrossed directions using stimuli defined by binocular disparity differences embedded in dynamic random-dot stereograms. Across three experiments, fixation was directed to a point on the display screen (which placed crossed stimuli in front of and uncrossed stimuli behind, the background dots of the stereogram), to a point in front of the display screen (which placed both crossed and uncrossed stimuli in front of the background dots), and to a point behind the display screen (which placed both crossed and uncrossed stimuli behind the background dots). Results showed that depth discrimination was always good when the stimuli appeared in front of the background dots of the stereogram, whereas discrimination was always poor when the stimuli appeared behind the background dots. These results suggest that differences between crossed and uncrossed stereopsis as reported in past research arose, in part, from effects related to occlusion.

Depth Perception↗

PubMed1999

The stereoscopic (cyclopean) motion aftereffect is selective for spatial frequency and orientation of disparity modulation.

Across two experiments, this study investigated the spatial frequency tuning and orientation tuning (both in the disparity domain) of the stereoscopic (cyclopean) motion aftereffect. In Experiment 1, observers adapted to a moving stereoscopic grating of a given cyclopean spatial frequency and tested for the motion aftereffect with a static grating of the same or different spatial frequency. Robust motion aftereffects were induced only when the spatial frequency of the adapt and test stimuli was the same. In Experiment 2, observers adapted to a moving stereoscopic grating of a given cyclopean orientation and tested for the motion aftereffect with a static grating of the same or different orientation. Robust motion aftereffects were induced only when the orientation of the adapt and test stimuli was the same. Together, these results suggest that the stereoscopic motion aftereffect is tuned for cyclopean spatial frequency and orientation which, in turn, suggest that the stereoscopic motion aftereffect is mediated by low-level oriented spatial-frequency mechanisms.

Adaptation, Physiological↗

PubMed1998

The cyclopean (stereoscopic) barber pole illusion.

Across two experiments, this study found that the barber pole illusion (i.e. grating pattern appearing to move in the direction of the long axis of a rectangular aperture) is perceived with stereoscopic (cyclopean) motion. The grating and aperture comprising the barber pole display were created from binocular disparity differences embedded in a dynamic random-dot stereogram or from luminance differences. In Experiment 1, observers viewed a square-wave grating moving through a rectangular aperture of 2:1 or 4:1 aspect ratio and indicated whether the grating appeared to move in a direction perpendicular to its orientation or in the direction of the long axis of the aperture. For both stereoscopic and luminance stimuli equally, the grating appeared to move in the direction of the aperture (i.e. the barber pole illusion) more often with the larger aspect ratio than with the smaller aspect ratio. The condition for which a stereoscopic grating moved through a luminance rectangular aperture was also examined: the grating appeared to move in the direction of the aperture (inter-attribute barber pole illusion). In Experiment 2, observers viewed a square-wave grating moving through a rectangular aperture of 3:1 aspect ratio whose sides were indented in order to change the local direction of motion of the line terminators. For both stereoscopic and luminance stimuli, the grating appeared to move more frequently in a direction perpendicular to its orientation with the indented aperture (i.e. the illusion was diminished). Thus, local velocity signals from moving stereoscopic line terminators play a role in the production of the barber pole illusion similar to that of luminance motion signals. This suggests that the generation and propagation of motion signals at cyclopean levels of vision play a part in the representation of coherently-moving rigid surfaces.

Depth Perception↗

PubMed1998

Multiple-feature discrimination faster than single-feature discrimination within the same object?

In the present study, we investigated whether judging the presence of multiple features within an object would be superior to judging the presence of only one feature. Feature discriminability and the number of features to discriminate within an object were varied. Specific features were judged as present or absent. Results showed that judging the presence of two or three features was faster than judging the presence of the less discriminable of these two or three features alone (multiple-feature benefits). These findings suggest that relevant features within an object activate (prime) a decision or response in a parallel, asynchronous fashion based on discriminability (Miller, 1982a). The ability of a response priming model, a response mapping model, and a template model to account for multiple-feature benefits is discussed.

Adult↗

PubMed1997

Direction-selective coding of stereoscopic (cyclopean) motion.

This study employed a selective adaptation paradigm and investigated thresholds for direction discrimination of translational stereoscopic motion (moving binocular disparity information). The stimuli were moving arrays of randomly positioned stereoscopic discs created from disparity embedded in dynamic random-element stereograms. When discrimination thresholds were measured across a range of base directions following adaptation in a fixed direction, discrimination thresholds were maximally elevated 20-30 deg away from adaptation and reduced in the same direction as adaptation. These results are consistent with a distributed-channel model of direction coding and indicate that the direction of stereoscopic motion is encoded by adaptable direction-selective mechanisms similar to those proposed for luminance-defined motion.

Adaptation, Ocular↗

PubMed1997

Direction discrimination of cyclopean (stereoscopic) and luminance motion.

This study compared direction discrimination of cyclopean (stereoscopic) and luminance motion involving stimuli equated for effective strength. The stimuli were random-walk cinematogram (RWC) displays whose signal and noise discs were created from binocular disparity differences embedded in a dynamic random-dot stereogram or from luminance differences. Experiment 1 measured global motion detection thresholds for cyclopean and luminance stimuli by manipulating the proportion of signal to noise discs. Detection thresholds for cyclopean motion were about 25% whereas detection thresholds for luminance motion were 5%, thus five times more cyclopean motion events than luminance events were necessary to elicit threshold responding. Experiment 2 measured thresholds for discriminating the direction of cyclopean and luminance motion under conditions of equal stimulus strength by presenting the motion displays at equal multiples of detection threshold. Direction discrimination thresholds (ranging from about 5-30 deg, depending upon conditions) were similar for cyclopean and luminance motion, thus the precision with which the pooling of local motion events in one direction can be discriminated from the pooling of events in a slightly different direction is the same for cyclopean and luminance stimuli. The finding that cyclopean motion information is pooled is consistent with the idea that the direction of cyclopean motion is coded in the responses of a population of directionally selective mechanisms.

Depth Perception↗

PubMed1996

Disparity tuning of the stereoscopic (cyclopean) motion aftereffect.

Across five experiments this study investigated the disparity tuning of the stereoscopic motion aftereffect (adaptation from moving retinal disparity). Adapting and test stimuli were moving and stationary stereoscopic grating patterns, respectively, created from dynamic random-dot stereograms. Observers adapted to moving stereoscopic grating patterns presented with a given disparity and viewed stationary test patterns presented with the same or differing disparity to examine whether the motion aftereffect is disparity contingent. Across experiments aftereffect duration was greatest when adapting motion and test pattern both were presented with zero disparity and in the plane of fixation. Aftereffect declined as disparity of adapting motion and/or test pattern increased away from fixation, even under conditions in which depth position of adapt and test was equal. This argues against a relative depth separation explanation of the decline, and instead suggests that the amount of adaptable substrate decreases away from fixation.

Adaptation, Physiological↗

PubMed1996

Enduring stereoscopic motion aftereffects induced by prolonged adaptation.

This study investigated the effects of prolonged adaptation on the recovery of the stereoscopic motion aftereffect (adaptation induced by moving binocular disparity information). The adapting and test stimuli were stereoscopic grating patterns created from disparity, embedded in dynamic random-dot stereograms. Motion aftereffects induced by luminance stimuli were included in the study for comparison. Adaptation duration was either 1, 2, 4, 8, 16, 32 or 64 min and the duration of the ensuing aftereffect was the variable of interest. The results showed that aftereffect duration was proportional to the square root of adaptation duration for both stereoscopic and luminance stimuli; on log-log axes, the relation between aftereffect duration and adaptation duration was a power law with the slope near 0.5 in both cases. For both kinds of stimuli, there was no sign of adaptation saturation even at the longest adaptation duration.

Adaptation, Ocular↗

PubMed1994

Properties of the stereoscopic (cyclopean) motion aftereffect.

Across four experiments, this study investigated properties of the stereoscopic motion aftereffect (adaptation from moving retinal disparity information). The results showed that stereoscopic motion can induce an adaptation aftereffect across a wide range of conditions and observers, provided that the duration of adaptation is sufficiently long and a perceptually salient test pattern is viewed. Motion adaptation was found to transfer between the stereoscopic and luminance domains [replicating a previous report by Fox, Patterson and Lehmkuhle (1982) Investigative Ophthalmology and Visual Science (Suppl.), 22, 144], suggesting that motion perception from stereoscopic (second-order) and luminance (first-order) attributes is mediated by a common neural substrate.

Adaptation, Ocular↗