Biomedical subjects

Russell L Woods

Publications and source records attributed to Russell L Woods.

10 recordsLinked to original sources

PubMed2005

Thresholds vary between spatial and temporal forced-choice paradigms: the case of lateral interactions in peripheral vision.

Psychophysicists use spatial or temporal two-alternative forced-choice (2AFC) paradigms interchangeably. Thus, experiments with the same general goal are carried out using one or the other paradigm by distinct or even the same research groups. For example, this situation has occurred both in studies on visual sensitivity in dyslexia and in studies on lateral interactions in peripheral vision. Conflicting results in either field (e.g. whether or not dyslexics have a visual deficit and whether or not peripheral detection is facilitated by the presence of flankers) appear to be resolved on the surmise that spatial and temporal 2AFC paradigms indeed produce different results. We designed experiments in which peripheral detection thresholds for Gabor patches (in the presence or absence of suprathreshold flankers) could be measured using completely equivalent spatial and temporal 2AFC paradigms so that any resultant difference can be unequivocally attributed to the effect of the paradigms themselves. The results showed that spatial 2AFC renders significantly lower sensitivity than temporal 2AFC when the target is presented along with suprathreshold flankers, but about the same sensitivity as temporal 2AFC when the target is presented alone. In the end, this resulted in statistically significant facilitation in peripheral vision only when measured with temporal 2AFC. Separate experiments at each of several peripheral locations revealed that the presence and magnitude of this effect varies not only with psychophysical paradigm but also with retinal locus.

Choice Behavior↗

PubMed2004

Facilitation of contrast detection in near-peripheral vision.

Foveal detection of a Gabor patch (target) is facilitated by collinear, displaced high-contrast flankers. Polat and Sagi reported that the same phenomenon occurred in the periphery, but no data were presented [Proc. Natl. Acad. Sci. 91 (1994) 1206]. Others have found no facilitation in a limited number of conditions tested. To resolve this apparent conflict, we measured lateral facilitation in the near-periphery using a range of stimulus parameters. We found facilitation for a range of target-flanker distances for peripheral eccentricities up to 6 degrees , but the magnitude of the effect was less than found in central vision. Facilitation varied across subjects and with spatial frequency. Flanker contrast had no effect over the range evaluated (10-80%). Equal facilitation was found for two global arrangements of the stimulus pattern. Facilitation was found using a temporal, but not a spatial two-alternative forced-choice paradigm, accounting for the different results among previous studies. This finding supports previous indications of the role of attention in altering such facilitation. The value of facilitation from lateral interactions for persons with central vision impairment, who have to shift their attention to a peripheral locus constantly, needs to be examined.

Adolescent↗

PubMed2004

Preferred retinal locus and reading rate with four dynamic text presentation formats.

BACKGROUND: Electronic display devices hold the potential to improve access to written material by people with low vision. For those with central field loss, the optimal form of electronic text presentation may vary according to the location of the preferred retinal locus, but this has never been investigated. In this study, we examined the relationship between preferred retinal locus location and reading rate for four electronic display formats (rapid serial visual presentation, horizontal scroll, vertical scroll, and page). METHODS: Short sentences were presented in each format to 35 low-vision (most with central field loss) and 14 age-matched control subjects. Subjects read aloud to determine maximum oral reading rate and read silently to determine preferred silent reading rate. RESULTS: With the exception of page format, maximum oral reading rates were faster than silent preferred reading rates for both groups of subjects. For the low-vision group, there was no significant difference in maximum oral reading rates between the four display formats; and when reading at a preferred silent rate, page format was faster than the other three formats. Though page format was read more quickly, half of the low-vision subjects preferred the horizontal-scroll format. Contrary to our predictions, there was no significant effect of preferred retinal locus location (vertical vs. lateral) on reading rate and no significant interaction between preferred retinal locus location and display format. CONCLUSIONS: The differences between maximum oral and preferred silent reading rates and the lack of a relationship between reading rates and preferred display format reinforce the importance of patient preference in the evaluation and selection of a device or display format during rehabilitation.

Adult↗

PubMed2004

Wideband enhancement of television images for people with visual impairments.

Wideband enhancement was implemented by detecting visually relevant edge and bar features in an image to produce a bipolar contour map. The addition of these contours to the original image resulted in increased local contrast of these features and an increase in the spatial bandwidth of the image. Testing with static television images revealed that visually impaired patients (n = 35) could distinguish the enhanced images and preferred them over the original images (and degraded images). Most patients preferred a moderate level of wideband enhancement, since they preferred natural-looking images and rejected visible artifacts of the enhancement. Comparison of the enhanced images with the originals revealed that the improvement in the perceived image quality was significant for only 22% of the patients. Possible reasons for the limited increase in perceived image quality are discussed, and improvements are suggested.

Activities of Daily Living↗

PubMed2004

Tracking the line of primary gaze in a walking simulator: modeling and calibration.

This article describes a system for tracking the line of primary gaze (LoPG) of participants as they view a large projection screen. Using a magnetic head tracker and a tracking algorithm, we find the onscreen location at which a participant is pointing a head-mounted crosshair. The algorithm presented for tracking the LoPG uses a polynomial function to correct for distortion in magnetic tracker readings, a geometric model for computing LoPG from corrected tracker measurements, and a method for finding the intersection of the LoPG with the screen. Calibration techniques for the above methods are presented. The results of two experiments validating the algorithm and calibration methods are also reported. Experiments showed an improvement in accuracy of LoPG tracking provided by each of the two presented calibration steps, yielding errors in LoPG measurements of less than 2 degrees over a wide range of head positions. Source code for the described algorithms can be downloaded from the Psychonomic Society Web archive, http://www.psychonomic.org/archive/.

Algorithms↗

PubMed2003

Contour integration in peripheral vision reduces gradually with eccentricity.

Hess and Dakin reported that normally-sighted subjects using peripheral vision (beyond 10 degrees ) were unable to detect paths of alternating-phase Gabors embedded within randomly positioned Gabors, but could detect same-phase paths. This result led them to propose a "fundamental difference" between central and peripheral visual processing. While we were able to replicate many of their results, our normally-sighted observers could detect alternating-phase paths beyond 10 degrees. We found that path detection decreased monotonically as a function of eccentricity (0 degrees -30 degrees ) for both alternating-phase and same-phase stimuli. As with most visual functions the more difficult path detection condition (alternating-phase) declined slightly faster. The results for the normally-sighted observers could not be explained by poor fixation. Three people with substantial central vision loss (i.e. they can only use peripheral vision) could see both same- and alternating-phase stimuli with eccentric viewing of 13 degrees -17 degrees. Therefore central and peripheral vision appear to use similar visual mechanisms to perform the task, there being no fundamental difference.

Adult↗

PubMed2003

Comparison of monochromatic ocular aberrations measured with an objective cross-cylinder aberroscope and a Shack-Hartmann aberrometer.

Repeated measures of wavefront aberrations were taken along the line-of-sight of seven eyes using two instruments: an objective, cross-cylinder aberroscope (OA) and a Shack-Hartmann (SH) aberrometer. Both instruments were implemented on the same optical table to facilitate interleaved measurements on the same eyes under similar experimental conditions. Variability of repeated measures of individual coefficients tended to be much greater for OA data than for SH data. Although Zernike coefficients obtained from a single measurement were generally larger when measured with the OA than with the SH, the averages across five trials were often smaller for the OA. The Zernike coefficients obtained from the two instruments were not significantly correlated. Radial modulation-transfer functions and point-spread functions derived from the two sets of measurements were similar for some subjects, but not all. When average Zernike coefficients were used to determine optical quality, the OA indicated superior optics in some eyes, but the reverse trend was true if Zernike coefficients from individual trials were used. Possible reasons for discrepancies between the OA and SH measurements include difference in sampling density, quality of data images, alignment errors, and temporal fluctuations. Multivariate statistical analysis indicated that the SH aberrometer discriminated between subjects much better than did the objective aberroscope.

Diagnostic Techniques, Ophthalmological↗

PubMed2002

Lateral interactions: size does matter.

Usually a high-contrast, co-local mask increases contrast threshold (inhibition). Interestingly, a laterally displaced mask (flanker) can facilitate contrast detection (Vision Research 33 (1993) 993; 34 (1994) 73). When spatial scaling of these flanker effects was implied, stimulus bandwidth was confounded with spatial frequency (lambda(-1)). Under conditions where at lower spatial frequencies, the size (standard deviation, sigma) of the Gabor patch was smaller (sigma<lambda) than higher spatial frequencies (sigma=lambda), the effect appeared scale invariant. We replicated the original results for all conditions. However, when Gabor size was fixed (sigma=lambda), facilitation changed with spatial frequency (range 2--13 cycles/deg). When Gabor size was varied (sigma=0.5-2 lambda), usually the combination of larger patch sizes and lower spatial frequencies caused inhibition. We were unable to find any conditions that demonstrated spatial scaling. The size, both lambda and sigma, of both stimulus and flankers, influenced contrast threshold. Also, facilitation reduced as contrast of the flankers was reduced to detection threshold. Some facilitation was apparent with sub-threshold flankers. These results need to be reconciled with current models of lateral interactions.

Adult↗

PubMed2002

Vision and mobility performance of subjects with age-related macular degeneration.

PURPOSE: To investigate the effects of age-related macular degeneration (ARMD) on mobility performance and to identify the vision determinants of mobility in subjects with ARMD. METHODS: Walking speed and the number of obstacle contacts made on a 79-m indoor mobility course were measured in 21 subjects with ARMD and 11 age-matched subjects with normal vision. The mobility measures were transformed to percentage preferred walking speed and contacts score. The vision functions assessed included binocular visual acuity, contrast sensitivity, and visual field. RESULTS: In this study, subjects with ARMD did not walk significantly slower or make significantly more obstacle contacts on the mobility course than the normally sighted subjects of similar age. Between 29% and 35% of the variance in the ARMD mobility performance was accounted for by visual field and contrast sensitivity measures. The most significant predictor of mobility performance scored as percentage preferred walking speed was the size of a binocular central scotoma. CONCLUSION: As the size of a binocular central scotoma increases, mobility performance decreases.

Aged↗

PubMed2002

Stereoacuity at distance and near.

PURPOSE: Because previous studies have reported conflicting evidence, we examined a possible difference in stereoacuity between distance and near, in particular using a random-dot display. We compared distance and near stereoacuities using identical presentation formats at the two distances. METHODS: Twelve young adults with low, stable refractive errors and apparently normal binocular vision participated. Stereoacuity was determined with a Mentor B-VAT II using Random Dot E (BVRDE) and Contour Circles (BVC) stereograms presented on a standard monitor (25 x 19.3 cm) at 518 cm (distance-habitual) and a small monitor (2.0 x 1.4 cm) at 40 cm (near-habitual). To examine whether accommodation-convergence influenced stereoacuity, testing at 40 cm was repeated with the addition of +2.50 DS lenses and base-in prisms (near-compensated) that created the same accommodation and convergence demands as for distance-habitual viewing. RESULTS: The two stereotests produced similar findings. Stereoacuity was not significantly different for distance-habitual and near-habitual viewing of the BVRDE (p = 0.43) and BVC (p = 0.79) stereotests. Near-compensated stereoacuity was worse than near-habitual (BVRDE, p = 0.005; BVC, p = 0.004) and distance-habitual (BVRDE, p = 0.05; BVC, p = 0.003) for both stereotests. Because near stereoacuity with yoked prisms (control condition) was the same as without prism (near-habitual), prism-induced optical distortions cannot account for the difference. CONCLUSIONS: Stereoacuity was not different at distance and near under normal viewing conditions. The conflict between subject knowledge of target proximity and the optically-induced relaxation of accommodation and convergence, or an inaccurate accommodative-convergence response, might have caused poor near-compensated stereoacuity.

Accommodation, Ocular↗