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P V McGraw

Publications and source records attributed to P V McGraw.

At least 19 recordsLinked to original sources
PubMed2006

The segregation and integration of colour in motion processing revealed by motion after-effects.

Analysis of the colour and motion of objects is widely believed to take place within segregated processing pathways in the primate visual system. However, it is apparent that this segregation cannot remain absolute and that there must be some capacity for integration across these sub-modalities. In this study, we have assessed the extent to which colour constitutes a separable entity in human motion processing by measuring the chromatic selectivity of two kinds of after-effect resulting from motion adaptation. First, the traditional motion after-effect, where prolonged inspection of a unidirectional moving stimulus results in illusory motion in the opposite direction, was found to exhibit a high degree of chromatic selectivity. The second type of after-effect, in which motion adaptation induces misperceptions in the spatial position of stationary objects, was completely insensitive to chromatic composition. This dissociation between the chromatic selectivities of these after-effects shows that chromatic inputs remain segregated at early stages of motion analysis, while at higher levels of cortical processing there is integration across chromatic, as well as achromatic inputs, to produce a unified perceptual output.

Adaptation, Physiological↗
PubMed2004

Sensory uncertainty governs the extent of audio-visual interaction.

Auditory signals have been shown to exert a marked influence on visual perception in a wide range of tasks. However, the mechanisms of these interactions are, at present, poorly understood. Here we present a series of experiments where a temporal cue within the auditory domain can significantly affect the localisation of a moving visual target. To investigate the mechanism of this interaction, we first modulated the spatial positional uncertainty of the visual target by varying its size. When visual positional uncertainty was low (small target size), auditory signals had little or no influence on perceived visual location. However, with increasing visual uncertainty (larger target sizes), auditory signals exerted a significantly greater influence on perceived visual location. We then altered the temporal profile of the auditory signal by modulating the spread of its Gaussian temporal envelope. Introducing this temporal uncertainty to the auditory signal greatly reduced its effect on visual localisation judgements. These findings support the view that the relative uncertainty in individual sensory domains governs the perceptual outcome of multisensory integration.

Acoustic Stimulation↗
PubMed2001

Is second-order spatial loss in amblyopia explained by the loss of first-order spatial input?

The purpose of the study was to determine whether amblyopes show detection loss for second-order spatial information, and if present, whether the loss is explained by the loss of first-order spatial input. We psychophysically determined detection thresholds for the amblyopic and non-amblyopic eyes of five adult amblyopes and the dominant eyes of three control observers. We found that four amblyopic eyes and two non-amblyopic eyes showed second-order loss relative to the control eyes. The second-order loss was greater than the first-order loss at the carrier spatial frequency (first-order input). The extra second-order loss indicates an early amplification of cortical neural loss that we speculate is due to deficient binocular input to second-order neurons.

Adult↗
PubMed2000

Vernier and contrast discrimination in central and peripheral vision.

The present paper asks whether Vernier offset discrimination is limited by the observer's sensitivity to local contrast change in both central and peripheral vision. To answer this question we compared Vernier discrimination and contrast discrimination thresholds (specified in the same units) for a pair of narrow ribbons of cosine gratings. Because the ribbons are narrow, both the offset information (for Vernier discrimination) and the contrast information (for contrast discrimination) are highly localized. We found that when the stimuli are narrow ribbons, the local contrast cue is the limiting factor in Vernier discrimination. However, our results also show that integration of information along the length of the gratings (the ribbon width) is: (i) different for Vernier and contrast discrimination, and (ii) for Vernier discrimination the integration of information along the length of the gratings differs qualitatively in central and peripheral vision. For narrow ribbons, the peripheral 'template' for ribbon Vernier acuity is not as well matched to the stimulus (in two-dimensional spatial frequency space) as the foveal 'template'.

Contrast Sensitivity↗
PubMed2000

Visual acuity measurements in a national sample of British elderly people.

BACKGROUND: Despite the fact that visual function has an important role in the quality of life in later years, very few studies have measured visual acuity in population based nationwide samples of British elderly people. Such measurements were carried out in the context of the national diet and nutrition survey of people aged 65 years or over (NDNS). METHODS: NDNS participants, who were living in 80 different randomly selected postcode areas of mainland Britain, were visited at their home by a nurse who measured visual acuity at 3 metres, using the Glasgow acuity card (GAC) method. In addition, a brief questionnaire related to ocular health was administered. RESULTS: Visual acuity was measured in 1362 NDNS participants who were not classified as mentally impaired. Visual impairment (using the WHO low vision criteria) was measured in 195 (14.3%) subjects. Prevalence of visual impairment increased significantly with age (65-74 years 3.1%; 75-84 years 11.6%; 85+ years 35.5%, p<0.001 for trend). Impaired vision was more common in subjects living in a nursing home (odds ratio adjusted for age 2.59 (95% CI 2.23 to 2. 96)) and in women (odds ratio adjusted for age 1.55 (95% CI 1.21 to 1.89)). 132 (9.7%) subjects had previously undergone cataract surgery and another 157 (11.5%) had been told that they currently had cataract. Vision improved 0.2 log units or more (at least one Snellen line) with the aid of a pinhole occluder in 289 subjects (21. 2%). CONCLUSION: Results of this nationwide, community based study confirm that problems with poor distance visual acuity exist in a substantial part of the elderly community, particularly in women and people living in nursing homes.

Aged↗
PubMed2000

Improving the reliability of visual acuity measures in young children.

Whilst the methodology of adult letter acuity measurement has been substantially refined over the last two decades, relatively little development has occurred in methods for quantifying letter acuity in young children. This study compares a recently developed visual acuity test (Glasgow Acuity Cards), which incorporates several key design features used in adult test charts to improve the sensitivity and reliability of visual acuity measurements. The equivalence of acuity measurements made with Glasgow Acuity Cards were compared with the Bailey-Lovie logMAR chart and Snellen chart in adults, and with traditional Single Letter Acuity and a modified Single Letter Acuity test in children. The test-retest reliability of acuity measurements made with Glasgow Acuity Cards and the Single Letter Acuity tests were also assessed in a large group of visually normal children. In addition, the ability of the pre-school letter acuity tests to detect differences in acuity between the two eyes, and to detect amblyopia were examined. Ninety-five percent of vision measurements made with the Bailey-Lovie chart and Glasgow Acuity Cards differ by less than 0.07 log unit. Furthermore, the sensitivity of Glasgow Acuity Cards to detecting changes in acuity longitudinally and inter-ocular differences in acuity is considerably greater as compared with traditional Single Letter Acuity tests. Improvements in paediatric acuity chart design are important for the effective detection and management of children with amblyopia.

Adolescent↗
PubMed1999

Perceptual distortions in the neural representation of visual space.

The visual mechanism by which human observers determine the separation between objects has long been of interest. This study examines the extent to which separation in visual space can be misperceived in foveal and extrafoveal vision. Foveally, vertical separations were consistently overestimated relative to horizontal separations, a result which is consistent with the well-documented horizontal-vertical illusion (HVI). Extrafoveally, much larger misrepresentations of visual space were perceived. In addition, separations tangential to fixation were consistently perceived as being greater than separations in a radial direction. These marked misperceptions of visual space which occur in extrafoveal vision take the form of a radial/tangential anisotropy combined with an overestimation of vertical distance. The results have important implications for meridional anisotropies which have previously been documented in a number of visual performance tasks.

Adult↗
PubMed1999

Alignment of separated patches: multiple location tags.

Gaussian and Gabor patches can be accurately localized; however, it is not yet clear which cues (or location tags) the visual system utilizes for localization. To determine the cues used in spatial alignment, we measured and modelled the perceived shifts for asymmetric Gaussian and Gabor patches over a wide range of separations, patch sizes and orientations. For Gaussian patches we observed perceived shifts that were generally consistent with that of the centroid of the envelope. For Gabor patches we found that the perceived shift depends on the carrier orientation (whether co-axial or orthoaxial with the patch arrangement), separation (in units of carrier wavelength) and patch size (number of cycles per standard deviation). Gabor patches with the carrier orthoaxial (horizontal) to the three vertically arranged patches, were similar to Gaussian patches. However, Gabor patches with the carrier coaxial (vertical) to the three vertically arranged patches resulted in perceived shifts that were consistent with a number of alternate localization primitives. The selection of primitives was dependent on mainly the separation and patch size. Our results support the suggestion that the visual system can use multiple tags for location (Hess et al., Vis Res 1994;34:2439-2451; Badcock et al., Vis Res 1996;36:1467-1472).

Cues↗
PubMed1999

Non-veridical size perception of expanding and contracting objects.

Observers were presented with various types of stimulus expansion and contraction which resulted in marked misperceptions of size. Firstly, the perceived size of an object which is changing in size is shown to be biased in the direction of the size change. Secondly, expansion or contraction of the internal texture of objects is found to influence their perceived size. Finally, an illusory texture manipulation in the form of a movement after-effect is shown to produce the same type of size misperception as a real expansion or contraction of internal texture. The spatio-temporal characteristics of these illusory size changes are investigated.

Humans↗
PubMed1999

Discriminating mirror symmetry in foveal and extra-foveal vision.

The ability to discriminate perfect from imperfect mirror symmetry was examined at the fovea and at eccentricities out to 10 degrees in the nasal visual field. A 2-AFC method of constant stimuli was employed in which a bilaterally symmetric pattern was presented in one interval and a degraded version of this symmetric pattern in the other. The subject's task was to decide which interval contained the perfectly symmetric pattern. Pattern size was varied by changing the viewing distance. Probit analysis revealed the degree of asymmetry corresponding to 75% correct performance. Given sufficient size scaling, perfectly symmetric stimuli can be discriminated from degraded symmetric stimuli in extra-foveal vision. Spatial scaling with an E2 value similar to that for positional acuity was successful in removing the eccentricity dependence for the task.

Fovea Centralis↗
PubMed1999

Spatial characteristics of the second-order visual pathway revealed by positional adaptation.

The visual system is thought to process luminance (first-order) and contrast (second-order) information by dedicated cortical streams. To explore the spatial characteristics of the second-order pathway, we examined the effect of adaptation on spatial localization in human subjects. We show that, unlike first-order adaptation, second-order positional adaptation via cortical mechanisms transfers across orientations but not across spatial frequencies. These results support physiological evidence that these two processing streams are distinct and suggest that the cortical mechanism mediating second-order positional adaptation maintains spatial frequency information but sums signals across orientations.

Adaptation, Ocular↗
PubMed1999

Functional visual loss in amblyopia and the effect of occlusion therapy.

PURPOSE: The aim of this study was to define the nature of functional visual loss in amblyopia and to identify those subjects whose amblyopia is chiefly due to one or more of the following deficits: abnormal contour interaction, abnormal eye movements, abnormal contrast perception, or positional uncertainty. METHODS: Fifty amblyopic children with a mean age of 5.6+/-1.3 years were referred from diverse sources. In addition to routine orthoptic and optometric evaluation the principal visual deficits in the amblyopic eye of each subject were identified using the following measures of visual acuity: high contrast linear, single optotype, repeat letter and low contrast linear, plus Vernier and displacement thresholds. These measures were repeated as the children underwent a prescribed occlusion therapy regime, after parental consent. RESULTS: All amblyopic subjects demonstrated a functional loss in each of the tests used, and occlusion therapy appeared to improve all aspects of the amblyopia. High contrast visual acuity was not always the primary deficit in visual function, and when amblyopic subjects were divided according to their primary visual loss, this visual function was found to show the greatest improvement with treatment. CONCLUSIONS: These results suggest that to successfully identify the primary visual deficit and monitor the success of occlusion therapy it is necessary to assess other aspects of visual function in amblyopia.

Amblyopia↗
PubMed1999

Contour interaction for high and low contrast optotypes in normal and amblyopic observers.

We investigate the influence of stimulus contrast upon contour interaction in normal and amblyopic subjects. Using a computer generated acuity task, flanked and unflanked acuities were measured psychometrically at both high contrast (80%) and low contrast (6%), in a group of 19 normal and 11 amblyopic subjects. The crowding ratio for high contrast letters was found to be significantly higher than that for low contrast letters. The extent of the crowding zone was measured at high and low contrast by varying the separation of the optotype and flanking bars. The crowding zone measurement was repeated for the high contrast optotypes using dioptric blur. The position of the flanking contours was found to have a significant effect on letter resolution at high contrast but no significant effect was demonstrable at low contrast. With the addition of dioptric blur the effect of contour interaction became negligible at high contrast. These findings support the hypothesis that the crowding effect is: (1) similar in normal and amblyopic eyes when tested at threshold; (2) is contrast dependent appearing only for high contrast optotypes.

Adult↗
PubMed1998

The effect of suprathreshold contrast on stimulus centroid and its implications for the perceived location of objects.

Using an alignment task, we investigate the role of suprathreshold contrast upon the perceived location of asymmetric Gaussian-windowed stimuli. A model which extracts the centroid of the stimulus envelope between limits defined by contrast threshold accounts well for the observed variation in perceived position. This finding helps to explain previous discrepancies in the literature regarding the validity of stimulus centroid as a determinant of visual location.

Contrast Sensitivity↗
PubMed1998

Geometric representation of the mechanisms underlying human curvature detection.

Combined manipulation of blur, line length and contrast reveal two distinct processes involved in curvature detection. When line length is small relative to blur, thresholds are almost directly proportional to blur and independent of line length. When line length is large relative to blur thresholds are directly proportional to line length and independent of blur. The aspect ratio (line length/blur) of curved contours represents a scale-invariant metric which forms the decisive factor in determining curvature performance.

Contrast Sensitivity↗