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D H Foster

Publications and source records attributed to D H Foster.

At least 37 records · Page 2Linked to original sources
PubMed1996

Qualitative cues in the discrimination of affine-transformed minimal patterns.

An important factor in judging whether two retinal images arise from the same object viewed from different positions may be the presence of certain properties or cues that are 'qualitative invariants' with respect to the natural transformations, particularly affine transformations, associated with changes in viewpoint. To test whether observers use certain affine qualitative cues such as concavity, convexity, collinearity, and parallelism of the image elements, a 'same-different' discrimination experiment was carried out with planar patterns that were defined by four points either connected by straight line segments (line patterns) or marked by dots (dot patterns). The first three points of each pattern were generated randomly; the fourth point fell on their diagonal bisector. According to the position of that point, the patterns were concave, triangular (three points being collinear), convex, or parallel sided. In a 'same' trial, an affine transformation was applied to one of two identical patterns; in a 'different' trial, the affine transformation was applied after the point lying on the diagonal bisector was perturbed a short, fixed distance along the bisector, inwards for one pattern and outwards for the other. Observers' ability to discriminate 'same' from 'different' pairs of patterns depended strongly on the position of the fourth, displaced, point: performance varied rapidly when the position of the displaced point was such that the patterns were nearly triangular or nearly parallel sided, consistent with observers using the hypothesised qualitative cues. The experimental data were fitted with a simple probabilistic model of discrimination performance that used a combination of these qualitative cues and a single quantitative cue.

Adult↗
PubMed1996

Dependence of relational colour constancy on the extraction of a transient signal.

Human observers can correctly attribute changes in the appearance of a scene either to changes in the incident light or to changes in the spectral-reflectance properties of the scene. This ability was assessed as a function of the time course of illuminant and spectral-reflectance changes. Observers were presented with computer simulations of Mondrian patterns of 49 randomly selected Munsell papers. On each trial a Mondrian pattern was presented for 1 s; the pattern then changed either instantaneously or gradually into another Mondrian pattern, also presented for 1 s, which was related to the first either by an illuminant change or by an illuminant change accompanied by additional changes in the spectral-reflectance functions of the individual papers. Illuminant and spectral-reflectance change were applied linearly in time (with respect to CIE coordinates) over intervals ranging from 0 to 7 s. Observers indicated whether there was a spectral-reflectance change. They were able to make reliable discriminations between illuminant and spectral-reflectance changes both when the changes were applied instantaneously and when they were applied gradually over time, but performance worsened progressively as the duration of the changes increased, that is, as their rate decreased. It is suggested that discrimination in this task depends on the extraction of a low-level transient signal which is generated in response to rapid changes in scene appearance and which is progressively attenuated as changes occur more and more gradually.

Adult↗
PubMed1995

Artificial neural networks simulating visual texture segmentation and target detection in line-element images.

Measurements were made of human observers' performance both in segmenting regions of line-elements and in detecting line-element targets in stimuli containing several orientations. Performance was modelled by four artificial neural networks constructed from processing units trained to mimic the gross functionality of certain loosely defined classes of cortical cells. Model 1 contained modules sensitive to absolute orientation only, and it provided a poor fit to the human-performance data. Model 2 contained modules sensitive to orientation contrast: the outputs of these modules could be suppressed with fields of uniformly oriented line-elements. Model 3 contained orientation-contrast-sensitive modules of a different type: their outputs could be suppressed with fields of randomly oriented line-elements. Models 2 and 3 both successfully processed line-element arrays with orientation heterogeneities, but these models still provided inadequate fits to the human-performance data. Model 4 contained both types of orientation-contrast-sensitive modules; this model was able to account for human performance in the segmentation and detection tasks, both qualitatively and quantitatively.

Contrast Sensitivity↗
PubMed1995

Orientation contrast vs orientation in line-target detection.

This study concerns the roles of absolute and relative orientation in determining detectability of a line-element target in a background field of uniformly oriented line elements. Target detectability was determined as a function of background-field orientation, sampled at 5 deg intervals, for three levels of orientation contrast--the difference between target and background orientations--sampled at 10, 20 and 30 deg. Stimulus displays were presented briefly and followed by a mask. There were 10 observers, whose detection performance was quantified by the discrimination index d' from signal detection theory. Target detectability was found to depend both on absolute orientation, represented by background-field orientation, and on orientation contrast. At each level of orientation contrast, performance was best when the background field, not the target element, was vertical or horizontal. These data are difficult to explain by general models of orientation discrimination based on simple orientation opponency between local line-sensitive filter units; three other models specifically concerned with target detection are briefly considered.

Adult↗
PubMed1995

Comparison of colour discrimination and electroretinography in evaluation of visual pathway dysfunction in aretinopathic IDDM patients.

The slow progression of diabetic retinopathy makes it difficult to assess the effects of intervention therapy. There is thus a need for surrogate markers of visual change in diabetes. Colour vision tests and electroretinography (ERG) may be useful in this regard; yet little is known of their relative performance in the assessment of visual dysfunction in diabetes. The aim of the present study was to compare colour discrimination (100 hue test) and ERG indices (oscillatory potentials (OP) and pattern ERG (PERG)) in the evaluation of aretinopathic IDDM patients. Colour discrimination was abnormal in 10 aretinopathic IDDM patients when compared with nine age matched controls; mean square root 100 hue error scores were 10.38 (SD 2.89) versus 4.77 (1.87) respectively, p < 0.01. OP implicit times of the ERG were also abnormal; for example, for right eye, mean OP1 implicit time for diabetics versus OP1 implicit time for controls was 20.1 (2.0) versus 18.6 (1.4) ms, p = 0.03. Comparison of the two techniques suggested that the 100 hue test was more sensitive and more specific than ERG OP implicit times in the detection of diabetic visual dysfunction in these patients.

Adult↗
PubMed1995

Effect of short term changes in blood glucose on visual pathway function in insulin dependent diabetes.

BACKGROUND: Visual pathway function is abnormal in patients with insulin dependent diabetes mellitus (IDDM) without retinopathy, yet the mechanism underlying this abnormality is unknown. It is hypothesised that short term changes in blood glucose level affect visual pathway function in IDDM. METHODS: Colour discrimination was measured in 10 uncomplicated aretinopathic IDDM patients during hyperinsulinaemic clamp, with the Farnsworth Munsell 100 hue test (100 hue test). After stable euglycaemia, patients were made hyperglycaemic (14 mmol/l), maintained euglycaemic (5 mmol/l), and rendered hypoglycaemic (2.5 mmol/l), in random order, on separate occasions at least 1 week apart. RESULTS: Short term (1-2 hours) changes in blood glucose did not affect colour discrimination: mean (SD) 100 hue error score at 2.5 mmol/l was 34 (22) compared with 35 (33) at 5 mmol/l, and 39 (28) at 14 mmol/l. CONCLUSION: These data suggest that short term (1-2 hours) changes in blood glucose are not the mechanism for visual pathway dysfunction in aretinopathic IDDM patients.

Adult↗
PubMed1995

Optimized model of oriented-line-target detection using vertical and horizontal filters.

A line-element target differing sufficiently in orientation from a background of line elements can be visually detected easily and quickly; orientation thresholds for such detection are lowest when the background elements are all vertical or all horizontal. A simple quantitative model of this performance was constructed from three processing stages: (1) linear filtering by two classes of anisotropic filters, (2) nonlinear point transformation, and (3) estimation of a signal-to-noise ratio based on responses to images with and without a target. A Monte Carlo optimization procedure (simulated annealing) was used to determine the model parameter values required for providing an accurate description of psychophysical data on orientation increment thresholds.

Humans↗
PubMed1994

Relational colour constancy from invariant cone-excitation ratios.

Quantitative measurements of perceptual colour constancy show that human observers have a limited and variable ability to match coloured surfaces in scenes illuminated by different light sources. Observers can, however, make fast and reliable discriminations between changes in illuminant and changes in the reflecting properties of scenes, a discriminative ability that might be based on a visual coding of spatial colour relations. This coding could be provided by the ratios of cone-photoreceptor excitations produced by light from different surfaces: for a large class of pigmented surfaces and for surfaces with random spectral reflectances, these ratios are statistically almost invariant under changes in illumination by light from the sun and sky or from a planckian radiator. Cone-excitation ratios offer a possible, although not necessarily unique, basis for perceptual colour constancy in so far as it concerns colour relations.

Color↗
PubMed1994

Optic neuritis. Differential losses of luminance and chromatic function near a scotoma.

Visual sensitivity to achromatic and chromatic stimulus flashes was determined at sites just inside, on the boundary and just outside scotomata in 11 patients with recovered optic neuritis. The colour of the flashes and the size of the steady background on which they appeared were such that detection was more likely to be mediated by either the large-diameter, magnocellular fibres or the small-diameter, parvocellular fibres of the anterior visual pathway. The spacing of the test sites ranged from 0.5 degrees to 4 degrees visual angle, depending on the shape and location of the scotomata. The greatest differences in sensitivity were between sites just inside and just outside the scotoma and in response to achromatic stimuli more likely to involve the magnocellular fibres. This effect may be due to the size of magnocellular fibres or to their relatively smaller numbers.

Adult↗
PubMed1994

Extent and duration of practice effects on performance with the Farnsworth-Munsell 100-Hue test.

The extent and persistence of practice effects on serial performance in the Farnsworth-Munsell 100-Hue test (100-Hue test) were evaluated in an experiment in which six subjects performed the 100-Hue test up to 17 times over six weeks, and then once more after 7 months. A practice effect occurred which was highly statistically significant for the group as a whole (P < 0.001) and for three individual subjects (P < 0.05). A practice effect was still evident 7 months after the last test performance. Error scores were reduced almost to zero after 5-10 retests so that it was unclear whether the effects of practice had disappeared or whether failure to improve further was a 'floor effect'. As a control against a floor effect, a second experiment was performed in which subjects' performance was impaired by placing neutral density filters in front of their eyes (thus artificially raising their 100-Hue error score). Under these conditions, error scores continued to fall and were halved after 17 tests (P < 0.03). It is concluded that practice has a large effect on 100-Hue test performance which continues over many retests and for many months after testing.

Adult↗
PubMed1994

Effect of diabetes associated increases in lens optical density on colour discrimination in insulin dependent diabetes.

Optical density (OD) of the crystalline lens has been shown in non-diabetics to increase linearly with age over the first five decades and at an increased rate thereafter; in insulin dependent diabetic (IDDM) patients, lens OD increases with age and with duration of diabetes at a rate similar to that in non-diabetics over the age of 60 years. Recently, it has been established that colour discrimination is abnormal in a majority of young patients with uncomplicated IDDM and angiographically normal retinas. Colour discrimination loss was attributed to functional abnormalities in the retina or neural pathways; yet the possibility exists that increases in lens OD may account for part or all of the colour discrimination loss in IDDM. In the present study, colour discrimination was compared in aretinopathic IDDM patients and age-matched controls, and then in a group of aretinopathic IDDM patients individually matched to controls with respect to lens OD. Colour discrimination was significantly worse in diabetic patients than in age-matched controls, and was significantly worse when diabetic patients were compared with controls matched for OD. The magnitude of the difference in 100 hue error score between diabetic patients and OD matched controls was, however, considerably less than the difference between diabetic patients and age-matched controls. These data suggest that colour discrimination loss in aretinopathic IDDM patients cannot be explained solely on the basis of diabetes induced increases in lens OD, but must involve abnormalities of the retina or its neural connections.

Adolescent↗
PubMed1994

Viewpoint--invariance of contour-curvature discrimination over extended performance levels.

A traditional approach to the problem of human object recognition is to assume that the visual system represents an object in terms of invariant quantities. This study considers a generalization of this approach to the problem of visually recognizing differences in shape, specifically between contour markings on a planar surface, as the position of the surface varies in space. For a given 'shape cue', perceived differences in contour may be quantified by threshold values--Weber fractions--at a particular criterion level of performance. A necessary theoretical condition for the Weber fraction to be constant with the relative viewpoint of the observer is that the cue should be a relative invariant under the natural spatial transformations of the image. Some data are reviewed showing how statistically efficient some cues are in explaining the observed discriminability of symmetric curved contours at a fixed criterion performance level of 75%. A new, fuller analysis is presented showing that the efficiency of the most efficient cue satisfying the invariance condition is maintained over a wide range of criterion performance levels, from 53% to 92% correct, corresponding to discrimination index values of 0.1 to 2.0. Over these levels, Weber's law was found empirically to hold almost exactly.

Animals↗
PubMed1993

Abnormalities of visual function in hereditary motor and sensory neuropathy.

Visual function was studied in a group of 15 patients with hereditary motor and sensory neuropathy (HMSN). Psychophysical measures of luminance and chromatic threshold and temporal contrast sensitivity were undertaken, together with visual evoked potentials (VEPs), visual fields and clinical neuro-ophthalmological examination. A patchy loss of visual function was found in individual cases of HMSN. In the group analysis there was evidence of a selective loss of luminance threshold and temporal contrast sensitivity at low temporal frequencies; the VEP P100 latency was not significantly prolonged. The losses of visual function in HMSN were discussed and compared with visual losses in multiple sclerosis, which had been detected using identical experimental techniques.

Adolescent↗
PubMed1993

The cue for contour-curvature discrimination.

Seven potential geometric cues for contour-curvature discrimination were tested: curvature, turning-angle, arc-length, arc-length-divided-by-chord-length, maximum-deviation (sag), mean-deviation and area. Three experiments were performed, each requiring the discrimination of two simultaneously presented, 1-sec-duration, curved-line stimuli, whose chord-lengths ranged from 12 to 48 arcmin visual angle and whose curvatures ranged from 0 to 0.13 arcmin-1. Experiments 1 and 2 determined for each cue the smallest detectable increment (the increment threshold) as a function of cue value, for a set of spatial transformations of the stimulus (one- and two-dimensional scalings) equivalent to changes in viewing distance and direction. In accordance with statistical estimation theory, the "best" cue was defined as the most efficient one, that is, the one which best accounted for the variance in the data. As a control, Expt 3 compared increment-threshold functions for circular and elliptical arcs of constant chord-length and circular arcs of constant arc-length. Over all three experiments, only sag and its linear approximation, mean-deviation, accounted well for the variance in the data; sag provided the best predictor, and its increment-threshold function satisfied Weber's law over almost all of the stimulus range. Additionally, sag has a special theoretical property (shared only with mean-deviation and area): the relationship it defines (in proportional terms) between curved contours in an image of an object or a scene is constant and independent of viewing distance and direction.

Adult↗
PubMed1992

An operational approach to colour constancy.

Colour constancy is traditionally defined as the invariance of perceived surface colours under changes in the spectral composition of the illuminant. Existing quantitative studies show that, by this definition, human subjects show poor colour constancy. A different and complementary aspect of colour constancy is considered which is concerned with the ability of a subject to attribute correctly changes in the colour appearance of a scene either to changes in reflecting properties of the surfaces that make up the scene, or to changes in the spectral composition of the illuminant. Data are presented showing that, if the changes in the appearance of a scene were sufficiently great, subjects were capable of making the required discriminations highly reliably, and without scrutiny.

Color Perception↗
PubMed1992

Pars interarticularis stress and disc degeneration in cricket's potent strike force: the fast bowler.

Cricket fast bowlers are the potent strike force in a multidiscipline team. They subject their spines to repetitive sagittal plane and rotatory movements over many years. The effect of this repetitive stress has not previously been analysed. This study examined 20 former fast bowlers to determine the incidence of spondylolysis, spondylolisthesis and degenerative change. Fast bowlers are noted to have an increased incidence of spondylolysis. A mixed front/side bowling style involving more lumbar hyperextension or rotation has significant association with spondylolysis when compared with side-on bowling styles. There was a high incidence of radiological thoracolumbar degenerative facet joint and disc disease in former fast bowlers.

Adult↗
PubMed1992

Immediate colour constancy.

Colour constancy is traditionally interpreted as the stable appearance of the colour of a surface despite changes in the spectral composition of the illumination. When colour constancy has been assessed quantitatively, however, by observers making matches between surfaces illuminated by different sources, its completeness has been found to be poor. An alternative operational approach to colour constancy may be taken which concentrates instead on detecting the underlying chromatic relationship between the parts of a surface under changes in the illuminant. Experimentally the observer's task was to determine whether a change in the appearance of a surface was due to a change in its reflecting properties or to a change in the incident light. Observers viewed computer simulations of a row of three Mondrian patterns of Munsell chips. The centre pattern was a reference pattern illuminated by a simulated, spatially uniform daylight; one of the outer patterns was identical but illuminated by a different daylight; and the other outer pattern was equivalent but not obtainable from the centre pattern by such a change in illuminant. Different patterns and different shifts in daylight were generated in each experimental trial. The task of the observer was to identify which of the outer patterns was the result of an illuminant change. Observers made reliable discriminations of the patterns with displays of durations from several seconds to less than 200 ms, and, for one observer, with displays of 1 ms. By these measures, human observers appear capable of colour constancy that is extremely rapid, and probably preattentive in origin.

Color Perception↗