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D H Foster

Publications and source records attributed to D H Foster.

At least 73 records · Page 4Linked to original sources
PubMed1986

Horizontal-vertical structure in the visual comparison of rigidly transformed patterns.

Visual recognition of patterns reflected or rotated through 180 degrees (point-inverted) depends critically on their positional symmetry and separation in the field. A possible explanatory scheme suggested a description of internal pattern representation structures and simple internal operations that naturally involved a horizontal-vertical reference system. Predictions of the scheme were tested here in three experiments. Subjects made same-different judgments on pairs of random-dot patterns briefly presented in various arrangements and related by reflection, point-inversion, or identity transformation, or paired at random. Experiment 1 tested reflected patterns and verified the importance of orientation of the reflection axis relative to display-configuration axis. Experiment 2 demonstrated an oblique effect of configuration on performance with reflected patterns, but not with identical or point-inverted patterns. Experiment 3 demonstrated a vertical shift effect of configuration on performance with point-inverted patterns, but not with identical or reflected patterns. We concluded that in same-different pattern comparisons, a horizontal-vertical reference system appears fundamental in determining the nature of and operations upon internal pattern representations.

Adult↗
PubMed1986

Discrete and continuous modes of curved-line discrimination controlled by effective stimulus duration.

In previous experiments two extreme modes of visual discrimination performance have been investigated by measuring small differences in pattern shape at points along a continuum of pattern shapes. These two modes, associated with discrete and continuous encoding processes, were obtained by simultaneously manipulating the number of pattern components in the display and the effective duration of the display. In this experiment, discrimination performance was measured for a fixed number of pattern components, namely three, and variable display time course. The stimuli used were curved lines drawn from a continuum with curvature parameter s. There were three stimulus time courses: (1) 2-s stimulus display, (2) 100-ms stimulus display, and (3) 100-ms stimulus display followed by a post-stimulus mask. Discrimination performance declined smoothly and monotonically with s for (1), but varied non-monotonically with s revealing a central peak for (3). Performance for (2) was intermediate between that for (1) and that for (3). A reduction in effective stimulus duration alone was thus sufficient to cause a transition from continuous to discrete modes of discrimination performance, a result which may be compatible with an explanation of variable discrimination modes based on a method of successive internal approximations of the stimulus.

Adult↗
PubMed1985

Multiple sclerosis: abnormalities in luminance, chromatic, and temporal function at multiple retinal sites.

Visual function was assessed in a group of patients with multiple sclerosis (MS) and in a group of matched normal controls. In these patients the disease was relatively mild. For each subject, measures of a range of psychophysical visual functions were carried out at multiple sites in each eye. Previous reports have only included some of these functions. Here, luminance threshold, two-flash resolution, perceptual latency, luminance critical flicker frequency (CFF), and chromatic CFF were all measured. Variabilities of these functions and correlation between chromatic and luminance CFFs were also evaluated. For both the MS group and the normal control group, the correlations between pairs of visual parameters were not overall significantly greater than chance level. The MS group did give a significantly reduced value relative to the normal group for luminance CFF and for the gradient of the plot of chromatic CFF against luminance CFF. This group was then subdivided according to history of visual involvement. The subgroup with previous visual symptoms had significant impairment for luminance threshold, variability of luminance threshold, luminance CFF, variability of two-flash resolution, and for the gradient of the plot of chromatic CFF against luminance CFF. The subgroup without previous visual symptoms showed no significant impairment for any individual parameter, although the gradient of the plot of chromatic CFF against luminance CFF was lower than normal.

Adult↗
PubMed1985

Internal representations and operations in the visual comparison of transformed patterns: effects of pattern point-inversion, position symmetry, and separation.

A scheme for visual pattern recognition is described. It is supposed, amongst other things, that patterns are internally represented by the visual system in terms of local features, spatial-order relations between local features, and global spatial relations specifying approximate pattern position with respect to the point of fixation. It is further supposed that there are two distinct types of internal operation that may be applied to the components of internal representations in the process of pattern comparison: typically a discrete spatial-order-reversal operation and a continuous position-shift operation. Some general predictions of the scheme are tested against data obtained in an experiment using random-dot patterns that were subjected to rigid transformations and presented at various locations along the horizontal meridian. Patterns were presented sequentially, in pairs, to subjects in a "same-different" comparison task. Pattern pairs were to be responded to as "same" if they were identical or related by point-inversion (planar rotation through 180 degrees) or responded to as "different". Extending earlier findings, the present results showed that "same"-detection performance for identical and point-inverted patterns depended differentially on the distance between the patterns and the symmetry of the pattern positions about the point of fixation in a manner consistent with the predictions of the scheme.

Adult↗
PubMed1985

Selective internal operations in the recognition of locally and globally point-inverted patterns.

Performance in discriminating rotated 'same' patterns from 'different' patterns may decrease with rotation angle up to about 90 degrees and then increase with angle up to 180 degrees. This anomalously improved performance under 180 degrees pattern rotation or point-inversion can be explained by assuming that patterns are internally represented in terms of local features and their spatial-order relations ('left of', 'above', etc.), and that, in pattern comparison, an efficient internal sense-reversal operation occurs (transforming 'left of' to 'right of', etc.). Previous experiments suggested that local features and spatial relations could not be efficiently separated in some pattern-comparison tasks. This hypothesis was tested by measuring 'same-different' discrimination performance under four transformation: point-inversion 1 of the whole pattern, point-inversion 1F of local features alone, point-inversion 1P of local-feature positions alone, and identity transformation Id. The results suggested that internal sense-reversal operations could be applied selectively and efficiently, provided that local features were well separated. Under this condition performances for 1F and 1 were about the same whereas performance for 1P was significantly worse, the latter performance resulting possibly from an attempt to apply internal global and local sense-reversal operations serially.

Adult↗
PubMed1985

Nonselective losses in foveal chromatic and luminance sensitivity in multiple sclerosis.

A psychophysical technique involving simple increment threshold measurements was used to determine foveal chromatic and luminance sensitivity in patients with multiple sclerosis (MS) and in matched normal controls. The patient group showed substantial and nonselective losses in chromatic and luminance sensitivity relative to the normal control group, and these losses were significantly correlated with each other over individual patients. It is suggested that impairment of foveal visual function due to demyelination is not more specific to fibers carrying chromatic information than to fibers carrying luminance information.

Adult↗
PubMed1984

Characterization of discrete and continuous modes of visual pattern discrimination.

Discrete and continuous modes of visual pattern discrimination performance are analyzed using a model for the investigation of discrete internal pattern representations described in previous papers (Foster, 1980a, b). A simple quantitative criterion is derived to characterize the two kinds of visual discrimination performance. Values predicted by this criterion are then compared with values obtained from experimental data.

Cybernetics↗
PubMed1983

Test and field spectral sensitivities of colour mechanisms obtained on small white backgrounds: action of unitary opponent-colour processes?

It has been shown that for human foveal vision the test spectral sensitivity curve obtained in the presence of a large white background exhibits peaks at about 440, 530 and 610 nm and a small dip or notch at about 580 nm. Additionally, field spectral sensitivity curves for the medium- and long-wavelength sensitive colour mechanisms when derived in the presence of a small monochromatic background (auxiliary field) spatially coincident with the test field are sharper than the corresponding Stiles's II mechanisms and peak at about 530 and 605 nm. The short-wavelength sensitive colour mechanism shows no such effect, the peak remaining at about 440 nm. The test spectral sensitivity curve obtained on a large white background and the sharpened field spectral sensitivity curves obtained on a spatially coincident, monochromatic auxiliary field have each been interpreted in terms of an opponent-process theory of colour vision: the response of the non-opponent luminance system is considered to be reduced by selective achromatic adaptation (with the large white background) or by selective contour masking (with the spatially coincident auxiliary field). The present study combined the two techniques of achromatic and spatial adaptation: a small white auxiliary field spatially coincident with the test field was used in measurements of both test and field spectral sensitivities. Under these conditions, it was found that the test spectral sensitivity curve showed more clearly defined peaks and a deeper notch at about 580 nm than when obtained on the large white background and that the test curve was well fitted by the upper envelope of the short-wavelength and sharpened medium- and long-wavelength field spectral sensitivity curves, suggesting the possibility that three unitary opponent-colour mechanisms may underlie test and field spectral sensitivities. A test of this hypothesis is proposed concerning the effect of auxiliary-field chromaticity on the position of the notch at about 580 nm in the test spectral sensitivity curve.

Adult↗
PubMed1983

Visual discrimination, categorical identification, and categorical rating in brief displays of curved lines: implications for discrete encoding processes.

Visual discrimination, categorical identification, and categorical rating measurements were made on sets of curved-line stimuli drawn from a theoretically uniform continuum with curvature parameter s. In Experiment 1, discriminability of pairs of curved lines separated by a constant distance on the s scale was measured at successive points along the scale. Curved lines were presented four at a time in a 100-msec display, which was followed by a random-dot mask. Discrimination performance was found to vary nonsmoothly with s. In Experiment 2, a categorical identification task was performed in which subjects labeled the curved-line stimuli of Experiment 1 straight, just curved, and more than just curved. From these data, a theoretical discrimination performance was computed that was closely congruent to the discrimination performance of Experiment 1. In Experiment 3, three different categorical rating scales with two, three, and four intervals were tested and each was shown to be less effective than the categorical identification scale for predicting discrimination performance. Mean ratings were, however, highly linear with s, suggesting that the curved-line continuum was psychometrically uniform. Experiment 4 provided further evidence for the uniformity of the curved-line continuum by measuring conventional acuity for curvature. Two rather than four curved lines were presented in each display; duration was increased to 2 sec; and the poststimulus mask was omitted. Acuity was found to vary linearly with s. It was concluded that under conditions in which attention is distributed over a number of elements in the field and in which viewing and effective visual processing time are restricted, performance in discriminating curved-line stimuli may be determined by relatively coarse, discrete visual processes.

Adult↗
PubMed1982

Rod- and cone-mediated visual function in multiple sclerosis.

Visual thresholds and perceptual latencies were determined in patients with multiple sclerosis (MS) and in normal control subjects. Measurements were made under light- and dark-adapted conditions, with stimuli chosen to stimulate rod and cone receptors selectively. More abnormalities in perceptual latency and luminance threshold were recorded in the light-adapted condition than in the dark-adapted condition, but this result was not specific to the rod or cone systems. Possible underlying pathophysiological processes are discussed, and it is suggested that reduced conduction velocity in the demyelinated visual pathway is the most likely explanation of the observed perceptual delays and that there is no evident retinal contribution.

Adult↗
PubMed1982

Abnormalities of chromatic and luminance critical flicker frequency in multiple sclerosis.

Critical flicker frequency (CFF) was measured for stimuli varying in chromaticity only and in luminance only for patients with multiple sclerosis (MS) and for matched normal controls. The two CFF measurements showed different underlying linear interdependencies for the two groups, consistent with a greater loss of temporal luminance function than of temporal chromatic function in MS patients. These results are discussed in relation to the pathophysiology of demyelinated nerve fibers. It is suggested that demyelination affects all types of nerve fiber unselectively; in particular, no support is found for the notion of a special vulnerability of fibers carrying time-varying chromatic information.

Color Perception↗
PubMed1981

Multiple sclerosis. Luminance threshold and measurements of temporal characteristics of vision.

Luminance threshold, perceptual latency, double-flash resolution, and critical flicker frequency were examined in patients with multiple sclerosis (MS) and in normal control subjects. The intensities of the stimuli used to test the temporal properties of vision were equalized with respect to individual luminance thresholds. In eight patients with MS, all the properties tested showed abnormality, double-flash resolution being most commonly affected. Retinal sites were not however, uniformly abnormal according to these measures. We conclude that abnormal temporal properties of vision in patients with MS are not a simple functional consequence of altered luminance thresholds.

Adult↗
PubMed1980

A description of discrete internal representation schemes for visual pattern discrimination.

A general description of a class of schemes for pattern vision is outlined in which the visual system is assumed to form a discrete internal representation of the stimulus. These representations are discrete in that they are considered to comprise finite combinations of "components" which are selected from a fixed and finite repertoire, and which designate certain simple pattern properties or features. In the proposed description it is supposed that the construction of an internal representation is a probabilistic process. A relationship is then formulated associating the probability density functions governing this construction and performance in visually discriminating patterns when differences in pattern shape are small. Some questions related to the application of this relationship to the experimental investigation of discrete internal representations are briefly discussed.

Humans↗
PubMed1980

A spatial perturbation technique for the investigation of discrete internal representations of visual patterns.

A technique is proposed for the investigation of discrete internal representations assumed to be formed by the visual system in response to pattern stimuli. The proposed technique involves applying a local L-parameter group of spatial transformations to a pattern to generate a "continuum" of patterns. The visual discriminability of pairs of perturbed patterns corresponding to small fixed increments in the transformation parameter is determined at various points in the parameter range. By means of a simple rule, the characteristics of this discrimination performance are then related to the probability density functions assumed to underlie the hypothesized discrete internal representations. Two experimental applications of the proposed technique are described. The first is concerned with a discrete internal representation involving the specification of the colinearity or noncolinearity of the points in a pattern; the second is concerned with a discrete internal representation involving the specification of the acuteness or obtuseness of the angle between two lines in a pattern.

Humans↗
PubMed1980

Irrelevance of local position information in visual adaptation to random arrays of small geometric elements.

Experiments were carried out to test whether spatial relationships between local pattern features are important in the production of pattern-specific visual adaptation effects. Evidence was found for sensitivity to the shape of local pattern features, but not to their relative positions. These findings are contrasted with recent results for texture discrminination where the visual system can make use of local position information.

Adaptation, Ocular↗