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.

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.

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.

Horizontal-vertical filters in early vision predict anomalous line-orientation identification frequencies.

A characteristic of early visual processing is a reduction in the effective number of filter mechanisms acting in parallel over the visual field. In the detection of a line target differing in orientation from a background of lines, performance with brief displays appears to be determined by just two classes of orientation-sensitive filter, with preferred orientations close to the vertical and horizontal. An orientation signal represented as a linear combination of responses from such filters is shown to provide a quantitative prediction of the probability density function for identifying the perceived orientation of a target line. This prediction was confirmed in an orientation-matching experiment, which showed that the precision of orientation estimates was worst near the vertical and horizontal and best at about 30 degrees each side of the vertical, a result that contrasts with the classical oblique effect in vision, when scrutiny of the image is allowed. A comparison of predicted and observed frequency distributions showed that the hypothesized orientation signal was formed as an opponent combination and horizontal and vertical filter responses.

Asymmetries in oriented-line detection indicate two orthogonal filters in early vision.

Visual detection of a line target differing in orientation from a background of lines may be achieved speedily and effortlessly. Such performance is assumed to occur early in vision and to involve filter mechanisms acting in parallel over the visual field. This study establishes orientational limits on this performance and analytically derives some generic properties of the underlying filters. It was found that, in brief displays, target orientation detection thresholds increased approximately linearly with background orientation, from minima at 0 degree (vertical) and 90 degrees, whereas background orientation detection thresholds decreased approximately linearly with target orientation, from maxima at 0 degree and 90 degrees. Target and background threshold functions were exactly antisymmetric. These data are shown to indicate a model of early line processing dominated by two classes of orientation-sensitive filter with axes close to the vertical and horizontal and orientation-tuning half-widths each of approximately 30 degrees at half-height.

Detecting natural changes of cone-excitation ratios in simple and complex coloured images.

Ratios of excitations in each cone-photoreceptor class produced by light reflected from pairs of surfaces in a scene are almost invariant under natural illuminant changes. The stability of these spatially defined ratios may explain the remarkable ability of human observers to efficiently discriminate illuminant changes from changes in surface reflectances. Spatial cone-excitation ratios are not, however, exactly invariant. This study is concerned with observers' sensitivity to these invariance violations. Simulations of Mondrian paintings with either 49 or two natural surfaces under Planckian illuminants were presented as images on a computer-controlled display in a two-interval experimental design: in one interval, the surfaces underwent an illuminant change; in the other interval, the surfaces underwent the same change but the images were then corrected so that, for each cone class, ratios of excitations were preserved exactly. Although the intervals with corrected images corresponded individually to highly improbable natural events, observers systematically misidentified them as containing the illuminant changes, the probability of error increasing as the violation of invariance in the other interval increased. For the range of illuminants and surfaces tested, sensitivity to violations of invariance was found to depend on cone class: it was greatest for long-wavelength-sensitive cones and least for short-wavelength-sensitive cones. Spatial cone-excitation ratios, or some closely related quantities, seem to be the cues preferred by observers for making inferences about surface illuminant changes.

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.

Multiple groups of orientation-selective visual mechanisms underlying rapid orientated-line detection.

Visual search for an edge or line element differing in orientation from a background of other edge or line elements can be performed rapidly and effortlessly. In this study, based on psychophysical measurements with ten human observers, threshold values of the angle between a target and background line elements were obtained as functions of background-element orientation, in brief masked displays. A repeated-loess analysis of the threshold functions suggested the existence of several groups of orientation-selective mechanisms contributing to rapid orientated-line detection; specifically, coarse, intermediate and fine mechanisms with preferred orientations spaced at angles of approximately 90 degrees, 35 degrees, and 10 degrees-25 degrees, respectively. The preferred orientations of coarse and some intermediate mechanisms coincided with the vertical or horizontal of the frontoparallel plane, but the preferred orientations of fine mechanisms varied randomly from observer to observer, possibly reflecting individual variations in neuronal sampling characteristics.

Interaction between rod and cone systems in dichoptic visual masking.

The brightness of a brief flash of light is reduced by the suitable presentation of a second flash in an adjacent region of the visual field. This making effect (metacontrast) can be induced dichoptically, that is with the test flash presented to one eye and the masking flash to the other. By a suitable choice of wavelengths and conditioning field, the test flash may be arranged to effectively stimulate only rod receptors and the masking flash only cone receptor. A dichoptic masking effect is still obtained.

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.

Effects of 4-aminopyridine in patients with multiple sclerosis.

4-Aminopyridine (4-AP) was administered to two groups of patients with multiple sclerosis (MS). The first group consisted of 5 patients with labile visual symptoms, 2 of whom had arcuate scotomata. 4-AP improved visual performance of most patients in this group and reduced the size of scotomata. The second group consisted of 5 patients with the spinal form of MS who were in a stable state; in this group 4-AP had little effect clinically or on tests of visual function.

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.

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.

Visual gap and offset discrimination and its relation to categorical identification in brief line-element displays.

Visual processing was investigated in judgments of relative line position. Stimulus continua were generated by bisecting a straight line and displacing the segments. Experiment 1 measured discrimination of pairs of longitudinally displaced segments at equal steps along the continuum. At long (2 s) durations discrimination fell smoothly, but at short (100 ms) durations it was sharp-peaked. In Experiment 2 the short-duration stimuli were labeled with subsets of the labels no gap, just a gap, and more than just a gap. Theoretical discrimination performances were computed and the one based on no gap and just a gap closely fitted observed performance. Experiments 3 and 4 were similar to 1 and 2, with lateral replacing longitudinal displacement. Similar "categorical" performance was obtained. It was concluded that there are discrete mechanisms for early detection of relative line position and that 2 labels can be used to characterize performance in each direction.

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.

Minimal information to determine affine shape equivalence.

Participants judged the affine equivalence of 2 simultaneously presented 4-point patterns. Performance level (d') varied between 1.5 and 2.7, depending on the information available for solving the correspondence problem (insufficient in Experiment 1a, superfluous in Experiment 1b, and minimal in Experiments 1c, 2a, 2b) and on the exposure time (unlimited in Experiments 1 and 2a and 500 ms in Experiment 2b), but it did not vary much with the complexity of the affine transformation (rotation and slant in Experiment 1 and same plus tilt in Experiment 2). Performance in Experiment 3 was lower with 3-point patterns than with 4-point patterns, whereas blocking the trials according to the affine transformation parameters had little effect. Determining affine shape equivalence with minimal-information displays is based on a fast assessment of qualitatively or quasi-invariant properties such as convexity/ concavity, parallelism, and collinearity.

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.

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.

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.

Detecting changes of spatial cone-excitation ratios in dichoptic viewing.

Spatial ratios of cone excitations produced by light reflected by different surfaces in a scene may provide the cue for discriminating changes in illuminant from changes in surface reflectances. To test whether these ratios can be computed across the two eyes, observers were presented with simulations on a computer-controlled monitor of pairs of juxtaposed or separated Munsell surfaces undergoing an illuminant change with a small change in cone-excitation ratios or a change with constant cone-excitation ratios. Surfaces were viewed either binocularly or dichoptically. Observers reliably discriminated the two changes in both viewing conditions, although less well dichoptically. Cone-excitation ratios, which may in principle be computed retinally, may also be computed cortically.