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.

Reference frame for rapid visual processing of line orientation.

Detection of a uniquely oriented line element in a background field of uniformly oriented line elements depends on the orientation of the background field. Is the orientational reference frame for this anisotropy entirely dependent on the orientations of structures outside the line-element display, the spatial regularity of the stimulus elements, and the direction of gravity? The effects of these potential cues were investigated in target-detection experiments with brief displays. The anisotropy was found whether or not gravitational or visual cues defined an orientational reference frame. Stimulus orientation may be coded with respect to the retina or body axis in rapid visual processing.

Parallel detection of violations of color constancy.

The perceived colors of reflecting surfaces generally remain stable despite changes in the spectrum of the illuminating light. This color constancy can be measured operationally by asking observers to distinguish illuminant changes on a scene from changes in the reflecting properties of the surfaces comprising it. It is shown here that during fast illuminant changes, simultaneous changes in spectral reflectance of one or more surfaces in an array of other surfaces can be readily detected almost independent of the numbers of surfaces, suggesting a preattentive, spatially parallel process. This process, which is perfect over a spatial window delimited by the anatomical fovea, may form an early input to a multistage analysis of surface color, providing the visual system with information about a rapidly changing world in advance of the generation of a more elaborate and stable perceptual representation.

Colour constancy from temporal cues: better matches with less variability under fast illuminant changes.

To test whether temporal transient cues could improve colour-constancy estimates, surface-colour matches were made across two Mondrian patterns illuminated by different daylights: the patterns were presented either in the same position in an alternating sequence or, as a control, simultaneously side-by-side. The degree of colour constancy was significantly higher with sequential stimulus presentation than with simultaneous presentation, in the best condition reaching 0.87 on a scale of 0 to 1 for matches averaged over 20 observers. The variance between observers was also markedly reduced with sequential stimulus presentation. The visual system appears to have mechanisms not requiring adaptation that can provide almost unbiased information about surface colour under changing illuminants.

Correlation of MRI lesions with visual psychophysical deficit in secondary progressive multiple sclerosis.

The aim of this work was, first, to clarify the nature of the relationship between the sensory deficit in the demyelinated visual pathway and morphological changes revealed by MRI and, secondly, to test whether there was a preferential effect of demyelination for either the magnocellular or parvocellular pathway in established multiple sclerosis. Twenty-four patients with secondary progressive multiple sclerosis were studied psychophysically and by MRI of the optic nerve and brain. MRI was performed with a Phillips (0.5T) scanner. Visual pathway MRI lesion load was evaluated independently using the total optic nerve lesion length and lesion area seen on STIR (short inversion time inversion recovery) images of the optic nerve and the total post-chiasmal lesion area on T(1)-, T(2)- and proton-density-weighted images of the brain. Psychophysical tests determined 75%-seeing thresholds for horizontal gratings consisting of isoluminant red and green sinusoids of the same spatial frequency combined out-of-phase for preferential stimulation of the parvocellular system and in-phase for preferential stimulation of the magnocellular system. It was found that, in this group of patients, visual psychophysical loss was significantly correlated with lesion area seen on proton density MRI sequences of the post-chiasmal visual pathway, and that the parvocellular pathway was more affected than the magnocellular pathway, especially at lower spatial frequencies.

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.

Asymmetries of saccadic eye movements in oriented-line-target search.

Visual search for a line-element target differing sufficiently in orientation from a background of line elements can be performed rapidly, effortlessly, and without eye movements. There is, however, a response asymmetry: detection is better with an oblique target element in vertical or horizontal background elements than when these orientation are interchanged. If the underlying visual mechanisms also provide an input to the oculomotor system, then a similar asymmetry should be observed in eye-movement behaviour. To test this hypothesis, an experiment was undertaken in which eye movements were recorded while subjects searched for a line-element target in background of line elements; orientations were chosen from the range 0 degree, 30 degrees, 60 degrees, and 90 degrees to the vertical. Data from three subjects showed that (1) latencies for the initial saccade, (2) angular errors in initial-saccade direction, and (3) manual response times depended similarly on the combination of target- and background-element orientations, performance being better for 30 degrees or 60 degrees targets in 0 degree or 90 degrees backgrounds than vice-versa. The early orientation-selective mechanisms responsible for the rapid detection of oriented-line targets are probably the same as those providing signals for saccadic eye movements.

Relational color constancy in achromatic and isoluminant images.

Relational color constancy, which refers to the constancy of perceived relations between surface colors under changes in illuminant, may be based on the computation of spatial ratios of cone excitations. As this activity need occur only within rather than between cone pathways, relational color constancy might be assumed to be based on relative luminance processing. This hypothesis was tested in a psychophysical experiment in which observers viewed simulated images of Mondrian patterns undergoing colorimetric changes that could be attributed either to an illuminant change or to a nonilluminant change; the images were isoluminant, achromatic, or unmodified. Observers reliably discriminated the two types of changes in all three conditions, implying that relational color constancy is not based on luminance cues alone. A computer simulation showed that in these isoluminant and achromatic images spatial ratios of cone excitations and of combinations of cone excitations were almost invariant under illuminant changes and that discrimination performance could be predicted from deviations in these ratios.

Human sensitivity to phase perturbations in natural images: a statistical framework.

Fourier-phase information is important in determining the appearance of natural scenes, but the structure of natural-image phase spectra is highly complex and difficult to relate directly to human perceptual processes. This problem is addressed by extending previous investigations of human visual sensitivity to the randomisation and quantisation of Fourier phase in natural images. The salience of the image changes induced by these physical processes is shown to depend critically on the nature of the original phase spectrum of each image, and the processes of randomisation and quantisation are shown to be perceptually equivalent provided that they shift image phase components by the same average amount. These results are explained by assuming that the visual system is sensitive to those phase-domain image changes which also alter certain global higher-order image statistics. This assumption may be used to place constraints on the likely nature of cortical processing: mechanisms which correlate the outputs of a bank of relative-phase-sensitive units are found to be consistent with the patterns of sensitivity reported here.

Limitations of rapid parallel processing in the detection of long and short oriented line targets.

The purpose of this study was to determine whether the detectability of a uniquely oriented line element in a field of uniformly oriented line elements depends on element length. Displays containing various numbers of elements were presented briefly and followed by a mask. The length and orientation of the elements were varied. With longer (1.0-deg) elements, detection performance varied little with the number of elements present. With shorter (0.25-deg) elements, performance worsened as the element number increased, especially when the uniformly oriented elements were oblique. It seems that rapid spatially parallel processes facilitate detection of targets in many-element displays of long elements but not of short elements.

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.

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.

Four issues concerning colour constancy and relational colour constancy.

Four issues concerning colour constancy and relational colour constancy are briefly considered: (1) the equivalence of colour constancy and relational colour constancy; (2) the dependence of relational colour constancy on ratios of cone excitations due to light from different reflecting surfaces, and the association of such ratios with von Kries' coefficient rule; (3) the contribution of chromatic edges to colour constancy and relational colour constancy; and (4) the effects of instruction and observer training. It is suggested that cognitive factors affect colour constancy more than relational colour constancy, which may be an inherently more robust phenomenon.

Gender and hedging: from sex differences to situated practice.

In a reanalysis of women's language, Holmes (1995) has argued that women's use of hedges expresses interpersonal warmth and not, as many researchers have maintained, linguistic tentativeness. It is typically men, she suggests, who employ hedges to convey imprecision and incertitude. In this study, we investigated the use of the hedges sort of and you know in a sample of South African students. Holmes's method of analysis was applied to hedging behavior in 52 dyadic conversations. The study consisted of a 2 (Speaker Gender: Male/Female) x 2 (Audience Gender: Male/Female) x 2 (CONDITION: Competitive/Noncompetitive) between-subjects experimental design. The results showed that contextual influences eclipsed the effects of gender: in fact, no main effects were found for speaker gender. Fewer hedges were deployed in the competitive condition than in the noncompetitive condition. Moreover, perhaps reflecting differences in social status, both sexes used sort of to express tentativeness more frequently when talking to male addressees. When speaking to female addressees, on the other hand, men deployed facilitative you know hedges more readily than women.

Bootstrap estimates of the statistical accuracy of thresholds obtained from psychometric functions.

Estimates of the accuracy of a threshold obtained from a psychometric function are often based on asymptotic theory. When the number of trails is small, however, these estimates may be untrustworthy. A computer program is described that uses a move reliable bootstrap approach to obtaining estimates of the standard deviation and confidence limits of a threshold and of the slope and spread of the psychometric function for any criterion level of performance.

Binary masks yielding gaussian light distributions in Maxwellian view.

Gaussian light distributions are important stimuli for vision research, but are difficult to produce for Maxwellian viewing conditions. In this study, two binary Maxwellian-view masks which produce smooth light distributions on the retina were considered: one was an out-of-focus circular aperture; the other was an in-focus pattern of opaque squares with a prescribed size distribution. The diffraction images of the two masks on the retina were computed numerically and shown to be well described by gaussian functions. As an experimental test, a mask with the desired pattern of opaque squares was manufactured by evaporation of chromium over a quartz substrate; its quality, as assessed by micro-transmittance measurements, was found to be adequate for most visual applications.

Thoracolumbar disc degeneration in young fast bowlers in cricket: a follow-up study.

OBJECTIVE: To determine the progression of thoracolumbar disc degeneration in young fast bowlers in cricket. DESIGN: Prospective fast bowling technique and MRI follow-up study. BACKGROUND: Previous studies on high-performance young fast bowlers have found that lumbar spine pathology was related to the mixed bowling technique. METHODS: Nineteen young male fast bowlers (mean age 13.6 years) underwent MRI scans to detect the presence of intervertebral disc abnormalities. Subjects were also filmed laterally (200 Hz) and from directly above (100 Hz) whilst bowling two maximum velocity deliveries (session 1). Subjects were tested using an identical methodology 2.7 years later (session 2). RESULTS: At session 1, the incidence of thoracolumbar disc degeneration was 21%; however, at session 2, the incidence significantly (P = 0.008) increased to 58%. Furthermore the increase in the incidence of back pain between session 1 and session 2 was also significant (P = 0.002). The progression of disc degeneration was found to be significantly (P = 0.015) related to the group of fast bowlers who utilized the mixed technique during both session 1 and 2 when compared to those who used this technique during one session only. CONCLUSIONS: Thoracolumbar disc degeneration and back pain increase significantly during the time period examined in this study. Further, bowlers who utilize the mixed bowling technique stand a greater chance of developing degenerative changes of the spine.

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.