Interactions between coincident and orthogonal cues to texture boundaries.

Arrays with horizontal or vertical texture boundaries formed by element orientation and length cues were displayed, and the texture boundary formed by one cue was specified as the target. The boundaries formed by the two cues were coincident on some trials and orthogonal on others. Observers' accuracy in reporting the orientation of the target boundary was improved by a coincident nontarget boundary and was worsened by an orthogonal one. Conditional mutual information measures are used to show how effects due to contextual modulation can be distinguished from effects due to additive combination of the cues. The results of five experiments are interpreted as evidence that the transmission of information about specific texture boundary cues is modulated by task context but not by a coincident or orthogonal boundary in another cue. We therefore distinguish between the effects of "context," as shown by the effects of any variable not called the target, and "modulatory contextual effects," as shown by the effects of one variable on the transmission of information specifically about another.

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.

A method for increasing the scoring efficiency of the Farnsworth-Munsell 100-Hue test.

This paper describes a method for scoring the Farnsworth-Munsell 100-Hue test, based on maximum-likelihood estimation, which in theory reduces test-to-test variability in scores and which is therefore better able to discriminate between different levels of overall colour discrimination than is the original Farnsworth scoring system. Error scores produced by the method are directly comparable to error scores produced by the traditional scoring system. It is hoped that this work will provoke further consideration of the efficiency of the scoring system as far as test-to-test variability is concerned, including the efficient detection of polarity in the subject's hue discrimination function.

Orientation dependence of human line-length judgements matches statistical structure in real-world scenes.

A set of 42 photographs of real-world scenes were filtered by convolution with a Laphacian-of-Gaussian kernel. The linear density of zero-crossings along notional lines in the filtered images was measured as a function of the orientation of those lines. The zero-crossing density was found to be greatest along near-vertical lines and least along horizontal lines. The peaks in this function were much broader than the troughs, and there was weak evidence of a small decline in density along lines at and very close to vertical. Psychophysical measurements of the apparent length of a line as a function of its orientation (by the current author and others) show very similar variation, subject to an overall scaling factor. This close correspondence is taken as strong evidence for some sort of environmental determination of the orientation dependence of human length judgements.

A second box-end scoring artifact in the Farnsworth-Munsell 100-hue test.

PURPOSE: To investigate and describe a hitherto unreported scoring artifact in the Farnsworth-Munsell 100-hue Test, arising from the grouping of the caps into four boxes, which causes caps near the ends of a box to score less than caps near the center of a box. This artifact is in addition to a previously reported one, which causes caps near the end of a box to score more than caps in the center of the box. METHODS: Two different statistical simulations were used to generate synthetic cap sequences, which were scored in the normal way. RESULTS: For error scores less than about 500, the new artifact, which depresses scores at the ends of boxes, was found to dominate the pattern of scores. CONCLUSION: The existing published correction for the box-end scoring artifact is inappropriate for scores less than about 500, and therefore should be applied cautiously.

A model for the observer on the Farnsworth-Munsell 100-hue test.

PURPOSE: To use a theoretical model of the observer on the Farnsworth-Munsell 100-hue test to estimate the magnitude of random variation in 100-Hue test error scores. METHODS: The model was based upon classical signal detection theory. Results from the model were obtained by computer simulation. RESULTS: There is a fairly regular relationship between mean test scores over many tests and the standard deviation of those scores. This relationship is for practical purposes unaffected by polarity in the observer's hue discrimination and by changes in the detailed assumptions of the model. CONCLUSION: The model provides a flexible tool for further theoretical research into the 100-hue test.

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.

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.

A table of d' for M-alternative odd-man-out forced-choice procedures.

There is a class of multiple-alternative forced-choice psychophysical procedures in which the subject is required to identify which one of a number of stimuli differs from the remaining stimuli, without having to consider the direction or sign of the difference. A table giving values of the discrimination index d' as a function of the proportion of correct responses in such odd-man-out procedures was generated by numerical simulation. The table covers experiments with 3 to 32 alternative stimuli.

The use of fractal image statistics in the estimation of lateral spatial extent.

It has been argued that the characteristics of many commonly occurring surface textures are such that the resulting luminance distributions have the statistical properties of fractals, over a wide range of spatial scales. We show that, when fractal luminance distributions are spatially filtered, the spatial density of zero-crossings obtained is inversely proportional to the scale of filtering, and is not strongly dependent on the fractal dimension of the pattern used. We propose that this predictable property of natural images could provide a basis for the estimation of lateral spatial extent by counting zero-crossings within an interval at a variety of spatial scales, and averaging over spatial scale. We carried out experiments to compare the relative apparent lateral extents of fractal patterns and patterns of equally spaced bars, as a function of the number of bars. The results are in good agreement with theory.