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B Moulden

Publications and source records attributed to B Moulden.

17 recordsLinked to original sources
PubMed1992

A comparison between colour and luminance contrast in a spatial linking task.

We report experiments which compare the ability of subjects to employ colour vs luminance contrast as a basis for discriminating the degree of collinearity of random element string pairs. The purpose of the study was to determine the extent to which spatial integration mechanisms could utilize colour contrast. In order to probe directly the processes of spatial integration per se, it was necessary to control for any differences in the efficiency with which the visual system utilized colour and luminance contrast to locate the positions of the individual elements in the test stimuli. To do this we first established the "equivalent" luminance contrast of an isochromatic stimulus which produced equal performance to an isoluminant stimulus in a 2 element per string alignment task. This equated the colour defined and luminance defined stimuli for local positional acuity. We then measured performance for both isoluminant and equivalent luminance contrast stimuli for strings consisting of 2, 4, 8 and 16 elements. This tested for any differences in the processes of spatial integration. For both unmasked stimuli and stimuli embedded in luminance noise, there was no consistent trend favouring either luminance or colour contrast as the number of elements in the stimuli was increased. We conclude that the visual system is able to employ colour contrast as efficiently as luminance contrast for collinearity judgements, thus implicating a general role for colour vision in spatial integration tasks.

Color Perception↗
PubMed1992

A multi-channel approach to brightness coding.

A model of brightness coding is presented which is shown to predict the appearance of a number of classical brightness phenomena. The model is known as MIDAAS which stands for Multiple Independent Descriptions Averaged Across Scale. In common with many other approaches to brightness perception MIDAAS imputes to local feature detectors a central role in the computation of brightness. It also explicitly recognises the crucial importance to brightness perception of feature detectors operating at different spatial scales. The unique and definitive feature of the model however is the supposition that each scale of spatial filtering operates as if to generate its own description of the pattern of brightness relationships in the image. The final percept is then provided by the composite of those individual brightness descriptions. It is shown that MIDAAS provides a good account of a variety of Mach band phenomena, the conditions under which the Missing Fundamental illusion is observed, the effect of occluding bars on the apparent contrast of step edges, the Chevreul illusion, simultaneous brightness contrast and the non-linear appearance of high contrast sinusoidal gratings. The advantages of MIDAAS over other approaches to brightness perception is discussed, as well as its current limitations.

Contrast Sensitivity↗
PubMed1992

Spatial gating effects on judged motion of gratings in apertures.

Wallach has described in qualitative terms the movement of lines behind apertures. We related the data he obtained to the aperture problem, constructed a model of movement perception, and carried out tests of the model. Experiment 1 was a parametric study, and showed the conditions under which a reliable illusion (the barber pole illusion) of diagonal movement of lines along an aperture could be obtained, and when fluctuating judgements or veridical percepts were obtained. On the basis of this study a dipole model was constructed. The model was further developed and tested. In experiment 2 the effects of total area of stimulation were examined: diagonal gratings were viewed behind multiple apertures. In experiment 3 the effects of local signs were examined: diagonal gratings were viewed in an aperture which had edges cut in small steps and stairs, with the risers parallel to the grating, and the treads parallel to the direction of motion of the grating. Experiment 4 was designed to test a prediction about the motion aftereffect of dots near and far from the point of fixation, and the results confirmed the model. It was concluded that the model accounts for the barber pole illusion and, generally, for the movement of gratings in apertures.

Adult↗
PubMed1991

White's effect and assimilation.

White's effect is a phenomenon in which grey bars replacing segments of the white phase of a square-wave grating appear darker than those replacing segments of the black phase. The direction of the brightness difference is consistent with brightness assimilation rather than with brightness contrast. We present data from two experiments which measure the degree of the brightness difference in stimuli consisting of just three inducing bars and a single grey test bar, as a function of various spatial manipulations of the inducing and test bars. The spatial manipulations were chosen to maximise the opportunity for assimilation effects to manifest themselves. The results do not support the view that assimilation is an important component of the effect. The data are shown to be consistent with our model of brightness induction in which both a local and a more spatially extensive contrast mechanism operate to produce White's effect.

Color Perception↗
PubMed1991

A model for contrast discrimination with incremental and decremental test patches.

Whittle [Vision Research, 26, 1677 (1986)] has shown that the metric of contrast W = delta L/Lmin (delta L = difference in luminance between test patch and background, Lmin = the smaller of the luminance of the background or test patch) is able to provide a unifying description of the pattern of contrast discrimination thresholds for pairs of test patches set against a common background. In particular the metric W unifies the pattern of discrimination thresholds for both increment and decrement pairs. We argue that while W provides a good mathematical description of Whittle's data it is functionally implausible since it implies that the component of the stimulus which sets the adaptational level for increments is different from that which sets the adaptational level for decrements. We argue that the metric G = ln(L/Lb) (L = test patch luminance, Lb = background luminance) is physiologically more plausible than W and show that G can provide at least as good a fit as W to Whittle's data when incorporated in a transfer function of the form RG = kG1-n, with n set to 0.69. The fit to the data can be improved still further if a parameter representing the non-linearity in the gain-luminance function at low luminances is included in the RG equation. The theoretical implications for retinal gain mechanisms are discussed.

Contrast Sensitivity↗
PubMed1991

The local border mechanism in grating induction.

Both White's effect and the grating induction effect are examples of brightness contrast phenomena. Models to account for these effects have either explicitly rejected local border mechanisms (such as retinal ganglion cells) in favour of cortical mechanisms, or explicitly rejected elongated cortical filters in favour of local mechanisms. We have argued that any viable model must include both classes of mechanism. In this paper we present some novel versions of induction effects, and describe the explanatory power of a model couched solely in terms of the operation of local spatial filters. The model employs filters at different spatial scales whose outputs are then averaged. Using this approach it is possible to give a good account not only for the novel demonstrations we present, but also for the pattern of results reported by others concerning various manipulations of the spatial parameters of induction displays.

Contrast Sensitivity↗
PubMed1991

Polarity specific adaptation to motion in the human visual system.

Three experiments investigated polarity specific adaptation to movement. Experiment 1 tested for temporal polarity specific adaptation, using counterphase sawtooth gratings as adapting and test stimuli. Each counterphase grating contained oppositely moving sawtooth components, and was thus balanced for direction, but both components of the adapting grating created only one polarity of luminance change over time, whereas the components of the test grating presented different signs. After adaptation, only the test component containing the unadapted temporal change was visible. A second experiment, using an analogous procedure, found evidence for spatial polarity specific adaptation. Experimental results can be explained by motion detectors which preserve information about spatial and temporal polarity. A third experiment found that spatial and temporal polarity specific adaptation differ in their dependence on temporal frequency.

Adaptation, Ocular↗
PubMed1990

The standard deviation of luminance as a metric for contrast in random-dot images.

Michelson's contrast, C, is an excellent metric for contrast in images with periodic luminance profiles, such as gratings, but is not suitable for images consisting of isolated stimulus elements, eg single bars; other metrics have been devised for such stimuli. But what metric should be used for random-dot images such as are commonly used in stereograms and kinematograms? Previously the standard deviation (SD) of the luminances (equivalent to the root mean square, RMS, of the amplitudes) has been taken as a measure of contrast, but on little more than intuitive grounds. The validity of this speculative usage is tested. Experiments are described in which a wide range of random-dot images of various compositions was used and the adapting power of these images measured. This was taken as an index of their visual effectiveness. The contrast and contrast-reducing effects of the stimuli were expressed in terms of six candidate metrics, including SD, to discover which would give the most lawful description of the experimental data. The usefulness and generality of the SD measure were confirmed. The effects of mean luminance were also measured and a general expression that would take them into account was derived. Finally, on the basis of computational modelling in which spatial filters with properties approximating those of retinal ganglion cells were used, a possible theoretical account for the success of the SD metric is offered.

Adult↗
PubMed1990

Light-dark asymmetries in the Craik-Cornsweet-O'Brien illusion and a new model of brightness coding.

With the aid of a matching technique, the magnitude of induced brightness in bars bordered with Craik-Cornsweet-O'Brien (CCOB) edges was investigated as a function of the width and amplitude of those edges. Data were collected for stimuli with the sloping part of the edge on both the inside and outside of the bar, and also for stimuli with both positive-going and negative-going edges. The results confirmed previous reports that induced brightness was greater for CCOB stimuli with negative-going, as opposed to positive-going, edges and greater for CCOB stimuli whose edges contained outer, as opposed to inner gradients. A model of brightness coding is offered to provide an explanation for the specific anisotropies observed, as well as the general effects of stimulus amplitude and width on induced brightness. The model assumes that a symbolic description of brightness is generated separately from each of a number of different-sized 2DG (second difference of a Gaussian) filters, and the resulting brightness profile obtained by averaging across the separate descriptions. The ability of other brightness models to account for the data is also discussed.

Contrast Sensitivity↗
PubMed1989

White's effect: a dual mechanism.

White (1979) has described a phenomenon in which grey bars replacing segments of the white phase of a square-wave grating appear darker than identical grey bars replacing segments of the black phase of the grating. We have investigated the properties of this effect with a view to discovering the underlying mechanisms. Four experiments are reported which reveal the effects of the heights and widths of both the flank and coaxial inducing bars upon the brightness of the grey bars. The results show that two processes, one the local corner effect, and one a spatially extensive process (possibly involving filters with elongated end-zones) operate to produce the effect. The implications of the findings for models of brightness perception are discussed and suggestions are made for further experiments.

Form Perception↗
PubMed1989

An orientation anisotropy in induced brightness.

It is shown that an orientation anisotropy exists for the magnitude of induced brightness in a cruciform stimulus consisting of a grey test patch positioned at the intersection of two inducing bars, one black and one white, oriented at right angles to each other. When the cruciform was oriented such that the white bar was horizontal, the grey patch appeared darker than when the same cruciform was oriented such that the white bar was vertical. The contribution of the black and white inducing bars towards the brightness of the test patch was investigated. A simple mathematical function, which took into account both the contribution of the two component inducing bars and the orientation anisotropy, was fitted to the data. No consistent orientation anisotropy was found with inducing stimuli at oblique orientations.

Attention↗
PubMed1989

Modelling visual detection: luminance response non-linearity and internal noise.

Two experiments that investigate the effect of various display factors on the detectability of a thin line signal in random visual noise are described. Three statistical decision models are described, together with their ability to account for the results. The first is an "ideal detector" model, the second an "energy integrator" model, and the third a model based upon the operation of retinal ganglion cells which incorporates a gain control mechanism. The ideal detector model fails to give a good account of human performance, whereas the other two models provide a good fit to the data. The digital Laplacian with gain control model has the slight advantage over the energy integrating model in being able to account for a small superiority in the detection of dark as opposed to bright signals. Finally, both models require the inclusion of an estimate of the internal noise of the human visual system to account for the pattern of performance observed under changing conditions of display contrast.

Adult↗
PubMed1989

Corner effect in induced hue: evidence for chromatic band-pass filters.

An experiment is described which investigates the spatial determinants of the apparent difference in hue between the central grey patches of chromatic 'H' pattern pairs, an effect similar to that first demonstrated by Wright (1969, The Measurement of Colour, Hilger, London) in coloured gratings. The hue difference is shown to be analogous to the brightness difference in achromatic 'H' patterns demonstrated by Moulden and Kingdom (1989, Vision Res. 29, 1245-1259). The origin of both effects is argued to be the presence of the corner intersections in the 'H' patterns, which are powerful stimuli for cells with circularly-symmetric, centre-surround organization. It is suggested that the results of the experiment with the chromatic 'H' patterns implicates the operation of cells with a spectrally double-opponent, rather than single-opponent receptive field organization.

Color Perception↗
PubMed1979

The Münsterberg illusion and 'irradiation'.

The Münsterberg illusion is a compelling phenomenon for which there is no generally accepted explanation. It is suggested that two different contour-shifting effects operate in this illusion, and these are named the 'symmetrical effect' and the 'corner effect' respectively. It is further suggested that both of these effects are caused by 'irradiation', and an attempt is made to cast some light upon the phenomenon of irradiation, which is itself only poorly understood.

Humans↗