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Biomedical subjects

R J Watt

Publications and source records attributed to R J Watt.

At least 19 recordsLinked to original sources
PubMed1997

The computation of orientation statistics from visual texture.

This paper examines how observers estimate the overall orientation of spatially disorganised textures containing variable orientation. Experiments used asymmetrical distributions of orientations to separate the predictions from different models of average orientation estimation. Stimuli were composed of two spatially intermingled sets of oriented patches, each set having Gaussian distributed element orientation. The threshold separation of the means of the two sets was determined for a variety of tasks. Discrimination of these textures from a reference composed of two sets with the same mean orientation was well predicted by discrimination of orientation variability. A single interval judgement of which set contained more elements required a greater separation of the set orientations and suggested that the sets must be resolved in the orientation domain for independent representation of their properties. That resolution is required to perform this task further suggests that orientational skew is not coded. Threshold offsets for judgement of average orientation were re-expressed as shifts of four candidate features for coding the central tendency of texel orientations. Comparison with similar thresholds for single distributions of orientations indicated that average orientation is assigned to the centroid of a set of orientation measures.

Discrimination, Psychological↗
PubMed1997

The combination of filters in early spatial vision: a retrospective analysis of the MIRAGE model.

Since the discovery of spatial-frequency-tuned channels in the visual system, most theories attempting to account for pattern encoding have assumed that the filters can be independently accessed and flexibly combined. We review here an alternative model, 'MIRAGE', in which the filters are inflexibly combined before pattern analysis. In the MIRAGE model the half-wave rectified outputs of all spatial-frequency channels are combined before locating spatial zero-bounded regions in the neural image, which serve as the spatial primitives for pattern analysis. We describe the evidence that led to this model, and review recent evidence on the rules of filter combination.

Humans↗
PubMed1994

Computational analysis of early visual mechanisms.

An important class of mechanism, image filtering, is normally used to model the first stages in the human visual process. A formal examination of the general computational properties of image filtering, looking at the logic of what image filtering should accomplish and how filtered images can be described, shows that image filtering does not make any useful information explicit. To do this, a further stage of post-filtering processing, termed image description, is necessary. With these image descriptions it is possible to establish relationships between the structures that emerge and the filters used. Three properties emerge that are of benefit: the structures are simply shaped when filters are used that are modestly orientation tuned; the responses are suitably primitive; and the response structures show spatial clusterings that can be used to identify certain classes of object. Within this framework, the properties of a range of oriented filters were examined in a series of computational experiments. Given a particular form of image description, the optimal filters in terms of response structure simplicity, primitiveness and flexibility show a degree of orientation selectivity corresponding to that obtained in mammalian vision.

Animals↗
PubMed1994

Detection of bilateral symmetry using spatial filters.

When bilaterally symmetric images are spatially filtered and thresholded, a subset of the resultant 'blobs' cluster around the axis of symmetry. Consequently, a quantitative measure of blob alignment can be used to code the degree of symmetry and to locate the axis of symmetry. Four alternative models were tested to examine which components of this scheme might be involved in human detection of symmetry. Two used a blob-alignment measure, operating on the output of either isotropic or oriented filters. The other two used similar filtering schemes, but measured symmetry by calculating the correlation of one half of the pattern with a reflection of the other. Simulations compared the effect of spatial jitter, proportion of matched to unmatched dots and width or location of embedded symmetrical regions, on models' detection of symmetry. Only the performance of the oriented filter + blob-alignment model was consistent with human performance in all conditions. It is concluded that the degree of feature co-alignment in the output of oriented filters is the cue used by human vision to perform these tasks. The broader computational role that feature alignment detection could play in early vision is discussed, particularly for object detection and image segmentation. In this framework, symmetry is a consequence of a more general-purpose grouping scheme.

Computer Simulation↗
PubMed1990

Regional distribution of the mechanisms that underlie spatial localization.

In order to understand the regional distribution of the mechanisms which underlie localization accuracy we (1) chose a task which is known to involve localization accuracy (2) optimized stimulus parameters for ecentric loci and (3) determined how two key spatial factors which affect localization accuracy vary as a function of ecentricity. These involve Gaussian blur and Gaussian jitter. These results suggest that there are three different functions with ecentricity for the mechanisms underlying this task which we relate to the spatial properties of the retina, namely mean cone density, receptoral convergence and regularity.

Contrast Sensitivity↗
PubMed1989

The Weber relation for position is not an artefact of eccentricity.

We have measured the spatial threshold for discriminating the length of arcs falling on isoeccentric circles in the periphery out to 4 deg. The just-noticeable-difference rises with the arc length in the range 10-500 arc min, repeating the usual finding with foveally presented straight-line stimuli. Our results do not support a recent suggestion that the Weber relation is mainly due to greater positional uncertainty in the peripheral retina.

Humans↗
PubMed1989

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.

Humans↗
PubMed1989

How are spatial filters used in fovea and parafovea?

The information used in the output of the visual filters was examined for a blur discrimination task for which it had previously been claimed that the best human performance was with pedestal blurs of intermediate magnitude. This conclusion was verified for parameters leading to asymptotic performance for foveal and parafoveal locations. The nature of the computations underlying these visual discriminations was then assessed experimentally and by simulations. The results suggest that a frequency-based model in its simplest form cannot explain these findings. A model based on the spatial properties of filter outputs is discussed.

Computer Simulation↗
PubMed1987

Shape recognition in amblyopia.

The results of recent investigations of supra-threshold spatial vision in amblyopia have indicated that the amblyopic eye may be more disadvantaged than acuity or contrast detection measurements have suggested. We report here on the sensitivity of the amblyopic eye when performing tasks requiring the recognition of small spatial differences in thin bright line stimuli displayed in a variety of 2-D configurations. Amblyopes were able to perform these tasks, but their amblyopic eye was generally poorer than their normal eye by a multiplicative factor. It is argued that this pattern of results indicates that spatial scrambling rather than blurring is an appropriate model for amblyopia.

Adult↗
PubMed1987

Spatial information and uncertainty in anisometropic amblyopia.

Anisometropic amblyopes were found to have a reduced sensitivity for shape discrimination. The introduction of positional jitter in the elements of the display had a profound effect on the performance of the normal eye, but not on that of the amblyopic eye. On the other hand the introduction of gaussian blur affects the performance of both eyes to the same degree. We conclude that raised spatial uncertainty due to metrical scrambling is a suitable model for anisometropic amblyopia.

Adult↗
PubMed1987

The detection of deviation from straightness in lines.

A wide range of differently shaped perturbations were introduced into long thin straight lines, and threshold amplitude for their detection was measured. This amplitude threshold varies over a 20-fold range, depending on the shape of the cue, but can be economically expressed as just one numerical value, irrespective of the cue shape. This quantity is the area of the largest bump in the target around a least squares regression line axis, and its value is 0.3 arc min2. This value can be related to a fundamental spatial error of 3 arc sec (standard deviation) which is the limiting constraint on shape sensitivity.

Discrimination, Psychological↗
PubMed1987

Scanning from coarse to fine spatial scales in the human visual system after the onset of a stimulus.

The manner in which the spatial characteristics of simple discrimination tasks change with time after the onset of a stimulus were examined. The experiments measured the improvements in sensitivity to the length, orientation, curvature, and stereoscopic depth of short lines that accrue with increased exposure durations. These improvements can be consistently interpreted in terms of a change of the spatial scale of analysis from coarse to fine over a period of at least 1000 msec. Variations in visual resolution acuity over the same period are negligible, and it is concluded that the changes in spatial characteristics concern the range of spatial filters in operation. This range progressively shrinks after stimulus presentation.

Humans↗
PubMed1986

Feature-based image segmentation in human vision.

Experiments are described which indicate that the integration of high-precision shape information along a bright line is blocked by the presence of certain image features. All the features involved have three properties: (1) they are points where contours are not smooth (i.e. not twice differentiable) within the limits set by the finite space constants of visual processes; (2) they are all points that are emphasized in the responses of certain classes of circularly symmetric bandpass spatial filter; and (3) they are all significant for three-dimensional shape analysis. The results are interpreted as implying an inflexible segmentation of the contour image before detailed shape analysis.

Form Perception↗
PubMed1985

A theory of the primitive spatial code in human vision.

MIRAGE, a theory for the primitive coding of the (1D) spatial distribution of luminance changes by the human visual system is developed from a theoretical examination of the practical problems associated with the characterization of such changes. The main novel feature of the theory is that the multiplicity of spatial filters in human vision is assumed to exist principally to transmit a broad bandwidth signal of considerable redundancy: the filters are not assumed to be marked with their centre frequency or bandwidth, and are not analyzed independently. The theory is largely independent of the particular filter transfer function form. MIRAGE is applied to a range of one-dimensional luminance patterns, and demonstrates several well-known brightness illusions, and a structured grouping principle. It is finally shown to be supported by a wide range of psychophysical data.

Filtration↗
PubMed1985

Vernier acuity: interactions between length effects and gaps when orientation cues are eliminated.

It is shown that Vernier acuity is nearly proportional to line length (in the range 5-20 arc min), provided that the orientation of the target is randomized from trial to trial and provided that no gap interrupts the line. The effect of a pair of gaps, one in each of the Vernier bars leads to the conclusion that line terminations dissect the target into independent parts prior to the visual analysis of shape and orientation cues, which are only assessed between such features.

Adult↗
PubMed1985

Image segmentation at contour intersections in human focal vision.

It would be efficient for the visual image to be broken initially into small segments, each of which could be analyzed separately before being related to the others to build a representation of the whole. The places in the image where such segmentation should occur will depend on the image and will not be fixed points in retinal space. In this study, it is shown that high-precision-shape information cannot be integrated across contour intersections. It is suggested that contour intersections are a particular feature of the retinal image at which segmentation inevitably occurs.

Form Perception↗
PubMed1984

Spatial frequency interference effects and interpolation in vernier acuity.

Discrete spatial sampling of sinusoidal gratings has a detrimental effect upon vernier acuity for such stimuli if the sampling rate is less than 20 c/deg. We have examined the possibility that interpolation failure is due to masking by spatial frequency components near to the signal frequency. In an explicit masking paradigm, vernier acuity for a sine wave grating was found to be adversely affected by the presence of a masking grating falling within a critical band of frequencies near the test target. In the sampled stimulus, removal of sampling replicas similar in frequency to the test improved acuity, but the extent of the residual interference by high frequency components could not be predicted from the masking data. The high frequency interference effect depended on fixed phase relations between frequency components, creating narrow bright bars in the spatial luminance profile, and was greatly reduced by phase randomizing the sampling replicas. Various explanations of this nonlinear behaviour are discussed, including the Marr-Hildreth theory of edge detection.

Form Perception↗