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R J Watt

Publications and source records attributed to R J Watt.

At least 19 recordsLinked to original sources
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↗
PubMed1984

Further evidence concerning the analysis of curvature in human foveal vision.

Data are presented which indicate that a length limit of 20 min arc applies to the highly accurate discrimination of curvature difference. Within this constraint and the previously reported orientational restriction (Watt and Andrews, 1982, Vision Res. 22, 449-460), curvature may determine the level of performance.

Form Perception↗
PubMed1984

Towards a general theory of the visual acuities for shape and spatial arrangement.

A wide range of pattern acuity measurements are presented, and detailed comparisons are made between different acuities for different stimuli and tasks. The most striking finding is that only a small number of cues and mechanisms are involved, and there are thus considerable inhomogeneities in spatial perception, even under focused attention and foveal viewing. A theory for pattern acuity is elaborated, bringing the pattern acuity phenomena into a more general scheme for the visual perception of spatial pattern. Four distinct mechanisms are required to account for all the data. Two of these analyse the shape of small segments of contour using high grade position and slope information; the other two relate the positions of these segments also using position and slope information, but of lower grade.

Form Perception↗
PubMed1984

Spatial filters and the localization of luminance changes in human vision.

Relative location acuities similar to vernier acuity were measured for a variety of different luminance profiles and contrasts. The results are interpreted in terms of the extraction of zero-bounded centroids in the responses of a limited range of bandpass spatial filters, with centre frequencies between 3.5 and 30 c/deg. The process by which the outputs of the filters are combined is examined, and MIRAGE, a specific proposal, tested and found to be supported. Analysis of the results obtained at lower contrasts suggests that the major source of noise lies not in the amplification of the filters, but is due to the location of the centroids being distorted by spurious noise zero-crossings. The pattern of noise intrusion is readily explained in terms of MIRAGE.

Form Perception↗
PubMed1984

Intensity-response nonlinearities and the theory of edge localization.

If an edge having a particular blur profile is exchanged for an edge at the same location but having a different blur profile, apparent movement between the two edges may be seen. The spatial extent of this movement was measured by a cancellation technique. Implications for the theory of edge location are discussed, including the possibility that spatial filtering is preceded by a nonlinear transduction stage.

Filtration↗