Biomedical subjects

S C Dakin

Publications and source records attributed to S C Dakin.

25 records · Page 2Linked to original sources

PubMed1998

Spatial-frequency tuning of visual contour integration.

We examine the mechanism that subserves visual contour detection and particularly its tuning for the spatial frequency of contour components. We measured the detection of contours composed of Gabor micropatterns within a field of randomly oriented distractor elements. Distractors were randomly assigned one of two spatial frequencies, and elements lying along the contour alternated between these values. We report that the degree of tolerable spatial-frequency difference between successive contour elements is inversely proportional to the orientation difference between them. Spatial-frequency tuning (half-width at half-height) for straight contours is approximately 1.3 octaves but, for contours with a 30 degrees difference between successive elements, drops to approximately 0.7 octaves. Integration of curved contours operates at a narrower bandwidth. Much orientation information in natural images arises from edges, and we propose that this narrowing of tuning is related to the reduction in interscale support that accompanies increasing edge curvature.

Humans↗

PubMed1997

Absence of contour linking in peripheral vision.

Human foveal vision is subserved initially by groups of spatial, temporal and orientational 'filters', the outputs of which are combined to define perceptual objects. Although a great deal is known about the filtering properties of individual cortical cells, relatively little is known about the nature of this 'linking' process. One recent approach has shown that the process can be thought of in terms of an association field whose strength is determined conjointly by the orientation and distance of the object. Here we describe a fundamental difference in this feature-linking process in central and peripheral parts of the visual field, which provides insight into the ways that foveal and peripheral visual perception differ. In the fovea, performance can be explained only by intercellular linking operations whereas in the periphery intracellular filtering will suffice. This difference represents a substantial economy in cortical neuronal processing of peripheral visual information and may allow a recent theory of intercellular binding to be tested.

Form Perception↗

PubMed1997

The spatial mechanisms mediating symmetry perception.

This paper examines the role of spatial frequency and orientation tuned channels in the perception of visual symmetry. Subjects discriminated between band-pass filtered, white noise textures that either did or did not contain vertical bilateral symmetry (VBS, i.e., around a vertical midline) as a function of the spatial phase disruption imposed on the images. Resistance to phase noise is largely scale-invariant for isotropically filtered images, but horizontally filtered images are consistently more noise-resistant than vertical. However, when stimuli are rotated through 90 deg (horizontal bilateral symmetry, HBS) performance is better with vertically filtered images suggesting a general advantage for orientations orthogonal to the axis of symmetry. At these orientations symmetry may be signaled directly by clusters of features along the axis. Our data further suggest that the established disadvantage for HBS may be attributable to an over-reliance on the output of horizontal filters. We compare models which exploit feature clustering around the axis by measuring the co-alignment in the output of oriented filters. Models using filters oriented orthogonal to the axis of symmetry predict the psychophysical performance for isotropic patterns and for patterns filtered orthogonal to the axis. For patterns filtered parallel to the axis, our data suggest that visual attention may play a role.

Humans↗

PubMed1997

The detection of structure in glass patterns: psychophysics and computational models.

Experiments are reported which examine the judgement of the mean orientation of textures composed either of short lines or dipoles (Glass patterns). The effects of element length, density, and orientation variation are described. Psychophysical data are compared with predictions from four schemes for extracting features from Glass patterns: token matching, isotropic filtering, oriented filtering, and "adaptive" filtering (selection of local peak output from multiply oriented filters). Glass patterns are spatially broadband but only contain orientation structure at a narrow range of scales making them suitable for examining how filter size is selected for texture processing. A criterion for scale selection is proposed: that local variation of feature orientation should be minimized. Simulations indicate that neither models using isotropic filtering nor token matching achieve human levels of performance on certain tasks. Adaptive filtering, operating at a scale selected using the criterion described, provides good agreement with the psychophysical data reported and is a practical scheme for deriving features using oriented filters.

Humans↗

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

Glass patterns: some contrast effects re-evaluated.

The relative contrast of features is known to be important in determining if they can be grouped. Two manipulations of feature contrast have previously been used to criticise models of visual grouping based on spatial filtering: high-pass filtering and reversal of contrast polarity. The effects of these manipulations are considered in the context of the perception of Glass patterns. It is shown that high-pass filtering elements, whilst destroying structure in the output of low-pass filters, do not significantly disrupt the output of locally band-pass filters. The finding that subjects can perceive structure in Glass patterns composed of high-pass features therefore offers no evidence against such spatial filtering mechanisms. Band-pass filtering models are shown to explain the rotation of perceived structure in Glass patterns composed of opposite contrast features. However, structure is correctly perceived in patterns composed of two 'interleaved' opposite contrast patterns, which is problematic for oriented filtering mechanisms. Two possible explanations are considered: nonlinear contrast transduction prior to filtering, and integration of local orientation estimates from first-order and second-order mechanisms.

Contrast Sensitivity↗

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↗