The detection of direction-defined and speed-defined spatial contours: one mechanism or two?

Spatial and Temporal Selectivity of Translational Glass Patterns Assessed With the Tilt After-Effect

Glass patterns (GPs) have been widely employed to investigate the mechanisms underlying processing of global form from locally oriented cues. The current study aimed to psychophysically investigate the level at which global orientation is extracted from translational GPs using the tilt after-effect (TAE) and manipulating the spatiotemporal properties of the adapting pattern. We adapted participants to translational GPs and tested with sinewave gratings. In Experiment 1, we investigated whether orientation-selective units are sensitive to the temporal frequency of the adapting GP. We used static and dynamic translational GPs, with dynamic GPs refreshed at different temporal frequencies. In Experiment 2, we investigated the spatial frequency selectivity of orientation-selective units by manipulating the spatial frequency content of the adapting GPs. The results showed that the TAE peaked at a temporal frequency of ∼30 Hz, suggesting that orientation-selective units responding to translational GPs are sensitive to high temporal frequencies. In addition, TAE from translational GPs peaked at lower spatial frequencies than the dipoles' spatial constant. These effects are consistent with form-motion integration at low and intermediate levels of visual processing.

Speed tuning properties of mirror symmetry detection mechanisms

The human visual system is often tasked with extracting image properties such as symmetry from rapidly moving objects and scenes. The extent to which motion speed and symmetry processing mechanisms interact is not known. Here we examine speed-tuning properties of symmetry detection mechanisms using dynamic dot-patterns containing varying amounts of position and local motion-direction symmetry. We measured symmetry detection thresholds for stimuli in which symmetric and noise elements either drifted with different relative speeds, were relocated at different relative temporal frequencies or were static. We also measured percentage correct responses under two stimulus conditions: a segregated condition in which symmetric and noise elements drifted at different speeds, and a non-segregated condition in which the symmetric elements drifted at two different speeds in equal proportions, as did the noise elements. We found that performance (i) improved gradually with increasing the difference in relative speed between symmetric and noise elements, but was invariant across relative temporal frequencies/lifetime duration differences between symmetric and noise elements, (ii) was higher in the segregated compared to non-segregated conditions, and in the moving compared to the static conditions. We conclude that symmetry detection mechanisms are broadly tuned to speed, with speed-selective symmetry channels combining their outputs by probability summation.

The role of motion and number of element locations in mirror symmetry perception

The human visual system has specialised mechanisms for encoding mirror-symmetry and for detecting symmetric motion-directions for objects that loom or recede from the observers. The contribution of motion to mirror-symmetry perception has never been investigated. Here we examine symmetry detection thresholds for stationary (static and dynamic flicker) and symmetrically moving patterns (inwards, outwards, random directions) with and without positional symmetry. We also measured motion detection and direction-discrimination thresholds for horizontal (left, right) and symmetrically moving patterns with and without positional symmetry. We found that symmetry detection thresholds were (a) significantly higher for static patterns, but there was no difference between the dynamic flicker and symmetrical motion conditions, and (b) higher than motion detection and direction-discrimination thresholds for horizontal or symmetrical motion, with or without positional symmetry. In addition, symmetrical motion was as easy to detect or discriminate as horizontal motion. We conclude that whilst symmetrical motion per se does not contribute to symmetry perception, limiting the lifetime of pattern elements does improve performance by increasing the number of element-locations as elements move from one location to the next. This may be explained by a temporal integration process in which weak, noisy symmetry signals are combined to produce a stronger signal.

Cortical spatiotemporal dimensionality reduction for visual grouping

The visual systems of many mammals, including humans, are able to integrate the geometric information of visual stimuli and perform cognitive tasks at the first stages of the cortical processing. This is thought to be the result of a combination of mechanisms, which include feature extraction at the single cell level and geometric processing by means of cell connectivity. We present a geometric model of such connectivities in the space of detected features associated with spatiotemporal visual stimuli and show how they can be used to obtain low-level object segmentation. The main idea is to define a spectral clustering procedure with anisotropic affinities over data sets consisting of embeddings of the visual stimuli into higher-dimensional spaces. Neural plausibility of the proposed arguments will be discussed.

Cue combination in a combined feature contrast detection and figure identification task

Target figures defined by feature contrast in spatial frequency, orientation or both cues had to be detected in Gabor random fields and their shape had to be identified in a dual task paradigm. Performance improved with increasing feature contrast and was strongly correlated among both tasks. Subjects performed significantly better with combined cues than with single cues. The improvement due to cue summation was stronger than predicted by the assumption of independent feature specific mechanisms, and increased with the performance level achieved with single cues until it was limited by ceiling effects. Further, cue summation was also strongly correlated among tasks: when there was benefit due to the additional cue in feature contrast detection, there was also benefit in figure identification. For the same performance level achieved with single cues, cue summation was generally larger in figure identification than in feature contrast detection, indicating more benefit when processes of shape and surface formation are involved. Our results suggest that cue combination improves spatial form completion and figure-ground segregation in noisy environments, and therefore leads to more stable object vision.

Grouping local orientation and direction signals to extract spatial contours: empirical tests of "association field" models of contour integration

Over the last decade or so a great deal of psychophysical research has attempted to delineate the principles by which local orientations and motions are combined across space to facilitate the detection of simple spatial contours. This has led to the development of "association field" models of contour detection which suggest that the strength of linking between neighbouring elements in an image, is determined by the degree to which they aligned along smooth (first-order) curves. To test this assumption we used a path detection paradigm to compare the ability of observers to identify the presence of contours defined by either spatial orientation, motion direction or by specific combinations of both types of visual attribute. The relative alignment of the local orientations and/or directions with respect to the axis of the depicted contour was systematically varied. For orientation-defined contours detection was best when the elements were aligned along (parallel with) the contour axis, approached chance levels for obliquely oriented elements and then improved for elements that were orthogonal to the contour axis (i.e., performance was a U-shaped function of degree of orientation misalignment). This pattern of results was found for both straight and curved contours and is not readily explicable in terms of current association field theories. For motion-defined contours, however, performance simply deteriorated as the relative directions of the constituent path elements were progressively misaligned with respect to the contour. Thus the rules by which local orientations are linked to define spatial contours are qualitatively different from those used for linking local directions and each may be mediated by distinct visual mechanisms. When both orientation and motion cues were simultaneously available, contour detection performance was generally enhanced, in a manner that is consistent with probability summation. We suggest that association field models of orientation linking may need to be extended in light of the present findings.

The spatial frequency and orientation selectivity of the mechanisms that extract motion-defined contours

The human visual system can undertake a specialized form of motion integration, one that enables the presence of extended spatial contours to be disambiguated from their backgrounds. We have shown previously that the visual system can selectively integrate local motion signals when their directions are along spatial contours and its efficiency is inversely related to the curvature of the contour involved (Ledgeway, T., & Hess, R. F. (2002). Vision Research, 42, 653-659). This integration primarily involves the direction, rather than the speed, of local motion signals. In the present study, we sought to investigate both the spatial frequency and orientation tuning of this specialized contour integration process, using a path detection paradigm. The results show that the tuning for spatial frequency is very broad, in line with previous studies that have examined this issue. In contrast, the orientation selectivity of the mechanism mediating contour extraction under these conditions is relatively narrowband. Thus, spatial frequency but not orientation pooling appears to take place prior to the extraction of motion-defined contours, a situation that is different from that previously shown for spatial contours composed of static, oriented elements.