Publications

2005
Dakin SC, Mareschal I, Bex PJ. An oblique effect for local motion: psychophysics and natural movie statistics. Journal of Vision [Internet]. 2005;5:878–87. Publisher's VersionAbstract
Human perception of visual motion is thought to involve two stages–estimation of local motion (i.e., of small features) and global motion (i.e., of larger objects)–identified with cortical areas V1 and {MT}, respectively. We asked if poor discrimination of oblique compared to cardinal directions (the oblique effect for motion; {OEM)} reflects a deficit in local or in global motion processing. We used an equivalent noise ({EN)} paradigm–where one measures direction discrimination thresholds in the presence of directional variability–to quantify local and global limits. We report that the {OEM} diminishes with increasing directional variability, indicating that global motion processing (the number of local motion signals pooled) is equal across all directions and that the {OEM} is attributable to anisotropies in local motion processing. To investigate the origin of this effect, we measured local motion statistics from natural movies (filmed from the point of view of a walking observer). This analysis reveals that the distribution of local directional energy on the oblique directions tends to be broader, and frequently more asymmetric, than on the cardinal directions. If motion detectors are optimized to deal with our visual world then such anisotropies likely explain the local nature of the {OEM.}
Bex PJ, Dakin SC. Spatial interference among moving targets. Vision Research [Internet]. 2005;45:1385–98. Publisher's VersionAbstract
Peripheral vision for static form is limited both by reduced spatial acuity and by interference among adjacent features ('crowding'). However, the visibility of acuity-corrected image motion is relatively constant across the visual field. We measured whether spatial interference among nearby moving elements is similarly invariant of retinal eccentricity and assessed if motion integration could account for any observed sensitivity loss. We report that sensitivity to the direction of motion of a central target-highly visible in isolation-was strongly impaired by four drifting flanking elements. The extent of spatial interference increased with eccentricity. Random-direction flanks and flanks whose directions formed global patterns of rotation or expansion were more disruptive than flanks forming global patterns of translation, regardless of the relative direction of the target element. Spatial interference was low-pass tuned for spatial frequency and broadly tuned for temporal frequency. We show that these results challenge the generality of models of spatial interference that are based on retinal image quality, masking, confusions between target and flanks, attentional resolution limits or (simple) "averaging" of element parameters. Instead, the results suggest that spatial interference is a consequence of the integration of meaningful image structure within large receptive fields. The underlying connectivity of this integration favours low spatial frequency structure but is broadly tuned for speed.
2004
Simmers AJ, Bex PJ. The representation of global spatial structure in amblyopia. Vision Research [Internet]. 2004;44:523–33. Publisher's VersionAbstract
Visual processing is thought to involve initial local analyses that are subsequently integrated globally to derive functional representations of structure that extends over large areas of visual space. Amblyopia is a common deficit in spatial vision that could be based on either unreliable local estimates of image structure, irregularities in global image integration or a combination of errors at both these stages. The purpose of this study was to quantify the integration of local spatial information in amblyopia with global orientation discrimination and inter-ocular matching tasks. Stimuli were composed of pseudo-random arrays of highly visible and resolvable features (Gabor patches) whose local orientation and position were drawn from global distributions whose mean and variance statistics were systemically varied. Global orientation discrimination thresholds in both the amblyopic and fellow eye were elevated. The orientational and positional variances perceived by the amblyopic eye were matched by stimuli with higher variances perceived in the fellow eye. It would appear that amblyopes are able to integrate orientation information across visual space but the global representation of local structure shows greater variability compared to normal. It is this increased spatial uncertainty that underlies the spatial deficit in amblyopia.
2003
Hess RF, Pointer JS, Simmers A, Bex P. Border distinctness in amblyopia. Vision Research [Internet]. 2003;43:2255–64. Publisher's VersionAbstract
On the basis of the contrast sensitivity loss in amblyopia which mainly affects higher spatial frequencies, one would expect amblyopes to perceive sharp edges as blurred. We show that they perceive sharp edges as sharp and have veridical edge blur perception. Contrary to the currently accepted view, this suggests that the amblyopic visual system is not characterized by a blurred visual representation.
Fiser J, Bex PJ, Makous W. Contrast conservation in human vision. Vision Research [Internet]. 2003;43:2637–48. Publisher's VersionAbstract
Visual experience, which is defined by brief saccadic sampling of complex scenes at high contrast, has typically been studied with static gratings at threshold contrast. To investigate how suprathreshold visual processing is related to threshold vision, we tested the temporal integration of contrast in the presence of large, sudden changes in the stimuli such occur during saccades under natural conditions. We observed completely different effects under threshold and suprathreshold viewing conditions. The threshold contrast of successively presented gratings that were either perpendicularly oriented or of inverted phase showed probability summation, implying no detectable interaction between independent visual detectors. However, at suprathreshold levels we found complete algebraic summation of contrast for stimuli longer than 53 ms. The same results were obtained during sudden changes between random noise patterns and between natural scenes. These results cannot be explained by traditional contrast gain-control mechanisms or the effect of contrast constancy. Rather, at suprathreshold levels, the visual system seems to conserve the contrast information from recently viewed images, perhaps for the efficient assessment of the contrast of the visual scene while the eye saccades from place to place.
Dakin SC, Bex PJ. Natural image statistics mediate brightness 'filling in'. Proceedings of The Royal Society of London [Internet]. 2003;270:2341–2348. Publisher's VersionAbstract
Although the human visual system can accurately estimate the reflectance (or lightness) of surfaces under enormous variations in illumination, two equiluminant grey regions can be induced to appear quite different simply by placing a light- dark luminance transition between them. This illusion, the Craik-Cornsweet-{O'Brien} ({CCOB)} effect, has been taken as evidence for a low-level 'filling-in' mechanism subserving lightness perception. Here, we present evidence that the mechanism responsible for the {CCOB} effect operates not via propagation of a neural signal across space but by amplification of the low spatial frequency ({SF)} structure of the image. We develop a simple computational model that relies on the statistics of natural scenes actively to reconstruct the image that is most likely to have caused an observed series of responses across {SF} channels. This principle is tested psychophysically by deriving classification images ({CIs)} for subjects' discrimination of the contrast polarity of {CCOB} stimuli masked with noise. {CIs} resemble 'filled-in' stimuli; i.e. observers rely on portions of the stimuli that contain no information per se but that correspond closely to the reported perceptual completion. As predicted by the model, the filling- in process is contingent on the presence of appropriate low {SF} structure.
Simmers AJ, Bex PJ, Hess RF. Perceived blur in amblyopia. Investigative Ophthalmlogy and Visual Science [Internet]. 2003;44:1395–400. Publisher's VersionAbstract
{PURPOSE:} The well-documented fact that visual acuity and contrast sensitivity in amblyopia are attenuated at high spatial frequencies predicts that amblyopes should perceive objects as blurred, because they do not have the high spatial frequency information necessary to represent sharp edges adequately. In the current study, the representation of blur in amblyopia with blur-discrimination and blur-matching tasks was explored in a series of experiments. {METHODS:} Monocular blur-discrimination thresholds were measured in a spatial two-alternative forced-choice procedure. Observers were required to discriminate which edge (right or left) appeared to be the lesser blurred. Observers also interocularly matched edges that were identical with those used in the blur-discrimination tasks, with the exception that they were viewed dichoptically at all times. {RESULTS:} Blur-discrimination thresholds were elevated in both the amblyopic and fellow fixing eyes but were within the normal range for interocular matching thresholds. {CONCLUSIONS:} The results suggest that blur is veridically represented in the amblyopic visual system. The surprising result is that all amblyopes, even those with the most severe visual loss, veridically matched all blurred edges, including the sharpest ones. This implies that amblyopes are able to represent levels of blur that are defined by spatial structure beyond their resolution limit.
Dakin SC, Bex PJ. Response to Wilson & Wilkinson: Evidence For Global Processing but No Evidence for Specialised Detectors in the Visual Processing of Glass Patterns. Vision Research. 2003;43:565–566.
Bex PJ, Simmers AJ, Dakin SC. Grouping local directional signals into moving contours. Vision Research [Internet]. 2003;43:2141–53. Publisher's VersionAbstract
We consider how local motion signals are combined to represent the movements of spatially extensive objects. A series of band-pass target dots, whose collective motion defined a moving contour, was positioned within a field of randomly moving noise dots. The visibility of the contours did not depend on the direction of movement relative to local contour orientation unless the contour was constrained to pass through fixation, suggesting that a previously reported advantage for collinear motion trajectories depends on the probability of detecting any of the target elements rather than the integrated contour. Contour visibility was invariant of the spatial frequency of the elements, but it did depend on the speed, number and spacing of elements defining it, as well as the angle and spatial frequency difference between adjacent elements. Local averaging of directional signals is not sufficient to explain these results. The visibility of these moving contours identifies narrow-band grouping processes that are sensitive to the shape defined by the directions of the elements forming the contour.
Bex PJ, Dakin SC. Motion detection and the coincidence of structure at high and low spatial frequencies. Vision Research [Internet]. 2003;43:371–83. Publisher's VersionAbstract
We used filtered random dot kinematograms and natural images to examine how motion detection depends the relative locations of structures defined at low and high spatial frequencies. The upper displacement limit of motion (D(max)), the lower displacement limit (D(min)) and motion coherence thresholds were unaffected by the degree of spatial coincidence between high and low spatial frequency structures i.e. whether they were consistent or inconsistent with a single feature. However motion detection was possible between band-pass filtered random dot patterns whose peak frequencies were separated by up to 4 octaves. The first result implicates spatial frequency selective motion detectors that operate independently. The second result implicates a motion system that can integrate the displacements of edges defined by widely separated spatial frequencies. Both are required to account for the two results, and they appear to operate under very similar conditions.
Bex PJ, Dakin SC, Simmers AJ. The shape and size of crowding for moving targets. Vision Research [Internet]. 2003;43:2895–904. Publisher's VersionAbstract
Our ability to identify alphanumeric characters can be impaired by the presence of nearby features, especially when the target is presented in the peripheral visual field, a phenomenon is known as crowding. We measured the effects of motion on acuity and on the spatial extent of crowding. In line with many previous studies, acuity decreased and crowding increased with eccentricity. Acuity also decreased for moving targets, but the absolute size of crowding zones remained relatively invariant of speed at each eccentricity. The two-dimensional shape of crowding zones was measured with a single flanking element on each side of the target. Crowding zones were elongated radially about central vision, relative to tangential zones, and were also asymmetrical: a more peripheral flanking element crowded more effectively than a more foveal one; and a flanking element that moved ahead of the target crowded more effectively than one that trailed behind it. These results reveal asymmetrical space-time dependent regions of visual integration that are radially organised about central vision.
2002
Bex PJ, Dakin SC. Comparison of the spatial-frequency selectivity of local and global motion detectors. Journal of the Optical Society of America [Internet]. 2002;19:670–7. Publisher's VersionAbstract
Convergent physiological and behavioral evidence indicates that the initial receptive fields responsible for motion detection are spatially localized. Consequently, the perception of global patterns of movement (such as expansion) requires that the output of these local mechanisms be integrated across visual space. We have differentiated local and global motion processes, with mixtures of coherent and incoherent moving patterns composed of bandpass filtered dots, and have measured their spatial-frequency selectivity. We report that local motion detectors show narrow-band spatial-frequency tuning (i.e., they respond only to a narrow range of spatial frequencies) but that global motion detectors show broadband spatial-frequency tuning (i.e., they integrate across a broad range of spatial frequencies), with a preference for low spatial frequencies.
Dakin SC, Bex PJ. Role of synchrony in contour binding: some transient doubts sustained. Journal of the Optical Society of America [Internet]. 2002;19:678–86. Publisher's VersionAbstract
The temporal correlation hypothesis proposes that neurons signal mutual inclusion in complex features, such as extended contours, by phase-locking their firing [C. M. Gray and W. Singer, Proc. Nat]. Acad. Sci. {USA} 86, 1698 (1989)]. Although this hypothesis remains controversial, a number of recent psychophysical studies have suggested that temporal correlation among features can indeed promote perceptual grouping. In particular, subjects are better at detecting extended visual contours embedded within a field of distractor elements when a small delay is present between a cycling presentation of the contour and the background [Nature 394, 179 (1988)]. We have replicated this finding and examined three potentially confounding factors. First, we controlled local density and used more curved contours composed of bandpass elements to confirm that the effect was associated with contour integration and not with the operation of coarse-scale spatial filters. Second, we minimized the effects of saccadic eye movements (which could combine with the flicker of the asynchronous display to introduce motion cues at the contour location) both by using a fixation marker that was visible only when observers made a saccade (allowing them to reject these trials) and by retinally stabilizing the stimulus. We report that eye movements contribute to the effect. Third, we asked if either visible persistence or transients at the onset and the offset of the asynchronous stimuli might contribute to the effect. We report that the effect is largely abolished by the inclusion of prestimulus and poststimulus masks and is entirely abolished by ramping the contrast of the stimulus on and off. Neither ramping, masking, nor stabilization should specifically disrupt a contour-binding scheme based on temporal synchrony, and we conclude that it is the transient component at the onset and the offset of these stimuli that is responsible for the reported advantage for asynchronous presentation.
Bex PJ, Makous W. Spatial frequency, phase, and the contrast of natural images. Journal of the Optical Society of America [Internet]. 2002;19:1096–106. Publisher's VersionAbstract
We examined contrast sensitivity and suprathreshold apparent contrast with natural images. The spatial-frequency components within single octaves of the images were removed (notch filtered), their phases were randomized, or the polarity of the images was inverted. Of Michelson contrast, root-mean-square ({RMS)} contrast, and band-limited contrast, {RMS} contrast was the best index of detectability. Negative images had lower apparent contrast than their positives. Contrast detection thresholds showed spatial-frequency-dependent elevation following both notch filtering and phase randomization. The peak of the spatial-frequency tuning function was approximately 0.5-2 cycles per degree (c/deg). Suprathreshold contrast matching functions also showed spatial-frequency-dependent contrast loss for both notch-filtered and phase-randomized images. The peak of the spatial-frequency tuning function was approximately 1-3 c/deg. There was no detectable difference between the effects of phase randomization and notch filtering on contrast sensitivity. We argue that these observations are consistent with changes in the activity within spatial-frequency channels caused by the higher-order phase structure of natural images that is responsible for the presence of edges and specularities.
Dakin SC, Bex PJ. Summation of concentric orientation structure: seeing the Glass or the window?. Vision Research [Internet]. 2002;42:2013–20. Publisher's VersionAbstract
Rotational Glass patterns are discriminable from noise at substantially lower signal-to-noise levels than translational patterns, a finding that has been attributed to the operation of concentrically tuned units in cortical area V4 (Wilson, Wilkinson, & Asaad, Vis. Res. 37 (17) (1997) 2325; Wilson & Wilkinson, Vis. Res. 38 (19) (1998) 2933). Under experimental conditions similar to Wilson et al. we found this advantage to be largely contingent on the pattern being viewed through a circular aperture. Because rotation of a random dot set cannot lead to the presence of unmatched dots at the boundary of a circular aperture, the integrity of low spatial frequency information at the boundary reliably indicates the presence of rotational, but not translational, structure. When we removed this cue, either using a square aperture or surrounding a round aperture with noise dots, none of the nine subjects tested showed any statistically significant advantage for rotational Glass patterns (although at least two did take longer to master the task with translational compared to rotational patterns). We go on to show generally similar patterns of global integration for both rotational and translational patterns. We conclude that this paradigm presently offers no concrete psychophysical evidence for specialised concentric orientation detectors.
2001
Simmers AJ, Bex PJ. Deficit of visual contour integration in dyslexia. Investigative Ophthalmlogy and Visual Science [Internet]. 2001;42:2737–42. Publisher's VersionAbstract
{PURPOSE:} The visual processing of text occurs spontaneously in most readers. Dyslexic persons, however, often report both somatic symptoms and perceptual distortions when trying to read. It is possible that the perceptual distortions experienced by those with dyslexia reflect a disturbance in the basic mechanisms supporting perceptual organization at the early stages of visual processing. Integration of information over extended areas of visual space can be measured psychophysically in a task that requires the detection of a path defined by aligned, spatially narrow-band elements on a dense field of otherwise similar elements that are randomly oriented and positioned. In the present study a contour integration task was used to investigate such perceptual organization in dyslexia. {METHODS:} The detection of contours or paths composed of Gabor micropatterns was performed within a field of randomly oriented distracter elements in a 2-alternate forced choice ({AFC)} task. The stimuli were manipulated by randomly varying both the density of the background noise elements and the number of elements that defined a path of constant length. {RESULTS:} In all observers, sensitivity to the paths increased with the number of target elements comprising the path, and subjects in both groups exhibited similar trends in relative density of the stimuli. However, in all conditions, dyslexic observers were two to three times less sensitive to path stimuli than the control group. {CONCLUSIONS:} In the present study the authors have described a visual deficit in a global integration task in dyslexia. The pattern of deficits reported suggest that abnormal cooperative associations may be present in dyslexia that are indicative of poor perceptual integration.
Dakin SC, Bex PJ. Local and global visual grouping: tuning for spatial frequency and contrast. Journal of Vision [Internet]. 2001;1:99–111. Publisher's VersionAbstract
Glass patterns are visual textures composed of a field of dot pairs (dipoles) whose orientations are determined by a simple geometrical transformation, such as a rotation. Detection of structure in these patterns requires the observer to perform local grouping (to find dipoles) and global grouping to combine their orientations into a percept of overall shape. We estimated the spatial frequency tuning of these grouping processes by measuring signal-to-noise detection thresholds for Glass patterns composed of spatially narrow-band elements. Local tuning was probed by varying the spatial frequency difference between the two elements comprising each dipole. Global tuning was estimated using dipoles containing one spatial frequency and then estimating masking as a function of the spatial frequency of randomly positioned noise elements. We report that the tuning of local grouping is band-pass (ie, it is responsive to a narrow range of spatial frequencies), but that tuning of global grouping is broad and low-pass (ie, it integrates across a broader range of lower spatial frequencies). Control experiments examined how the contrast and visibility of elements might contribute to these findings. Local grouping proved to be more resistant to local contrast variation than global grouping. We conclude that local grouping is consistent with the use of simple-oriented filtering mechanisms. Global grouping seems to depend more on the visibility of elements that can be affected by both spatial frequency and contrast.
Simmers AJ, Bex PJ, Smith FK, Wilkins AJ. Spatiotemporal visual function in tinted lens wearers. Investigative Ophthalmlogy and Visual Science [Internet]. 2001;42:879–84. Publisher's VersionAbstract
{PURPOSE.} Tinted lenses have been widely publicized as a successful new treatment for reading disorders and visual stress in children. The present study was designed to investigate a variety of visual deficits reported by children who experience high levels of visual stress and perceptual distortions when reading (Meares-Irlen syndrome; {MIS)} and to assess the improvements in visual comfort they report when tinted lenses are worn. {METHODS.} Twenty children (13.1 +/- 0.9 years of age) were recruited who had successfully worn tinted lenses for at least 6 months and were compared with an age-matched control group (12.6 +/- 2.2 years of age) of 21 children who were not lens wearers. A range of psychophysical tasks was adapted to identify specific anomalous visual perceptions. Spatiotemporal contrast sensitivity and contrast increment thresholds were used to investigate subjective reports of dazzle and hypercontrast, and a minimum motion perception (D(min)) and a motion-coherence task were used to assess subjective reports of visual instability and motion. {RESULTS.} In all viewing conditions (with versus without lens), no selective functional visual loss was demonstrated with any of the tasks used. Psychometric functions also revealed no significant difference between subject groups (control versus {MIS).} {CONCLUSIONS.} Under thorough psychophysical investigation, these results revealed no significant difference in visual function between subject group, and this finding is consistent with the absence of any effect of the tinted lenses in the group with {MIS.}
Bex PJ, Simmers AJ, Dakin SC. Snakes and ladders: the role of temporal modulation in visual contour integration. Vision Research [Internet]. 2001;41:3775–82. Publisher's VersionAbstract
We investigated temporal aspects of the cortical mechanisms supporting visual contour integration by measuring observers' efficiency at detecting fragmented contours, composed of Gabor micropatterns, embedded in a field of distractor elements. Gabors consisted of a static Gaussian enveloping a sinusoidal carrier which was temporally modulated by motion or counter-phase flicker. The elements forming the path could be oriented either parallel ('snakes') or perpendicular to the contour orientation ('ladders'). Sensitivity to contour structure (estimated by measuring the maximum tolerable element orientation jitter supporting contour detection) was increased when the elements were drifting or flickering. Snakes were more detectable than ladders under all conditions. The increase in sensitivity conferred by drifting carriers was present even when the elements in the same stimulus were drifting at a range of speeds spanning almost three octaves. These results lend further support to the notion that the contour integration system receives separate transient and sustained input.
1999
Bex PJ. Apparent speed and speed sensitivity during adaptation to motion. Journal of the Optical Society of America. 1999;16:2817–2824.Abstract
Adaptation, a change in response to a sustained stimulus, can be demonstrated in motion perception by velocity after effects- changes in the apparent speed of a moving pattern following adaptation. We measured changes in the apparent speed of sinusoidal gratings drifting at 4 or 7.5 deg/s during 30 s of adaptation followed by 30 s of recovery. The apparent speed of the patterns fell to approximately half the unadapted apparent speed, and the time constants of adaptation were much faster (5 s) than for recovery (22 s). Part of the loss of apparent speed (approximately 12%) was related to a loss of apparent contrast with adaptation. Sensitivity to speed increments and speed decrements increased during adaptation and was well described by a Weber fraction based on apparent speed. The results suggest that adaptation to motion, like light adaptation, may serve to improve an observer's sensitivity to the prevailing environment. (C) 1999 Optical Society of America [S0740- 3232(99)00612-2].