Publications

2012
Greenwood JA, Tailor VK, Sloper JJ, Simmers AJ, Bex PJ, Dakin SC. Visual acuity, crowding, and stereo-vision are linked in children with and without amblyopia. Investigative Ophthalmology & Visual Science. 2012;53:7655–7665.Abstract
Purpose: During development, the presence of strabismus and anisometropia frequently leads to amblyopia, a visual disorder characterised by interocular acuity differences. Though additional deficits in contrast sensitivity, crowding (the impaired recognition of closely-spaced objects), and stereo-acuity are common, the relationship between these abilities is unclear. Method: We measured the co-variation between these four abilities in children aged 4-9 (n=72) with strabismus, anisometropia, or mixed strabismus/anisometropia, and unaffected controls. Children reported the orientation of a target (a modified "Pac-Man", similar to Landolt-C stimuli) using four "ghosts" as references. Using a modified staircase procedure we measured threshold size (acuity), contrast-detection, foveal crowding (the minimum separation between target and ghost-flankers supporting accurate identification), and stereo-acuity (with random-dot stereogram ghosts). Results: Group averages revealed significant interocular differences (IODs) in acuity for all three clinical groups (0.2-0.3 log minutes), and significant crowding IODs for the strabismic and mixed groups (0.6 and 0.4 degrees, respectively). Nonetheless, crowding IODs were correlated with acuity IODs in all four groups (r values between 0.43-0.59 and p<.05, albeit p=0.07 in the mixed group). Similarly, the occurrence of stereo-blindness (most common in strabismic and mixed groups) was associated with a significant increase in IODs for both acuity and crowding (each p<.05). No correlations were found with contrast detection. Conclusions: Our results demonstrate an association between IODs in acuity and crowding, and furthermore between these IODs and the presence of stereo-vision. We suggest that the deficits derived from strabismus and anisometropia lay along a continuum with the abilities observed during normal development.
Cass J, Johnson A, Bex PJ, Alais D. Orientation-Specificity of Adaptation: Isotropic Adaptation Is Purely Monocular. PloS one. 2012;7:e47425.Abstract
Numerous studies have found that prolonged exposure to grating stimuli reduces sensitivity to subsequently presented gratings, most evidently when the orientations of the adapting and test patterns are similar. The rate of sensitivity loss varies with angular difference indicating both the presence and bandwidths of psychophysical ‘orientation channels’. Here we study the orientation dependency of contrast adaptation measured both monoptically and dichoptically. Earlier psychophysical reports show that orientation bandwidths are broader at lower spatial frequencies, and we confirm this with a simple von Mises model using 0.25 vs. 2 c.p.d. gratings. When a single isotropic (orientation invariant) parameter is added to this model, however, we find no evidence for any difference in bandwidth with spatial frequency. Consistent with cross-orientation masking effects, we find isotropic adaptation to be strongly low spatial frequency-biased. Surprisingly, unlike masking, we find that the effects of interocular adaptation are purely orientation-tuned, with no evidence of isotropic threshold elevation. This dissociation points to isotropic (or ‘cross-orientation’) adaptation being an earlier and more magnocellular-like process than that which supports orientation-tuned adaptation and suggests that isotropic masking and adaptation are likely mediated by separate mechanisms.
Wallis TSA, Bex PJ. Image correlates of crowding in natural scenes. Journal of Vision. 2012;12.Abstract
Visual crowding is the inability to identify visible features when they are surrounded by other structure in the peripheral field. Since natural environments are replete with structure and most of our visual field is peripheral, crowding represents the primary limit on vision in the real world. However, little is known about the characteristics of crowding under natural conditions. Here we examine where crowding occurs in natural images. Observers were required to identify which of four locations contained a patch of "dead leaves'' (synthetic, naturalistic contour structure) embedded into natural images. Threshold size for the dead leaves patch scaled with eccentricity in a manner consistent with crowding. Reverse correlation at multiple scales was used to determine local image statistics that correlated with task performance. Stepwise model selection revealed that local RMS contrast and edge density at the site of the dead leaves patch were of primary importance in predicting the occurrence of crowding once patch size and eccentricity had been considered. The absolute magnitudes of the regression weights for RMS contrast at different spatial scales varied in a manner consistent with receptive field sizes measured in striate cortex of primate brains. Our results are consistent with crowding models that are based on spatial averaging of features in the early stages of the visual system, and allow the prediction of where crowding is likely to occur in natural images.
Wiecek E, Jackson ML, Dakin SC, Bex P. Visual search with image modification in age-related macular degeneration. Investigative Ophthalmology & Visual Science. 2012;53:6600–6609.Abstract
Purpose. AMD results in loss of central vision and a dependence on low-resolution peripheral vision. While many image enhancement techniques have been proposed, there is a lack of quantitative comparison of the effectiveness of enhancement. We developed a natural visual search task that uses patients' eye movements as a quantitative and functional measure of the efficacy of image modification. Methods. Eye movements of 17 patients (mean age = 77 years) with AMD were recorded while they searched for target objects in natural images. Eight different image modification methods were implemented and included manipulations of local image or edge contrast, color, and crowding. In a subsequent task, patients ranked their preference of the image modifications. Results. Within individual participants, there was no significant difference in search duration or accuracy across eight different image manipulations. When data were collapsed across all image modifications, a multivariate model identified six significant predictors for normalized search duration including scotoma size and acuity, as well as interactions among scotoma size, age, acuity, and contrast (P < 0.05). Additionally, an analysis of image statistics showed no correlation with search performance across all image modifications. Rank ordering of enhancement methods based on participants' preference revealed a trend that participants preferred the least modified images (P < 0.05). Conclusions. There was no quantitative effect of image modification on search performance. A better understanding of low- and high-level components of visual search in natural scenes is necessary to improve future attempts at image enhancement for low vision patients. Different search tasks may require alternative image modifications to improve patient functioning and performance.
McIlreavy L, Fiser J, Bex PJ. Impact of simulated central scotomas on visual search in natural scenes. Optometry & Vision Science. 2012;89:1385–1394.Abstract
Purpose. In performing search tasks, the visual system encodes information across the visual field at a resolution inversely related to eccentricity and deploys saccades to place visually interesting targets upon the fovea, where resolution is highest. The serial process of fixation, punctuated by saccadic eye movements, continues until the desired target has been located. Loss of central vision restricts the ability to resolve the high spatial information of a target, interfering with this visual search process. We investigate oculomotor adaptations to central visual field loss with gaze-contingent artificial scotomas. Methods. Spatial distortions were placed at random locations in 25° square natural scenes. Gaze-contingent artificial central scotomas were updated at the screen rate (75 Hz) based on a 250 Hz eye tracker. Eight subjects searched the natural scene for the spatial distortion and indicated its location using a mouse-controlled cursor. Results. As the central scotoma size increased, the mean search time increased [F(3,28) = 5.27, p = 0.05], and the spatial distribution of gaze points during fixation increased significantly along the x [F(3,28) = 6.33, p = 0.002] and y [F(3,28) = 3.32, p = 0.034] axes. Oculomotor patterns of fixation duration, saccade size, and saccade duration did not change significantly, regardless of scotoma size. Conclusions. There is limited automatic adaptation of the oculomotor system after simulated central vision loss.
Wiecek E, Pasquale LR, Fiser J, Dakin S, Bex PJ. Effects of peripheral visual field loss on eye movements during visual search. Frontiers in psychology. 2012;3.Abstract
Natural vision involves sequential eye movements that bring the fovea to locations selected by peripheral vision. How peripheral visual field loss (PVFL) affects this process is not well understood. We examine how the location and extent of PVFL affects eye movement behavior in a naturalistic visual search task. Ten patients with PVFL and 13 normally sighted subjects with full visual fields (FVF) completed 30 visual searches monocularly. Subjects located a 4° × 4° target, pseudo-randomly selected within a 26° × 11° natural image. Eye positions were recorded at 50 Hz. Search duration, fixation duration, saccade size, and number of saccades per trial were not significantly different between PVFL and FVF groups (p > 0.1). A χ2 test showed that the distributions of saccade directions for PVFL and FVL subjects were significantly different in 8 out of 10 cases (p < 0.01). Humphrey Visual Field pattern deviations for each subject were compared with the spatial distribution of eye movement directions. There were no significant correlations between saccade directional bias and visual field sensitivity across the 10 patients. Visual search performance was not significantly affected by PVFL. An analysis of eye movement directions revealed patients with PVFL show a biased directional distribution that was not directly related to the locus of vision loss, challenging feed-forward models of eye movement control. Consequently, many patients do not optimally compensate for visual field loss during visual search.
Greenwood JA, Bex PJ, Dakin SC. Crowding follows the binding of relative position and orientation. Journal of vision. 2012;12.Abstract
Crowding—the deleterious influence of clutter on object recognition—disrupts the identification of visual features as diverse as orientation, motion, and color. It is unclear whether this occurs via independent feature-specific crowding processes (preceding the feature binding process) or via a singular (late) mechanism tuned for combined features. To examine the relationship between feature binding and crowding, we measured interactions between the crowding of relative position and orientation. Stimuli were a target cross and two flanker crosses (each composed of two near-orthogonal lines), 15 degrees in the periphery. Observers judged either the orientation (clockwise/counterclockwise) of the near-horizontal target line, its position (up/down relative to the stimulus center), or both. For single-feature judgments, crowding affected position and orientation similarly: thresholds were elevated and responses biased in a manner suggesting that the target appeared more like the flankers. These effects were tuned for orientation, with near-orthogonal elements producing little crowding. This tuning allowed us to separate the predictions of independent (feature specific) and combined (singular) models: for an independent model, reduced crowding for one feature has no effect on crowding for other features, whereas a combined process affects either all features or none. When observers made conjoint judgments, a reduction of orientation crowding (by increasing target–flanker orientation differences) increased the rate of correct responses for both position and orientation, as predicted by our combined model. In contrast, our independent model incorrectly predicted a high rate of position errors, since the probability of positional crowding would be unaffected by changes in orientation. Thus, at least for these features, crowding is a singular process that affects bound position and orientation values in an all-or-none fashion.
Wardle SG, Bex PJ, Cass J, Alais D. Stereoacuity in the periphery is limited by internal noise. Journal of Vision. 2012;12.Abstract
It is well-established that depth discrimination is finer in the fovea than the periphery. Here, we study the decline in depth discrimination thresholds with distance from the fovea using an equivalent noise analysis to separate the contributions of internal noise and sampling efficiency. Observers discriminated the mean depth of patches of “dead leaves” composed of ellipses varying in size, orientation, and luminance at varying levels of disparity noise between 0.05 and 13.56 arcmin and visual field locations between 0° and 9° eccentricity. At low levels of disparity noise, depth discrimination thresholds were lower in the fovea than in the periphery. At higher noise levels (above 3.39 arcmin), thresholds converged, and there was little difference between foveal and peripheral depth discrimination. The parameters estimated from the equivalent noise model indicate that an increase in internal noise is the limiting factor in peripheral depth discrimination with no decline in sampling efficiency. Sampling efficiency was uniformly low across the visual field. The results indicate that a loss of precision of local disparity estimates early in visual processing limits fine depth discrimination in the periphery.
2011
Dartt DA, Bex P, {D'Amore} P, Dana R, Mcloon L, Niederkorn J. Ocular Periphery and Disorders. 1st ed. Academic Press; 2011.
Dorr M, Bex P. A gaze-contingent display to study contrast sensitivity under natural viewing conditions, in Human Vision and Electronic Imaging XVI.Vol 7865. SPIE-IS&T; 2011:Y1–8.Abstract
Contrast sensitivity has been extensively studied over the last decades and there are well-established models of early vision that were derived by presenting the visual system with synthetic stimuli such as sine-wave gratings near threshold contrasts. Natural scenes, however, contain a much wider distribution of orientations, spatial frequencies, and both luminance and contrast values. Furthermore, humans typically move their eyes two to three times per second under natural viewing conditions, but most laboratory experiments require subjects to maintain central fixation. We here describe a gaze-contingent display capable of performing real-time contrast modulations of video in retinal coordinates, thus allowing us to study contrast sensitivity when dynamically viewing dynamic scenes. Our system is based on a Laplacian pyramid for each frame that efficiently represents individual frequency bands. Each output pixel is then computed as a locally weighted sum of pyramid levels to introduce local contrast changes as a function of gaze. Our GPU implementation achieves real-time performance with more than 100 fps on high-resolution video (1920 by 1080 pixels) and a synthesis latency of only 1.5ms. Psychophysical data show that contrast sensitivity is greatly decreased in natural videos and under dynamic viewing conditions. Synthetic stimuli therefore only poorly characterize natural vision.
Elze T, Song C, Stollhoff R, Jost J. Chinese Characters Reveal Impacts of Prior Experience on Very Early Stages of Perception. BMC Neuroscience. 2011;12:14.Abstract
Background: Visual perception is strongly determined by accumulated experience with the world, which has been shown for shape, color, and position perception, in the field of visuomotor learning, and in neural computation. In addition, visual perception is tuned to statistics of natural scenes. Such prior experience is modulated by neuronal top-down control the temporal properties of which had been subject to recent studies. Here, we deal with these temporal properties and address the question how early in time accumulated past experience can modulate visual perception. Results: We performed stimulus discrimination experiments and compared a group of Chinese participants with a German control group. The perception of our briefly presented visual objects (targets) was disturbed by masking stimuli which appeared in close spatiotemporal proximity. These masking stimuli were either intact or scrambled Chinese characters and did not overlap with the targets. In contrast to German controls, Chinese participants show substantial performance differences for real versus scrambled Chinese characters if these masking stimuli were presented as early as less than 100 milliseconds after the onset of the target. For Chinese observers, it even occured that meaningful masking stimuli enhanced target identification if they were shown at least 100 milliseconds after target onset while the same stimuli impaired recognition if presented in close temporal proximity to the target. The latter finding challenges interpretations of our data that solely rely on stimulus contours or geometric properties and emphasizes the impact of prior experience on the very early temporal dynamics of the visual system. Conclusions: Our findings demonstrate that prior experience which had been accummulated long before the experiments can modulate the time course of perception intriguingly early, namely already immediately after the perceptual onset of a visual event. This modulation cannot solely operate as a feedback in response to the visual event but is rather a permanent effect.
Dakin SC, Greenwood JA, Carlson TA, Bex PJ. Crowding is tuned for perceived (not physical) location. Journal of Vision [Internet]. 2011;11. Publisher's VersionAbstract
In the peripheral visual field, nearby objects can make one another difficult to recognize ( crowding) in a manner that critically depends on their separation. We manipulated the apparent separation of objects using the illusory shifts in perceived location that arise from local motion to determine if crowding depends on physical or perceived location. Flickering Gabor targets displayed between either flickering or drifting flankers were used to (a) quantify the perceived target–flanker separation and (b) measure discrimination of the target orientation or spatial frequency as a function of physical target–flanker separation. Relative to performance with flickering targets, we find that flankers drifting away from the target improve discrimination, while those drifting toward the target degrade it. When plotted as a function of perceived separation across conditions, the data collapse onto a single function indicating that it is perceived and not physical location that determines the magnitude of visual crowding. There was no measurable spatial distortion of the target that could explain the effects. This suggests that crowding operates predominantly in extrastriate visual cortex and not in early visual areas where the response of neurons is retinotopically aligned with the physical position of a stimulus.
Kane D, Bex P, Dakin S. Quantifying “the aperture problem” for judgments of motion direction in natural scenes. Journal of Vision. 2011;11.
Wallis TSA, Bex PJ. Visual Crowding is Correlated with Awareness. Current Biology. 2011;21:254–258.Abstract
Crowding by nearby features causes identification failures in the peripheral visual field. However, prominent visual features can sometimes fail to reach awareness [1, 2], causing scenes to be incorrectly interpreted. Here we examine whether awareness of the flanking features is necessary for crowding to occur. Flankers that were physically present were rendered perceptually absent with adaptation-induced blindness [3]. In a letter identification task, targets were presented unflanked or with up to four flanker letters. On each trial, observers reported both the number of letters they perceived and the identity of a target letter. This paradigm allowed trial-by-trial assessment of awareness and crowding, and ensured that both targets and flankers were fully attended. Target letter identification performance was dependent on correlated with the number of flanking letters that were perceived on a given trial, regardless of the number that were physically present. Our data demonstrate that crowding can be released when flanking elements at attended locations are suppressed from visual awareness.
Schneider N, Barth E, Bex P, Dorr M. An open-source low-cost eye-tracking system for portable real-time and offline tracking, in Proceedings of the 2011 Conference on Novel Gaze-Controlled Applications.; 2011:8:1-4.
2010
Bex PJ. Contrast Sensitivity. In Encyclopedia of the Eye Academic Press; 2010.
Greenwood JA, Bex PJ, Dakin SC. Crowding Changes Appearance. Current Biology [Internet]. 2010;20:496–501. Publisher's VersionAbstract
Crowding is the breakdown in object recognition that occurs in cluttered visual environments [1-4] and the fundamental limit on peripheral vision, affecting identification within many visual modalities [5-9] and across large spatial regions [10]. Though frequently characterized as a disruptive process through which object representations are suppressed [11, 12] or lost altogether [13-15], we demonstrate that crowding systematically changes the appearance of objects. In particular, target patches of visual noise that are surrounded ("crowded") by oriented Gabor flankers become perceptually oriented, matching the flankers. This was established with a change-detection paradigm: under crowded conditions, target changes from noise to Gabor went unnoticed when the Gabor orientation matched the flankers (and the illusory target percept), despite being easily detected when they differed. Rotation of the flankers (leaving target noise unaltered) also induced illusory target rotations. Blank targets led to similar results, demonstrating that crowding can induce apparent structure where none exists. Finally, adaptation to these stimuli induced a tilt aftereffect at the target location, consistent with signals from the flankers "spreading" across space. These results confirm predictions from change-based models of crowding, such as averaging [16], and establish crowding as a regularization process that simplifies the peripheral field by promoting consistent appearance among adjacent objects.
Murray S, Bex PJ. Perceived blur in naturally contoured images depends on phase. Frontiers in Psychology [Internet]. 2010;1:185. Publisher's VersionAbstract
Perceived blur is an important measure of image quality and clinical visual function. The magnitude of image blur varies across space and time under natural viewing conditions owing to changes in pupil size and accommodation. Blur is frequently studied in the laboratory with a variety of digital filters, without comparing how the choice of filter affects blur perception. We examine the perception of image blur in synthetic images composed of contours whose orientation and curvature spatial properties matched those of natural images but whose blur could be directly controlled. The images were blurred by manipulating the slope of the amplitude spectrum, Gaussian low-pass filtering or filtering with a Sinc function, which, unlike slope or Gaussian filtering, introduces periodic phase reversals similar to those in optically blurred images. For slope-filtered images, blur discrimination thresholds for over-sharpened images were extremely high and perceived blur could not be matched with either Gaussian or Sinc filtered images, suggesting that directly manipulating image slope does not simulate the perception of blur. For Gaussian- and Sinc-blurred images, blur discrimination thresholds were dipper-shaped and were well-fit with a simple variance discrimination model and with a contrast detection threshold model, but the latter required different contrast sensitivity functions for different types of blur. Blur matches between Gaussian- and Sinc-blurred images were used to test several models of blur perception and were in good agreement with models based on luminance slope, but not with spatial frequency based models. Collectively, these results show that the relative phases of image components, in addition to their relative amplitudes, determines perceived blur.
Dakin SC, Cass J, Greenwood JA, Bex PJ. Probabilistic, positional averaging predicts object-level crowding effects with letter-like stimuli. Journal of Vision. 2010;10:1–16.
Bex PJ. (In) Sensitivity to spatial distortion in natural scenes. Journal of Vision [Internet]. 2010;10:1–15. Publisher's VersionAbstract
The perception of object structure in the natural environment is remarkably stable under large variation in image size and projection, especially given our insensitivity to spatial position outside the fovea. Sensitivity to periodic spatial distortions that were introduced into one quadrant of gray-scale natural images was measured in a {4AFC} task. Observers were able to detect the presence of distortions in unfamiliar images even though they did not significantly affect the amplitude spectrum. Sensitivity depended on the spatial period of the distortion and on the image structure at the location of the distortion. The results suggest that the detection of distortion involves decisions made in the late stages of image perception and is based on an expectation of the typical structure of natural scenes