Publications

2009
Kane D, Bex PJ, Dakin SC. The aperture problem in contoured stimuli. Journal of Vision [Internet]. 2009;9:13 1–17. Publisher's VersionAbstract
A moving object elicits responses from V1 neurons tuned to a broad range of locations, directions, and spatiotemporal frequencies. Global pooling of such signals can overcome their intrinsic ambiguity in relation to the object's direction/speed (the "aperture problem"); here we examine the role of low-spatial frequencies ({SF)} and second-order statistics in this process. Subjects made a {2AFC} fine direction-discrimination judgement of 'naturally' contoured stimuli viewed rigidly translating behind a series of small circular apertures. This configuration allowed us to manipulate the scene by randomly switching which portion of the stimulus was presented behind each aperture or by occluding certain spatial frequency bands. We report that global motion integration is (a) largely insensitive to the second-order statistics of such stimuli and (b) is rigidly broadband even in the presence of a disrupted low {SF} component.
Bex PJ, Solomon SG, Dakin SC. Contrast sensitivity in natural scenes depends on edge as well as spatial frequency structure. Journal of Vision [Internet]. 2009;9:1 1–19. Publisher's VersionAbstract
The contrast sensitivity function is routinely measured in the laboratory with sine-wave gratings presented on homogenous gray backgrounds; natural images are instead composed of a broad range of spatial and temporal structures. In order to extend channel-based models of visual processing to more natural conditions, we examined how contrast sensitivity varies with the context in which it is measured. We report that contrast sensitivity is quite different under laboratory than natural viewing conditions: adaptation or masking with natural scenes attenuates contrast sensitivity at low spatial and temporal frequencies. Expressed another way, viewing stimuli presented on homogenous screens overcomes chronic adaptation to the natural environment and causes a sharp, unnatural increase in sensitivity to low spatial and temporal frequencies. Consequently, the standard contrast sensitivity function is a poor indicator of sensitivity to structure in natural scenes. The magnitude of masking by natural scenes is relatively independent of local contrast but depends strongly on the density of edges even though neither greatly affects the local amplitude spectrum. These results suggest that sensitivity to spatial structure in natural scenes depends on the distribution of local edges as well as the local amplitude spectrum.
Dakin SC, Bex PJ, Cass JR, Watt RJ. Dissociable effects of attention and crowding on orientation averaging. Journal of Vision [Internet]. 2009;9:28 1–16. Publisher's VersionAbstract
It has been proposed that visual crowding-the breakdown in recognition that occurs when objects are presented in cluttered scenes-reflects a limit imposed by visual attention. We examined this idea in the context of an orientation averaging task, having subjects judge the mean orientation of a set of oriented signal elements either in isolation, or "crowded" by nearby randomly oriented elements. In some conditions, subjects also had to perform an attentionally demanding secondary task. By measuring performance at different levels of signal orientation variability, we show that crowding increases subjects' local uncertainty (about the orientation of individual elements) but that diverting attention reduces their global efficiency (the effective number of elements they can average over). Furthermore, performance with the same stimulus-sequence, presented multiple times, reveals that crowding does not induce more stimulus-independent variability (as would be predicted by some accounts based on attention). We conclude that crowding and attentional load have dissociable perceptual consequences for orientation averaging, suggesting distinct neural mechanisms for both. For the task we examined, attention can modulate the effects of crowding by changing the efficiency with which information is analyzed by the visual system but since crowding changes local uncertainty, not efficiency, crowding does not reflect an attentional limit.
Cass J, Stuit S, Bex P, Alais D. Orientation bandwidths are invariant across spatiotemporal frequency after isotropic components are removed. Journal of Vision [Internet]. 2009;9:17 1–14. Publisher's VersionAbstract
It is well established that mammalian visual cortex possesses a large proportion of orientation-selective neurons. Attempts to measure the bandwidth of these mechanisms psychophysically have yielded highly variable results ( approximately 6 degrees -180 degrees ). Two stimulus factors have been proposed to account for this variability: spatial and temporal frequency; with several studies indicating broader bandwidths at low spatial and high temporal frequencies. We estimated orientation bandwidths using a classic overlay masking paradigm across a range of spatiotemporal frequencies (0.5, 2, and 8 c.p.d.; 1.6 and 12.5 Hz) with target and mask presented either monoptically or dichoptically. A standard three-parameter Gaussian model (amplitude and width, mean fixed at 0 degrees ) confirms that bandwidths generally increase at low spatial and high temporal frequencies. When incorporating an additional orientation-untuned (isotropic) amplitude component, however, we find that not only are the amplitudes of isotropic and orientation-tuned components highly dependent upon stimulus spatiotemporal frequency, but orientation bandwidths are highly invariant ( approximately 30 degrees half width half amplitude). These results suggest that previously reported spatiotemporally contingent bandwidth effects may have confounded bandwidth with isotropic (so-called cross-orientation) masking. Interestingly, the magnitudes of all monoptically derived parameter estimates were found to transfer dichoptically suggesting a cortical locus for both isotropic and orientation-tuned masking.
Greenwood JA, Bex PJ, Dakin SC. Positional averaging explains crowding with letter-like stimuli. Proceedings of the National Academy of Sciences {USA} [Internet]. 2009;106:13130–5. Publisher's VersionAbstract
Visual crowding is a breakdown in object identification that occurs in cluttered scenes, a process that represents the principle restriction on visual performance in the periphery. When crowded objects are presented experimentally, a key finding is that observers frequently report nearby flanking items instead of the target. This observation has led to the proposal that crowding reflects increased noise in the positional code for objects; although how the presence of nearby objects might disrupt positional encoding remains unclear. We quantified this disruption using cross-like stimuli, where observers judged whether the horizontal target line was positioned above or below the stimulus midpoint. Overall, observers were poorer at judging position in the presence of crowding flankers. However, offsetting horizontal lines in the flankers also led observers to report that the horizontal line in the target was shifted in the same direction, an effect that held for subthreshold flanker offsets. In short, crowding induced both random and systematic errors in observers' judgment of position, with or without the detection of flanker structure. Computational modeling reveals that perceived position in the presence of flankers follows a weighted average of noisy target- and flanker-line positions, rather than a substitution of flanker-features into the target, as has been proposed previously. Together, our results suggest that crowding is a preattentive process that uses averaging to regularize the noisy representation of position in the periphery.
Crossland MD, Bex PJ. Spatial alignment over retinal scotomas. Investigative Ophthalmlogy and Visual Science [Internet]. 2009;50:1464–9. Publisher's VersionAbstract
{PURPOSE:} Perceptual completion can mask the presence of physiological and pathologic retinal scotomas. This psychophysical study used a spatial alignment task to examine the processes underlying this perceptual completion. Similarities between the completion of pathologic and physiological scotomas would be consistent with large-scale reorganization of the visual system in eye disease {METHODS:} In five control subjects with no eye disease, Vernier alignment thresholds were measured over the physiological blind spot at the optic nerve head and over equally eccentric temporal retina. For nine subjects with retinal scotomas, alignment thresholds were measured over the maximum vertical extent of the larger scotoma in one eye and at an equal separation and eccentricity in the eye with a smaller or no scotoma {RESULTS:} In control subjects, alignment thresholds were better over the physiological blind spot than over equally eccentric temporal retina (P {\textless} 0.05). Alignment thresholds were no better over pathologic retinal scotomas than more intact, equally eccentric retina (P = 0.9) {CONCLUSIONS:} These quantitative differences implicate different mechanisms for perceptual completion over pathologic and physiological retinal scotomas. Filling in across pathologic scotomas appears to involve higher level image processing-based mechanisms that operate even when their input is interrupted. Filling-in at the optic nerve head involves additional low-level processes that may be hardwired, in which receptive fields span the blind spot and support fine orientation discriminations. These results argue against low-level reorganization of the visual system in people with retinal disease.
Cass J, Alais D, Spehar B, Bex PJ. Temporal whitening: transient noise perceptually equalizes the 1/f temporal amplitude spectrum. Journal of Vision [Internet]. 2009;9:12 1–19. Publisher's VersionAbstract
Naturally occurring luminance distributions are approximately 1/f in their spatial and temporal amplitude spectra. By systematically varying the spatio-temporal profile of broadband noise stimuli, we demonstrate that humans invariably overestimate the proportion of high spatial and temporal frequency energy. Critically, we find that that the strength of this bias is of a magnitude that predicts a perceptually equalized response to the spatio-temporal fall off in the natural amplitude spectrum. This interpretation is supported by our finding that the magnitude of this transient response bias, while evident across a broad range of narrowband spatial frequencies (0.25-8 cycles/deg), decreases above 2 cycles/deg, which itself compensates for the increase in temporal frequency energy previously observed at high spatial frequencies as a consequence of small fixational eye movements (M. Rucci, R. Iovin, M. Poletti, & F. Santini, 2007). Additional temporal masking and adaptation experiments reveal a transiently biased asymmetry. Whereas temporal frequencies {\textgreater}4 Hz mask and adapt 1- and 15-Hz targets, lower masking and adaptation frequencies have much less effect on sensitivity to 15-Hz compared with 1-Hz targets. These results imply that the visual system over-represents its transient input to an extent that predicts an equalized temporal channel response to the low-frequency-biased structure of natural scenes.
2008
Mareschal I, Bex P, Dakin S. Local motion processing limits fine direction discrimination in the periphery. Vision Research [Internet]. 2008;48:1719–1725. Publisher's Version
Crossland MD, Bex PJ. The twinkle aftereffect is pre-cortical and is independent of filling-in. Journal of Vision [Internet]. 2008;8:13 1–10. Publisher's VersionAbstract
A real or artificial scotoma within a dynamic noise field fills in within a few seconds. When the dynamic noise is replaced with a homogenous field, a twinkling after effect ({TwAE)} is induced exclusively in the location of the former scotoma. We are employing the appearance of the {TwAE} to perform rapid perimetry in patients with retinal scotomas. To analyze the loci within the visual system and the mechanisms of filling-in and the {TwAE}, we examined their orientation tuning, inter-ocular transfer, and threshold versus contrast functions by measuring contrast detection thresholds for stimuli presented in areas that were filled-in or contained the {TwAE.} For filling-in, detection thresholds were narrowly tuned for orientation, transferred interocularly, and rose monotonically with the contrast of a surround pattern. These results indicate that surround suppression and filling-in involve inhibitory processes originating at cortical stages of visual processing. Threshold versus contrast functions were weakly dipper-shaped for the {TwAE}, did not transfer inter-ocularly, and were not tuned for orientation. These results indicate that the {TwAE} involves additive noise that is pre-cortical in origin and that it is distinct from filling-in.
2007
Falkenberg HK, Rubin GS, Bex PJ. Acuity, crowding, reading and fixation stability. Vision Research [Internet]. 2007;47:126–35. Publisher's VersionAbstract
People with age-related macular disease frequently experience reading difficulty that could be attributed to poor acuity, elevated crowding or unstable fixation associated with peripheral visual field dependence. We examine how the size, location, spacing and instability of retinal images affect the visibility of letters and words at different eccentricities. Fixation instability was simulated in normally sighted observers by randomly jittering single or crowded letters or words along a circular arc of fixed eccentricity. Visual performance was assessed at different levels of instability with forced choice measurements of acuity, crowding and reading speed in a rapid serial visual presentation paradigm. In the periphery: (1) acuity declined; (2) crowding increased for acuity- and eccentricity-corrected targets; and (3), the rate of reading fell with acuity-, crowding- and eccentricity-corrected targets. Acuity and crowding were unaffected by even high levels of image instability. However, reading speed decreased with image instability, even though the visibility of the component letters was unaffected. The results show that reading performance cannot be standardised across the visual field by correcting the size, spacing and eccentricity of letters or words. The results suggest that unstable fixation may contribute to reading difficulties in people with low vision and therefore that rehabilitation may benefit from fixation training.
Falkenberg HK, Bex PJ. Contextual modulation of the motion aftereffect. Journal of Experimental Psychology: Human Perception & Performance. 2007;33:257–270.
Langley K, Bex PJ. Contrast adaptation implies two spatiotemporal channels but three adapting processes. Journal of Experimental Psychology: Human Perception & Performance [Internet]. 2007;33:1283–96. Publisher's VersionAbstract
The contrast gain control model of adaptation predicts that the effects of contrast adaptation correlate with contrast sensitivity. This article reports that the effects of high contrast spatiotemporal adaptors are maximum when adapting around 19 Hz, which is a factor of two or more greater than the peak in contrast sensitivity. To explain the discrepancy, the predictions made by parallel versus cascaded models of spatiotemporal processing are compared. It is demonstrated that a parallel two-temporal channel model, in which the adaptive attenuation of a channel is proportional to its response, cannot explain the effects of adaptation on threshold contrast but that a cascaded model can. The cascaded model suggests that the visual system temporally encodes spatiotemporal signals via an adaptive transient encoding process that lies in cascade with an adaptable two-temporal channel system of sustained versus transient processes. ({PsycINFO} Database Record (c) 2007 {APA}, all rights reserved).
Bex {PJ }, Dakin {SC }, Mareschal I. Contrast Gain Control in Natural Scenes. Journal of Vision. 2007;7:12, 1–12.Abstract
Behavioral and electrophysiological studies of visual processing routinely employ sine wave grating stimuli, an approach that has led to the development of models in which the first stage of cortical visual processing acts as a bank of narrowband local filters whose responses vary with the contrast of preferred structure falling within their receptive fields. The relevance of this approach to natural vision is currently being challenged. We examine the contrast response of the human visual system to natural scenes. The results support a narrowband approach to visual processing but require its elaboration. Unlike grating patterns, the contrast response to natural scenes depends on the phase structure at remote spatial scales, but over a limited spatial region. The results suggest that contrast gain control acts within, but not across, cortical hypercolumns and serves to reduce the difference between the responses of detectors in regions of high and low contrast. This process tends to normalize the response of the visual system across natural scenes, which contain uneven contrast distributions.
Crossland MD, Dakin SC, Bex PJ. Illusory stimuli can be used to identify retinal blind spots. {PLoS} {ONE} [Internet]. 2007;2:e1060. Publisher's VersionAbstract
{BACKGROUND:} Identification of visual field loss in people with retinal disease is not straightforward as people with eye disease are frequently unaware of substantial deficits in their visual field, as a consequence of perceptual completion ("filling-in") of affected areas. {METHODOLOGY:} We attempted to induce a compelling visual illusion known as the induced twinkle after-effect ({TwAE)} in eight patients with retinal scotomas. Half of these patients experience filling-in of their scotomas such that they are unaware of the presence of their scotoma, and conventional campimetric techniques can not be used to identify their vision loss. The region of the {TwAE} was compared to microperimetry maps of the retinal lesion. {PRINCIPAL} {FINDINGS:} Six of our eight participants experienced the {TwAE.} This effect occurred in three of the four people who filled-in their scotoma. The boundary of the {TwAE} showed good agreement with the boundary of lesion, as determined by microperimetry. {CONCLUSION:} For the first time, we have determined vision loss by asking patients to report the presence of an illusory percept in blind areas, rather than the absence of a real stimulus. This illusory technique is quick, accurate and not subject to the effects of filling-in.
Bex PJ, Langley K. The perception of suprathreshold contrast and fast adaptive filtering. Journal of Vision [Internet]. 2007;7:1–23. Publisher's VersionAbstract
We examine how the perceived contrast of dynamic noise images depends upon temporal frequency ({TF)} and mean luminance. A novel stepwise suprathreshold matching paradigm shows that both threshold and suprathreshold contrast sensitivity functions may be described by an inverted-U shape as a function of {TF.} The shape and the peak {TF} of the tuning function vary with the conditions under which it is measured. Spatiotemporal vision is weakly band-pass at low luminance levels (0.8 cd/m2) but becomes more strongly band-pass at high luminances (40–400 cd/m2). The peak temporal frequencies of the band-pass functions increase with the mean luminance and contrast of the test signals. As a function of increasing image contrast, our results demonstrate that the visual system broadens the spatiotemporal bandwidth of its signal detection mechanisms, especially at high mean luminances. Our results are shown to be consistent with an adaptable signal transmission system in which early luminance-dependent gain control mechanisms, in combination with on-line estimates of contrast via the autocorrelation function lead to an adaptive enhancement of spatiotemporal vision at high temporal frequencies.
Falkenberg HK, Bex PJ. Sources of Motion-Sensitivity Loss in Glaucoma. Investigative Ophthalmology & Visual Science [Internet]. 2007;48:2913–2921. Publisher's Version
2006
Mareschal I, Dakin SC, Bex PJ. Dynamic properties of orientation discrimination assessed by using classification images. Proceedings of the National Academy of Sciences of the United States of America [Internet]. 2006;103:5131–6. Publisher's VersionAbstract
Recent physiological studies indicate that the tuning properties of neurons under acute preparation in primary visual cortex can change over time. We used a psychophysical reverse correlation paradigm to examine the potential repercussions of this neuronal property for human observers' ability to discriminate the orientation of targets over time. Observers were required to identify the orientation of a Gabor target presented within dynamic white noise. Frames from the noise movies were pooled to compute dynamic classification images ({CIs)} associated with the observers' discrimination performance, which then were fit with a weighted difference-of-Gabor function. Best-fitting templates were temporally bandpass, tuned to more oblique orientations than the stimulus but, crucially, did not change over time. The results suggest that the template for orientation discrimination is selected within the first 50 ms of stimulus onset and that, unlike the response of single cells, there is no measurable dynamic component to either orientation or spatial frequency tuning of human orientation discrimination.
Bex PJ, Falkenberg HK. Resolution of complex motion detectors in the central and peripheral visual field. Journal of the Optical Society of America [Internet]. 2006;23:1598–607. Publisher's VersionAbstract
We examine how local direction signals are combined to compute the focus of radial motion ({FRM)} in random dot patterns and examine how this process changes across the visual field. Equivalent noise analysis showed that a loss in {FRM} accuracy was largely attributable to an increase in local motion detector noise with little or no change in efficiency across the visual field. The minimum separation for discriminating the foci of two overlapping optic flow patterns increased in the periphery faster than predicted from the resolution for a single {FRM.} This behavior requires that observers average numerous local velocities to estimate the {FRM}, which enables resistance to internal and external noise and endows the system with the property of position invariance. However, such pooling limits the precision with which multiple looming objects can be discriminated, especially in the peripheral visual field.
2005
Bex PJ, Dakin SC, Mareschal I. Critical band masking in optic flow. Network: Computation in Neural Systems [Internet]. 2005;16:261–84. Publisher's VersionAbstract
Visual processing has been widely investigated with narrow band stimuli at low contrasts. We used a masking paradigm to examine how visual sensitivity under these conditions compares with the perception of the direction of heading in real scenes (i.e., with dynamic natural images at high contrasts). We first confirmed and extended previous studies showing biases in the amplitude distribution for spatial frequency, temporal frequency, speed and direction in dynamic natural movies. We then measured contrast thresholds for identification of the direction of motion for an observer traveling at various speeds. In spite of differences in contrast sensitivity and large non-uniformities in the amplitude content of the stimuli, contrast thresholds were relatively invariant of spatial frequency and completely invariant of temporal frequency, speed and direction. Our results suggest that visual processing normalises responses to supra-threshold structure at different spatial and temporal frequencies within natural stimuli and so equates their effective visibility.
Dakin SC, Mareschal I, Bex PJ. Local and global limitations on direction integration assessed using equivalent noise analysis. Vision Research [Internet]. 2005;45:3027–49. Publisher's VersionAbstract
We used an equivalent noise ({EN)} paradigm to examine how the human visual system pools local estimates of direction across space in order to encode global direction. Observers estimated the mean direction (clockwise or counter-clockwise of vertical) of a field of moving band-pass elements whose directions were drawn from a wrapped normal distribution. By measuring discrimination thresholds for mean direction as a function of directional variance, we were able to infer both the precision of observers' representation of each element's direction (i.e., local noise) as well as how many of these estimates they were averaging (i.e., global pooling). We estimated {EN} for various numbers of moving elements occupying regions of various sizes. We report that both local and global limits on direction integration are determined by the number of elements present in the display (irrespective of their density or the size of region they occupy), and we go on to show how this dependence can be understood in terms of neural noise. Specifically, we use Monte Carlo simulations to show that a maximum-likelihood operator, operating on pooled directional signals from visual cortex corrupted by Poisson noise, accounts for psychophysical data across all conditions tested, as well as motion coherence thresholds (collected under similar experimental conditions). A population vector-averaging scheme (essentially a special case of {ML} estimation) produces similar predictions but out-performs subjects at high levels of directional variability and fails to predict motion coherence thresholds.