Pattern randomness aftereffect

Vision Research, 1999

The present paper addresses whether topographical jitter or undersampling might limit pattern perception in foveal, peripheral and strabismic amblyopic vision. In the first experiment, we measured contrast thresholds for detecting and identifying the orientation (up, down, left, right) of E-like patterns comprised of Gabor samples. We found that detection and identification thresholds were both degraded in peripheral and amblyopic vision; however, the orientation identification/detection threshold ratio was approximately the same in foveal, peripheral and amblyopic vision. This result is somewhat surprising, because we anticipated that a high degree of uncalibrated topographical jitter in peripheral and amblyopic vision would have affected orientation identification to a greater extent than detection. In the second experiment, we investigated the tolerance of human and model observers to perturbation of the positions of the samples defining the pattern when its contrast was suprathreshold, by measuring a 'jitter threshold' (the amount of jitter required to reduce performance from near perfect to 62.5% correct). The results and modeling of our jitter experiments suggest that pattern identification is highly robust to positional jitter. The positional tolerance of foveal, peripheral and amblyopic vision is equal to about half the separation of the features and the close similarity between the three visual systems argues against extreme topographical jitter. The effects of jitter on human performance are consistent with the predictions of a 'template' model. In the third experiment we determined what fraction of the 17 Gabor samples are needed to reliably identify the orientation of the E-patterns by measuring a 'sample threshold' (the proportion of samples required for 62.5% correct performance). In foveal vision, human observers are highly efficient requiring only about half the samples for reliable pattern identification. Relative to an ideal observer model, humans perform this task with 85% efficiency. In contrast, in both peripheral vision and strabismic amblyopia more samples are required. The increased number of features required in peripheral vision and strabismic amblyopia suggests that in these visual systems, the stimulus is underrepresented at the stage of feature integration.

Perception & Psychophysics, 1997

Vision Research, 2004

Spatial and temporal regularities commonly exist in natural visual scenes. The knowledge of the probability structure of these regularities is likely to be informative for an efficient visual system. Here we explored how manipulating the spatio-temporal prior probability of stimuli affects human orientation perception. Stimulus sequences comprised four collinear bars (predictors) which appeared successively towards the foveal region, followed by a target bar with the same or different orientation. Subjects' orientation perception of the foveal target was biased towards the orientation of the predictors when presented in a highly ordered and predictable sequence. The discrimination thresholds were significantly elevated in proportion to increasing prior probabilities of the predictors. Breaking this sequence, by randomising presentation order or presentation duration, decreased the thresholds. These psychophysical observations are consistent with a Bayesian model, suggesting that a predictable spatio-temporal stimulus structure and an increased probability of collinear trials are associated with the increasing prior expectation of collinear events. Our results suggest that statistical spatio-temporal stimulus regularities are effectively integrated by human visual cortex over a range of spatial and temporal positions, thereby systematically affecting perception.

nature neuroscience, 2002

Visual perception involves coordination between sensory sampling of the world and active interpretation of the sensory data. Human perception of objects and scenes is normally stable and robust, but it falters when one is presented with patterns that are inherently ambiguous or contradictory. Under such conditions, vision lapses into a chain of continually alternating percepts, whereby a viable visual interpretation dominates for a few seconds and is then replaced by a rival interpretation. This multistable vision, or 'multistability' , is thought to result from destabilization of fundamental visual mechanisms, and has offered valuable insights into how sensory patterns are actively organized and interpreted in the brain 1,2 . Despite a great deal of recent research and interest in multistable perception, however, its neurophysiological underpinnings remain poorly understood. Physiological studies have suggested that disambiguation of ambiguous patterns draws on activity within the visual cortex 3-10 , but how this activity ultimately contributes to perceptual solution is not yet known. Even less clear is the nature of the perceptual alternation process itself. Traditional views hold that it is an automatic consequence of incompatible, antagonistic stimulus representations in the sensory visual cortex 11,12 . Recent evidence challenges this notion, suggesting instead that perceptual alternations are initiated outside the primarily sensory areas 13,14 (for a review, see ref. 15).

The relationship between the processing of orientations by the human visual system has been related to the orientation content of the natural environment; horizontal orientations, while predominant in natural environments, are perceived less well than vertical and oblique orientations are perceived best, though they are least prevalent in the natural world. This 'horizontal effect' has further extended the well-studied relationship between visual encoding and natural scene statistics as the differential perception of orientations in broadband scenes inversely matches their differential representation in the natural environment. However, the original hypothesis that this relationship may have evolved across millennia in order to make the visual system an efficient informationtransmitting system has been called into question by research showing the modification of orientation perception by exposure to altered environments and studies showing a later development of adult-like orientation processing. Recent work into the effects of adaptation on visual encoding of the natural environment have led me to the conclusion that the relationship between the statistics of the natural world and visual encoding is, in a way, much simpler than previously posited; rather than being adapted over millennia to whiten the typical natural scene anisotropy, the visual system adjusts processing v dynamically to match the current visual environment. The project presented here details how the statistics of the recently viewed environment affect the way that the visual brain processes information. To assess the effect of recent exposure on broadband orientation processing, the orientation content subjects viewed was modified via fast Fourier transform (FFT) filtering of their environment in near-real-time. Results show that experience in an altered environment modifies anisotropic processing: observers' orientation perception changes from matching the typical environmental distribution to matching that of the recently experienced atypical environment. The results of these experiments can be predicted by assuming that observers' biases of perception are probabilistic and rely on an internal model that matches the recently experienced environmental distribution. This change in perception indicates not only that orientation processing is plastic, but that it is related in a predictable way to an observer's recent visual environment. vi

Vision research, 1999

Deciding whether a novel visual pattern is the same as or different from a previously seen reference is easier if both stimuli are presented to the same rather than to different locations in the field of view (Foster & Kahn (1985). Biological Cybernetics, 51, 305-312; Dill & Fahle (1998). Perception and Psychophysics, 60, 65-81). We investigated whether pattern symmetry interacts with the effect of translation. Patterns were small dot-clouds which could be mirror-symmetric or asymmetric. Translations were displacements of the visual pattern symmetrically across the fovea, either left-right or above-below. We found that same-different discriminations were worse (less accurate and slower) for translated patterns, to an extent which in general was not influenced by pattern symmetry, or pattern orientation, or direction of displacement. However, if the displaced pattern was a mirror image of the original one (along the trajectory of the displacement), then performance was largely invari...

Psychological Research, 2008

It has been speculated that visual symmetry perception from dynamic stimuli involves mechanisms diVerent from those for static stimuli. However, previous studies found no evidence that dynamic stimuli lead to active temporal processing and improve symmetry detection. In this study, four psychophysical experiments investigated temporal processing in symmetry perception using both dynamic and static stimulus presentations of dot patterns. In Experiment 1, rapid successive presentations of symmetric patterns (e.g., 16 patterns per 853 ms) produced more accurate discrimination of orientations of symmetry axes than static stimuli (single pattern presented through 853 ms). In Experiments 2-4, we conWrmed that the dynamic-stimulus advantage depended upon presentation of a large number of unique patterns within a brief period (853 ms) in the dynamic conditions. Evidently, human vision takes advantage of temporal processing for symmetry perception from dynamic stimuli.

Symmetry

Recent studies have shown that limiting the lifetime of pattern elements improves symmetry detection, potentially by increasing the number of element locations. Here, we investigate how spatial relocation, luminance contrast modulation and lifetime duration of elements affect symmetry perception in dynamic stimuli. Stimuli were dynamic dot-patterns containing varying amounts of symmetry about a vertical axis. Symmetrical matched-pairs were: (i) relocated to multiple successive, but random locations (i.e., multiple locations condition); (ii) relocated between the same two locations (i.e., two locations condition); (iii) not, relocated, but their luminance contrast was modulated at different temporal frequencies (i.e., one location condition), and (iv) not relocated, but a single pattern was presented at full contrast (i.e., static condition). In the dynamic conditions, we varied the elements' lifetime duration and temporal frequency of contrast modulation. We measured symmetry detection thresholds using a two-interval forced choice procedure. Our results show improved performance for the multiple locations condition compared to two-location and static conditions, suggesting a cumulative process whereby weak symmetry information is integrated by spatiotemporal filters to increase overall symmetry signal strength. Performance also improved for the static, contrast modulated patterns, but this was explained by a reduction in perceived density. This suggests that different mechanisms mediate symmetry detection in dynamic stimuli and static contrast modulated patterns.

We present a statistical account for the subjective probability of alternation in people's perception of randomness. By examining the spatio-temporal distances between pattern events, specifically, the frequency and delay of binary patterns in a Markov chain, we obtain some normative measures to calibrate people's expectation of randomness. We suggest that it can be fruitful to study subjective randomness in the context of human object representation and perception of time and space.

Vision Research, 1996