Area summation in human vision at and above detection threshold

The initial image-processing stages of visual cortex are well suited to a local (patchwise) analysis of the viewed scene. But the world's structures extend over space as textures and surfaces, suggesting the need for spatial integration. Most models of contrast vision fall shy of this process because (i) the weak area summation at detection threshold is attributed to probability summation (PS) and (ii) there is little or no advantage of area well above threshold. Both of these views are challenged here. First, it is shown that results at threshold are consistent with linear summation of contrast following retinal inhomogeneity, spatial filtering, nonlinear contrast transduction and multiple sources of additive Gaussian noise. We suggest that the suprathreshold loss of the area advantage in previous studies is due to a concomitant increase in suppression from the pedestal. To overcome this confound, a novel stimulus class is designed where: (i) the observer operates on a constant retinal area, (ii) the target area is controlled within this summation field, and (iii) the pedestal is fixed in size. Using this arrangement, substantial summation is found along the entire masking function, including the region of facilitation. Our analysis shows that PS and uncertainty cannot account for the results, and that suprathreshold summation of contrast extends over at least seven target cycles of grating

The human visual system is optimised for processing the spatial information in natural visual images

AbstractA fundamental tenet of visual science is that the detailed properties of visual systems are not capricious accidents, but are closely matched by evolution and neonatal experience to the environments and lifestyles in which those visual systems must work [1–5]. This has been shown most convincingly for fish [6] and insects [7]. For mammalian vision, however, this tenet is based more upon theoretical arguments [8–11] than upon direct observations [12,13]. Here, we describe experiments that require human observers to discriminate between pictures of slightly different faces or objects. These are produced by a morphing technique that allows small, quantifiable changes to be made in the stimulus images. The independent variable is designed to give increasing deviation from natural visual scenes, and is a measure of the Fourier composition of the image (its second-order statistics). Performance in these tests was best when the pictures had natural second-order spatial statistics, and degraded when the images were made less natural. Furthermore, performance can be explained with a simple model of contrast coding, based upon the properties of simple cells [14–17] in the mammalian visual cortex. The findings thus provide direct empirical support for the notion that human spatial vision is optimised to the second-order statistics of the optical environment

Summation of perceptual cues in natural visual scenes

Natural visual scenes are rich in information, and any neural system analysing them must piece together the many messages from large arrays of diverse feature detectors. It is known how threshold detection of compound visual stimuli (sinusoidal gratings) is determined by their components' thresholds. We investigate whether similar combination rules apply to the perception of the complex and suprathreshold visual elements in naturalistic visual images. Observers gave magnitude estimations (ratings) of the perceived differences between pairs of images made from photographs of natural scenes. Images in some pairs differed along one stimulus dimension such as object colour, location, size or blur. But, for other image pairs, there were composite differences along two dimensions (e.g. both colour and object-location might change). We examined whether the ratings for such composite pairs could be predicted from the two ratings for the respective pairs in which only one stimulus dimension had changed. We found a pooling relationship similar to that proposed for simple stimuli: Minkowski summation with exponent 2.84 yielded the best predictive power (r=0.96), an exponent similar to that generally reported for compound grating detection. This suggests that theories based on detecting simple stimuli can encompass visual processing of complex, suprathreshold stimuli

Magnetisation Studies of Geometrically Frustrated Antiferromagnets SrLn2O4, with Ln = Er, Dy and Ho

We present the results of susceptibility \chi(T) and magnetisation M(H) measurements performed on single crystal samples of the rare-earth oxides SrLn2O4 (Ln = Er, Dy and Ho). The measurements reveal the presence of magnetic ordering transition in SrHo2O4 at 0.62 K and confirm that SrEr2O4 orders magnetically at 0.73 K, while in SrDy2O4 such a transition is absent down to at least 0.5 K. The observed ordering temperatures are significantly lower than the Curie-Weiss temperatures, \theta_{CW}, obtained from the high-temperature linear fits to the 1/\chi(T) curves, which implies that these materials are subject to geometric frustration. Strong anisotropy found in the \chi(T) curves for a field applied along the different crystallographic directions is also evident in the M(H) curves measured both above and below the ordering temperatures. For all three compounds the magnetisation plateaux at approximately one third of the magnetisation saturation values can be seen for certain directions of applied field, which is indicative of field-induced stabilisation of a collinear {\it two-up one-down} structure.Comment: 6 pages, 6 figure

Methods and Mechanisms of Motion Dazzle

‘Motion dazzle’ is a phenomenon where high contrast patterns are hypothesised to cause errors in speed and direction perception of targets that are in motion. Motion dazzle is relevant to both ecological questions, such as why striped patterning may have evolved in animals, and also for camouflage design for human purposes. I have used an interdisciplinary approach to address questions about motion dazzle in human subjects, combining techniques from psychophysics and behavioural ecology. I have focused on what aspects of a target are important in creating motion dazzle effects and what specific perceptual effects are seen, with a view to understanding the phenomenon at a mechanistic level. Using touch screen technology, I have replicated recent work that shows that targets with striped markings are relatively difficult for humans to capture, and have also shown that both overall luminance match of a target to the background, the distribution of ‘features’ on a target, and the orientation of target stripes relative to target motion are crucial in determining capture success. I have also made measurements of speed and direction judgements for targets with different patterning using psychophysical techniques. Speed judgments do not seem to be affected systematically by static striped patterning; however, subjects do show directional errors based on static stripe orientation relative to the direction of motion. Motion dazzle may therefore be an effective camouflage strategy caused by several different misperceptions, not all of which have been predicted previously

Organization of the visual cortex in human albinism

In albinism there is an abnormal projection of part of the temporal retina to the visual cortex contralateral to the eye. This projection, together with the normally routed fibers from nasal retina, provides a cortical hemisphere with visual input from more than the normal hemifield of visual space. In many mammalian models of albinism, a possible sensory mismatch in the visual cortex is avoided either by reorganization of the thalamocortical connections to give the abnormal input an exclusive cortical representation, or by the abnormal input being substantially suppressed. In this study we examine, with fMRI, how the human visual cortex topographically maps its input in albinism. We find that the input from temporal retina is not substantially suppressed and forms a retinotopic mapping that is superimposed on the mapping of the nasal retina in striate and extrastriate areas. The abnormal routing of temporal fibers is not total, with the line of decussation shifting to between 6 and 14degrees into temporal retina. Our results indicate that the abnormal input to visual cortex in human albinism does not undergo topographic reorganization between the thalamus and cortex. Furthermore, the abnormal input is not significantly suppressed in either striate or extrastriate areas. The topographic mapping that we report in human does not conform, therefore, to the commonly observed patterns in other mammals but takes the form of the "true albino" pattern that has been reported rarely in cat and in the only other individual primate studied

Perception of differences in natural-image stimuli:why is peripheral viewing poorer than foveal?

Visual Difference Predictor (VDP) models have played a key role in digital image applications such as the development of image quality metrics. However, little attention has been paid to their applicability to peripheral vision. Central (i.e., foveal) vision is extremely sensitive for the contrast detection of simple stimuli such as sinusoidal gratings, but peripheral vision is less sensitive. Furthermore, crowding is a well-documented phenomenon whereby differences in suprathreshold peripherally viewed target objects (such as individual letters or patches of sinusoidal grating) become more difficult to discriminate when surrounded by other objects (flankers). We examine three factors that might influence the degree of crowding with natural-scene stimuli (cropped from photographs of natural scenes): (1) location in the visual field, (2) distance between target and flankers, and (3) flanker-target similarity. We ask how these factors affect crowding in a suprathreshold discrimination experiment where observers rate the perceived differences between two sequentially presented target patches of natural images. The targets might differ in the shape, size, arrangement, or color of items in the scenes. Changes in uncrowded peripheral targets are perceived to be less than for the same changes viewed foveally. Consistent with previous research on simple stimuli, we find that crowding in the periphery (but not in the fovea) reduces the magnitudes of perceived changes even further, especially when the flankers are closer and more similar to the target. We have tested VDP models based on the response behavior of neurons in visual cortex and the inhibitory interactions between them. The models do not explain the lower ratings for peripherally viewed changes even when the lower peripheral contrast sensitivity was accounted for; nor could they explain the effects of crowding, which others have suggested might arise from errors in the spatial localization of features in the peripheral image. This suggests that conventional VDP models do not port well to peripheral vision