Visual contrast perception in visual snow syndrome reveals abnormal neural gain but not neural noise.

Visual snow syndrome is a neurological condition characterized by a persistent visual disturbance, visual snow, in conjunction with additional visual symptoms. Cortical hyperexcitability is a potential pathophysiological mechanism, which could be explained by increased gain in neural responses to visual input. Alternatively, neural noise in the visual pathway could be abnormally elevated. We assessed these two potential competing neural mechanisms in our studies of visual contrast perception. Cortical hyperexcitation also occurs in migraine, which commonly co-occurs with visual snow syndrome. Therefore, to determine whether the effect of visual snow syndrome can be distinguished from interictal migraine, we recruited four participant groups: controls, migraine alone, visual snow syndrome alone and visual snow syndrome with migraine. In the first experiment, we estimated internal noise in 20 controls, 21 migraine participants and 32 visual snow syndrome participants (16 with migraine) using a luminance increment detection task. In the second experiment, we estimated neural contrast gain in 21 controls, 22 migraine participants and 35 visual snow syndrome participants (16 with migraine) using tasks assessing sensitivity to changes in contrast from a reference. Contrast gain and sensitivity were measured for the putative parvocellular and 'on' and 'off' magnocellular pathways, respectively. We found that luminance increment thresholds and internal noise estimates were normal in both visual snow syndrome and migraine. Contrast gain measures for putative parvocellular processing and contrast sensitivity for putative off magnocellular processing were abnormally increased in visual snow syndrome, regardless of migraine status. Therefore, our results indicate that visual snow syndrome is characterized by increased neural contrast gain but not abnormal neural noise within the targeted pathways.

Contribution of higher-order structure to perception of mirror symmetry: Role of shapes and corners.

Visual mirror symmetry is a global feature that is dependent on specific low-level relationships between component elements. Initially conceptualized as virtual lines between paired elements, it has been suggested that higher-order structure between pairs of symmetric elements forming virtual four cornered shapes may also be important for strengthening the percept of mirror symmetry. We utilize corner elements, formed by joining two Gabor elements along a central midline creating vertices with variable internal angles, in a temporal integration paradigm. This allows us to specifically manipulate the presence and type of higher-order versus lower-order structure in patterns with symmetrically placed elements. We show a significant contribution of higher-order structure to the salience of visual symmetries compared with patterns with only lower-order structures. We also find that although we are more sensitive to patterns with higher-order structure, there is no difference in the temporal processing of higher-order versus lower-order patterns. These findings have important implications for existing spatial filter models of symmetry perception that rely on lower-order structures alone and reinforces the need for elaborated models that can more readily capture the complexities of real-world symmetries.

Analysis of shape uses local apparent position rather than physical position.

Objects are often identified by the shapes of their boundaries. Here, by measuring threshold amplitudes for detection of sinusoidal modulation of local position, orientation and centrifugal speed in a closed path of Gabor patches, we show that the positions of such boundaries are misperceived to accommodate local illusions of orientation context and motion induced positional bias. These two types of illusion are shown to occur independently, but the misperception of position is additive. We conclude that, in the analysis of shape, the visual system uses the apparent rather than the veridical boundary conformation.

Daily vision testing can expose the prodromal phase of migraine.

Background Several visual tasks have been proposed as indirect assays of the balance between cortical inhibition and excitation in migraine. This study aimed to determine whether daily measurement of performance on such tasks can reveal perceptual changes in the build up to migraine events. Methods Visual performance was measured daily at home in 16 non-headache controls and 18 individuals with migraine using a testing protocol on a portable tablet device. Observers performed two tasks: luminance increment detection in spatial luminance noise and centre surround contrast suppression. Results Luminance thresholds were reduced in migraine compared to control groups ( p < 0.05), but thresholds did not alter across the migraine cycle; while headache-free, centre-surround contrast suppression was stronger for the migraine group relative to controls ( p < 0.05). Surround suppression weakened at around 48 hours prior to a migraine attack and strengthened to approach their headache-free levels by 24 hours post-migraine (main effect of timing, p < 0.05). Conclusions Daily portable testing of vision enabled insight into perceptual performance in the lead up to migraine events, a time point that is typically difficult to capture experimentally. Perceptual surround suppression of contrast fluctuates during the migraine cycle, supporting the utility of this measure as an indirect, non-invasive assay of the balance between cortical inhibition and excitation.

Convergent evidence for global processing of shape.

There is an ongoing debate over whether there is convincing evidence in support of global contour integration in shape discrimination tasks, particularly when using radial frequency (RF) patterns as stimuli (Baldwin, Schmidtmann, Kingdom, & Hess, 2016). The objection lies in the previous use of high-threshold theory (HTT), rather than signal detection theory (SDT) to model the probability summation estimates of observer thresholds to determine whether integration of information is occurring around the contour. Here we used a discrimination at threshold method to establish evidence of global processing of two frequently used RF patterns (RF3 and RF5) that does not require mathematical modeling. To provide a bridge between current and past research we examined the two proposed methods, finding that HTT produced probability summation estimates that were more conservative than SDT (when an appropriate number of channels was used to generate estimates). We found no difference in observer thresholds when an RF pattern was presented as the only test stimulus in a block of trials or when two RF patterns were interleaved, and no evidence for a decrease in psychometric slopes with increasing numbers of stimulus elements. These findings are contrary to what is predicted by SDT for a stimulus whose detection conforms to probability summation. Therefore, our results find no evidence that supports probability summation, further demonstrating the importance of using random phase RF patterns while measuring integration around a contour and providing strong evidence for global shape processing around low frequency RF patterns.

Visual search reveals a critical component to shape.

Objects are often identified by the shape of their contours. In this study, visual search tasks were used to reveal a visual dimension critical to the analysis of the shape of a boundary-defined area. Points of maximum curvature on closed paths are important for shape coding and it was shown here that target patterns are readily identified among distractors if the angle subtended by adjacent curvature maxima at the target pattern's center differs from that created in the distractors. A search asymmetry, indicated by a difference in performance in the visual search task when the roles of target and distractor patterns are reversed, was found when the critical subtended angle was only present in one of the patterns. Performance for patterns with the same subtended angle but differing local orientation and curvature was poor, demonstrating insensitivity to differences in these local features of the patterns. These results imply that the discrimination of objects by the shape of their boundaries relies on the relative positions of their curvature maxima rather than the local properties of the boundary from which these positions are derived.

The effect of spatiotemporal displacement on the integration of shape information.

Within a natural scene it is not uncommon for an object's shape to be revealed over time. We investigated whether the same integration of shape information that happens around a fully visible contour also happens when that information is distributed over time. In a two-interval forced-choice task, observers discriminated between a radial frequency (RF) pattern and a circle that were revealed either using an implicit slit or traced out by a dot's motion; and a line and a modulated line that were either contour-defined or motion-defined. First, with presentation times of approximately 1 s, we found no difference in the strength of integration when comparing a freely visible contour to one that (a) moved behind a slit; (b) was revealed by a moving slit; or (c) revealed piecemeal by a slit appearing at random locations (Experiment 1). Changing the duration of presentation (250-4,000 ms) had no effect on strength of integration or threshold for detection within the moving slit condition (Experiment 2). Considering these results, Experiment 3 revisited integration for a dot tracing out an RF path (Or, Thabet, Wilkinson, & Wilson, 2011), finding removal of a change in speed cue increased the strength of integration to that found in Experiments 1 and 2 of the current study. The pattern of results for modulated lines was different from RF patterns; however, within these conditions, there was no difference in strength of integration between contour-defined and motion-defined stimuli. Our results suggest motion-defined patterns are processed as form from motion.

Integration of shape information occurs around closed contours but not across them.

Scenery and complex objects can be reduced to a combination of shapes, so it is pertinent to examine if the integration of information found occurring around simple contours also occurs across them. Baldwin, Schmidtmann, Kingdom, and Hess (2016) investigated this idea using radial frequency (RF) patterns, distributing information around a single contour or across four contours. However, their use of a restricted number of locations for this information may have influenced their results (see Green, Dickinson, & Badcock, 2017). The current study revisits their paradigm using random-phase (spatial uncertainty) presentation of RF patterns with 11 observers. Results provide strong evidence for the integration of information around single contours but not across them. These findings are contrary to the lack of integration found by Baldwin et al. (2016) within a single contour, but do provide support for their suggestion that improvement in performance when adding information to separate RF patterns is a function of probability summation. Similar to Green et al. (2017), it suggests the importance of using random-phase RF patterns when measuring integration.

Suprathreshold contrast summation over area using drifting gratings.

This study investigated contrast summation over area for moving targets applied to a fixed-size contrast pedestal-a technique originally developed by Meese and Summers (2007) to demonstrate strong spatial summation of contrast for static patterns at suprathreshold contrast levels. Target contrast increments (drifting gratings) were applied to either the entire 20% contrast pedestal (a full fixed-size drifting grating), or in the configuration of a checkerboard pattern in which the target increment was applied to every alternate check region. These checked stimuli are known as "Battenberg patterns" and the sizes of the checks were varied (within a fixed overall area), across conditions, to measure summation behavior. Results showed that sensitivity to an increment covering the full pedestal was significantly higher than that for the Battenberg patterns (areal summation). Two observers showed strong summation across all check sizes (0.71°-3.33°), and for two other observers the summation ratio dropped to levels consistent with probability summation once check size reached 2.00°. Therefore, areal summation with moving targets does operate at high contrast, and is subserved by relatively large receptive fields covering a square area extending up to at least 3.33° × 3.33° for some observers. Previous studies in which the spatial structure of the pedestal and target covaried were unable to demonstrate spatial summation, potentially due to increasing amounts of suppression from gain-control mechanisms which increases as pedestal size increases. This study shows that when this is controlled, by keeping the pedestal the same across all conditions, extensive summation can be demonstrated.

Global processing of random-phase radial frequency patterns but not modulated lines.

Previously, researchers have used circular contours with sinusoidal deformations of the radius (radial frequency [RF] patterns) to investigate the underlying processing involved in simple shape perception. On finding that the rapid improvement in sensitivity to deformation as more cycles of modulation were added was greater than expected from probability summation across sets of local independent detectors, they concluded that global integration of contour information was occurring. More recently, this conclusion has been questioned by researchers using a method of calculating probability summation derived from signal detection theory (Baldwin, Schmidtmann, Kingdom, & Hess, 2016). They could not distinguish between global integration and probability summation. Furthermore, it has been argued that RF patterns and lines are processed in a similar manner (Mullen, Beaudot, & Ivanov, 2011; Schmidtmann & Kingdom, 2017). The current study investigates these claims using fixed-phase (in which the local elements have spatial certainty) and random-phase (in which the local elements have spatial uncertainty) RF patterns and modulated lines. Thresholds were collected from eight naïve observers and compared to probability summation estimates calculated using methods derived from both high threshold theory and signal detection theory. The results indicate global processing of random-phase RF patterns and evidence for an interaction between local and global cues for fixed-phase RF patterns. They also show no evidence of global integration with modulated line stimuli. The results provide further evidence for the global processing of random-phase RF patterns and indicate that RF patterns and modulated lines are processed differently.

Separate banks of information channels encode size and aspect ratio.

Size and aspect ratio are ecologically important visual attributes. Relative size confers depth, and aspect ratio is a size-invariant cue to object identity. The mechanisms of their analyses by the visual system are uncertain. In a series of three psychophysical experiments we show that adaptation causes perceptual repulsion in these properties. Experiment 1 shows that adaptation to a square causes a subsequently viewed smaller (larger) test square to appear smaller (larger) still. Experiment 2 reveals that a test rectangle with an aspect ratio (height/width) of two appears more slender after adaptation to rectangles with aspect ratios less than two, while the same test stimulus appears more squat after adaptation to a rectangle with an aspect ratio greater than two. Significantly, aftereffect magnitudes peak and then decline as the sizes or aspect ratios of adaptor and test diverge. Experiment 3 uses the results of Experiments 1 and 2 to show that the changes in perceived aspect ratio are due to adaptation to aspect ratio rather than adaptation to the height and width of the stimuli. The results are consistent with the operation of distinct banks of information channels tuned for different values of each property. The necessary channels have log-Gaussian sensitivity profiles, have equal widths when expressed as ratios, are labeled with their preferred magnitudes, and are distributed at exponentially increasing intervals. If an adapting stimulus reduces each channel's sensitivity in proportion to its activation then the displacement of the centroid of activity due to a subsequently experienced test stimulus predicts the measured size or aspect ratio aftereffect.

Temporal synchrony is an effective cue for grouping and segmentation in the absence of form cues.

The synchronous change of a feature across multiple discrete elements, i.e., temporal synchrony, has been shown to be a powerful cue for grouping and segmentation. This has been demonstrated with both static and dynamic stimuli for a range of tasks. However, in addition to temporal synchrony, stimuli in previous research have included other cues which can also facilitate grouping and segmentation, such as good continuation and coherent spatial configuration. To evaluate the effectiveness of temporal synchrony for grouping and segmentation in isolation, here we measure signal detection thresholds using a global-Gabor stimulus in the presence/absence of a synchronous event. We also examine the impact of the spatial proximity of the to-be-grouped elements on the effectiveness of temporal synchrony, and the duration for which elements are bound together following a synchronous event in the absence of further segmentation cues. The results show that temporal synchrony (in isolation) is an effective cue for grouping local elements together to extract a global signal. Further, we find that the effectiveness of temporal synchrony as a cue for segmentation is modulated by the spatial proximity of signal elements. Finally, we demonstrate that following a synchronous event, elements are perceptually bound together for an average duration of 200 ms.

Larger receptive fields revealed using Battenberg stimuli to assess contrast summation with moving patterns.

This study reevaluated the summation extent for moving stimuli using the Battenberg summation paradigm (Meese, 2010), which aims to circumvent internal noise changes with increasing stimulus size by holding display size constant. In the checkerboard stimulus, the size of the checks (luminance-modulated drifting gratings) was varied to measure dependence on signal area. Experiment 1 was a contrast detection task that used either signal checks alternating with uniform, mean luminance, checks (single-motion) or alternate checks containing gratings moving in opposite directions (opposing-motion). The latter was designed to test whether summation extent changes when segregating regions based on motion direction. Results showed summation over a square summation area with a side length of 3.33°, much larger than previous estimates of less than 1° for similar stimuli (Anderson & Burr, 1991). This was found for both motion combinations, providing no evidence that summation extent differs when segregating patterns based on direction, at contrast detection threshold. These results are in close agreement with those obtained for static patterns (Meese, 2010) and support the same underlying summation model. Experiment 2 was a contrast increment detection task conducted to determine whether differences in summation extent arise under suprathreshold contrast conditions. There was no dependence on check size for either condition across the range of sizes tested. This supports the suggestion that segmentation mechanisms dominate perception under high-contrast conditions, a potential adaptive strategy employed by the visual system.

In unison: First- and second-order information combine for integration of shape information.

The modulation of orientation around radial frequency (RF) patterns and RF textures is globally processed in both cases. This psychophysical study investigates whether the combination-a textured RF path obtained by applying an RF texture to an RF contour-is processed like a texture or a contour when making judgements about shape. Unlike RF textures, the impression of a closed flow was not required for global integration of textured RF paths, suggesting that these paths were processed as second-order, or contrast-defined contours. Luminance-defined (LD) RF paths were shown to globally integrate but with thresholds approximately half of those for the proposed second-order textured paths. The next experiment investigated whether this benefit was due to LD stimuli possessing double the amount of information (first- and second-order information). A mixed three-part contour composed of two different second-order texture components and an LD component was then employed to determine how the different cues combined. The mixed path thresholds matched predictions derived from a linear combination of first- and second-order cues. The conclusion is that the shape of isolated contours is processed using both first- and second-order information equally and that the contribution of texture is to carry additional second-order signal.

Discrete annular regions of texture contribute independently to the analysis of shape from texture.

Radial frequency (RF) textures (created by applying a sinusoidal modulation of orientation to an otherwise circular texture) have been shown to be globally processed. RF textures differ from RF patterns (paths deformed from circular by a sinusoidal modulation in radius) in that the elements need not be constrained to a specific path. In the natural environment, objects differ from their background in texture, and a bounding contour can mark this textural change. This study examines the extent to which modulation of texture sums across space and whether the inclusion of a boundary between two areas provides a segmentation cue that limits the area over which summation occurs. RF textures were split into two annular regions and signal introduced to inner, outer, or both annuli Thresholds for the detection of RF modulation of orientation were not affected by the presence of a boundary. Further, it was found that the thresholds matched predictions for the independent contribution of the inner and outer areas to performance and that changing the relative phase of the modulation in the inner and outer annuli had no impact on performance, implying independent integration within the two annuli. Finally, integration of modulation information within the annuli was confirmed to ensure these results do apply to textures that are globally processed.

Global shape processing: A behavioral and electrophysiological analysis of both contour and texture processing.

The number of corners on the boundary of a closed contour is thought to be particularly critical for shape detection and discrimination. The aim of the current study was to examine the relative contribution of the number of corners and the angle between corners to shape discrimination in complex visual scenes as well as to determine the time course and neural substrates of global shape processing based on the presence or absence of these specific features. In Experiment 1, event-related potentials were recorded while participants discriminated between two radial frequency (RF) patterns with the same maximum local curvature defining corners but varying arrangements of those corners. The results showed that the angle separating corners was more critical than the overall number of corners for discrimination performance. An enhanced negativity (posterior N220) over the occipital lobe was elicited following the presentation of an RF with three modulation cycles (RF3) but not following a circle, suggesting that the posterior N220 is sensitive to variation in curvature on a contour. In Experiment 2, we confirm the primary effect of the presence of corners on the amplitude of the posterior N220 component and extend the stimuli to include shapes defined by texture. Source localization on the N170 and N220 components was conducted in Experiment 2, and a source in cortical area V4 was identified. These findings suggest that corners contain vital information for the discrimination of shapes. Additionally, this study shows that the perceptual characteristics and neuroanatomical substrates can be detected using electrophysiological measures.

The broad orientation dependence of the motion streak aftereffect reveals interactions between form and motion neurons.

The extended integration time of visual neurons can lead to the production of the neural equivalent of an orientation cue along the axis of motion in response to fast-moving objects. The dominant model argues that these motion streaks resolve the inherent directional uncertainty arising from the small size of receptive fields in V1, by combining spatial orientation with motion signals in V1. This model was tested in humans using visual aftereffects, in which adapting to a static grating causes the perceived direction of a subsequently presented motion stimulus to be tilted away from the adapting orientation. We found that a much broader range of orientations produced aftereffects than predicted by the current model, suggesting that these orientation cues influence motion perception at a later stage than V1. We also found that varying the spatial frequency of the adaptor changed the aftereffect from repulsive to attractive for motion-test but not form-test stimuli. Finally, manipulations of V1 excitability, using transcranial stimulation, reduced the aftereffect, suggesting that the orientation cue is dependent on V1. These results can be accounted for if the orientation information from the motion streak, gathered in V1, enters the motion system at a later stage of motion processing, most likely V5. A computational model of motion direction is presented incorporating gain modifications of broadly tuned motion-selective neurons by narrowly tuned orientation-selective cells in V1, which successfully accounts for the extant data. These results reinforce the suggestion that orientation places strong constraints on motion processing but in a previously undescribed manner.

Modulated textures with shape structures implied by a closed flow are processed globally.

Radial frequency (RF) patterns, shapes deformed from circular by a sinusoidal modulation of radius, have been used to demonstrate global integration of shape information around a closed path by showing that the modulation depth required to detect shape deformation decreases rapidly as larger segments of the contour are modulated. In this psychophysical study we use a field of Gabor patches to examine integration of shape information in sampled RF patterns either alone or placed within an orientation-noise background and show that orientation-noise can be disregarded during the integration of modulation information. We also examine integration in modulated textures with local orientations that flow parallel or perpendicular to an underlying RF shape-structure. In using modulated textures comprising of elements with a random radial position but with orientation modulated such that it conforms to the local orientation of an RF pattern (RF texture) we demonstrate integration around texture patterns that imply shape. Texture patterns with element orientations locally orthogonal (RFO textures) to those of RF textures, however, exhibit a rate of decrease in modulation threshold, which is substantially reduced. When the textures are scrambled by permuting the polar positions of the patches the rate of decrease in threshold with increasing number of patches modulated in orientation is reduced for RF textures but not RFO textures. Detection of modulation in both scrambled textures is shown to be consistent with the detection of local cues. We conclude that implied closure in a modulated flow appears to be critical for global integration of textures.

Tolerance for local and global differences in the integration of shape information.

Shape is a critical cue to object identity. In psychophysical studies, radial frequency (RF) patterns, paths deformed from circular by a sinusoidal modulation of radius, have proved valuable stimuli for the demonstration of global integration of local shape information. Models of the mechanism of integration have focused on the periodicity in measures of curvature on the pattern, despite the fact that other properties covary. We show that patterns defined by rectified sinusoidal modulation also exhibit global integration and are indistinguishable from conventional RF patterns at their thresholds for detection, demonstrating some indifference to the modulating function. Further, irregular patterns incorporating four different frequencies of modulation are globally integrated, indicating that uniform periodicity is not critical. Irregular patterns can be handed in the sense that mirror images cannot be superimposed. We show that mirror images of the same irregular pattern could not be discriminated near their thresholds for detection. The same irregular pattern and a pattern with four cycles of a constant frequency of modulation completing 2π radians were, however, perfectly discriminated, demonstrating the existence of discrete representations of these patterns by which they are discriminated. It has previously been shown that RF patterns of different frequencies are perfectly discriminated but that patterns with the same frequency but different numbers of cycles of modulation were not. We conclude that such patterns are identified, near threshold, by the set of angles subtended at the center of the pattern by adjacent points of maximum convex curvature.

Role of form information in motion pooling and segmentation.

Traditional theories of visual perception have focused on either form or motion processing, implying a functional separation. However, increasing evidence indicates that these features interact at early stages of visual processing. The current study examined a well-known form-motion interaction, where a shape translates along a circular path behind opaque apertures, giving the impression of either independently translating lines (segmentation) or a globally coherent, translating shape. The purpose was to systemically examine how low-level motion information and form information interact to determine which percept is reported. To this end, we used a stimulus with boundaries comprising multiple, spatially-separated Gabor patches with three to eight sides. Results showed that shapes with four or fewer sides appeared to move in a segmented manner, whereas those with more sides were integrated as a solid shape. The separation between directions, rather than the total number of sides, causes this switch between integrated or segmented percepts. We conclude that the change between integration and segmentation depends on whether local motion directions can be independently resolved. We also reconcile previous results on the influence of shape closure on motion integration: Shapes that form open contours cause segmentation, but with no corresponding enhanced sensitivity for shapes forming closed contours. Overall, our results suggest that the resolution of the local motion signal determines whether motion segmentation or integration is perceived with only a small overall influence of form.