Induction of Kanizsa Contours Requires Awareness of the Inducing Context

It remains unknown to what extent the human visual system interprets information about complex scenes without conscious analysis. Here we used visual masking techniques to assess whether illusory contours (Kanizsa shapes) are perceived when the inducing context creating this illusion does not reach awareness. In the first experiment we tested perception directly by having participants discriminate the orientation of an illusory contour. In the second experiment, we exploited the fact that the presence of an illusory contour enhances performance on a spatial localization task. Moreover, in the latter experiment we also used a different masking method to rule out the effect of stimulus duration. Our results suggest that participants do not perceive illusory contours when they are unaware of the inducing context. This is consistent with theories of a multistage, recurrent process of perceptual integration. Our findings thus challenge some reports, including those from neurophysiological experiments in anaesthetized animals. Furthermore, we discuss the importance to test the presence of the phenomenal percept directly with appropriate methods

A new method for mapping perceptual biases across visual space

How we perceive the environment is not stable and seamless. Recent studies found that how a person qualitatively experiences even simple visual stimuli varies dramatically across different locations in the visual field. Here we use a method we developed recently that we call multiple alternatives perceptual search (MAPS) for efficiently mapping such perceptual biases across several locations. This procedure reliably quantifies the spatial pattern of perceptual biases and also of uncertainty and choice. We show that these measurements are strongly correlated with those from traditional psychophysical methods and that exogenous attention can skew biases without affecting overall task performance. Taken together, MAPS is an efficient method to measure how an individual's perceptual experience varies across space

Spatial Heterogeneity in Bistable Figure-Ground Perception

The appearance of visual objects varies substantially across the visual field. Could such spatial heterogeneity be due to undersampling of the visual field by neurons selective for stimulus categories? Here, we show that which parts of a bistable vase-face image observers perceive as figure and ground depends on the retinal location where the image appears. The spatial patterns of these perceptual biases were similar regardless of whether the images were upright or inverted. Undersampling by neurons tuned to an object class (e.g., faces) or variability in general local versus global processing cannot readily explain this spatial heterogeneity. Rather, these biases could result from idiosyncrasies in low-level sensitivity across the visual field

Neuroplasticity and amblyopia: vision at the balance point

Purpose of review: New insights into triggers and brakes of plasticity in the visual system are being translated into new treatment approaches which may improve outcomes not only in children, but also in adults. / Recent findings: Visual experience-driven plasticity is greatest in early childhood, triggered by maturation of inhibitory interneurons which facilitate strengthening of synchronous synaptic connections, and inactivation of others. Normal binocular development leads to progressive refinement of monocular visual acuity, stereoacuity and fusion of images from both eyes. At the end of the ‘critical period’, structural and functional brakes such as dampening of acetylcholine receptor signalling and formation of perineuronal nets limit further synaptic remodelling. Imbalanced visual input from the two eyes can lead to imbalanced neural processing and permanent visual deficits, the commonest of which is amblyopia. / Summary: The efficacy of new behavioural, physical and pharmacological interventions aiming to balance visual input and visual processing have been described in humans, and some are currently under evaluation in randomised controlled trials. Outcomes may change amblyopia treatment for children and adults, but the safety of new approaches will need careful monitoring, as permanent adverse events may occur when plasticity is re-induced after the end of the critical period

Inferior Occipital Gyrus Is Organized along Common Gradients of Spatial and Face-Part Selectivity

The ventral visual stream of the human brain is subdivided into patches with categorical stimulus preferences, like faces or scenes. However, the functional organization within these areas is less clear. Here, we used functional magnetic resonance imaging and vertex-wise tuning models to independently probe spatial and face-part preferences in the inferior occipital gyrus (IOG) of healthy adult males and females. The majority of responses were well explained by Gaussian population tuning curves for both retinotopic location and the preferred relative position within a face. Parameter maps revealed a common gradient of spatial and face-part selectivity, with the width of tuning curves drastically increasing from posterior to anterior IOG. Tuning peaks clustered more idiosyncratically but were also correlated across maps of visual and face space. Preferences for the upper visual field went along with significantly increased coverage of the upper half of the face, matching recently discovered biases in human perception. Our findings reveal a broad range of neural face-part selectivity in IOG, ranging from narrow to “holistic.” IOG is functionally organized along this gradient, which in turn is correlated with retinotopy

Population receptive field estimates for motion-defined stimuli

The processing of motion changes throughout the visual hierarchy, from spatially restricted ‘local motion’ in early visual cortex to more complex large-field ‘global motion’ at later stages. Here we used functional magnetic resonance imaging (fMRI) to examine spatially selective responses in these areas related to the processing of random-dot stimuli defined by differences in motion. We used population receptive field (pRF) analyses to map retinotopic cortex using bar stimuli comprising coherently moving dots. In the first experiment, we used three separate background conditions: no background dots (dot-defined bar-only), dots moving coherently in the opposite direction to the bar (kinetic boundary) and dots moving incoherently in random directions (global motion). Clear retinotopic maps were obtained for the bar-only and kinetic-boundary conditions across visual areas V1-V3 and in higher dorsal areas. For the global-motion condition, retinotopic maps were much weaker in early areas and became clear only in higher areas, consistent with the emergence of global-motion processing throughout the visual hierarchy. However, in a second experiment we demonstrate that this pattern is not specific to motion-defined stimuli, with very similar results for a transparent-motion stimulus and a bar defined by a static low-level property (dot size) that should have driven responses particularly in V1. We further exclude explanations based on stimulus visibility by demonstrating that the observed differences in pRF properties do not follow the ability of observers to localise or attend to these bar elements. Rather, our findings indicate that dorsal extrastriate retinotopic maps may primarily be determined by the visibility of the neural responses to the bar relative to the background response (i.e. neural signal-to-noise ratios) and suggests that claims about stimulus selectivity from pRF experiments must be interpreted with caution

Subjective size perception depends on central visual cortical magnification in human v1.

In the Ebbinghaus illusion, the context surrounding an object modulates its subjectively perceived size. Previous work implicates human primary visual cortex (V1) as the neural substrate mediating this contextual effect. Here we studied in healthy adult humans how two different types of context (large or small inducers) in this illusion affected size perception by comparing each to a reference stimulus without any context. We found that individual differences in the magnitudes of the illusion produced by either type of context were correlated with V1 area defined through retinotopic mapping using functional MRI. However, participants' objective ability to discriminate the size of objects presented in isolation was unrelated to illusion strength and did not correlate with V1 area. Control analyses showed no correlations between behavioral measures and the overall V1 area estimated probabilistically on the basis of neuroanatomy alone. Therefore, subjective size perception correlated with variability in central cortical magnification rather than the anatomical extent of primary visual cortex. We propose that such changes in subjective perception of size are mediated by mechanisms that scale with the extent to which an individual's V1 selectively represents the central visual field

Individual differences in visual salience vary along semantic dimensions

What determines where we look? Theories of attentional guidance hold that image features and task demands govern fixation behavior, while differences between observers are interpreted as a “noise-ceiling” that strictly limits predictability of fixations. However, recent twin studies suggest a genetic basis of gaze-trace similarity for a given stimulus. This leads to the question of how individuals differ in their gaze behavior and what may explain these differences. Here, we investigated the fixations of >100 human adults freely viewing a large set of complex scenes containing thousands of semantically annotated objects. We found systematic individual differences in fixation frequencies along six semantic stimulus dimensions. These differences were large (>twofold) and highly stable across images and time. Surprisingly, they also held for first fixations directed toward each image, commonly interpreted as “bottom-up” visual salience. Their perceptual relevance was documented by a correlation between individual face salience and face recognition skills. The set of reliable individual salience dimensions and their covariance pattern replicated across samples from three different countries, suggesting they reflect fundamental biological mechanisms of attention. Our findings show stable individual differences in salience along a set of fundamental semantic dimensions and that these differences have meaningful perceptual implications. Visual salience reflects features of the observer as well as the image

Feature-location effects in the Thatcher illusion

Face perception is impaired for inverted images, and a prominent example of this is the Thatcher illusion: “Thatcherized” (i.e., rotated) eyes and mouths make a face look grotesque, but only if the whole face is seen upright rather than inverted. Inversion effects are often interpreted as evidence for configural face processing. However, recent findings have led to the alternative proposal that the Thatcher illusion rests on orientation sensitivity for isolated facial regions. Here, we tested whether the Thatcher effect depends not only on the orientation of facial regions but also on their visual-field location. Using a match-to-sample task with isolated eye and mouth regions we found a significant Feature × Location interaction. Observers were better at discriminating Thatcherized from normal eyes in the upper compared to the lower visual field, and vice versa for mouths. These results show that inversion effects can at least partly be driven by nonconfigural factors and that one of these factors is a match between facial features and their typical visual-field location. This echoes recent results showing feature–location effects in face individuation. We discuss the role of these findings for the hypothesis that spatial and feature tuning in the ventral stream are linked