Internal curvature signal and noise in low- and high-level vision

Similarity in motion binds and bends judgments of aspect ratio

It is well known that objects become grouped in perceptual organization when they share some visual feature, like a common direction of motion. Less well known is that grouping can change how people perceive a set of objects. For example, when a pair of shapes consistently share a common region of space, their aspect ratios tend to be perceived as more similar (are attracted toward each other). Conversely, when shapes are assigned to different regions in space their aspect ratios repel from each other. Here we examine whether the visual system produce both attractive and repulsive distortions when the state of grouping between a pair of shapes changes on a moment-to-moment basis. Observers viewed a pair of ellipses that differed in terms of how flat or tall they were and reported the aspect ratio of one ellipse from the pair. Each ellipse was defined by a cloud of coherently-moving dots, and the dots within the two ellipses had either the same or different directions of motion, varying from trial-to-trial. We found that the cued ellipse's aspect ratio was reported to be repelled from the aspect ratio of the uncued ellipse when the shapes had different directions of motion compared to when they had the same direction of motion. These results suggest that the visual system can adaptively alter visual experience based on grouping, in particular, repelling the appearance of objects when they do not appear to go together, and it can do so quickly and flexibly.

Human Symmetry Uncertainty Detected by a Self-Organizing Neural Network Map

Symmetry in biological and physical systems is a product of self-organization driven by evolutionary processes, or mechanical systems under constraints. Symmetry-based feature extraction or representation by neural networks may unravel the most informative contents in large image databases. Despite significant achievements of artificial intelligence in recognition and classification of regular patterns, the problem of uncertainty remains a major challenge in ambiguous data. In this study, we present an artificial neural network that detects symmetry uncertainty states in human observers. To this end, we exploit a neural network metric in the output of a biologically inspired Self-Organizing Map Quantization Error (SOM-QE). Shape pairs with perfect geometry mirror symmetry but a non-homogenous appearance, caused by local variations in hue, saturation, or lightness within and/or across the shapes in a given pair produce, as shown here, a longer choice response time (RT) for “yes” responses relative to symmetry. These data are consistently mirrored by the variations in the SOM-QE from unsupervised neural network analysis of the same stimulus images. The neural network metric is thus capable of detecting and scaling human symmetry uncertainty in response to patterns. Such capacity is tightly linked to the metric’s proven selectivity to local contrast and color variations in large and highly complex image data.

Ensemble perception includes information from multiple spatial scales

Most visual scenes contain information at different spatial scales, including the local and global, or the detail and gist. Global processes have become increasingly implicated in research examining summary statistical perception, initially as the output of ensemble coding, and more recently as a gating mechanism for selecting which information is included in the averaging process itself. Yet local and global processing are known to be rapidly integrated by the visual system, and it is plausible that global-level information, like spatial organization, may be included as an input during ensemble coding. We tested this hypothesis using an ensemble shape-perception task in which observers evaluated the mean aspect ratios of sets of ellipses. In addition to varying the aspect ratios of the individual shapes, we independently varied the spatial arrangements of the sets so that they had either flat or tall organizations at the global level. We found that observers made precise summary judgments about the average aspect ratios of the sets by integrating information from multiple shapes. More importantly, global flat and tall organizations were incorporated into ensemble judgments about the sets; summary judgments were biased in the directions of the global spatial arrangements on each trial. This global-to-local integration even occurred when the global organizations were masked. Our results demonstrate that the process of summary representation can include information from both the local and global scales. The gist is not just an output of ensemble representation - it can be included as an input to the mechanism itself.

Perceptual averaging of facial expressions requires visual awareness and attention

Humans, as highly social animals, are regularly exposed to the faces of conspecifics-often more than one at a time. This feature of social living is important for understanding face perception, not just because it means that information from faces is available in bulk, but also because it changes the way individuals are perceived. For instance, when two faces are seen nearby one another, they tend to look like each other. This phenomenon of perceptual averaging is robust when both faces are seen and attended. But in everyday life, some faces may not receive the full benefit of attention, or they may not be visible at all. We evaluated whether perceptual averaging of relatively complex and simple information on faces, including facial expression and head orientation, can still occur even in these circumstances. In particular, we used object-substitution masking (OSM) and a dual-task designed to disrupt visual awareness and attention, respectively, during evaluations of briefly presented face pairs. Disruptions of awareness or attention eliminated averaging of facial expression, whereas orientation averaging persisted in spite of these challenges. These results demonstrate boundary conditions for the process of perceptual averaging. More generally, they provide insight into how the visual system processes multitudes of objects, both simple and complex, both with and without attention and awareness.

Object-substitution masking weakens but does not eliminate shape interactions

At any moment, some objects in the environment are seen clearly, whereas others go unnoticed. Whether or not these gaps in awareness are actually problematic may depend on the extent that information about unseen objects is lost. Determining when and how visual awareness and visual processing become linked is thus of great importance. Previous research using object-substitution masking (OSM) demonstrated that relatively simple visual features, such as size or orientation, are still processed even when they are not visible. Yet this does not appear to be the case for more complex features like faces. This suggests that, during OSM, disruptions of visual processing and awareness may tend to co-occur beginning at some intermediate stage along the ventral pathway. We tested this hypothesis by evaluating the extent to which OSM disrupted the perception and processing of two-dimensional objects. Specifically, we evaluated whether an unseen shape's aspect ratio would influence the appearance of another shape that was briefly visible nearby. As expected, the aspect ratios of two shapes appeared to be more similar to each other when both were visible. This averaging effect was weakened, but not eliminated, when one ellipse in each pair received OSM. These shape interactions persisted even when one ellipse from each pair was invisible. When combined with previous work, these results suggest that during object-substitution masking, disruptions of visual processing tend to strengthen with increases in stimulus complexity, becoming more tightly bound to the mechanisms of visual awareness at intermediate stages of visual analysis.

2D Geometry Predicts Perceived Visual Curvature in Context-Free Viewing

Planar geometry was exploited for the computation of symmetric visual curves in the image plane, with consistent variations in local parameters such as sagitta, chordlength, and the curves' height-to-width ratio, an indicator of the visual area covered by the curve, also called aspect ratio. Image representations of single curves (no local image context) were presented to human observers to measure their visual sensation of curvature magnitude elicited by a given curve. Nonlinear regression analysis was performed on both the individual and the average data using two types of model: (1) a power function where y (sensation) tends towards infinity as a function of x (stimulus input), most frequently used to model sensory scaling data for sensory continua, and (2) an “exponential rise to maximum” function, which converges towards an asymptotically stable level of y as a function of x. Both models provide satisfactory fits to subjective curvature magnitude as a function of the height-to-width ratio of single curves. The findings are consistent with an in-built sensitivity of the human visual system to local curve geometry, a potentially essential ground condition for the perception of concave and convex objects in the real world.

Effects of awareness on numerosity adaptation

Numerosity perception is a process involving several stages of visual processing. This study investigated whether distinct mechanisms exist in numerosity adaptation under different awareness conditions to characterize how numerosity perception occurs at each stage. The status of awareness was controlled by masking conditions, in which monoptic and dichoptic masking were proposed to influence different levels of processing. Numerosity adaptation showed significant aftereffects when the participants were aware (monoptic masking) and unaware (dichoptic masking) of adaptors. The interocular transfer for numerosity adaptation was distinct under the different awareness conditions. Adaptation was primarily binocular when participants were aware of stimuli and was purely monocular when participants were unaware of adaptors. Moreover, numerosity adaptation was significantly reduced when the adaptor dots were clustered into chunks with awareness, whereas clustering had no effect on unaware adaptation. These results show that distinct mechanisms exist in numerosity processing under different awareness conditions. It is suggested that awareness is crucial to numerosity cognition. With awareness, grouping (by clustering) influences numerosity coding through altered object representations, which involves higher-level cognitive processing.

Human decision making based on variations in internal noise: an EEG study

Perceptual decision making is prone to errors, especially near threshold. Physiological, behavioural and modeling studies suggest this is due to the intrinsic or ‘internal’ noise in neural systems, which derives from a mixture of bottom-up and top-down sources. We show here that internal noise can form the basis of perceptual decision making when the external signal lacks the required information for the decision. We recorded electroencephalographic (EEG) activity in listeners attempting to discriminate between identical tones. Since the acoustic signal was constant, bottom-up and top-down influences were under experimental control. We found that early cortical responses to the identical stimuli varied in global field power and topography according to the perceptual decision made, and activity preceding stimulus presentation could predict both later activity and behavioural decision. Our results suggest that activity variations induced by internal noise of both sensory and cognitive origin are sufficient to drive discrimination judgments.

Laughter exaggerates happy and sad faces depending on visual context

Laughter is an auditory stimulus that powerfully conveys positive emotion. We investigated how laughter influenced the visual perception of facial expressions. We presented a sound clip of laughter simultaneously with a happy, a neutral, or a sad schematic face. The emotional face was briefly presented either alone or among a crowd of neutral faces. We used a matching method to determine how laughter influenced the perceived intensity of the happy, neutral, and sad expressions. For a single face, laughter increased the perceived intensity of a happy expression. Surprisingly, for a crowd of faces, laughter produced an opposite effect, increasing the perceived intensity of a sad expression in a crowd. A follow-up experiment revealed that this contrast effect may have occurred because laughter made the neutral distractor faces appear slightly happy, thereby making the deviant sad expression stand out in contrast. A control experiment ruled out semantic mediation of the laughter effects. Our demonstration of the strong context dependence of laughter effects on facial expression perception encourages a reexamination of the previously demonstrated effects of prosody, speech content, and mood on face perception, as they may be similarly context dependent.

Sounds exaggerate visual shape

While perceiving speech, people see mouth shapes that are systematically associated with sounds. In particular, a vertically stretched mouth produces a /woo/ sound, whereas a horizontally stretched mouth produces a /wee/ sound. We demonstrate that hearing these speech sounds alters how we see aspect ratio, a basic visual feature that contributes to perception of 3D space, objects and faces. Hearing a /woo/ sound increases the apparent vertical elongation of a shape, whereas hearing a /wee/ sound increases the apparent horizontal elongation. We further demonstrate that these sounds influence aspect ratio coding. Viewing and adapting to a tall (or flat) shape makes a subsequently presented symmetric shape appear flat (or tall). These aspect ratio aftereffects are enhanced when associated speech sounds are presented during the adaptation period, suggesting that the sounds influence visual population coding of aspect ratio. Taken together, these results extend previous demonstrations that visual information constrains auditory perception by showing the converse - speech sounds influence visual perception of a basic geometric feature.

Reference repulsion in the categorical perception of biological motion

Perceiving biological motion is important for understanding the intentions and future actions of others. Perceiving an approaching person's behavior may be particularly important, because such behavior often precedes social interaction. To this end, the visual system may devote extra resources for perceiving an oncoming person's heading. If this were true, humans should show increased sensitivity for perceiving approaching headings, and as a result, a repulsive perceptual effect around the categorical boundary of leftward/rightward motion. We tested these predictions and found evidence for both. First, observers were especially sensitive to the heading of an approaching person; variability in estimates of a person's heading decreased near the category boundary of leftward/rightward motion. Second, we found a repulsion effect around the category boundary; a person walking approximately toward the observer was perceived as being repelled away from straight ahead. This repulsive effect was greatly exaggerated for perception of a very briefly presented person or perception of a chaotic crowd, suggesting that repulsion may protect against categorical errors when sensory noise is high. The repulsion effect with a crowd required integration of local motion and human form, suggesting an origin in high-level stages of visual processing. Similar repulsive effects may underlie categorical perception with other social features. Overall, our results show that a person's direction of walking is categorically perceived, with improved sensitivity at the category boundary and a concomitant repulsion effect.

Awareness becomes necessary between adaptive pattern coding of open and closed curvatures

Visual pattern processing becomes increasingly complex along the ventral pathway, from the low-level coding of local orientation in the primary visual cortex to the high-level coding of face identity in temporal visual areas. Previous research using pattern aftereffects as a psychophysical tool to measure activation of adaptive feature coding has suggested that awareness is relatively unimportant for the coding of orientation, but awareness is crucial for the coding of face identity. We investigated where along the ventral visual pathway awareness becomes crucial for pattern coding. Monoptic masking, which interferes with neural spiking activity in low-level processing while preserving awareness of the adaptor, eliminated open-curvature aftereffects but preserved closed-curvature aftereffects. In contrast, dichoptic masking, which spares spiking activity in low-level processing while wiping out awareness, preserved open-curvature aftereffects but eliminated closed-curvature aftereffects. This double dissociation suggests that adaptive coding of open and closed curvatures straddles the divide between weakly and strongly awareness-dependent pattern coding.