Simple actions modulate context-dependent visual size perception at late processing stages.

A simple button press towards a prime stimulus enhances subsequent visual search for objects that match the prime. The present study investigated whether this action effect is a general phenomenon across different task domains, and the underlying neural mechanisms. The action effect was measured in an unspeeded size-matching task, with the presentation of the central target and the surrounding inducers of the Ebbinghaus illusion together to one eye or separately to each eye, and when repetitive TMS was applied over right primary motor cortex (M1). The results showed that a prior key-press significantly reduced the illusion effect compared to passive viewing. Notably, the action effect persisted with dichoptic presentation of the Ebbinghaus configuration, but disappeared with the right M1 disruption. These results suggest that action guides visual perception to influence human behavior, which mainly affects the late visual processing stage and probably relies on feedback projections from the motor cortex.

The contribution of semantic distance knowledge to size constancy in perception and grasping when visual cues are limited.

To achieve a stable perception of object size in spite of variations in viewing distance, our visual system needs to combine retinal image information and distance cues. Previous research has shown that, not only retinal cues, but also extraretinal sensory signals can provide reliable information about depth and that different neural networks (perception versus action) can exhibit preferences in the use of these different sources of information during size-distance computations. Semantic knowledge of distance, a purely cognitive signal, can also provide distance information. Do the perception and action systems show differences in their ability to use this information in calculating object size and distance? To address this question, we presented 'glow-in-the-dark' objects of different physical sizes at different real distances in a completely dark room. Participants viewed the objects monocularly through a 1-mm pinhole. They either estimated the size and distance of the objects or attempted to grasp them. Semantic knowledge was manipulated by providing an auditory cue about the actual distance of the object: "20 cm", "30 cm", and "40 cm". We found that semantic knowledge of distance contributed to some extent to size constancy operations during perceptual estimation and grasping, but size constancy was never fully restored. Importantly, the contribution of knowledge about distance to size constancy was equivalent between perception and action. Overall, our study reveals similarities and differences between the perception and action systems in the use of semantic distance knowledge and suggests that this cognitive signal is useful but not a reliable depth cue for size constancy under restricted viewing conditions.

Network dynamics underlying alterations in apparent object size.

A target circle surrounded by small circles looks larger than an identical circle surrounded by large circles (termed as the Ebbinghaus illusion). While previous research has shown that both early and high-level visual regions are involved in the generation of the illusion, it remains unclear how these regions work together to modulate the illusion effect. Here, we used functional MRI and dynamic causal modelling to investigate the neural networks underlying the illusion in conditions where the focus of attention was manipulated via participants directing their attention to and maintain fixation on only one of the two illusory configurations at a time. Behavioural findings confirmed the presence of the illusion. Accordingly, functional MRI activity in the extrastriate cortex accounted for the illusory effects: apparently larger circles elicited greater activation than apparently smaller circles. Interestingly, this spread of activity for size overestimation was accompanied by a decrease in the inhibitory self-connection in the extrastriate region, and an increase in the feedback connectivity from the precuneus to the extrastriate region. These findings demonstrate that the representation of apparent object size relies on feedback projections from higher- to lower-level visual areas, highlighting the crucial role of top-down signals in conscious visual perception.

Linear perspective cues have a greater effect on the perceptual rescaling of distant stimuli than textures in the virtual environment.

The presence of pictorial depth cues in virtual environments is important for minimising distortions driven by unnatural viewing conditions (e.g., vergence-accommodation conflict). Our aim was to determine how different pictorial depth cues affect size constancy in virtual environments under binocular and monocular viewing conditions. We systematically removed linear perspective cues and textures of a hallway in a virtual environment. The experiment was performed using the method of constant stimuli. The task required participants to compare the size of 'far' (10 m) and 'near' (5 m) circles displayed inside a virtual environment with one or both or none of the pictorial depth cues. Participants performed the experiment under binocular and monocular viewing conditions while wearing a virtual reality headset. ANOVA revealed that size constancy was greater for both the far and the near circles in the virtual environment with pictorial depth cues compared to the one without cues. However, the effect of linear perspective cues was stronger than textures, especially for the far circle. We found no difference between the binocular and monocular viewing conditions across the different virtual environments. We conclude that linear perspective cues exert a stronger effect than textures on the perceptual rescaling of far stimuli placed in the virtual environment, and that this effect does not vary between binocular and monocular viewing conditions.

Temporal features of size constancy for perception and action in a real-world setting: A combined EEG-kinematics study.

A stable representation of object size, in spite of continuous variations in retinal input due to changes in viewing distance, is critical for perceiving and acting in a real 3D world. In fact, our perceptual and visuo-motor systems exhibit size and grip constancies in order to compensate for the natural shrinkage of the retinal image with increased distance. The neural basis of this size-distance scaling remains largely unknown, although multiple lines of evidence suggest that size-constancy operations might take place remarkably early, already at the level of the primary visual cortex. In this study, we examined for the first time the temporal dynamics of size constancy during perception and action by using a combined measurement of event-related potentials (ERPs) and kinematics. Participants were asked to maintain their gaze steadily on a fixation point and perform either a manual estimation or a grasping task towards disks of different sizes placed at different distances. Importantly, the physical size of the target was scaled with distance to yield a constant retinal angle. Meanwhile, we recorded EEG data from 64 scalp electrodes and hand movements with a motion capture system. We focused on the first positive-going visual evoked component peaking at approximately 90 ms after stimulus onset. We found earlier latencies and greater amplitudes in response to bigger than smaller disks of matched retinal size, regardless of the task. In line with the ERP results, manual estimates and peak grip apertures were larger for the bigger targets. We also found task-related differences at later stages of processing from a cluster of central electrodes, whereby the mean amplitude of the P2 component was greater for manual estimation than grasping. Taken together, these findings provide novel evidence that size constancy for real objects at real distances occurs at the earliest cortical stages and that early visual processing does not change as a function of task demands.

Visual illusions in young people reporting psychotic-like experiences.

BACKGROUND AND OBJECTIVES: A disruption in the co-ordination of bottom-up and top-down processing is thought to underlie anomalous perceptual experiences in psychosis. Visual illusions represent a valuable methodology in exploring this disruption. Here, we examined visual illusions in a group of young people having psychotic-like experiences. We also examined the relationship between illusion susceptibility and appraisal of psychotic-like experiences as well as depression, anxiety and stress levels. METHOD: 25 young people reporting psychotic-like experiences and 53 healthy participants performed an adjustment task that measured susceptibility to a battery of 13 visual illusions. Levels of depression, anxiety and stress were quantified in both groups. The clinical group also completed measures examining frequency, appraisals and emotional responses to psychotic-like experiences. RESULTS: A general increase of illusion susceptibility was found in the clinical group compared to the control group. However, when depression, anxiety and stress levels were controlled for, this difference disappeared. Stress turned out to be the best predictor of illusion susceptibility in the clinical group, whereas anomalous experiences, depression and anxiety were unrelated to overall illusion strength. LIMITATIONS: This study is limited to young participants reporting significant mental health difficulties and psychotic-like experiences. Findings should be replicated in an Ultra High Risk (prodromal) group. CONCLUSIONS: Increased levels of stress explained the enhanced vulnerability to illusions in the clinical group. This increased susceptibility suggests a perceptual style that relies too heavily on prior expectations at the expense of the true sensory evidence, potentially leading to an altered perceptual experience of the world.

Looking at the Ebbinghaus illusion: differences in neurocomputational requirements, not gaze-mediated attention, explain a classic perception-action dissociation.

Perceiving and grasping an object present an animal with different sets of computational problems. The solution in primates entails the specialization of separate neural networks for visual processing with different object representations. This explains why the Ebbinghaus illusion minimally affects the grasping hand's in-flight aperture, which normally scales with target size, even though the size of the target disc remains misperceived. An attractive alternative account, however, posits that grasps are refractory to the illusion because participants fixate on the target and fail to attend to the surrounding context. To test this account, we tracked both limb and gaze while participants made forced-choice judgments of relative disc size in the Ebbinghaus illusion or did so in combination with grasping or manually estimating the size of one of the discs. We replicated the classic dissociation: grasp aperture was refractory to the measured illusory effect on perceived size, while judgments and manual estimates of disc size were not. Importantly, the number of display-wide saccades per second and the percentage of total fixation time or fixations directed at the selected disc failed to explain the dissociation. Our findings support the contention that object perception and goal-directed action rely on distinct visual representations. This article is part of a discussion meeting issue 'New approaches to 3D vision'.

Extracurricular sports activities modify the proprioceptive map in children aged 5-8 years.

The Chinese government has recently issued the strictest ever guideline to improve the compulsory education system. The new policy aims at reducing the burden of excessive homework and supplementary tutoring, whilst promoting extracurricular activities, including sports and arts, for primary and junior middle school students. To examine the impact that this reform might have on sensory development-which is critical for higher-order cognitive functions-we assessed proprioceptive abilities in children from 5 to 8 years of age. Proprioception refers to sensations of position and motion of the body in space and is mediated by activity in somatosensory and prefrontal cortical areas. By asking participants to perform position matching tasks in the forward-backward directions, we were able to compare the proprioceptive maps of children with and without regular sports training. We demonstrate that extracurricular sports activities can modify the proprioceptive map and improve proprioceptive acuity and stability in school-aged children.

Fading boundaries between the physical and the social world: Insights and novel techniques from the intersection of these two fields.

This review focuses on the subtle interactions between sensory input and social cognition in visual perception. We suggest that body indices, such as gait and posture, can mediate such interactions. Recent trends in cognitive research are trying to overcome approaches that define perception as stimulus-centered and are pointing toward a more embodied agent-dependent perspective. According to this view, perception is a constructive process in which sensory inputs and motivational systems contribute to building an image of the external world. A key notion emerging from new theories on perception is that the body plays a critical role in shaping our perception. Depending on our arm's length, height and capacity of movement, we create our own image of the world based on a continuous compromise between sensory inputs and expected behavior. We use our bodies as natural "rulers" to measure both the physical and the social world around us. We point out the necessity of an integrative approach in cognitive research that takes into account the interplay between social and perceptual dimensions. To this end, we review long-established and novel techniques aimed at measuring bodily states and movements, and their perception, with the assumption that only by combining the study of visual perception and social cognition can we deepen our understanding of both fields.

A review on various explanations of Ponzo-like illusions.

This article reviews theoretical and empirical arguments for and against various theories that explain the classic Ponzo illusion and its variants from two different viewpoints concerning the role of perceived depth in size distortions. The first viewpoint argues that all Ponzo-like illusions are driven by perceived depth. The second viewpoint argues that the classic Ponzo illusion is unrelated to depth perception. This review will give special focus to the first viewpoint and consists of three sections. In the first section, the role of the number of pictorial depth cues and previous experience in the strength of all Ponzo-like illusions are discussed. In the second section, we contrast the first viewpoint against the theories that explain the classic Ponzo illusion with mechanisms that are unrelated to depth perception. In the last section, we propose a Bayesian-motivated reconceptualization of Richard Gregory's misapplied size constancy theory that explains Ponzo-variant illusions in terms of prior information and prediction errors. The new account explains why some studies have provided inconsistent evidence for misapplied size constancy theory.

Developmental Trajectories of Size Constancy as Implicitly Examined by Simple Reaction Times.

It is still unclear whether size constancy is an innate ability or whether it develops with age. As many developmental studies are limited to the child's comprehension of the task instructions, here, an implicit measure of perceived size, namely, simple manual reaction time (RT), was opted for based on the assumption that perceptually bigger objects generate faster detection times. We examined size constancy in children (from 5 to 14 years of age) and adults using a simple RT approach. Participants were presented with pictures of tennis balls on a screen that was physically moved to two viewing distances. Visual stimuli were adjusted in physical size in order to subtend the same visual angle across distances, determining two conditions: a small-near tennis ball vs. a big-far tennis ball. Thanks to size constancy, the two tennis balls were perceived as different even though they were of equal size on the retina. Stimuli were also matched in terms of luminance. Participants were asked to react as fast as possible to the onset of the stimuli. The results show that the RTs reflected the perceived rather than the retinal size of the stimuli across the different age groups, such that participants responded faster to stimuli that were perceived as bigger than those perceived as smaller. Hence, these findings are consistent with the idea that size constancy is already present in early childhood, at least from the age of five, and does not require extensive visual learning.

The conceptual understanding of depth rather than the low-level processing of spatial frequencies drives the corridor illusion.

Our objective was to determine how different spatial frequencies affect the perceptual size rescaling of stimuli in the corridor illusion. Two experiments were performed using the method of constant stimuli. In experiment 1, the task required participants to compare the size of comparison and standard rings displayed over the same background image. ANOVA on the points of subject equality (PSEs) revealed that the perceived size of the top and bottom standard rings changed as a function of the availability of the high, medium, and low spatial frequency information. In experiment 2, the task required participants to compare the size of a comparison ring presented outside of the background image with a standard ring presented inside it. ANOVA on the PSEs revealed that the apparent size of the top and not the bottom standard ring changed depending on the availability of medium spatial frequency information. Eye-tracking revealed that the spatial frequency range of the background image in the periphery affected participants' eye positioning, which may explain why the effects of different spatial frequencies fluctuated across experiments. Nonetheless, when we consider these findings together, we propose that the conceptual understanding of depth plays a more important role in explaining the corridor illusion than the low-level processing of depth information extracted from different spatial frequencies along separate channels.

Interocular transfer effects of linear perspective cues and texture gradients in the perceptual rescaling of size.

Our objective was to determine whether the influence of linear perspective cues and texture gradients in the perceptual rescaling of stimulus size transfers from one eye to the other. In experiment 1, we systematically added linear perspective cues and texture gradients in a background image of the corridor illusion. To determine whether perceptual size rescaling takes place at earlier or later stages, we tested how the perceived size of top and bottom rings changed under binocular (rings and background presented to both eyes), monocular (rings and background presented to the dominant eye only), and dichoptic (rings and background presented separately to the dominant and nondominant eyes, respectively) viewing conditions. We found differences between viewing conditions in the perceived size of the rings when linear perspective cues, but not texture gradients, were presented. Specifically, linear perspective cues produced a stronger illusion under the monocular compared to the dichoptic viewing condition. Hence, there was partial interocular transfer from the linear perspective cues, suggesting a dominant role of monocular neural populations in mediating the corridor illusion. In experiment 2, we repeated similar procedures with a more traditional Ponzo illusion background. Contrary to findings from experiment 1, there was a full interocular transfer with the presence of the converging lines, suggesting a dominant role of binocular neural populations. We conclude that higher order visual areas, which contain binocular neural populations, are more involved in the perceptual rescaling of size evoked by linear perspective cues in the Ponzo compared to the corridor illusion.

Grip Constancy but Not Perceptual Size Constancy Survives Lesions of Early Visual Cortex.

Object constancies are central constructs in theories of visual phenomenology. A powerful example is "size constancy," in which the perceived size of an object remains stable despite changes in viewing distance [1-4]. Evidence from neuropsychology [5], neuroimaging [6-11], transcranial magnetic stimulation [12, 13], single-unit and lesion studies in monkey [14-20], and computational modeling [21] suggests that re-entrant processes involving reciprocal interactions between primary visual cortex (V1) and extrastriate visual areas [22-26] play an essential role in mediating size constancy. It is seldom appreciated, however, that object constancies must also operate for the visual guidance of goal-directed action. For example, when reaching out to pick up an object, the hand's in-flight aperture scales with size of the goal object [27-30] and is refractory to the decrease in retinal-image size with increased viewing distance [31-41] (Figure 1), a phenomenon we call "grip constancy." Does grip constancy, like perceptual constancy, depend on V1 or can it be mediated by pathways that bypass it altogether? We tested these possibilities in an individual, M.C., who has bilateral lesions encompassing V1 and much of the ventral visual stream. We show that her perceptual estimates of object size co-vary with retinal-image size rather than real-world size as viewing distance varies. In contrast, M.C. shows near-normal scaling of in-flight grasp aperture to object size despite changes in viewing distance. Thus, although early visual cortex is necessary for perceptual object constancy, it is unnecessary for grip constancy, which is mediated instead by separate visual inputs to dorsal-stream visuomotor areas [42-48].

The contribution of stereopsis in Emmert's law.

Size constancy is the ability to perceive objects as remaining constant in size regardless of their distance from the observer. Emmert's law demonstrates that viewing distance determines the perceived size of afterimages according to the amount of depth cues that are available. Using an afterimage paradigm, we examined to what extent removing stereopsis and other depth cues affects size-distance scaling. Thirty participants 'projected' afterimages onto a surface presented at different distances under binocular, monocular, and eyes-closed conditions. The perceived size of the afterimages closely followed the size-distance scaling predictions made by Emmert's law under binocular testing conditions, when all depth cues were available. In contrast, monocular testing decreased adherence to Emmert's law, while the eyes-closed condition resulted in a greater breakdown of size-distance scaling. Because we used an afterimage paradigm, this study provides the first demonstration of how perceived size is modulated by the availability of depth cues under conditions with a constant retinal image stimulus.

The contribution of linear perspective cues and texture gradients in the perceptual rescaling of stimuli inside a Ponzo illusion corridor.

We examined the influence of linear perspective cues and texture gradients in the perceptual rescaling of stimuli over a highly-salient Ponzo illusion of a corridor. We performed two experiments using the Method of Constant Stimuli where participants judged the size of one of two rings. In experiment 1, one ring was presented in the upper visual-field at the end of the corridor and the other in the lower visual-field at the front of the corridor. The perceived size of the top and bottom rings changed as a function of the availability of linear perspective and textures. In experiment 2, only one ring was presented either at the top or the bottom of the image. The perceived size of the top but not the bottom ring changed as a function of the availability of linear perspective and textures. In both experiments, the effects of the cues were additive. Perceptual rescaling was also stronger for the top compared to the bottom ring. Additional eye-tracking revealed that participants tended to gaze more in the upper than the lower visual-field. These findings indicate that top-down mechanisms provide an important contribution to the Ponzo illusion. Nonetheless, additional maximum likelihood estimation analyses revealed that linear perspective fulfilled a greater contribution in experiment 2, which is suggestive of a bottom-up mechanism. We conclude that both top-down and bottom-up mechanisms play important roles. However, the former seems to fulfil a more prominent role when both stimuli are presented in the image.

Changing the Real Viewing Distance Reveals the Temporal Evolution of Size Constancy in Visual Cortex.

Our visual system provides a distance-invariant percept of object size by integrating retinal image size with viewing distance (size constancy). Single-unit studies with animals have shown that some distance cues, especially oculomotor cues such as vergence and accommodation, can modulate the signals in the thalamus or V1 at the initial processing stage [1-7]. Accordingly, one might predict that size constancy emerges much earlier in time [8-10], even as visual signals are being processed in the thalamus. So far, the studies that have looked directly at size coding have either used fMRI (poor temporal resolution [11-13]) or relied on inadequate stimuli (pictorial illusions presented on a monitor at a fixed distance [11, 12, 14, 15]). Here, we physically moved the monitor to different distances, a more ecologically valid paradigm that emulates what happens in everyday life and is an example of the increasing trend of "bringing the real world into the lab." Using this paradigm in combination with electroencephalography (EEG), we examined the computation of size constancy in real time with real-world viewing conditions. Our study provides strong evidence that, even though oculomotor distance cues have been shown to modulate the spiking rate of neurons in the thalamus and in V1, the integration of viewing distance cues and retinal image size takes at least 150 ms to unfold, which suggests that the size-constancy-related activation patterns in V1 reported in previous fMRI studies (e.g., [12, 13]) reflect the later processing within V1 and/or top-down input from other high-level visual areas.

Perceptual size discrimination requires awareness and late visual areas: A continuous flash suppression and interocular transfer study.

We applied continuous flash suppression (CFS) during an interocular transfer paradigm to evaluate the importance of awareness and the contribution of early versus late visual structures in size recognition. Specifically, we tested if size judgements of a visible target could be influenced by a congruent or incongruent prime presented to the same or different eye. Without CFS, participants categorised a target as "small" or "large" more quickly when it was preceded by a congruent prime - regardless of whether the prime and target were presented to the same or different eye. Interocular transfer enabled us to infer that the observed priming was mediated by late visual areas. In contrast, there was no priming under CFS, which underscores the importance of awareness. We conclude that awareness and late visual structures are important for size perception and that any subconscious processing of the stimulus has minimal effect on size recognition.