Perception

Luminance ratios along shadow edges remain the same even when they cross reflectance borders. According to Gilchrist (1988, Perception & Psychophysics 43 415–424) this so-called ratio-invariance property is a crucial factor in the perception of shadows. However, Soranzo and Agostini (2004, Perception 33 1359–1368) suggested that in some conditions (named ‘impossible shadows’), a luminance pattern might still be perceived as a shadow even if the ratio-invariance property along its edge is violated. This can occur when an edge is collinear with another edge (contextual edge) which incorporates it, shares the same polarity, and generates a larger ratio. The hypothesis that impossible shadows are actually perceived as shadows is here tested by comparing the perceptual contrast of a luminance edge in the absence of a contextual edge (control condition) to that of both possible shadow edges (where the contextual and mediating edge share the same ratio) and impossible shadow edges (where the ratio of the contextual edge is larger rather than that at the mediating edge). We found that the perceived contrast of luminance edges shrinks in both possible and impossible shadow conditions rather than in the control condition. This evidence supports the hypothesis that a luminance pattern might be perceived as a shadow even when the ratio-invariance property is violated.

Previous studies of visual search for illusory figures have provided equivocal results, with efficient search for Kanizsa squares (eg Davis and Driver, 1994 Nature 371 291–293) contrasting with inefficient search for Kanizsa triangles (eg Grabowecky and Treisman, 1989 Investigative Ophthalmology and Visual Science 30 457). Here, we investigated whether shape orientation can explain these differences. The results from three experiments replicated previous findings: Kanizsa squares in experiment 1 could be detected more efficiently than Kanizsa triangles in experiment 2. In addition, when controlling for stimulus complexity in experiment 3, we found search for Kanizsa diamonds intermediate in efficiency. Taken together, these results suggest an oblique effect in search for Kanizsa figures with cardinal shape orientations leading to more efficient performance than oblique shape orientations. Our findings indicate that both shape orientation and stimulus complexity affect search for illusory figures.

The perception of visual illusions is a powerful diagnostic of implicit integration of global information. Many illusions occur when length, size, orientation, or luminance are misjudged because neighboring visuospatial information cannot be ignored. We asked if people with Williams syndrome (WS), a rare genetic disorder that results in severely impaired global visuospatial construction abilities, are also susceptible to the context of visual illusions. Remarkably, we found that illusions influenced WS individuals to the same degree as normal adults, although size discrimination was somewhat impaired in WS. Our results are evidence that illusions are a consequence of the brain's bias to implicitly integrate visual information, even in a population known to have difficulty in explicitly representing spatial relationships among objects. Moreover, these results suggest that implicit and non-implicit integration of spatial information have different vulnerabilities in abnormal development.

We investigated here how partly occluded stimuli affect early and late prehension kinematics. Stimuli were pictures of partly occluded cylindrical objects with indentations or protrusions at regular intervals along the contour. They diverged into two plausible completions: a local completion, comprising a linear continuation of the contour, and a global completion, revealing a continuation of the contour indentations. Convergent stimuli with one plausible completion acted as controls. Twenty-eight participants repeatedly performed a grasping movement in darkness, as if they grasped the earlier-seen partly occluded cylinder. The peak acceleration of the hand opening (occurring at about 20% of the movement time) and the maximum hand aperture (at about 60% of the movement time) were taken to reflect the representation of the width of the stimuli used for action. The results show that the grasping movements were affected not only by local but also by global stimulus properties of which the representation remained stable over time.

When the two eyes view dissimilar monocular stimuli, the resulting interocular suppression can spread beyond the region of explicit stimulus conflict: portions of one rival target will disappear even though there is no competing stimulation at the corresponding location in the other eye's view. In a series of experiments we examined whether this spread of suppression is spatially isotropic or governed by the configuration of the stimulus a portion of which is subject to suppression. Observers reported the incidence of stimulus disappearance at different locations along or nearby the contours of a large figure, part of which was suppressed by presentation of a continuous flash-suppression stimulus to a restricted region of the other eye. For all observers, suppression spread over several degrees along the contours of the figure, but tended not to spread to locations nearby but disconnected from the figure. Suppression spread effectively over a smoothly curved contour, and it spread around a sharp corner defined by two abutting contours, albeit less effectively. Suppression tended not to spread to features within the interior of a figure (a face), even if those features formed an integral part of the figure. A gap within a spatially extended stimulus arrested the spread of suppression, unless that gap appeared to arise from occlusion. Spread of suppression was unrelated to sensory eye dominance and was found with a more conventional binocular rivalry configuration, too. These findings implicate the involvement of neural circuitry in which inhibition propagates along paths of excitation beyond spatial regions of explicit interocular conflict.

Other-race faces are generally recognised more poorly than own-race faces. According to Levin's influential race-coding hypothesis, this other-race recognition deficit results from spontaneous coding of race-specifying information, at the expense of individuating information, in other-race faces. Therefore, requiring participants to code race-specifying information for all faces should eliminate the other-race effect by reducing recognition of own-race faces to the level of other-race faces. We tested this prediction in two experiments. Race coding was induced by requiring participants to rate study faces on race typicality (experiment 1) or to categorise them by race (experiment 2). Neither manipulation reduced the other-race effect, providing no support for the race-coding hypothesis. Instead, race-coding instructions marginally increased the other-race effect in experiment 1 and had no effect in experiment 2. These results do not support the race-coding hypothesis. Surprisingly, a control task of rating the attractiveness of study faces increased the other-race effect, indicating that deeper encoding of faces does not necessarily reduce the effect (experiment 1). Finally, the normally robust other-race effect was absent when participants were instructed to individuate other-race faces (experiment 2). We suggest that poorer recognition of other-race faces may reflect reduced perceptual expertise with such faces and perhaps reduced motivation to individuate them.

Apperceptive prosopagnosia is supposedly characterised by impaired configural processing, which could refer to either perception of spatial structure or holistic mechanisms. Ten prosopagnosic patients were tested with (i) dot patterns, to determine if manipulations of complexity, size, orientation, or the regularity of global structure generated effects consistent with the holistic hypothesis; and (ii) hierarchical letters, to probe for a global ‘whole-object’ processing deficit. With dot patterns (experiment 1) patients were impaired even for simple two-dot stimuli, but did better with more complex patterns, when size or orientation varied, or with a regular global structure. In experiment 2, they demonstrated normal latency effects of global-level processing. Apperceptive prosopagnosia, including that from lesions encompassing the right fusiform gyrus, is thus associated with a ‘configural deficit’ that impairs perception of spatial structure, not just for faces but also for non-facial patterns. While it cannot be concluded that holistic processing is entirely normal in these subjects, their performance shows significant modulation by whole-object structure, indicating that some whole-object processing is occurring in these patients.

Previous studies have shown that people's ability to detect, from memory, alterations in highly familiar faces is excellent. Indeed, just noticeable differences for the detection of small alterations in a recognition-memory task were not significantly different from the corresponding measures in a perceptual-discrimination task (Brédart and Devue, 2006 Perception 35 101–106; Ge et al, 2003 Perception 32 601–614). The object of the present study was to evaluate whether people's perceptual memory for body shapes of very familiar persons reaches the high level of precision that was reported for face memory. For one group of participants, the task was to detect body shape alterations (an increase or a decrease of 2% to 10% of the waist-to-hip ratio) on photographs depicting either themselves or a friend. For another group of participants who did not know the target persons, the task was to discriminate whether two photographs presented side by side were the same or not. Results showed that the detection of alterations was significantly better in the perceptual-discrimination task than in the recognition-memory tasks (for the participant's own body as well as for the friend's body). In conclusion, the high fidelity of perceptual memory for very familiar faces does not extend to familiar body shapes.

When a participant views a rubber hand being stroked by a paintbrush while his/her real hand is unseen and similarly stroked by another paintbrush, a misperception known as the rubber hand illusion occurs whereby tactile sensations are falsely referred to the non-body part. The purpose of the current study was to further examine the rubber hand illusion with conditions of movement. An apparatus was devised that would synchronise visual with felt movement in an active condition and a passive condition. An asynchronous condition was included as a control in which visual and felt movement were purposely disconnected. The three movement conditions (active, passive, and asynchronous) were statistically compared in order to assess our prediction that synchronous conditions of movement (especially active) would generate more reports of the illusion. The performance of the movement conditions was evaluated against a visual-tactile condition, which is a known contributor to the rubber hand illusion. Not only significantly more robust reports of the illusion were obtained when visual movement and felt movement were synchronised but there was also a trend toward stronger reports in the active condition rather than the passive condition. Interestingly, the pattern of results differed according to the particular question on the self-report.

Perception of self-tilt is affected by shearing force acting on the otolith organs in the ears, by pressure acting on the tactile receptors in the skin, and by visual pattern falling on the retinae. We examined how the vestibular, somatosensory, and visual inputs interact in judging self-tilt in roll. Each of fourteen observers, sitting in a chair and gazing at a rotation pattern in the frontal plane, was tilted to various angles and verbally judged to what extent his/her body was tilted. The independent variables examined were body tilt (0° to ±108°), and the rate (7.5° s⁻¹, 15° s⁻¹, and 30° s⁻¹) and direction (CW and CCW) of the visual rotating pattern. We found that (i) the sensory scale for self-tilt is represented by a third-order polynomial lacking the quadratic component, (ii) perceived self-tilt for the CW (or CCW) rotation of the visual pattern is displaced CCW (or CW), (iii) the linear and cubic components of the equation increase with an increase in the rate of rotation of the visual pattern, and (iv) if the body is tilted in concord with the direction of vection, the velocity of visual pattern is effective, but when the body is tilted in conflict with the direction of vection, it is less effective. These findings are interpreted in terms of vestibular and somatosensory suppressions exerted on the effects of vection.

The existence of graded structure in fruit and flower odour categories and its stability in different cultures is examined. Groups of students from France, the United States, and Vietnam performed a typicality rating task, a similarity judgment task, a membership verification task, a recognition memory task, a familiarity rating task, and a free identification task using a set of 40 odorants (20 fruit odorants and 20 flower odorants). Overall, our results demonstrate that fruit and flower odour categories possess graded structure. Moreover, principal component analyses of the data revealed the implication of typicality in a variety of cognitive tasks where typical odours receive a preferential processing compared to atypical ones. Finally, our results suggest that typicality can be predicted to a certain extent by experiential knowledge but that other determinants play a role in odour category structure. Altogether, this study confirms that graded structure is a universal property of categories and suggests that universals and cultural specifics can both constrain the emergence of odour category structures.

What are the natural constraints for the human body representation? Here I report a perceptual illusion where healthy individuals experience having two right arms, with both sensing touches applied to them. This effect reveals how visual and tactile signals from the body are integrated in a probabilistic fashion, resulting in a single limb being represented at two locations at the same time, giving rise to a perceptual duplication of this limb. This is an important observation because it suggests that even the gross morphology that we experience of ourselves is a construct resulting from dynamic and integrative processes in the perceptual systems.

If, after being in the dark for many minutes, one views an extended surface under dim (scotopic) illumination, one fails to see any hint of the dark spot at the center of gaze that might be expected from the absence of rods in the fovea. Here we report that, if the surface is suddenly completely darkened, one sees for a few seconds a relatively bright spot, about 2 deg in size, at the point of fixation. If the surface is now restored to its original brightness, a dark spot of similar size appears where one fixates, that again lasts for several seconds. All this can be observed with no elaborate apparatus.