Perception

Over the past few decades, different theories have been advanced to explain geometric-optical illusions based on various perceptual processes such as assimilation and/or contrast. Consistent with the contradictory effects of assimilation and contrast, Pressey's assimilation theory provided an explanation for the Müller-Lyer illusion, but failed to account for the Titchener (Ebbinghaus) illusion. A model that explains both Müller-Lyer and Titchener illusions according to a common underlying process may outline a unified explanation for a variety of geometric-optical illusions. In order to develop such a model, the concept of empty space is introduced as an area of the illusory figure that is not filled by line drawings. It was predicted that the magnitude of illusion would increase with the area of the empty space around the illusory figures. The effect of empty space on the magnitude of perceptual distortion was measured in Müller-Lyer figures, with outward arrowheads of different length. The results indicated an overestimation of the target stimulus in all of the figures. Nevertheless, consistent with the prediction of the present model, the horizontal line in the Müller-Lyer figure with the longest arrowheads appeared shorter than that with the shortest arrowheads, although the size contrast of these figures was the same. According to the analysis proposed in the present study, the area of empty space not only affects the magnitude of illusion but also serves as a contextual cue for the perceptual system to determine the direction of illusion (orientation). The functional relationships between the size contrast and empty space provide a common explanation for the Müller-Lyer, Titchener, and a variety of other geometric-optical illusions.

We carried out six experiments to find out whether simple manual reaction time (RT) to flux-equated visual stimuli of different size is modulated by size constancy or by the retinal angle subtended by the stimuli. We found that RT decreased with the increase in perceived stimulus size rather than retinal angle and that this relationship depended on the use of familiar 3-D-like stimuli and on the availability of other size-constancy cues. Thus, a stereotyped speeded motor response, such as that employed in a simple RT paradigm, is modulated by size constancy, as is the case with perceptual judgments. The present results provide original evidence on the relationship between simple RT and perception.

In the Freezing Rotation illusion a stimulus rotating with a constant velocity is perceived as stationary on the screen, when it is presented in front of a background pattern that moves with a sinusoidal velocity profile, during the phase in which stimulus and background rotate in the same direction. It has been suggested that this illusion is caused by the interfering effect of induced motion resulting from the relative motion between the centre and the surround. Since the magnitude of such an induced motion component presumably relates to the difference between background and centre velocities, the illusion itself should also be related to the amount of relative motion between the centre and the surround, and it should not occur when this difference is zero. We present evidence here that this hypothesis is incorrect. First, we show that the illusion also occurs when the background moves with a constant velocity instead of sinusoidally. Second, we show that the illusion consists of a fixed underestimation of centre velocity when the centre and the surround move in the same direction and a fixed overestimation of centre velocity when they move in opposite directions or when the background remains stationary. The amount of underestimation and overestimation of velocity is not related to the velocity difference between the centre and the surround. Some factors that may be relevant to the explanation of the illusion are discussed.

The question how channels tuned to different motion directions contribute to motion perception has been investigated by using motion adaptation to silence certain channels, and then measuring performance in a fine motion-discrimination task. To help constrain models of how the channels become integrated, we examined whether changes in performance stem from reduced accuracy (bias) or from reduced precision (sensitivity) in direction judgments. On a given trial, the observer first adapted to a field of dots moving coherently in a given direction (ranging ±180° from upward), then judged whether the motion of an ensuing test stimulus (ranging ±3°) was left or right of reference. Bias and sensitivity of the psychometric fits were computed for each adapter direction. Relative to baseline performance, post-adaptation judgments showed significant changes in sensitivity that were tightly correlated with overall performance. Meanwhile, bias shifts were found to be weaker and less systematic. Both performance and sensitivity suffered the largest losses at ±60°, with some enhancement at 180°. No similar trends were found in the domain of bias. A regression model, with precision as the sole predictor, captured 97% of the variation in performance; no gains were found in adding bias to the model. Our findings on fine motion-discrimination question the idealized notion of a pure feature detector, as the main impact of adaptation in such a system would be to bias direction judgments away from the adapted direction.

In three experiments, we examined the role of structural similarity and different types of motion on the efficiency of performing same – different shape judgments across changes in viewpoints. In all experiments, participants judged whether two novel, multi-part objects were structurally identical, and they were to ignore any viewpoint or motion differences between the objects. In experiment 1, participants were affected by viewpoint differences more for structurally similar than structurally distinct objects, but this interaction was mitigated by rigid motion. In experiments 2 and 3, we used only structurally similar objects that moved only some of their parts, either in a similar way between objects within a pair or in distinctive ways. Participants' recognition performance was facilitated by this articulated motion relative to both static and scrambled controls. We conclude that coherent motion facilitates generalisation across different views of dynamic objects under some conditions.

The time needed to detect changes in the colouration of a single moving stimulus becomes shorter with its increasing velocity (Kreegipuu et al, 2006 Vision Research 46 1848–1855). We examined the ability to detect colour change in moving chromatic bars or sinusoidal gratings through temporal order judgment (TOJ) and reaction time (RT) tasks to test whether the effect of velocity found in a previous study is universal and holds for different tasks and stimuli. The results demonstrate that the TOJ and simple RT to the colour change of a moving grating are insensitive to stimulus velocity. Therefore, we conclude that the process of comparison of the two internal representations of external events does not have access to temporal information precise enough to estimate the exact time when something enters our subjective awareness. The motion effect on colour-change perception seems to be confined to a single stimulus that moves across the visual field, to events that contain some spatial predictability, and to tasks that reflect the time of the change relatively directly.

Movements in virtual stereoscopic space tend to be difficult and slow. To shed some light on the origins of these difficulties, we studied open-loop pointing with targets presented in a mirror stereoscope. Whilst targets were placed in a virtual horizontal plane, movement end-points were located in an inclined plane. The inclination of this plane was a stable individual characteristic. Amplitude errors gave no evidence of a contraction bias. Open-loop movements had fairly straight trajectories, but closed-loop movements were strongly curved so that they ended in a horizontal plane rather than in an inclined one. Effects of closed-loop movements on subsequent open-loop movements (calibration effects) were only small. These findings reveal that movements in a virtual environment exhibit a number of characteristics also seen in real environments, but in addition they reflect specific visual illusions. Extensive calibration procedures are needed to overcome such errors for the reliance on closed-loop control of pointing to be relaxed.

To understand the way in which video-game play affects subsequent perception and cognitive strategy, two experiments were performed in which participants played either a fast-action game or a puzzle-solving game. Before and after video-game play, participants performed a task in which both speed and accuracy were emphasized. In experiment 1 participants engaged in a location task in which they clicked a mouse on the spot where a target had appeared, and in experiment 2 they were asked to judge which of four shapes was most similar to a target shape. In both experiments, participants were much faster but less accurate after playing the action game, while they were slower but more accurate after playing the puzzle game. Results are discussed in terms of a taxonomy of video games by their cognitive and perceptual demands.

Though the psychological literature is replete with information about the perception of faces presented at a full-frontal view, we know very little about how faces are perceived–and impressions formed–when viewed from other angles. We tested impressions of faces at full-frontal, three-quarter, and profile views. Judgments of personality (aggressiveness, competence, dominance, likeability, and trustworthiness) and physiognomy (attractiveness and facial maturity) were significantly correlated across full-frontal, three-quarter, and profile views of male faces. When under time pressure, with only a 50 ms exposure to each face, the correlations for profile with full-frontal and three-quarter view judgments of personality (but not physiognomy) dropped considerably. However, judgments of the full-frontal and three-quarter faces were significantly correlated across the self-paced and 50 ms viewing durations. These findings therefore show that perceptions of full faces lead to relatively similar interferences across both viewing angle and time.

Facial symmetry, averageness, sexual dimorphism, and skin colour/texture all serve as cues to attractiveness, but their role in the perception of health is less clear. This ambiguity could reflect the fact that these facial traits are not the only cues to health. We propose that adiposity is an important, but thus far disregarded, facial cue to health. Our results demonstrate two important prerequisites for any health cue. First, we show that facial adiposity, or the perception of weight in the face, significantly predicts perceived health and attractiveness. Second, we show that perceived facial adiposity is significantly associated with measures of cardiovascular health and reported infections. Perceived facial adiposity, or a correlate thereof, is therefore an important and valid cue to health that should be included in future studies.

The notions of symmetry group and camouflage are explained, and a technique is introduced to visually camouflage symmetry groups in natural textures. Two experiments were carried out to test discriminability of symmetry groups p1, p2, pm, and pg in 4-oddity and 5AFC designs with different or same texture per set of stimulus pictures, respectively. In 4-oddity, detection of p1 was enhanced by pm distractors and impaired by those of pg. In 5AFC, p2 proved discriminable against p1; pg did not. Findings are interpreted as qualifying broad claims about symmetry as an organisational principle for vision. Further investigations of visual discrimination of symmetry operations and groups are suggested.

There are numerous speculations about famous artists and how their perception was affected by their medical conditions. In this study, we examined how illness and hospitalisation affect children's art. A total of 157 paintings by 122 hospitalised children were interpreted by three reputed artists. Works of ill children were compared with those of a control group from an international art exhibition. We also comment on how diseases influenced the technique of famous artists to further examine the possible impact of illness on the artistry of hospitalised children. Developmental delay in painting was evident in the study group. Use of ready-made schemas was a common practice (55%). 56% of the work from patients older than eight years failed in perspective and site perception. The theme in 89% of the drawings had no human figures. Projection of self sufferings, prominent elementary lines, and reflection of distorted perception could be clearly recognised in various study materials; anxiety about assigned surgery was exhibited by scribbling. There exists a close relationship between medicine and humanities. The interpretation of fine art from a medical perspective may help to increase our appreciation of the suffering of an individual. It is obvious that diseases change the artistic style and inner perspective. The question is how do healthcare specialists view this?

More than three-quarters of a century ago Wertheimer and Benary demonstrated an ingenious and clear, though, interestingly, small effect: a grey triangle just inside an arm of a black cross on a white background appears slightly lighter than an identical triangle immediately adjacent to the cross, despite both triangles having the same perimeter exposure to black and white. Over a generation ago White discovered an apparently related, but far stronger effect: when short grey (test) bars are placed onto either black or white alternating long bars, the short test bars placed on the long black bars appear much lighter than those placed on the long white bars. A decade ago Spehar, Gilchrist, and Arend found that, enigmatically, if the short test bars in White's effect are the lightest stimulus in a figure, then the relative lightness of the test bars inverts compared with the standard version of White's effect. Here we show that the Wertheimer–Benary effect does not invert, but instead produces a very weak version of the standard effect. We also demonstrate a novel, nulled Wertheimer-Benary effect.

Ever since the assassination of US President Kennedy, numerous theories have circulated purporting that Lee Harvey Oswald, the accused assassin, acted as part of a larger criminal conspiracy. It has been suggested, for example, that incriminating photographs of Oswald were manipulated, and hence evidence of a broader plot. Specifically, it has been argued that the lighting and shadows in these photos are physically impossible. Because the visual system is often unable to reliably judge 3-D geometry and lighting, a detailed 3-D analysis of the Oswald photos to determine if claims of tampering are warranted.