Marco Bertamini

Amodal Completion of Partly Occluded Surfaces: Is There a Mosaic Stage?

Recent investigators have proposed that amodal completion is a sequential process requiring a preliminary mosaic stage. Results of 6 studies of the time course of completion processes show support for this mosaic-first view with pictorial displays, but not with displays involving occlusion specified by binocular parallax or when pictorial displays were observed monocularly. These results still do not rule out the mosaic-first view. A parallel model, however, can account more economically for the available data.

The Importance of Being Convex: An Advantage for Convexity when Judging Position

Perception of contour polarity was investigated in five experiments in which observers had to judge the vertical position of a vertex. When the vertex was perceived as convex, the level of performance as measured by reaction time and errors was higher than when the same vertex was perceived as concave. I conclude that contour polarity affects how observers perceive shape, and in particular part structure, and that the position of a part is more readily available than the position of a boundary between parts.

Implicit Affective Evaluation of Visual Symmetry

Symmetry and beauty are strongly linked, but is the positive response to visual symmetry automatic? We used the Implicit Association Test (IAT) to measure the valence of visual regularities in the absence of overt judgments. In our first experiment, participants classified dot patterns as random or having an axis of reflection, and words as positive or negative. When the same button was used to report reflection and positive words, responses were faster than when the same button was used to report reflection and negative words. We take this association to indicate an implicit preference for reflectional patterns. In subsequent experiments, a reflected pattern was preferred to a rotation or translation, and a rotational pattern was preferred to random patterns. In some cases these results were not in agreement with verbally reported preferences, but implicit preferences were always predicted by the speed at which patterns could be identified. We conclude that the IAT can unearth an automatic affective response produced by perceptual fluency.

The shape of holes

The shape of holes can be recognized as accurately as the shape of objects (Palmer, S. E. (1999). Vision science: photons to phenomenology. Cambridge, MA: MIT Press), yet the area enclosed by a hole is a background region, and it can be demonstrated that background regions are not represented as having shape. What is therefore the shape of a hole, if any? To resolve this apparent paradox, we suggest that the shape of a hole is available indirectly from the shape of the surrounding object. We exploited the fact that observers are faster at judging the position of convex vertices than concave ones (Perception 30 (2001) 1295), and using a figural manipulation of figure/ground we found a reversal of the relative speeds when the same contours were presented as holes instead of objects. If contours were perceived as belonging to the hole rather than the surrounding object then there would have been no qualitative difference in responses to the object and hole stimuli. We conclude that the contour bounding a hole is automatically assigned to the surrounding object, and that a change in perception of a region from object to hole always drastically changes the encoded information. We discuss the many interesting aspects of holes as a subject of study in different disciplines and predict that much insight especially about shape will continue to come from holes.

Detection of symmetry and perceptual organization: The way a lock-and-key process works

We studied the speed with which observers could detect symmetry in drawings that incorporated symmetric contours--related by reflection or translation--within single objects or across different objects. We asked observers to perform a speeded decision whether pairs of contours are the same, i.e., related by reflection or by translation, or different. When the contours belong to a single object, observers are faster to see the relation between contours when they are related by reflection than by translation. When the contours belong to different objects, observers are faster to see the relation between the contours when they are related by translation than by reflection. We tested whether this advantage of translation is due to a lock-and-key process. We first tested our hypothesis by manipulating the correspondence of the features, so as to make matching more difficult. This change did not produce the predicted pattern of results. We performed a second manipulation to change the appearance of the objects: we increased the prägnanz of the objects by changing the type of lines used to connect the contours. Results indicate that perceptual organization can alter detectability of symmetry.

Memory for position and dynamic representations

Memory for the position of an object is biased. When asked to judge whether an object has changed its position with respect to a position shown a few milliseconds before, observers tend to detect the displacement more often when the displacement is not in the expected direction (downward for a falling object). The hypothesis proposed by Freyd (1983, 1987) states that the internal representation of an object is intrinsically dynamic. Therefore, the forces perceived as acting on the object affect the representation. Quantitative predictions of this model were tested in three experiments by measuring memory distortion for the position of an object on an inclined plane. Angle of inclination and retention interval were varied. The results for different inclinations support the physical model. The time course of the memory distortion suggests a new view about the relation between this phenomenon and very short-term memory.

Bite-Size Science and Its Undesired Side Effects

Short and rapid publication of research findings has many advantages. However, there is another side of the coin that needs careful consideration. We argue that the most dangerous aspect of a shift toward “bite-size” publishing is the relationship between study size and publication bias. Findings based on a single study or a study based on a limited sample size are more likely to be false positive, because the false positive rate remains constant, whereas the true positive rate (the power) declines as sample size declines. Pressure on productivity and on novelty value further exacerbates the problem.

Symmetry perception and affective responses: a combined EEG/EMG study.

The perception and appreciation of visual symmetry have been studied in several recent EEG experiments. Although symmetry is known to be an important determinant of aesthetic preference, previous studies have concluded that evaluation does not occur spontaneously. These studies also found that symmetrical and random patterns do not differ in terms of early sensory processing, within 200 ms of stimulus onset. We presented participants with symmetrical or random abstract patterns, which they had to classify correctly. Contrary to previous work, we found that N1 amplitude was sensitive to all types of regularity, and P1 was sensitive to rotational symmetry. We also found that activity in the Zygomaticus Major, the facial muscle responsible for smiling, was greater for reflection patterns. However, we were able to reverse this effect by changing the task so that participants had to treat random patterns as the target stimuli. We conclude that participants spontaneously select reflectional symmetry as the target, and positive affective responses automatically follow from successful target detection. This work provides a new account of the neural mechanisms involved in visual symmetry perception.

Naive optics: Understanding the geometry of mirror reflections.

Paper-and-pencil tasks showed that many university students believed that when laterally approaching a mirror, they would see a reflection in the mirror before it was geometrically possible. Participants failed to adequately factor in the observers location in the room. However, when asked about the behavior of a ray of light, participants knew about the law of reflection. No differences between psychology and physics students were detected, suggesting that the phenomenon is widespread and refractory to training. The findings were replicated with observers making judgments about image locations in a real room using a pretend mirror. Possible heuristics about mirror reflection that might explain the data are discussed. Naive optics is a promising venue to further knowledge of how observers understand basic laws of physics.

Brain Activity in Response to Visual Symmetry

A number of studies have explored visual symmetry processing by measuring event related potentials and neural oscillatory activity. There is a sustained posterior negativity (SPN) related to the presence of symmetry. There is also functional magnetic resonance imaging (MRI) activity in extrastriate visual areas and in the lateral occipital complex. We summarise the evidence by answering six questions. (1) Is there an automatic and sustained response to symmetry in visual areas? Answer: Yes, and this suggests automatic processing of symmetry. (2) Which brain areas are involved in symmetry perception? Answer: There is an extended network from extrastriate areas to higher areas. (3) Is reflection special? Answer: Reflection is the optimal stimulus for a more general regularity-sensitive network. (4) Is the response to symmetry independent of view angle? Answer: When people classify patterns as symmetrical or random, the response to symmetry is view-invariant. When people attend to other dimensions, the network responds to residual regularity in the image. (5) How are brain rhythms in the two hemispheres altered during symmetry perception? Answer: Symmetry processing (rather than presence) produces more alpha desynchronization in the right posterior regions. Finally, (6) does symmetry processing produce positive affect? Answer: Not in the strongest sense, but behavioural measures reveal implicit positive evaluation of abstract symmetry.