Biomedical subjects

Marco Bertamini

Publications and source records attributed to Marco Bertamini.

17 recordsLinked to original sources

PubMed2006

Amodal completion and visual holes (static and moving).

Occlusion is a frequent occurrence in a cluttered world of opaque objects. Often information about the shape of partly occluded objects can be gathered from the visible portion of the object and in particular its contours. Here we address the case where a region of a surface is visible exclusively through an aperture (visual hole). We make several observations about the grouping of surface regions visible through holes, and the appearance of moving objects and holes. These observations support the view that holes are shape properties of the object-with-hole.

Form Perception↗

PubMed2006

A visual-haptic Necker cube reveals temporal constraints on intersensory merging during perceptual exploration.

When viewing a three-dimensional Necker cube with one eye, participants can experience illusory reversals even while they feel the cube with their hands. This surprising property of the visual-haptic Necker cube affords a unique opportunity to investigate temporal constraints on interactions between vision and touch during extended observation of a three-dimensional object. Our observers reported reversals while they viewed the cube and, at the same time, they either held it with two-finger grips, felt it with while their hands remained stationary, or actively explored it by moving one hand. Consistent with a multisensory approach to three-dimensional form perception, touch had a clear effect on both the number and the duration of illusory percepts. Additionally, when observers alternated between stationary and moving periods during exploration, transitions from stationary to moving-hand haptics played a crucial role in inhibiting illusory reversals. A temporal analysis of the probability of first reversals occurring after different types of motor transition revealed a "vetoing window" initiating approximately 2s after the transition and lasting at least another 1-2s. Implications for multisensory processes during exploration are discussed.

Form Perception↗

PubMed2006

Who owns the contour of a visual hole?

Holes are useful in the study of shape, contour curvature, and border ownership. Several authors have suggested that holes have figural or quasi-figural status. I discuss three criteria to test the evidence that holes behave more like figures than like ground: (i) holes perceived as such; (ii) similar performance for holes and figures; (iii) different performance for holes and other ground regions. Using these criteria, I review the literature and conclude that holes do not have figural status in relation to border ownership. I also argue that holes are ideal stimuli to study figure-ground organisation.

Contrast Sensitivity↗

PubMed2006

Errors in judging information about reflections in mirrors.

We investigated people's perception and knowledge of planar mirror reflections. People were accurate at deciding when they could first see their reflection as they approached a mirror from the side, but only if their reflection was visible. Most people stopped too early if the mirror was covered up. People also overestimated the size of the reflection of their face on the surface of a mirror if they were shown a covered mirror. Their accuracy improved somewhat if their reflection was visible but, unlike the first task, they still made striking errors. Perceptual feedback thus improved performance at predicting the behaviour of mirror reflections in both tasks but failed to eliminate errors in the second task. The overestimation of reflection size was not face-specific as it generalised to novel stimuli (paper ellipses) and it was found with both a matching response and for verbal size estimations. The early error in the first task appears to be due to an inaccurate belief that can be overridden by perceptual feedback. The overestimation in the second task is primarily caused by a powerful size-constancy effect.

Adolescent↗

PubMed2006

Visual search for a circular region perceived as a figure versus as a hole: evidence of the importance of part structure.

To study contour curvature polarity, we compared strictly convex regions (circular figures) with strictly concave regions (circular holes). We tested for an asymmetry between visual searches for concavities and those for convexities. We found that providing a preview of the background benefited search for concavities (holes) more than it did search for convexities (figures) and that for convex figures, nearer targets were responded to more quickly. Importantly, however, we failed to find any support for the hypothesis that concave targets are inherently more salient. We conclude that previous findings in the literature, which have been taken to indicate preferential processing of concavities, due to their increased salience, are more likely the result of an early computation of part structure based on concavities.

Depth Perception↗

PubMed2005

Detection of change in shape and its relation to part structure.

Using a change detection paradigm (Barenholtz, E., Cohen, E. H., Feldman, J., & Singh, M. (2003). Detection of change in shape: An advantage for concavities. Cognition, 89(1) 1-9), we measured sensitivity to the changes of shapes and in particular the difference between detecting a new convex or concave vertex. We conclude that concave vertices per se are not more salient, but changes in the sequence of convexities and concavities along a contour are salient. We argue that these changes are likely to signal a change in perceived part structure.

Form Perception↗

PubMed2005

On what people know about images on mirrors.

As observed by Gombrich [Gombrich, E. H. (1960). Art and illusion. Oxford: Phaidon Press], we confirm that most people are unaware of the size of their own image on mirrors. Specifically we have documented the knowledge that people have of the size of their own head and of the size of the mirror image of their own head. In addition we have explored naive beliefs about how the size of mirror images changes with distance. The main pattern of findings is consistent with a focus on target distance and a difficulty in factoring the observer's vantage point correctly when people reason about the problem. The issue of information about vantage point is discussed in relation to other literatures.

Adult↗

PubMed2005

Naive optics: acting on mirror reflections.

It is known that naive observers have striking misconceptions about mirror reflections. In 5 experiments, this article systematically extends the findings to graphic stimuli, to interactive visual tasks, and finally to tasks involving real mirrors. The results show that the perceptual knowledge of nonexpert adults is far superior to their conceptual knowledge. Whereas conceptual errors include the assumption of left-right reversals in mirror images and often blatant extensions of the boundary of mirror space, the perceptual context prevents most such errors. However, a consistent bias to misjudge objects in mirrors too far to the outside is demonstrable in all cases including tasks with real mirrors. The authors present a 2-stage hypothesis consisting of an implicit bias of judging the mirror surface to be turned toward the observer's line of sight followed by a normalization that becomes explicit.

Adult↗

PubMed2005

Boundary extension: the role of magnification, object size, context, and binocular information.

Boundary extension is a tendency to remember close-up scenes as if they extended beyond the occluding boundaries. The authors explored the contributing factors using brief retention intervals and computer-generated images. Boundary extension turns out to be more complex than previously thought and is not linked to the effects of image magnification and field-of-view changes. Although this is consistent with the idea that boundary extension is the product of the activation of a mental schema that provides information of what is likely to exist outside the picture boundaries, the authors also found that properties of the object at the center of the picture can affect boundary extension independently of the information at the boundaries. In a test of boundary extension using stereograms, the effect does not seem to depend on amount of perceived depth, suggesting a weaker link to perception of space than previously hypothesized.

Environment↗

PubMed2004

Illusory surfaces affect the integration of local motion signals.

A 2IFC paradigm was used to measure speed discrimination thresholds for pairs of Gabor patches. When one of these patches was phenomenally placed over an illusory surface (IS), we observed higher thresholds relative to control conditions without ISs. Additional controls demonstrated that this effect was due to the placement of the patch on a different phenomenal depth plane rather than to the mere presence of an IS. We conclude that (i) ISs can affect the long-range integration of local motion signals, and (ii) long-range motion integration obeys a coplanarity principle.

Depth Perception↗

PubMed2004

Early computation of contour curvature and part structure: evidence from holes.

We used holes to study unilateral border ownership and in particular the information carried by the sign of the curvature along the contour (ie the difference between convex and concave regions). When people perceive a hole, its shape has a reversed curvature polarity (ie a changed sign of curvature) compared to the same region perceived as an object. Bertamini (2001 Perception 30 1295-1310), and Bertamini and Croucher (2003 Cognition 87 33-54) suggested and found evidence to support the hypothesis that, because convex regions are perceived as parts, positional information is more readily available for convex regions. Therefore a change is predicted when a given region is perceived as either a hole or a figure. We confirm that finding in this study, using holes defined by binocular disparity. We conclude that a change from figure to hole always reverses the encoding of curvature polarity. In turn, polarity obligatorily affects perceived part structure and the processing of position.

Analysis of Variance↗

PubMed2003

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.

Form Perception↗

PubMed2003

Naive optics: Predicting and perceiving reflections in mirrors.

Undergraduate students predicted what would be made visible by a planar mirror. A paper-and-pencil task confirmed previous findings that when approaching a mirror from the side, participants expected to see their reflection in the mirror earlier than they actually would. This early response was found for all mirrors when the observer moved horizontally--even when the mirror was placed on the floor or the ceiling--but not when the observer moved vertically (in a lift). The data support the hypothesis that many people imagine the world in the mirror as rotated around the vertical axis. When participants had to judge manipulated mirror reflections according to their naturalness, a high degree of tolerance was found. In contrast to the prediction task, a rotation around the vertical axis was judged to be less natural than other distortions. The authors conclude that perceptual knowledge and predictive knowledge lead to different patterns of errors. ((c) 2003 APA, all rights reserved)

Adult↗

PubMed2003

The Venus effect: people's understanding of mirror reflections in paintings.

We propose the term 'Venus effect' for a common phenomenon in picture perception. It occurs when a picture shows an actor and a mirror that are not placed along the observer's line of sight, for instance a Venus admiring herself in a small mirror, and when the actor's reflection in the mirror is visible to the observer. In this situation, observers tend to report, incorrectly, that Venus is also seeing herself in the mirror in the same location as the observer. We discuss this using famous paintings as examples.

Adult↗

PubMed2002

No within-object advantage for detection of rotation.

It is known that in a detection task the type of rigid transformation to be detected (reflection vs. translation) interacts with the type of display (closed vs. open contours). The advantage for closed contours found with reflection is believed to be a general within-object advantage, whilst the advantage for open contours found with translation is an exception, described as a lock-and-key process (Acta Psychol. 95 (1997) 119). We tested rotation, using a reaction time paradigm, and found the same result as for translation. Moreover, we found that the critical factor is not the number of objects present, rather it is whether the comparison is made across a surface or across an aperture between surfaces. Post-experiment interviews did not confirm any difference for observers who reported using a conscious lock-and-key mental transformation. We speculate that seeing a translation or a rotation across a closed figure is difficult because the closure of the figure emphasises the mismatch of the contour polarities on the two sides of the figure. That is, there may be a closed object advantage for detecting a difference in polarity which interferes with the task of detecting a regularity in shape. Evidence from the analysis of foil rejection trials supports such a speculation.

Adult↗

PubMed2002

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 observer's 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.

Adolescent↗

PubMedDate not supplied

The chromatic input to global motion perception.

For over 30 years there has been a controversy over whether color-defined motion can be perceived by the human visual system. Some results suggest that there is no chromatic motion mechanism at all, whereas others do find evidence for a purely chromatic motion mechanism. Here we examine the chromatic input to global motion processing for a range of color directions in the photopic luminance range. We measure contrast thresholds for global motion identification and simple detection using sparse random-dot kinematograms. The results show a discrepancy between the two chromatic axes: whereas it is possible for observers to perform the global motion task for stimuli modulated along the red-green axis, we could not assess the contrast threshold required for stimuli modulated along the yellowish-violet axis. The contrast required for detection for both axes, however, are well below the contrasts required for global motion identification. We conclude that there is a significant red-green input to global motion processing providing further evidence for the involvement of the parvocellular pathway. The lack of S-cone input to global motion processing suggests that the koniocellular pathway mediates the detection but not the processing of complex motion for our parameter range.

Artifacts↗