laboratoire de physique statistique
laboratoire de physique statistique


Geometrical illusions are not always where you think they are: a review of some classical and less classical illusions, and ways to describe them - Ninio, Jacques

Abstract : Geometrical illusions are known through a small core of classical illusions that were discovered in the second half of the nineteenth century. Most experimental studies and most theoretical discussions revolve around this core of illusions, as though all other illusions were obvious variants of these. Yet, many illusions, mostly described by German authors at the same time or at the beginning of the twentieth century have been forgotten and are awaiting their rehabilitation. Recently, several new illusions were discovered, mainly by Italian authors, and they do not seem to take place into any current classification. Among the principles that are invoked to explain the illusions, there are principles relating to the metric aspects (contrast, assimilation, shrinkage, expansion, attraction of parallels) principles relating to orientations (regression to right angles, orthogonal expansion) or, more recently, to gestalt effects. Here, metric effects are discussed within a measurement framework, in which the geometric illusions are the outcome of a measurement process. There would be a main ``convexity'' bias in the measures: the measured value m(x) of an extant x would grow more than proportionally with x. This convexity principle, completed by a principle of compromise for conflicting measures can replace, for a large number of patterns, both the assimilation and the contrast effects. We know from evolutionary theory that the most pertinent classification criteria may not be the most salient ones (e.g., a dolphin is not a fish). In order to obtain an objective classification of illusions, I initiated with Kevin O'Regan systematic work on ``orientation profiles'' (describing how the strength of an illusion varies with its orientation in the plane). We showed first that the Zollner illusion already exists at the level of single stacks, and that it does not amount to a rotation of the stacks. Later work suggested that it is best described by an ``orthogonal expansion'' an expansion of the stacks applied orthogonally to the oblique segments of the stacks, generating an apparent rotation effect. We showed that the Poggendorff illusion was mainly a misangulation effect. We explained the hierarchy of the illusion magnitudes found among variants of the Poggendorff illusion by the existence of control devices that counteract the loss of parallelism or the loss of collinearity produced by the biased measurements. I then studied the trapezium illusion. The oblique sides, but not the bases, were essential to the trapezium illusion, suggesting the existence of a common component between the trapezium and the Zollner illusion. Unexpectedly, the trapeziums sometimes appeared as twisted surfaces in 3d. It also appeared impossible, using a nulling procedure, to make all corresponding sides of two trapeziums simultaneously equal. The square-diamond illusion is usually presented with one apex of the diamond pointing toward the square. I found that when the figures were displayed more symmetrically, the illusion was significantly reduced. Furthermore, it is surpassed, for all subjects, by an illusion that goes in the opposite direction, in which the diagonal of a small diamond is underestimated with respect to the side of a larger square. In general, the experimental work generated many unexpected results. Each illusory stimulus was compared to a number of control variants, and often, I measured larger distortions in a variant than in the standard stimulus. In the Discussion, I will stress what I think are the main ordering principle in the metric and the orientation domains for illusory patterns. The convexity bias principle and the orthogonal expansion principles help to establish unsuspected links between apparently unrelated stimuli, and reduce their apparently extreme heterogeneity. However, a number of illusions (e.g., those of the twisted cord family, or the Poggendorff illusions) remain unpredicted by the above principles. Finally, I will develop the idea that the brain is constructing several representations, and the one that is commonly used for the purpose of shape perception generates distortions inasmuch as it must satisfy a number of conflicting constraints, such as the constraint of producing a stable shape despite the changing perspectives produced by eye movements.
Why must visual stimuli be so poor? - Ninio, J.
PERCEPTION 4160 (2012) 
Ribosomal Kinetics and Accuracy: Sequence Engineering to the Rescue - Ninio, Jacques
Orientation profiles of the trapezium and the square-diamond geometrical illusions - Ninio, J.
PERCEPTION 4045 (2011) 
Alternation frequency thresholds for stereopsis as a technique for exploring stereoscopic difficulties - Rychkova, Svetlana and Ninio, Jacques
I-PERCEPTION 250-68 (2011) 

Abstract : When stereoscopic images are presented alternately to the two eyes, stereopsis occurs at F >= 1 Hz full-cycle frequencies for very simple stimuli, and F >= 3 Hz full-cycle frequencies for random-dot stereograms (eg Ludwig I, Pieper W, Lachnit H, 2007 ``Temporal integration of monocular images separated in time: stereopsis, stereoacuity, and binocular luster'' Perception \& Psychophysics 69 92-102). Using twenty different stereograms presented through liquid crystal shutters, we studied the transition to stereopsis with fifteen subjects. The onset of stereopsis was observed during a stepwise increase of the alternation frequency, and its disappearance was observed during a stepwise decrease in frequency. The lowest F values (around 2.5 Hz) were observed with stimuli involving two to four simple disjoint elements (circles, arcs, rectangles). Higher F values were needed for stimuli containing slanted elements or curved surfaces (about 1 Hz increment), overlapping elements at two different depths (about 2.5 Hz increment), or camouflaged overlapping surfaces (> 7 Hz increment). A textured cylindrical surface with a horizontal axis appeared easier to interpret (5.7 Hz) than a pair of slanted segments separated in depth but forming a cross in projection (8 Hz). Training effects were minimal, and F usually increased as disparities were reduced. The hierarchy of difficulties revealed in the study may shed light on various problems that the brain needs to solve during stereoscopic interpretation. During the construction of the three-dimensional percept, the loss of information due to natural decay of the stimuli traces must be compensated by refreshes of visual input. In the discussion an attempt is made to link our results with recent advances in the comprehension of visual scene memory.
Folded Sheet Versus Transparent Sheet Models for Human Symmetry Judgments - Ninio, Jacques
SYMMETRY-BASEL 3503-523 (2011) 

Abstract : As a contribution to the mysteries of human symmetry perception, reaction time data were collected on the detection of symmetry or repetition violations, in the context of short term visual memory studies. The histograms for reaction time distributions are rather narrow in the case of symmetry judgments. Their analysis was performed in terms of a simple kinetic model of a mental process in two steps, a slow one for the construction of the representation of the images to be compared, and a fast one, in the 50 ms range, for the decision. There was no need for an additional `mental rotation' step. Symmetry seems to facilitate the construction step. I also present here original stimuli showing a color equalization effect across a symmetry axis, and its counterpart in periodic patterns. According to a ``folded sheet model'', when a shape is perceived, the brain automatically constructs a mirror-image representation of the shape. Based in part on the reaction time analysis, I present here an alternative ``transparent sheet'' model in which the brain constructs a single representation, which can be accessed from two sides, thus generating simultaneously a pattern and its mirror-symmetric partner. Filtering processes, implied by current models of symmetry perception could intervene at an early stage, by nucleating the propagation of similar perceptual groupings in the two symmetric images.
Visual memory - Ninio, Jacques
PERCEPTION 39140-141 (2010) 
Stereoscopic memory beyond stimuli persistence: the multiplicative effect of binocular intervals - Rychkova, S. I. and Rabitchev, I. E. and Ninio, J.
PERCEPTION 39161 (2010) 
Frail Hypotheses in Evolutionary Biology - Ninio, Jacques
Recognising random shapes with inverted contrast polarity - Patri, J-F and Ninio, J.
PERCEPTION 38178 (2009) 
Alternation frequency thresholds for stereopsis reveal different types of stereoscopic difficulties - Rychkova, S. I. and Ninio, J.
PERCEPTION 3859 (2009) 
Paradoxical fusion of two images and depth perception with a squinting eye - Rychkova, S. I. and Ninio, J.
VISION RESEARCH 49530-535 (2009) 

Abstract : Some strabismic patients with inconstant squint can fuse two images in a single eye, and experience lustre and depth. One of these images is foveal and the other extrafoveal. Depth perception was tested on 30 such subjects. Relief was perceived mostly on the fixated image. Camouflaged continuous surfaces (hemispheres, cylinders) were perceived as bumps or hollows, without detail. Camouflaged rectangles could not be separated in depth from the background, while their explicit counterparts could. Slanted bars were mostly interpreted as frontoparallel near or remote bars. Depth responses were more frequent with stimuli involving inward rather than outward disparities, and were then heavily biased towards ``near'' judgements. All monocular fusion effects were markedly reduced after the recovery of normal stereoscopic vision following an orthoptic treatment. The depth effects reported here may provide clues on what stereoscopic pathways may or may not accomplish with incomplete retinal and misleading vergence information. (C) 2009 Elsevier Ltd. All rights reserved.
The science and craft of autostereograms - Ninio, Jacques
SPATIAL VISION 21185-200 (2008) 

Abstract : Autostereograms or SIRDS (Single Image Random-Dot Stereograms) are camouflaged stereograms which combine the Julesz random-dot stereogram principle with the wallpaper effect. They can represent any 3D shape on a single image having a quasi-periodic appearance. Rather large SIRDS can be interpreted in depth with unaided eyes. In the hands of computer graphic designers, SIRDS spread all over the world in 1992-1994, and these images, it was claimed, opened a new era of stereoscopic art. Some scientific, algorithmic and artistic aspects of these images are reviewed here. Scientifically, these images provide interesting cues on stereoscopic memory, and on the roles of monocular regions and texture boundaries in stereopsis. Algorithmically, problems arising with early SIRDS, such as internal texture repeats or ghost images are evoked. Algorithmic recommendations are made for gaining a better control on the construction of SIRDS. Problems of graphic quality (smoothness of the represented surfaces, or elimination of internal texture repeats) are discussed and possible solutions are proposed. Artistically, it is proposed that SIRDS should become less anecdotal, and more oriented towards simple geometric effects, which could be implemented on large panels in natural surrounds.
Symmetry influences colour perception: the transparent sheet model - Ninio, J.
PERCEPTION 37147 (2008) 
Depth perception following monocular fusion of two images with a squinting eye - Rychkova, S. I. and Ninio, J.
PERCEPTION 3715-16 (2008) 
Designing visually rich, nearly random textures - Ninio, Jacques
SPATIAL VISION 20561+ (2007) 

Abstract : Camouflaging textures containing as in real life edges at all orientations, were designed by computer, then manually, for use in stereoscopic vision studies. In the manual procedure, the starting point is either a set of photographs (for instance, of barks) or a manually produced first-generation texture. Then patches are cut zigzagging and assembled into successive generations of textures. The absence of extended edges - straight or curved - and the local heterogeneity of the texture are important camouflaging factors, allowing curved surfaces to be covered with these textures without visible join. Small areas of a texture often suggest a scene, but when the areas are assembled, the suggestive power is lost, and the statistical properties of the texture then dominate. However, when symmetry is introduced (as in the Rorschach test), meaningful scenes emerge again.
Mental kinetics in visual memory - Ninio, J.
PERCEPTION 3652 (2007) 
Errors and alternatives in prebiotic replication and catalysis - Ninio, Jacques
CHEMISTRY \& BIODIVERSITY 4622-632 (2007) 

Abstract : The work on nonenzymatic nucleic acid replication performed by Leslie Orgel and co-workers over the last four decades, now extended by work on artificial selection of RNA aptamers and ribozymes, is generating some pessimism concerning the `naked gene' theories of the origin of life. It is suggested here that the low probability of finding RNA aptamers and ribozymes within pools of random sequences is not as disquieting as the poor gain in efficiency obtained with increases in information content. As acknowledged by Orget and many other authors, primitive RNA replication and catalysis must have occurred within already complex and dynamic environments. I, thus, propose to pay attention to a number of possibilities that bridge the gap between `naked gene' theories, on one side, and metabolic theories in which complex systems self-propagate by growth and fragmentation, on the other side. For instance, one can de-emphasize nucleotide-by-nucleotide replication leading to long informational polymers, and view instead long random polymers as storage devices, from which shorter oligomers are excised. Catalytic tasks would be mainly performed by complexes associating two or more oligomers belonging to the same or to different chemical families. It is proposed that the problems of stability, binding affinity, reactivity, and specificity could be easier to handle by heterogeneous complexes of short oligomers than by long, single-stranded polymers. Finally, I point out that replication errors in a primitive replication context should include incorporations of alternative nucleotides with interesting, chemically reactive groups. In this way, an RNA sequence could be at the same time an inert sequence when copied without error, and a ribozyme, when a chemically reactive nucleotide is inadvertently introduced during replication.
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Doubts about quantal analysis - Ninio, Jacques