It is commonly admitted that an antigen, or a processed antigen, is first presented to a T cell by a macrophage or some other antigen-presenting cell, bearing an MHC molecule compatible with the T-cell receptor. The T cell thus activated, when encountering a B cell, tests for the presence of the same antigen, through a similar mechanism of MHC-dependent antigen presentation. When a B cell engages into a program of extensive somatic diversification of its immunoglobulin genes, there is a potential risk of increased mutation rate in other genes3 and, in particular, the MHC genes. A minority of B cells would be capable of antigen presentation. The majority of B cells, issued from extensive somatic diversification would have extremely limited capacities for antigen presentation. Each antigen may, in principle, be uniquely characterized by its set of binding energies to a large collection of antibodies.

Autostereograms or SIRDS (Single Image Random-Dot Stereograms) are camou aged stereograms which combine the Julesz random-dot stereogram principle with the wallpaper e ect. They can represent any 3d shape on a single image having a quasiperiodic 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 scienti c, algorithmic and artistic aspects of these images are reviewed here. Scienti cally, 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 anecdotical, and more oriented towards simple geometric e ects which could be implemented on large panels in natural surrounds.

Objective: Studying some temporal aspects of stereoscopic processing in order to clarify phenomena of stereoscopic persistence and investigate how the brain deals with stereoscopic stimuli that involve various types of clues or difficulties (simple versus complex, with or without oriented elements, with or without curvature, with explicit or camouflaged shapes, with or without disparity discontinuities). Method: Left and right images of 20 different stereo pairs were generated, one above the other, on a laptop computer display and were observed by the left and right eye of each subject separately wearing proper prism glasses. The stimuli were presented to six subjects under alternating presentation conditions (i) under pure alternation (ii) with an intercalated variable void interval V between the monocular left and right stimulus intervals (iii) with a variable temporal overlap of the two monocular stimuli, thus producing a binocular interval B between the purely monocular intervals. For each V, B, or null interval we determined the longest duration M of the monocular presentations that was compatible with stereopsis.Results: A modest increase in the duration of binocular intervals B makes it possible much larger increase in the duration of the purely monocular presentation M. The M’s are related to the binocular intervals B’s by a power law, of exponent around 1.3 - 2.5 depending on stimulus complexity. At large B’s, when M is increased, the subject experiences a transition from stable to pulsating stereopsis, then a transition from pulsating to no stereopsis. In an intermediate range of B values, the representative curve splits in 2 branches, an upper one separating stable or pulsating stereopsis from no stereopsis, and a lower branch separating stable from pulsating stereopsis. At some point, the subject reports stable stereopsis, but would be in a regime of “pulsating stereopsis” of which he/she is not aware. The results are represented according to a general phase diagram containing 4 phases. A pair of closely related stimuli can give rise, in some subjects, to strikingly different behaviours at large B’s. Conclusion: We suggest the existence of a rather stable stereoscopic memory store, allowing a stable 3d interpretation of scenes. Various types of data would decay at different rates and stereoscopic processing, sustained over several alternations, would require brief occasional updates.

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 Zöllner 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 Zöllner 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.

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.

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.

◆ Illusory rotation in the haptic perception of a moving bar A M L Kappers, Z M Kluit (Utrecht University, Helmholtz Institute; Utrecht University; e-mail: a.m.l.kappers@uu.nl) Haptic matching of the orientation of bars separated by a horizontal distance leads to large systematic deviations (eg Kappers and Koenderink, 1999 Perception 28 781–795). A bar on the right side has to be rotated clockwise in order to be perceived as parallel to a bar at the left side. This finding leads to the following intriguing question which we investigated in this study: Will a bar moving from left to right in a fixed orientation be perceived as rotating counterclockwise? Blindfolded subjects had to touch a bar that moved from left to right or from right to left while it was rotating clockwise or counterclockwise with different speeds or did not rotate. For each trial they had to decide whether the rotation was clockwise or counterclockwise. From psychometric curves fitted to the data, we could determine that the results were consistent with the findings in the static case: A bar moving from left to right has to rotate clockwise in order to be perceived as non-rotating (and vice versa). In other words, a translating bar causes the illusory perception of a rotation. [This research was supported by The Netherlands Organisation for Scientific Research (NWO)]

In the last decades, under the headings of “mutation strategies,” “evolvability,” or “soft inheritance,” many ideas have been advanced on mechanisms assumed to promote innovative evolution beyond what one may anticipate from the classical model of random mutation and selection. Many population geneticists find these ideas superficially seducing but mathematically unfounded. While agreeing with the need to critically evaluate such proposals in the light of population genetics, I will argue that population geneticists are not immune to criticism. For instance, the “infinite site model” introduced by Kimura makes the unrealistic assumption that any neutral mutation arises only once during a neutral fixation episode, which leads, I propose, to an underestimation of the neutral fixation rates in large populations. Critical parameters such as mutation and recombination rates, effective population sizes or beneficial/deleterious mutation ratios are assigned convenient values, which may seem ad hoc to people outside the field. The lack of concern for the subtleties of genetic mechanisms is also criticized. Phenomena such as compensatory mutations, recurrent mutations, hot spots, and polymorphism, which population geneticists treat in the mathematical context of neutral versus selective fixations, can instead be interpreted in terms of genetic mechanisms for producing complex mutational events. Finally, single nucleotide substitutions are often treated as the quasi-exclusive source of variations, yet they cannot help much once the genes are optimized with respect to these substitutions. I suggest that population geneticists should invest more effort in refining the numerical values of the critical parameters used in their models. They should take into account the recent proposals on how mutations arise. They should also pay more attention to phenotypic variations, and develop criteria to discriminate between proposed evolutionary mechanisms that can actually work, and others that cannot.

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.

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 Orgel 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.