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

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.

Alternation frequency thresholds for stereopsis as a technique for exploring stereoscopic difficulties.

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 & Psychophysics69 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 science and craft of autostereograms.

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.

Designing visually rich, nearly random textures.

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.

Errors and alternatives in prebiotic replication and catalysis.

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.

Multiple stages in codon-anticodon recognition: double-trigger mechanisms and geometric constraints.

Thirty years of kinetic studies on tRNA selection in the elongation cycle are reviewed, and confronted with results derived from various sources, including structural studies on the ribosome, genetic observations on ribosome and EF-Tu accuracy mutants, and codon-specific elongation rates. A coherent framework is proposed, which gives meaning to many puzzling effects. Ribosomal accuracy would be governed by a "double-trigger" principle, according to which the ribosome uses energy in the forward direction to create new configurations for tRNA selection, and energy in the backward direction to regain its initial configuration, in particular after a premature dissociation event.

How B cells and dendritic cells may cooperate in antigen purification.

The specificity of the immunological responses is achieved through the cooperation of three classes of cells: B and T lymphocytes, and dendritic cells (DCs). A critical, intensely studied interaction is that between DCs and T cells, during which the DC presents MHC-bound antigenic fragments to the T cell receptor (TCR). There has been recent excitement about the possibility of increasing the signal-to-noise ratio in the detection of cognate antigen-TCR couples, by the use of kinetic proofreading mechanisms.

Testing sequence effects in visual memory: clues for a structural model.

In order to probe the internal organization of visual temporary memory, systematic experiments were performed in which the subjects had to memorize a series of 2-5 images then pass recognition tests, either in all possible testing permutations, (in the case of 2-4 images) or in 20 selected permutations (in the case of 5 images). Over 300,000 tests were performed, generating more than 40,000 errors. The error-rates were found to follow simple rules.