Stereo vision by self-organization

We propose a new algorithm for stereoscopic depth perception, where the depth map is the momentary state of a dynamic process. To each image point we assign a set of possible disparity values. In a dynamic process with competition and cooperation, the correct disparity value is selected for each image point. Therefore, we solve the correspondence problem by a dynamic, self-organizing process, the structure of which shows analogies to the human visual system. The algorithm can be implemented in a massive parallel manner and yields good results for either artificial or natural images. Received: 1 July 1993/Accepted in revised form: 22 December 1993     

Evolution of color vision.

Color vision is achieved by comparing the inputs from retinal photoreceptor neurons that differ in their wavelength sensitivity. Recent studies have elucidated the distribution and phylogeny of opsins, the family of light-sensitive molecules involved in this process. Interesting new findings suggest that animals have evolved a strategy to achieve specific sensitivity through the mutually exclusive expression of different opsin genes in photoreceptors.     

Physical evidence for distinct mechanisms of translational control by upstream open reading frames.

The Saccharomyces cerevisiae GCN4 mRNA 5'-leader contains four upstream open reading frames (uORFs) and the CPA1 leader contains a single uORF. To determine how these uORFs control translation, we examined mRNAs containing these leaders in cell-free translation extracts to determine where ribosomes were loaded first and where they were loaded during steady-state translation. Ribosomes predominantly loaded first at GCN4 uORF1. Following its translation, but not the translation of uORF4, they efficiently reinitiated protein synthesis at Gcn4p. Adding purified eIF2 increased reinitiation at uORFs 3 or 4 and reduced reinitiation at Gcn4p. This indicates that eIF2 affects the site of reinitiation following translation of GCN4 uORF1 in vitro. In contrast, for mRNA containing the CPA1 uORF, ribosomes reached the downstream start codon by scanning past the uORF. Addition of arginine caused ribosomes that had synthesized the uORF polypeptide to stall at its termination codon, reducing loading at the downstream start codon, apparently by blocking scanning ribosomes, and not by affecting reinitiation. The GCN4 and CPA1 uORFs thus control translation in fundamentally different ways.     

Predicting internal exons by oligonucleotide composition and discriminant analysis of spliceable open reading frames.

A new method which predicts internal exon sequences in human DNA has been developed. The method is based on a splice site prediction algorithm that uses the linear discriminant function to combine information about significant triplet frequencies of various functional parts of splice site regions and preferences of oligonucleotides in protein coding and intron regions. The accuracy of our splice site recognition function is 97% for donor splice sites and 96% for acceptor splice sites. For exon prediction, we combine in a discriminant function the characteristics describing the 5'-intron region, donor splice site, coding region, acceptor splice site and 3'-intron region for each open reading frame flanked by GT and AG base pairs. The accuracy of precise internal exon recognition on a test set of 451 exon and 246693 pseudoexon sequences is 77% with a specificity of 79%. The recognition quality computed at the level of individual nucleotides is 89% for exon sequences and 98% for intron sequences. This corresponds to a correlation coefficient for exon prediction of 0.87. The precision of this approach is better than other methods and has been tested on a larger data set. We have also developed a means for predicting exon-exon junctions in cDNA sequences, which can be useful for selecting optimal PCR primers.