Forming classes by stimulus frequency: Behavior and theory

Visual classification is the way we relate to different images in our environment as if they were the same, while relating differently to other collections of stimuli (e.g., human vs. animal faces). It is still not clear, however, how the brain forms such classes, especially when introduced with new or changing environments. To isolate a perception-based mechanism underlying class representation, we studied unsupervised classification of an incoming stream of simple images. Classification patterns were clearly affected by stimulus frequency distribution, although subjects were unaware of this distribution. There was a common bias to locate class centers near the most frequent stimuli and their boundaries near the least frequent stimuli. Responses were also faster for more frequent stimuli. Using a minimal, biologically based neural-network model, we demonstrate that a simple, self-organizing representation mechanism based on overlapping tuning curves and slow Hebbian learning suffices to ensure classification. Combined behavioral and theoretical results predict large tuning overlap, implicating posterior infero-temporal cortex as a possible site of classification.

Flower color variation: A model for the experimental study of evolution

We review the study of flower color polymorphisms in the morning glory as a model for the analysis of adaptation. The pathway involved in the determination of flower color phenotype is traced from the molecular and genetic levels to the phenotypic level. Many of the genes that determine the enzymatic components of flavonoid biosynthesis are redundant, but, despite this complexity, it is possible to associate discrete floral phenotypes with individual genes. An important finding is that almost all of the mutations that determine phenotypic differences are the result of transposon insertions. Thus, the flower color diversity seized on by early human domesticators of this plant is a consequence of the rich variety of mobile elements that reside in the morning glory genome. We then consider a long history of research aimed at uncovering the ecological fate of these various flower phenotypes in the southeastern U.S. A large body of work has shown that insect pollinators discriminate against white phenotypes when white flowers are rare in populations. Because the plant is self-compatible, pollinator bias causes an increase in self-fertilization in white maternal plants, which should lead to an increase in the frequency of white genes, according to modifier gene theory. Studies of geographical distributions indicate other, as yet undiscovered, disadvantages associated with the white phenotype. The ultimate goal of connecting ecology to molecular genetics through the medium of phenotype is yet to be attained, but this approach may represent a model for analyzing the translation between these two levels of biological organization.

Self-assembly of a heteroduplex helicate from two different ligand strands and Cu(II) cations.

Cu(II) ions have been reacted with a 1/1 mixture of two linear ligands, one containing three 2,2'- bipyridine groups and the other three 2,2':6',2"-terpyridine groups. Absorption spectroscopy and fast atom bombardment mass spectrometry indicate the formation of a trinuclear complex containing one ligand of each kind. Determination of the crystal structure of this compound has confirmed that it is indeed a linear trinuclear complex in which two different ligands are wrapped in a helical fashion around the pentacoordinated metal ions. The central coordination geometry is trigonal bipyramidal; the two lateral Cu(II) ions are in a square pyramidal environment. Thus, a heteroduplex helicate is formed by the self-assembly of two different ligand strands and three specific metal ions induced by the coordination number and geometry of the latter. The self-assembly process may be considered to result from the reading of the steric and binding information present in the two ligands by Cu(II) ions through a pentacoordination algorithm. The same ligands have been shown earlier to yield homoduplex helicates from ions of tetrahedral and octahedral coordination geometry and strands of bidentate bipyridines and tridentate terpyridines, respectively. These two types of artificial double helical species may be related on one hand to the natural homoduplex nucleic acids and on the other hand to the DNA:RNA heteroduplex.

A method for distance determination in proteins using a designed metal ion binding site and site-directed spin labeling: evaluation with T4 lysozyme.

The use of molecular genetics to introduce both a metal ion binding site and a nitroxide spin label into the same protein opens the use of paramagnetic metalnitroxyl interactions to estimate intramolecular distances in a wide variety of proteins. In this report, a His-Xaa3-His metal ion binding motif was introduced at the N terminus of the long interdomain helix of T4 lysozyme (Lys-65 --> His/Gln-69 --> His) of three mutants, each containing a single nitroxide-labeled cysteine residue at position 71, 76, or 80. The results show that Cu(II)-induced relaxation effects on the nitroxide can be quantitatively analyzed in terms of interspin distance in the range of 10-25 A using Redfield theory, as first suggested by Leigh [Leigh, J.S. (1970) J. Chem. Phys. 52, 2608-2612]. Of particular interest is the observation that distances can be determined both under rigid lattice conditions in frozen solution and in the presence of motion of the spins at room temperature under physiological conditions. The method should be particularly attractive for investigating structure in membrane proteins that are difficult to crystallize. In the accompanying paper, the technique is applied to a polytopic membrane protein, lactose permease.