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K A Robbins

Publications and source records attributed to K A Robbins.

4 recordsLinked to original sources
PubMed1999

Modal behavior of cortical neural networks during visual processing.

The network behavior of cortical cells during the processing of a light flash was characterized in an isolated, but functionally intact, turtle visual system. Rapid changes in intracellular membrane potential were monitored optically using a voltage-sensitive dye (VSD). Spatially coherent changes in membrane potential were determined by subjecting high-speed movies of the VSD signals to Karhunen-Loéve decomposition. In all experimental trials analyzed (n > 50), coherent activity was restricted to a small number of similar spatial patterns or modes. At least four modes (M(1,1), M(1,2), M(2,1), and M(2,2)) have an organizational structure similar to the normal modes of a vibrating membrane (drum). This empirical observation of modal activity provides a useful framework for analyzing the macroscopic behavior of cortical networks.

Animals↗
PubMed1992

Structure-function analysis of hepatocyte growth factor: identification of variants that lack mitogenic activity yet retain high affinity receptor binding.

Hepatocyte growth factor (HGF) is a potent mitogen for parenchymal liver, epithelial and endothelial cells. Structurally, it has similarities to kringle-containing serine proteases, although it does not possess proteolytic activity. A structure-activity relationship study of human HGF was performed by functional analysis of HGF substitution and deletion variants. Analysis of HGF variants was accomplished by defining their ability to induce DNA synthesis on hepatocytes in primary culture and to compete with wild-type HGF for binding to a soluble form of the HGF receptor. Three groups of variants were made: (i) substitutions at the cleavage site, (ii) substitutions within the protease-like domain and (iii) deletions of the beta-chain and/or kringle domains. Our results show that: (i) single-chain HGF is a zymogen-like promitogen in that cleavage into a two-chain form is required for biological activity, however, the single chain form of HGF still retains substantial receptor binding capacity; (ii) certain mutations in the protease-like domain result in variants that are completely defective for mitogenic activity, yet exhibit apparent receptor binding affinities similar to wild-type HGF (Kd approximately 50-70 pM); and (iii) a variant containing the N-terminal 272 residues of mature HGF showed only a 4-fold increase in Kd when compared with wild-type HGF indicating that a primary receptor binding determinant is located within this sequence.

Animals↗
PubMed1976

A moment equation description of magnetic reversals in the earth.

Low order moments of the alpha dynamo equations in a geophysical regime are related to the dependent variables in a reversing disk dynamo. The link suggests that nonperiodic reversal is an intrinsic feature of the alpha dynamo equations provided the forcing is vigorous and the toroidal and poloidal magnetic fields are sufficiently out of phase. The nonuniformities in reversal frequency are then attributable to subcritical instability.

Journal Article↗