Department of Bioscience & Bioengineering

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Patterns of thalamocortical connections were related to architectonically defined subdivisions of the pulvinar complex and the dorsolateral geniculate nucleus (LGN) in tree shrews (Tupaia belangeri). Tree shrews are of special interest because they are considered close relatives of primates, and they have a highly developed visual system. Several distinguishable tracers were injected within and across cortical visual areas in individual tree shrews in order to reveal retinotopic patterns and cortical targets of subdivisions of the pulvinar. The results indicate that each of the three architectonic regions of the pulvinar has a distinctive pattern of cortical connections and that one of these divisions is further divided into two regions with different patterns of connections. Two of the pulvinar nuclei have similar retinotopic patterns of projections to caudal visual cortex. The large central nucleus of the pulvinar (Pc) projects to the first and second visual areas, V1 and V2, and an adjoining temporal dorsal area (TD) in retinotopic patterns indicating that the upper visual quadrant is represented dorsal to the lower quadrant in Pc. The smaller ventral nucleus (Pv) which stains darkly for the Cat-301 antigen, projects to these same cortical areas, with a retinotopic pattern. Pv also projects to a temporal anterior area, TA. The dorsal nucleus (Pd), which densely expresses AChE, projects to posterior and ventral areas of temporal extrastriate cortex, areas TP and TPI. A posterior nucleus, Pp, projects to anterior areas TAL and TI, of the temporal lobe, as well as TPI. Injections in different cortical areas as much as 6 mm apart labeled overlapping zones in Pp and double-labeled some cells. These results indicate that the visual pulvinar of tree shrews contains at least four functionally distinct subdivisions, or nuclei. In addition, the cortical injections revealed that the LGN projects topographically and densely to V1 and that a significant number of LGN neurons project to V2 and TD. © 2003 Wiley-Liss, Inc.

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Journal of Comparative Neurology

The Visual Pulvinar in Tree Shrews II. Projections of Four Nuclei to Areas of Visual Cortex

Impulses efficiently propagate into nominally dry granular beds and backscatter from buried inclusions in such beds may be potentially exploited to image shallow buried objects (SBOs). However, reliable imaging of SBOs requires “cleaning up” of surface vibrations, and, in addition to three-dimensional (3D) particle dynamics simulations, a phenomenological model to parametrize the bed surface may be useful for field applications. We introduce a 1D mean-field-like toy model with two parameters, which allows one to model surface vibrations, is consistent with experiments in a granular bed, and can help estimate the approximate signal transmission properties of the bed. © 2003 The American Physical Society.

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Physical Review E - Statistical Physics, Plasmas, Fluids, and Related Interdisciplinary Topics

Impulse backscattering in granular beds: Introducing a toy model

Shock absorption traditionally exploits visco-elastic devices, fluid viscous dampers and friction dampers, or "soft" materials. Recent work on impulse propagation in granular assemblies suggests that it may be possible to absorb shock waves using a different variety of shock absorbers and thus unlock the possibility of building structures that are significantly more shock absorbent than is currently possible, an issue of relevance to concerns of security. These new shock absorbers are composed of granular materials and exploit the nonlinear energy propagation properties in assemblies of granular materials. They have the potential to partially recover the energy of the absorbed shock waves for useful purposes, a property that might allow one to design devices to convert the incident energy to other useful forms. The basic physics of the "tapered chain shock absorber" is briefly discussed in this work.

Materials Research Society Symposium - Proceedings

Energy absorption and recovery in tapered granular chains: Small chains and low tapering

The first draft of the human genome has revealed enormous variability in the global distribution of Alu repeat elements. There are regions such as the four homeobox gene clusters, which are nearly devoid of these repeats that contrast with repeat dense regions in other transcriptionally active regions of the genome. Our analysis of the completely sequenced chromosomes 21 and 22 revealed a striking bias in Alu distribution. These elements are more clustered in genes which are involved in metabolism, transport, and signaling processes. In contrast, they are significantly fewer in genes coding for information pathway components as well as structural proteins. This bias in Alu distribution is independent of the effect of Alu density of the flanking genomic region and is also not affected by the GC content of the gene and its upstream and downstream regions. The relative proportions of Alu subfamilies (Alu J, Alu S, and Alu Y) are not significantly different in genes with high Alu density belonging to the functional categories of transport, metabolism, and signaling. However, in the structural proteins and information genes, these proportions are lower than the other three categories. We suggest that Alu elements might be involved in regulatory mechanisms and are therefore differentially selected in primate genomes.

Molecular Biology and Evolution

Nonrandom distribution of Alu elements in genes of various functional categories: Insight from analysis of human chromosomes 21 and 22

Tree shrews are highly visual mammals closely related to primates. They have a large visual pulvinar complex, but its organization and relation to visual cortex is only partly known. We processed brain sections through the pulvinar with seven different procedures in an effort to reveal histologically distinct compartments. The results revealed three major subdivisions. A dorsal subdivision, Pd, stains darkly for acetylcholinesterase (AChE) and occupies the dorsoposterior one-third of the pulvinar complex. A ventral subdivision, Pv, stains darkly when processed with the Cat-301 antibody and occupies the ventroanterior fifth of the pulvinar complex along the brachium of the superior colliculus. Unexpectedly, part of Pv is ventral to the brachium. A large central subdivision, Pc, stains moderately dark for AChE and cytochrome oxidase (CO), and very light for Cat-301. Pc includes about half of the pulvinar complex, with parts on both sides of the brachium of the superior colliculus. These architectonic results demonstrate that the pulvinar complex of tree shrews is larger and has more subdivisions than previously described. The complex resembles the pulvinar of primates by having a portion ventral to the brachium and by having histochemically distinct nuclei; the number of nuclei is less than in primates, however. © 2003 Wiley-Liss, Inc.

The Visual Pulvinar in Tree Shrews I. Multiple Subdivisions Revealed through Acetylcholinesterase and Cat-301 Chemoarchitecture

A two-parameter model to describe impulse backscattering at the surface of a granular assembly is proposed. The model is robust, and, at the same time, sensitive to changes in the distribution of p across the layers. The model qualitatively reproduces the time evolution of average energy of the surface grains of a complex, heterogeneous medium such as granular beds, following the generation of a normal impulse at the bed surface.

Physical Review E - Statistical, Nonlinear, and Soft Matter Physics

The impulse backscattering based detection and imaging of shallow buried objects was discussed. The analyses show that it is possible to use impulses to detect and image buried objects in granular beds. The simulations also suggest that the sensors used to detect backscattering data in granular beds must be able to gather time data at intervals of 10-4 seconds or less.

Impulse backscattering based detection and imaging of shallow buried objects

We consider a chain of monodisperse elastic grains of radius R where the grains are barely in contact. The grains repel upon contact via the Hertz-type potential, [formula presented] [formula presented] where [formula presented] is the grain-grain overlap, [formula presented] where [formula presented] denotes the displacement of grain i from its original equilibrium position. This being a computational study, we consider n to be arbitrary. Our dynamical simulations build on several earlier studies by Nesterenko, Coste, and Sen and co-workers that have shown that an impulse propagates as a solitary wave of fixed spatial extent, [formula presented] through a chain of grains. Here, we develop on a recent study by Manciu, Sen, and Hurd [Phys. Rev. E 63, 016614 (2001)] that shows that colliding solitary waves in the chains of interest spawn a well-defined hierarchy of multiple secondary solitary waves (SSWs) that carry ∼0.5% or less of the energy of the original solitary waves. We show that the emergence of SSWs is a complex process where nonlinear forces and the discreteness of the grains lead to the partitioning of the available energy into hierarchies of SSWs. The process of formation of SSWs involves length scales and time scales that are controlled by the strength of the nonlinearity in the system. To the best of our knowledge, there is no formal theory that describes the dynamics associated with the formation of SSWs. Calculations for cases where the Hertz-type potential can be symmetric in the overlap parameter δ, i.e., where δ can be both positive and negative, suggest that the formation of secondary solitary waves may be a fundamental property of certain discrete, nonlinear systems. © 2002 The American Physical Society.

Secondary solitary wave formation in systems with generalized Hertz interactions

Solutions to Newton's equations for particles in one-body potentials of form V1(xi) = ∑p A(i)2pxi2p, where p &gt; 0 and an integer, can be regarded as generators of infinite sequences of correlated frequencies {Ωm}. Simple nonlinear potentials can hence provide efficient ways of generating correlated information. Two-body potentials V2(xi - xj) can provide ways to communicate aspects of that information between the correlated frequency sequences. Temperature and noise can play a role in introducing a time scale across which the frequencies retain their identity. Introduction of explicit time dependence in the energy terms might be appropriate for constructing top down versions of toy models for the brain, something that is lacking at the present time. The richness of the nonlinear system along with the effects of heat baths, external noise and time dependence allows for the possibility of describing aging effects, processing of information "templates" in the brain and of the development of correlations between such "templates". In short, nonlinearity, interactions, noise effects and introduction of time-dependent energies might allow for the construction of "top-down" models of the brain with the eventual goal of possibly unifying the neurological, molecular biological, biochemical and psychiatric approaches toward studying the brain. © 2002 Elsevier Science B.V. All rights reserved.

Physica A: Statistical Mechanics and its Applications

Proposal for a nonlinear top-down toy model of the brain

The Mori-Lee treatment of linear response theory demonstrates that the Laplace transform of any relaxation function of a dynamical variable can be expressed as a continued fraction. For certain simple nonlinear systems, the continued fraction representation of the relaxation functions cannot be evaluated perturbatively. It turns out that many body systems with even a single on-site nonlinearity can dramatically alter the relaxation behavior in such systems. We argue that nonperturbative continued fractions in the Mori-Lee formalism are necessarily associated with systems that exhibit relaxation that is sensitive to the presence of nonlinearities. © 2002 Elsevier Science B.V. All rights reserved.