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1.
The gaze control system governs distinct gaze behaviors, including visual fixation and gaze reorientations. Transitions between
these gaze behaviors are frequent and smooth in healthy individuals. This study models these gaze-behavior transitions for
different numbers of gaze degrees of freedom. Eye/head gaze behaviors have twice the number of degrees of freedom as eye-only
gaze behaviors. Each gaze behavior is observable in the system dynamics and is correlated with neuronal behaviors in several,
coordinated neural centers, including the vestibular nuclei. The coordination among the neural centers establishes a sensorimotor
state which maintains each gaze behavior. This study develops a mathematical framework for synthesizing the coordination among
neural centers in gaze sensorimotor states and focuses on the role of vestibular nuclei neurons in gaze sensorimotor state
transitions.
Received: 17 December 1999 / Accepted in revised form: 3 May 2001 相似文献
2.
Justin Garson 《Biology & philosophy》2011,26(4):547-565
Despite the voluminous literature on biological functions produced over the last 40 years, few philosophers have studied the
concept of function as it is used in neuroscience. Recently, Craver (forthcoming; also see Craver 2001) defended the causal role theory against the selected effects theory as the most appropriate theory of function for neuroscience.
The following argues that though neuroscientists do study causal role functions, the scope of that theory is not as universal
as claimed. Despite the strong prima facie superiority of the causal role theory, the selected effects theory (when properly developed) can handle many cases from neuroscience
with equal facility. It argues this by presenting a new theory of function that generalizes the notion of a ‘selection process’
to include processes such as neural selection, antibody selection, and some forms of learning—that is, to include structures
that have been differentially retained as well as those that have been differentially reproduced. This view, called the generalized
selected effects theory of function, will be defended from criticism and distinguished from similar views in the literature. 相似文献
3.
A neural network architecture based on the neural anatomy and function of retinal neurons in tiger salamander and mudpuppy
retinae is proposed to study basic aspects of early visual information processing. The model predictions for the main response
characteristics of retinal neurons are found to be in agreement with neurophysiological data, including the antagonistic role
of horizontal cells in the outer plexiform layer. The examination of possible γ-aminobutyric acid (GABA) action from horizontal
cells suggests that GABAA alone, GABAB alone, or their weighted combination can generate the response characteristics observed in bipolar cells.
Received: 25 June 2002 / Accepted: 28 January 2003 /
Published online: 20 May 2003
Acknowledgements. The authors would like to thank an anonymous reviewer for valuable comments.
Correspondence to: S. X. Yang (e-mail: syang@uoguelph.ca) 相似文献
4.
Neuronal network modelling of the effects of anaesthetic agents on somatosensory pathways 总被引:3,自引:0,他引:3
The whole question of consciousness, awareness and depth of anaesthesia is both timely, little understood and deeply challenging.
Models of the underlying neural pathway mechanisms/dynamics are necessary for understanding the interactions involved and
their structure and function. A neuronal network of the somatosensory pathways is proposed in this paper based on experimental
information and physiological investigation into anaesthesia. Existing mathematical neuronal models from the literature have
been modified and then employed to describe the dynamics of the proposed pathway network. Effects of anaesthetic agents on
the cortex were simulated in the model which describes the evoked cortical responses. By comparison with responses from anaesthetised
rats, the model's responses are able to describe the dynamics of typical responses. Thus, the proposed model promises to be
valuable for investigating the mechanisms of anaesthesia on the cortex and the effects of brain lesions.
Received: 4 March 2002 / Accepted in revised form: 8 July 2002
Correspondence to: D. A. Linkens (e-mail: d.linkens@sheffield.ac.uk, Tel.: +44-114-2225133, Fax: +44-114-2731729)
Acknowledgements. C.H. Ting was supported by a postgraduate scholarship from the University of Sheffield. 相似文献
5.
6.
A theory of hippocampal memory based on theta phase precession 总被引:10,自引:0,他引:10
Yamaguchi Y 《Biological cybernetics》2003,89(1):1-9
The neural dynamics of the hippocampal network for memory encoding of novel temporal sequences is proposed based on the theta
rhythm modulated firing of place cells called theta phase precession. It is hypothesized that theta phase precession is generated
at the entorhinal cortex by phase locking between local field theta oscillation and neural oscillators and that the hippocampal
closed network with feedforward and backward projections employ theta phase precession to create selectivity in the associative
connections needed for temporal sequence storage. Our analyses and computer experiments reveal that the phase precession generated
by phase locking instantaneously endows stable phase relations among neural activities in the successively changing neural
population. It is concluded that theta phase precession provides biologically plausible dynamics for the memory encoding of
novel temporal sequences as episodic events.
Received: 18 December 2002 / Accepted: 18 March 2003 /
Published online: 20 May 2003
Correspondence to: Y. Yamaguchi (e-mail: yokoy@brain.riken.go.jp, Fax: +81-48-4676938)
Acknowledgements. The author would like to express acknowledgement to Drs. McNaughton and Skaggs for their discussion and comment and to Dr.
Amari for his continuous encouragement. Further thanks are given to Mr. Haga and Dr. Wu for their discussion and cooperation. 相似文献
7.
Magarshak et al. represented an RNA molecule as a complex vector and an RNA secondary structure Γ as a complex matrix S
Γ in such a way that the molecule represented by was compatible with the secondary structure Γ if and only if . They only considered Watson-Crick base pairs and their representation cannot be extended to allow for G⋅U pairs. In this paper we study a generalization of Magarshak's representation that allows for these pairs, and in particular
we provide a family of algebraic structures where that generalization can be carried out. We also show that this representation
can be used to compare secondary structures, through transfer matrices which transform the representation of one secondary
structure into the representation of the other.
Received: 10 December 2001 / Revised version: 7 May 2002 /
Published online: 28 February 2003
Key words or phrases: RNA secondary structure – Algebra – Finite field 相似文献
8.
Rolando Brawer Facundo D. Batista Oscar R. Burrone Daniel O. Sordelli M. C. Cerquetti 《Archives of microbiology》1998,169(6):530-533
A temperature-sensitive mutant of Salmonella typhimurium was isolated earlier after transposon mutagenesis with Tn10d Tet. The mutant D220 grows well at 28 °C but has a lower growth rate and forms filaments at 37 °C. Transposon-flanking fragments
of mutant D220 DNA were cloned and sequenced. The transposon was inserted in the dam gene between positions 803 and 804 (assigned allele number: dam-231 : : Tn10d Tet) and resulted in a predicted ten-amino-acid-shorter Dam protein. The insertion created a stop codon that led to a truncated
Dam protein with a temperature-sensitive phenotype. The insertion dam-231 : : Tn10d Tet resulted in a dam“leaky” phenotype since methylated and unmethylated adenines in GATC sequences were present. In addition, the dam-231 : : Tn10d Tet insertion rendered dam mutants temperature-sensitive for growth depending upon the genetic background of the S. typhimurium strain. The wild-type dam gene of S. typhimurium exhibited 82% identity with the Escherichia coli dam gene. 相似文献
9.
Sensory experience alters the functional organization of cortical networks. Previous studies using behavioral training motivated
by aversive or rewarding stimuli have demonstrated that cortical plasticity is specific to salient inputs in the sensory environment.
Sensory experience associated with electrical activation of the basal forebrain (BasF) generates similar input specific plasticity.
By directly engaging plasticity mechanisms and avoiding extensive behavioral training, BasF stimulation makes it possible
to efficiently explore how specific sensory features contribute to cortical plasticity. This review summarizes our observations
that cortical networks employ a variety of strategies to improve the representation of the sensory environment. Different
combinations of receptive-field, temporal, and spectrotemporal plasticity were generated in primary auditory cortex neurons
depending on the pitch, modulation rate, and order of sounds paired with BasF stimulation. Simple tones led to map expansion,
while modulated tones altered the maximum cortical following rate. Exposure to complex acoustic sequences led to the development
of combination-sensitive responses. This remodeling of cortical response characteristics may reflect changes in intrinsic
cellular mechanisms, synaptic efficacy, and local neuronal connectivity. The intricate relationship between the pattern of
sensory activation and cortical plasticity suggests that network-level rules alter the functional organization of the cortex
to generate the most behaviorally useful representation of the sensory environment.
Received: 14 January 2002 / Accepted: 15 March 2002
Correspondence to: M.P. Kilgard (e-mail: kilgard@utdallas.edu, Tel.: +1-972-8832345, Fax: +1-972-8832491) 相似文献
10.
A model is presented that allows prediction of the probability for the formation of appositions between the axons and dendrites
of any two neurons based only on their morphological statistics and relative separation. Statistics of axonal and dendritic
morphologies of single neurons are obtained from 3D reconstructions of biocytin-filled cells, and a statistical representation
of the same cell type is obtained by averaging across neurons according to the model. A simple mathematical formulation is
applied to the axonal and dendritic statistical representations to yield the probability for close appositions. The model
is validated by a mathematical proof and by comparison of predicted appositions made by layer 5 pyramidal neurons in the rat
somatosensory cortex with real anatomical data. The model could be useful for studying microcircuit connectivity and for designing
artificial neural networks.
Received: 11 February 2002 / Accepted: 5 November 2002 / Published online: 20 February 2003
Correspondence to: H. Markram (e-mail: Henry.Markram@epfl.ch Tel.: +41-21-6939537, Fax: +41-21-6935350)
Acknowledgements. This study was supported by the National Alliance for Autism Research, the Minerva Foundation, the US Navy, the Ebner Center
for Biomedical Research, and the Edith Blum Foundation. 相似文献
11.
Neural oscillation is one of the most extensively investigated topics of artificial neural networks. Scientific approaches
to the functionalities of both natural and artificial intelligences are strongly related to mechanisms underlying oscillatory
activities. This paper concerns itself with the assumption of the existence of central pattern generators (CPGs), which are the plausible neural architectures with oscillatory capabilities, and presents a discrete and generalized
approach to the functionality of locomotor CPGs of legged animals. Based on scheduling by multiple edge reversal (SMER), a primitive and deterministic distributed algorithm, it is shown how oscillatory building block (OBB) modules can be created and, hence, how OBB-based networks can be formulated as asymmetric Hopfield-like neural networks
for the generation of complex coordinated rhythmic patterns observed among pairs of biological motor neurons working during
different gait patterns. It is also shown that the resulting Hopfield-like network possesses the property of reproducing the
whole spectrum of different gaits intrinsic to the target locomotor CPGs. Although the new approach is not restricted to the
understanding of the neurolocomotor system of any particular animal, hexapodal and quadrupedal gait patterns are chosen as
illustrations given the wide interest expressed by the ongoing research in the area.
Received: 14 June 2002 / Accepted: 18 February 2003 /
Published online: 20 May 2003
Correspondence to: Z. Yang (e-mail: zhijun.yang@ed.ac.uk)
Acknowledgements. This work was partially supported by CNPq, the Brazilian Research Agency, under support number 143032/96-8. We are grateful
for the helpful discussions with Prof. V.C. Barbosa, Dr. A.E. Xavier, Dr. M.S. Dutra, and Dr. A.F.R. Araújo. The donations
of FPGA hardware and software from XILINX Incorporation under the order No. XUP2930 and XUP3576 are also highly appreciated. 相似文献
12.
Robert P. Hausinger 《Journal of biological inorganic chemistry》1997,2(3):279-286
The five known nickel-dependent enzymes include urease, hydrogenase, carbon monoxide dehydrogenase (and CO dehydrogenase/acetyl-coenzyme
A synthase), methyl-S–coenzyme M reductase, and one class of superoxide dismutase. Consistent with their disparate functions, these Ni enzymes
have distinct metallocenter structures that vary in Ni coordination geometry, number and types of metals, and the presence
of additional components. Sophisticated cellular Ni processing systems have been devised to allow for specific and functional
incorporation of Ni into these proteins. This review highlights several themes that are common to the enzyme activation processes
and summarizes current concepts related to the enzyme-specific Ni assembly pathways.
Received, accepted: 3 April 1997 相似文献
13.
Lindström T 《Journal of mathematical biology》2002,45(5):396-418
In this paper we derive and analyze a discrete version of Rosenzweig's (Am. Nat. 1973) food-chain model. We provide substantial
analytical and numerical evidence for the general dynamical patterns of food chains predicted by De Feo and Rinaldi (Am. Nat.
1997) remaining largely unaffected by this discretization. Our theoretical analysis gives rise to a classification of the
parameter space into various regions describing distinct governing dynamical behaviors. Predator abundance has a local optimum
at the edge of chaos.
Received: 13 August 1999 / Revised version: 12 March 2002 / Published online: 17 October 2002
Mathematics Subject Classification (1991): 92D40
Keywords or phrases: Discrete food-chain – Discrete Hopf (Neimark-Sacker) bifurcation – Pulsewise birth processes – Mean yield maximization –
Nicholson-Bailey model 相似文献
14.
Statistical inferences are essentially important in analyzing neural spike trains in computational neuroscience. Current approaches
have followed a general inference paradigm where a parametric probability model is often used to characterize the temporal
evolution of the underlying stochastic processes. To directly capture the overall variability and distribution in the space
of the spike trains, we focus on a data-driven approach where statistics are defined and computed in the function space in
which spike trains are viewed as individual points. To this end, we at first develop a parametrized family of metrics that
takes into account different warpings in the time domain and generalizes several currently used spike train distances. These
new metrics are essentially penalized L
p
norms, involving appropriate functions of spike trains, with penalties associated with time-warping. The notions of means
and variances of spike trains are then defined based on the new metrics when p = 2 (corresponding to the “Euclidean distance”). Using some restrictive conditions, we present an efficient recursive algorithm,
termed Matching-Minimization algorithm, to compute the sample mean of a set of spike trains with arbitrary numbers of spikes.
The proposed metrics as well as the mean spike trains are demonstrated using simulations as well as an experimental recording
from the motor cortex. It is found that all these methods achieve desirable performance and the results support the success
of this novel framework. 相似文献
15.
Pisolithus tinctorius (Pers.) Coker and Couch [Syn. = P. arhizus (Scop.: Pers.) Rauschert] (Pt) is a widespread ectomycorrhizal basidiomycete forming mycorrhizas with a variety of hosts. Developmental and functional
aspects of the symbiosis are well documented and thus Pt has been adopted as a model organism for investigations of the molecular basis of ectomycorrhizal interactions. In this review
of the current state of knowledge of interactions between Pt and its hosts we demonstrate that Pt displays much intraspecific heterogeneity of host specificity, physiology and the benefits the fungus can impart upon the
host plant. It is not clear at present how far such heterogeneity reflects systematic segregation within Pt.
Accepted: 20 May 1997 相似文献
16.
The urine concentrating mechanism of mammals and birds depends on a counterflow configuration of thousands of nearly parallel
tubules in the medulla of the kidney. Along the course of a renal tubule, cell type may change abruptly, resulting in abrupt
changes in the physical characteristics and transmural transport properties of the tubule. A mathematical model that faithfully
represents these abrupt changes will have jump discontinuities in model parameters. Without proper treatment, such discontinuities
may cause unrealistic transmural fluxes and introduce suboptimal spatial convergence in the numerical solution to the model
equations. In this study, we show how to treat discontinuous parameters in the context of a previously developed numerical
method that is based on the semi-Lagrangian semi-implicit method and Newton's method. The numerical solutions have physically
plausible fluxes at the discontinuities and the solutions converge at second order, as is appropriate for the method.
Received: 13 November 2001 / Revised version: 28 June 2002 / Published online: 26 September 2002
This work was supported in part by the National Institutes of Health (National Institute of Diabetes and Digestive and Kidney
Diseases, grant DK-42091.)
Mathematics Subject Classification (2000): 65-04, 65M12, 65M25, 92-04, 92C35, 35-04, 35L45
Keywords or phrases: Mathematical models – Differential equations – Mathematical biology – Kidney – Renal medulla – Semi-Lagrangian semi-implicit 相似文献
17.
Inhibitory control of neural differentiation in mammalian cells 总被引:2,自引:0,他引:2
In Xenopus embryos, a truncated type II activin receptor (Δ1XAR1), capable of blocking signals by several transforming growth factor
(TGF)-β family members, can induce neural tissue suggesting neural fate is under inhibitory control. Activin and bone morphogenetic
protein 4 (BMP4) can act as neural inhibitors but only BMP4 can induce epidermis in Xenopus ectodermal cells. We have used the pluripotent mouse embryonal carcinoma cell line P19 to examine whether the mechanisms of
ectodermal cell fate decisions are conserved among vertebrates. We show that a P19 cell line expressing Δ1XAR1 will differentiate
into neurons. In addition, BMP4 inhibits retinoic acid (RA)-induced neural differentiation of P19 cells and induces keratin
expression. These results suggest that in mammals as in amphibians neural fate is under inhibitory control and BMP4 can alter
ectodermal differentiation.
Received: 23 September 1996 / Accepted: 8 January 1997 相似文献
18.
In this article, a neural model for generating and learning a rapid ballistic movement sequence in two-dimensional (2D) space
is presented and evaluated in the light of some considerations about handwriting generation. The model is based on a central
nucleus (called a planning space) consisting of a fully connected grid of leaky integrators simulating neurons, and reading
an input vector Ξ (t) which represents the external movement of the end effector. The movement sequencing results in a succession of motor strokes
whose instantiation is controlled by the global activation of the planning space as defined by a competitive interaction between
the neurons of the grid. Constraints such as spatial accuracy and movement time are exploited for the correct synchronization
of the impulse commands. These commands are then fed into a neuromuscular synergy whose output is governed by a delta lognormal
equation. Each movement sequence is memorized originally as a symbolic engram representing the sequence of the principal reference
points of the 2D movement. These points, called virtual targets, correspond to the targets of each single rapid motor stroke
composing the movement sequence. The task during the learning phase is to detect the engram corresponding to a new observed
movement; the process is controlled by the dynamics of the neural grid.
Received: 16 March 1995/Accepted in revised form: 25 July 1995 相似文献
19.
Human infants rapidly learn new skills and customs via imitation, but the neural linkages between action perception and production are not well understood. Neuroscience studies in adults suggest that a key component of imitation–identifying the corresponding body part used in the acts of self and other–has an organized neural signature. In adults, perceiving someone using a specific body part (e.g., hand vs. foot) is associated with activation of the corresponding area of the sensory and/or motor strip in the observer’s brain–a phenomenon called neural somatotopy. Here we examine whether preverbal infants also exhibit somatotopic neural responses during the observation of others’ actions. 14-month-old infants were randomly assigned to watch an adult reach towards and touch an object using either her hand or her foot. The scalp electroencephalogram (EEG) was recorded and event-related changes in the sensorimotor mu rhythm were analyzed. Mu rhythm desynchronization was greater over hand areas of sensorimotor cortex during observation of hand actions and was greater over the foot area for observation of foot actions. This provides the first evidence that infants’ observation of someone else using a particular body part activates the corresponding areas of sensorimotor cortex. We hypothesize that this somatotopic organization in the developing brain supports imitation and cultural learning. The findings connect developmental cognitive neuroscience, adult neuroscience, action representation, and behavioral imitation. 相似文献
20.
Edoardo Beretta Fortunata Solimano Yasuhiro Takeuchi 《Journal of mathematical biology》1996,35(1):1-19
A mathematical model for the delivery of drug directly to the macrophages by using the phagocytosis of senescent red blood
cells is proposed. The model is based on the following assumption: At time t=0 a preassigned red blood cell population n(0, a)=φ(a), a>0, loaded by the drug, is injected in the blood circulation. Among the cells of that population only those with an age a≧ā (ā=120 days) will be phagocytosed by macrophages. Of course, the lifetime of the drug must be higher than ā. Within the
red blood cells it cannot be metabolized, neither can it diffuse through their membranes. The emphasis of the paper is on
the mathematical properties and on the formulation of the control problem.
Received 15 December 1994; received in revised form 20 July 1995 相似文献