bluuebunny/crossref_metadata_2025 huggingface dataset:
Room-temperature operation of photopumped monolithic InP vertical-cavity laser with two air-gap Bragg reflectors
Authors: Chitica N. , Strassner M.
Published: March 2003

We demonstrate a long wavelength (λ=1.56 μm) vertical-cavity laser built on a low-loss resonator formed by two InP/air-gap Bragg reflectors. The monolithic, InP-based structure uses a periodic gain active region with six strain-compensated quantum wells. The photopumped vertical-cavity laser requires record low power density of only 370 W/cm2 to reach threshold at 25 °C. The equivalent threshold current density is estimated to be as low as 400 A/cm2. Continuous-wave operation is demonstrated up to 32 °C despite the low heat conductivity of the reflectors. The emission is single mode and a power of up to 110 μW has been coupled into a single-mode fiber.
Electron transfer to oriented molecules: Surprising steric effect in t-butyl bromide
Authors: Harris Sean A. , Brooks Philip R.
Published: March 2003

Collisions between neutral K atoms and oriented t-butyl bromide molecules produce the ions K+ and Br− at energies high enough to separate charged particles (≳4 eV). Ions are detected by coincidence tof mass spectrometry for orientation of the t-butyl bromide such that the K atom attacks either the Br end or the t-butyl end of the molecule. At high energies the steric asymmetry factor is larger than that for CH3Br. But at energies near threshold, the steric asymmetry factor reverses sign and attack at the t-butyl end becomes more reactive than attack at the Br end. The electron is apparently transferred into different orbitals at different ends.
Z-scan study on the nonlinear refractive index of copper nanocluster composite silica glass
Authors: Battaglin G. , Calvelli P. , Cattaruzza E. , Gonella F. , Polloni R. , Mattei G. , Mazzoldi P.
Published: March 2003

We used the Z-scan technique for measuring the nonlinear refractive index n2 of a thin composite film formed by copper nanoparticles embedded in silica glass. By varying the number of pulses of the laser shot, we evidenced heating effects induced by the laser during measurements. We were able to estimate the nonthermal refractive-index value, n2=(3.0±0.3)×10−12 cm2/W.
Force based displacement measurement in micromechanical devices
Authors: O’Shea S. J. , Ng C. K. , Tan Y. Y. , Xu Y. , Tay E. H. , Chua B. L. , Tien N. C. , Tang X. S. , Chen W. T.
Published: March 2003

We demonstrate how force detection methods based on atomic force microscopy can be used to measure displacement in micromechanical devices. We show the operation of a simple microfabricated accelerometer, the proof mass of which incorporates a tip which can be moved towards an opposing surface. Both noncontact operation using long range electrostatic forces and tapping mode operation are demonstrated. The displacement sensitivity of the present device using feedback to control the tip-surface separation is approximately 1 nm.
Photocatalysis of sculptured thin films of TiO2
Authors: Suzuki Motofumi , Ito Tadayoshi , Taga Yasunori
Published: March 2003

Enhanced surface reaction efficiency has been demonstrated in the photocatalysis of obliquely deposited TiO2 thin films with variously shaped columns such as zigzag, cylinder, and helix. The columnar thickness and spacing play an important role in the enhancement of the effective surface area, while the columnar shape is less important. The optimum morphology for a surface reaction has been obtained at the deposition angle α=70°, where the photocatalytic activity is 2.5 times larger than that at α=0°. The morphology controlled obliquely deposited thin films are applicable for solar cell, electro- and photochromic devices besides photocatalyst.
Theoretical study of sulfur–hydrogen–vacancy complex in diamond
Authors: Miyazaki Takehide , Okushi Hideyo , Uda Tsuyoshi
Published: March 2003

We present an ab initio study of sulfur (S)–hydrogen (H)–vacancy (V) complexes in diamond. An S–H–V defect may become a much shallower donor than an isolated substitutional S defect when S in the complex is either three or five connected. Upon annealing the S-doped crystal, preferential formation of other deep-level defects would deactivate the shallow S-complex donors.
Relationship between kinetics and thermodynamics of supercooled liquids
Authors: Mohanty Udayan , Craig Nathaniel , Fourkas John T.
Published: March 2003

We show that a single, reasonable assumption about the probability of rearrangements in a supercooled liquid necessarily implies a direct connection between kinetic and thermodynamic observables in these media. The prediction is tested against available experimental data for supercooled liquids spanning the range of observed kinetics.
Specular reflection enhancement and reduction from the surface of a photorefractive crystal in the presence of a “pump” beam
Authors: Zhang Jiasen , Yoshikado Shin , Aruga Tadashi
Published: March 2003

We demonstrate the enhancement and reduction of the specular reflection from the surface of a photorefractive crystal in the presence of a “pump” beam. The pump beam could be the phase conjugate beam of the input beam or another laser beam. The contribution of the reflection grating is also demonstrated in a 45°-cut crystal. The change of the specular reflection with respect to the intensity ratio of the input beam to the pump beam is measured.
Spin and superconducting fluctuations in the copper–oxygen planes of a quasi-two-dimensional high-temperature superconductor
Authors: Sergeeva G. G. , Gonchar V. Yu. , Voitsenya A. V.
Published: March 2003

The features of the two-dimensional superconducting and spin fluctuations in the copper–oxygen planes of quasi-two-dimensional high-Tc superconductors at T<T*, where T* is the charge ordering temperature, are studied in the two-dimensional (2D) XY model of Berezinskiı̆, Kosterlitz, and Thouless. It is shown that the coexistence of three phases (metallic, almost insulating, and superconducting) in the copper–oxygen planes, with an independent distribution in each plane, enhances the semiconductor character of the charge transport along the ĉ axis and leads to two channels of charge tunneling with different temperature dependence of the tunneling probability tc(T). The expressions obtained for the temperature dependence of the resistivity ρc(T) can be used to determine the temperature of the 2D XY ordering of the copper spins in the insulating stripes and the dimensionality of the superconducting transition from the results of measurements.
Influence of a large biquadratic interaction on the phase states of a non-Heisenberg biaxial ferromagnet
Authors: Fridman Yu. A. , Kosmachev O. A.
Published: March 2003

A study is made of the phase states of a biaxial non-Heisenberg ferromagnet with a large biquadratic interaction in an external magnetic field. It is shown that in such systems the formation of canted phases is energetically unfavorable but that the quadrupolar and ferromagnetic phases can exist in them. The phase transitions between these phases are studied, and it is shown that they are first-order transitions. The field of the first-order transition is found. The phase stability fields are determined from an analysis of the spectra of coupled magnetoelastic waves.
Phonon scattering by structural defects in solid p-H2 and in p-H2–o-D2 solutions
Authors: Korolyuk O. A. , Gorodilov B. Ya. , Krivchikov A. I. , Raenko A. V. , Jeżowski A.
Published: March 2003

A study is made of the influence of structural defects on the thermal conductivity in parahydrogen crystals and in parahydrogen–orthodeuterium solutions. The defects in the crystals are generated by means of a thermal shock. The temperature dependence of the thermal conductivity is analyzed in the framework of the Callaway relaxation model in the Debye approximation for the phonon spectrum with allowance for phonon–phonon scattering processes and Rayleigh scattering on isotopic defects and structural defects such as dislocations and low-angle boundaries. The effect produced in the sample by the thermal shock is found to depend on the deuterium concentration. In pure parahydrogen an increase in the dislocation density is observed, and in parahydrogen–orthodeuterium solutions an increase in the density of low-angle boundaries. The change in the density of low-angle boundaries in the solutions after the thermal shock depends linearly on the concentration of o-D2.
The initiation of bursts in thalamic neurons and the cortical control of thalamic sensitivity
Authors: Destexhe Alain , Sejnowski Terrence J.
Published: March 2003

Thalamic neurons generate high–frequency bursts of action potentials when a low–threshold (T–type) calcium current, located in soma and dendrites, becomes activated. Computational models were used to investigate the bursting properties of thalamic relay and reticular neurons. These two types of thalamic cells differ fundamentally in their ability to generate bursts following either excitatory or inhibitory events. Bursts generated with excitatory inputs in relay cells required a high degree of convergence from excitatory inputs, whereas moderate excitation drove burst discharges in reticular neurons from hyperpolarized levels. The opposite holds for inhibitory rebound bursts, which are more difficult to evoke in reticular neurons than in relay cells. The differences between the reticular neurons and thalamocortical neurons were due to different kinetics of the T–current, different electrotonic properties and different distribution patterns of the T–current in the two cell types. These properties enable the cortex to control the sensitivity of the thalamus to inputs and are also important for understanding states such as absence seizures.
Novel neuronal and astrocytic mechanisms in thalamocortical loop dynamics
Authors: Crunelli Vincenzo , Blethyn Kate L. , Cope David W. , Hughes Stuart W. , Parri H. Rheinallt , Turner Jonathan P. , Tòth Tibor I. , Williams Stephen R.
Published: March 2003

In this review, we summarize three sets of findings that have recently been observed in thalamic astrocytes and neurons, and discuss their significance for thalamocortical loop dynamics.

(i) A physiologically relevant ‘window’ component of the low–voltage–activated, T–type Ca
2+
current (
I
Twindow
) plays an essential part in the slow (less than 1 Hz) sleep oscillation in adult thalamocortical (TC) neurons, indicating that the expression of this fundamental sleep rhythm in these neurons is not a simple reflection of cortical network activity. It is also likely that
I
Twindow
underlies one of the cellular mechanisms enabling TC neurons to produce burst firing in response to novel sensory stimuli.

(ii) Both electrophysiological and dye–injection experiments support the existence of gap junction–mediated coupling among young and adult TC neurons. This finding indicates that electrical coupling–mediated synchronization might be implicated in the high and low frequency oscillatory activities expressed by this type of thalamic neuron.

(iii) Spontaneous intracellular Ca
2+
([Ca
2+
]
i
) waves propagating among thalamic astrocytes are able to elicit large and long–lasting
N
–methyl–D–aspartate–mediated currents in TC neurons. The peculiar developmental profile within the first two postnatal weeks of these astrocytic [Ca
2+
]
i
transients and the selective activation of these glutamate receptors point to a role for this astrocyte–to–neuron signalling mechanism in the topographic wiring of the thalamocortical loop.

As some of these novel cellular and intracellular properties are not restricted to thalamic astrocytes and neurons, their significance may well apply to (patho)physiological functions of glial and neuronal elements in other brain areas.
Thalamocortical control of feed–forward inhibition in awake somatosensory ‘barrel’ cortex
Authors: Swadlow Harvey A .
Published: March 2003

Intracortical inhibition plays a role in shaping sensory cortical receptive fields and is mediated by both feed–forward and feedback mechanisms. Feed–forward inhibition is the faster of the two processes, being generated by inhibitory interneurons driven by monosynaptic thalamocortical (TC) input. In principle, feed–forward inhibition can prevent targeted cortical neurons from ever reaching threshold when TC input is weak. To do so, however, inhibitory interneurons must respond to TC input at low thresholds and generate spikes very quickly. A powerful feed–forward inhibition would sharpen the tuning characteristics of targeted cortical neurons, and interneurons with sensitive and broadly tuned receptive fields could mediate this process. Suspected inhibitory interneurons (SINs) with precisely these properties are found in layer 4 of the somatosensory (S1) ‘barrel’ cortex of rodents and rabbits. These interneurons lack the directional selectivity seen in most cortical spiny neurons and in ventrobasal TC afferents, but are much more sensitive than cortical spiny neurons to low–amplitude whisker displacements. This paper is concerned with the activation of S1 SINs by TC impulses, and with the consequences of this activation. Multiple TC neurons and multiple S1 SINs were simultaneously studied in awake rabbits, and cross–correlation methods were used to examine functional connectivity. The results demonstrate a potent, temporally precise, dynamic and highly convergent/divergent functional input from ventrobasal TC neurons to SINs of the topographically aligned S1 barrel. Whereas the extensive pooling of
convergent
TC inputs onto SINs generates sensitive and broadly tuned inhibitory receptive fields, the potent TC
divergence
onto many SINs generates sharply synchronous activity among these elements. This TC feed–forward inhibitory network is well suited to provide a fast, potent, sensitive and broadly tuned inhibition of targeted spiny neurons that will suppress spike generation following all but the most optimal feed–forward excitatory inputs.
Spike timing and visual processing in the retinogeniculocortical pathway
Authors: Usrey W. Martin
Published: March 2003

Although the visual response properties of neurons along the retinogeniculocortical pathway have been studied for decades, relatively few studies have examined how individual neurons along the pathway communicate with each other. Recent studies in the cat (
Felis domestica
) now show that the strength of these connections is very dynamic and spike timing plays an important part in determining whether action potentials will be transferred from pre– to postsynaptic cells. This review explores recent progress in our understanding of what role spike timing has in establishing different patterns of geniculate activity and how these patterns ultimately drive the cortex.
Pre–attentive segmentation and correspondence in stereo
Authors: Zhaoping Li
Published: March 2003

Traditional stereo grouping models have focused on the problem of stereo correspondence between monocular inputs. Recent physiological data revealed that the disparity selective V2 cells increase their responses when (random–dot stereograms) stimuli within their receptive fields are at or near the boundary of a depth surface. Such highlights to depth (non–luminance) edges are seemingly not computationally required for the correspondence problem. Computationally, these highlights make the boundaries of a depth surface more salient, serving pre–attentive segmentation (between depth planes) and attracting visual attention. In special cases, they enable the psychophysically observed perceptual pop–out of a target from a background of visually identical distractors at a different depth. To achieve the highlights, mutual inhibition between disparity selective cells that are tuned to the same or similar depths is required. However, such mutual inhibition would impede the computation for the correspondence problem, which requires mutual excitation between the same cells. In this work, I introduce a computational model that, I believe, is the first to address both stereo correspondence and pre–attentive stereo segmentation. The computational mechanisms in the model are based on intracortical interactions in V2. I will demonstrate that the model captures the following physiological and psychophysical phenomena: (i) depth–edge highlighting; (ii) disparity capture; (iii) pop–out; and (iv) transparency.
The influence of the corticothalamic projection on responses in thalamus and cortex
Authors: Wörgötter Florentin , Eyding Dirk , Macklis Jeffrey D. , Funke Klaus
Published: March 2003

We review results on the in vivo properties of neurons in the dorsal lateral geniculate nucleus (dLGN) that receives its afferent input from the retina and projects to the visual cortex. In addition, the dLGN receives input from the brain stem and from a rather strong corticothalamic back–projection, which originates in layer 6 of the visual cortex. We compare the behaviour of dLGN cells during spontaneous changes of the frequency contents of the electroencephalograph (EEG) (which are mainly related to a changing brain stem influence), with those that are obtained when experimentally silencing the corticothalamic feedback. The spatial and temporal response properties of dLGN cells are compared during these two conditions, and we report that the neurons behave similarly during a synchronized EEG state and during inactive corticothalamic feedback. In both situations, dLGN cells are rather phasic and their remaining tonic activity is temporally dispersed, indicating a hyperpolarizing effect. By means of a novel method, we were able to chronically eliminate a large proportion of the corticothalamic projection neurons from the otherwise intact cortex. In this condition, we found that cortical cells also lose their EEG specific response differences but, in this instance, probably due to a facilitatory (depolarizing) plasticity reaction of the remaining network.
Excitatory inputs to spiny cells in layers 4 and 6 of cat striate cortex
Authors: Bannister N.J. , Nelson J.C. , Jack J.J.B.
Published: March 2003

The principal target of lateral geniculate nucleus in the cat visual cortex is the stellate neurons of layer 4. In previously reported work with intracellular recording and extracellular stimulation in slices of visual cortex, three general classes of fast excitatory synaptic potentials (EPSPs) in layer 4a spiny stellate neurons were identified. One of these classes, characterized by large and relatively invariant amplitudes (mean 1.7 mV, average coefficient of variation (CV) 0.083) were attributed to the action of geniculate axons because, unlike the other two classes, they could not be matched by intracortical inputs, using paired recording. We have examined in detail the properties of this synaptic input in twelve examples, selecting for study those EPSPs where there was secure extracellular stimulation of the single fibre input to a pair of stimuli 50 ms apart. In our analysis, we conclude that the depression that these inputs show to the second stimulus is entirely postsynaptic, since the evidence strongly suggests that the probability of transmitter release at the synaptic site(s) remains 1.0 for both stimuli. We argue that the most plausible explanation for this postsynaptic depression is a reduction in the average probability of opening the synaptic channels. Using a simple biochemical analysis (c.f. Sigworth plot), it is then possible to calculate the number of synaptic channels and their probability of opening, for each of the 12 connections. The EPSPs had a mean amplitude of 1.91 mV (±1.3 mV SD) and a mean CV of 0.067 (± 0.022). The calculated number of channels ranged from 20 to 158 (59.4 ± 48.7) and their probability of opening to the first EPSP had an average of 0.83 (± 0.09), with an average depression of the probability to 0.60 for the second EPSP.

Geniculate afferents also terminate in layer 6. Intracellular recordings were also made in the upper part of this layer and a total of 51 EPSPs were recorded from pyramidal cells of three principal types. Amongst this dataset we sought EPSPs with similar properties to those characterized in layer 4a. Three examples were found, which is a much lower percentage (6%) than the incidence of putative geniculate EPSPs found in layer 4a (42%).
The role of the thalamus in the flow of information to the cortex
Authors: Sherman S. Murray , Guillery R. W.
Published: March 2003

The lateral geniculate nucleus is the best understood thalamic relay and serves as a model for all thalamic relays. Only 5–10% of the input to geniculate relay cells derives from the retina, which is the driving input. The rest is modulatory and derives from local inhibitory inputs, descending inputs from layer 6 of the visual cortex, and ascending inputs from the brainstem. These modulatory inputs control many features of retinogeniculate transmission. One such feature is the response mode, burst or tonic, of relay cells, which relates to the attentional demands at the moment. This response mode depends on membrane potential, which is controlled effectively by the modulator inputs. The lateral geniculate nucleus is a first–order relay, because it relays subcortical (i.e. retinal) information to the cortex for the first time. By contrast, the other main thalamic relay of visual information, the pulvinar region, is largely a higher–order relay, since much of it relays information from layer 5 of one cortical area to another. All thalamic relays receive a layer–6 modulatory input from cortex, but higher–order relays in addition receive a layer–5 driver input. Corticocortical processing may involve these corticothalamocortical ‘re–entry’ routes to a far greater extent than previously appreciated. If so, the thalamus sits at an indispensable position for the modulation of messages involved in corticocortical processing.
Rate and timing in cortical synaptic plasticity
Authors: Nelson Sacha B. , Sjöström Per Jesper , Turrigiano Gina G.
Published: March 2003

Debate has raged over the past few years as to whether cortical neurons transmit information primarily in their average firing rates or in the precise timing of their spikes. Here, we address the related question of which features of spike trains control plasticity at cortical synapses. Using paired recording in slices we have developed a quantitative and predictive description of the joint dependence of cortical plasticity on the rate and relative timing of pre– and postsynaptic firing. The results hold important implications for which parts of the neural code are most readily stored for later retrieval.
Target and temporal pattern selection at neocortical synapses
Authors: Thomson Alex M. , Bannister A. Peter , Mercer Audrey , Morris Oliver T.
Published: March 2003

We attempt to summarize the properties of cortical synaptic connections and the precision with which they select their targets in the context of information processing in cortical circuits. High–frequency presynaptic bursts result in rapidly depressing responses at most inputs onto spiny cells and onto some interneurons. These ‘phasic’ connections detect novelty and changes in the firing rate, but report frequency of maintained activity poorly. By contrast, facilitating inputs to interneurons that target dendrites produce little or no response at low frequencies, but a facilitating–augmenting response to maintained firing. The neurons activated, the cells they in turn target and the properties of those synapses determine which parts of the circuit are recruited and in what temporal pattern. Inhibitory interneurons provide both temporal and spatial tuning. The ‘forward’ flow from layer–4 excitatory neurons to layer 3 and from 3 to 5 activates predominantly pyramids. ‘Back’ projections, from 3 to 4 and 5 to 3, do not activate excitatory cells, but target interneurons. Despite, therefore, an increasing complexity in the information integrated as it is processed through these layers, there is little ‘contamination’ by ‘back’ projections. That layer 6 acts both as a primary input layer feeding excitation ‘forward’ to excitatory cells in other layers and as a higher–order layer with more integrated response properties feeding inhibition to layer 4 is discussed.
A new interpretation of thalamocortical circuitry
Authors: Adams Paul , Cox Kingsley
Published: March 2003

Almost all the information that is needed to specify thalamocortical and neocortical wiring derives from patterned electrical activity induced by the environment. Wiring accuracy must be limited by the anatomical specificity of the cascade of events triggered by neural activity and culminating in synaptogenesis. We present a simple model of learning in the presence of plasticity errors. One way to achieve learning specificity is to build better synapses. We discuss an alternative, circuit–based, approach that only allows plasticity at connections that support highly selective correlations. This circuit resembles some of the more puzzling aspects of thalamocorticothalamic circuitry.
Glutamate receptor functions in sensory relay in the thalamus
Authors: Salt T. E.
Published: March 2003

It is known that glutamate is a major excitatory transmitter of sensory and cortical afferents to the thalamus. These actions are mediated via several distinct receptors with postsynaptic excitatory effects predominantly mediated by ionotropic receptors of the α–amino–3–hydroxy–5–methyl–4–isoxazolepropionate (AMPA) and
N
–methyl–D–aspartate varieties (NMDA). However, there are also other kinds of glutamate receptor present in the thalamus, notably the metabotropic and kainate types, and these may have more complex or subtle roles in sensory transmission. This paper describes recent electrophysiological experiments done
in vitro
and
in vivo
which aim to determine how the metabotropic and kainate receptor types can influence transmission through the sensory thalamic relay. A particular focus will be how such mechanisms might operate under physiological conditions.
Recordings, behaviour and models related to corticothalamic feedback
Authors: Gerstein G. L. , Kirkland K. L. , Musial P. G. , Talwar S. K.
Published: March 2003

In this paper, we review recent work on aspects of corticothalamic interactions in the auditory and in the visual systems. There are gross similarities in the arrangements of these systems, but considerable contrasts in the processing computations and in the effects of corticothalamic feedback.
Analysing neuronal correlates of the comparison of two sequentially presented sensory stimuli
Authors: Brody Carlos D. , Hernández Adrián , Zainos Antonio , Lemus Luis , Romo Ranulfo
Published: March 2003

In a typical sequential sensory discrimination task, subjects are required to make a decision based on comparing a sensory stimulus against the memory trace left by a previous stimulus. What is the neuronal substrate for such comparisons and the resulting decisions? This question was studied by recording neuronal responses in a variety of cortical areas of awake monkeys (
Macaca mulatta
), trained to carry out a vibrotactile sequential discrimination task. We describe methods to analyse responses obtained during the comparison and decision phases of the task, and describe the resulting findings from recordings in secondary somatosensory cortical area (S2). A subset of neurons in S2 become highly correlated with the monkey's decision in the task.
Thalamic circuitry and thalamocortical synchrony
Authors: Jones Edward G.
Published: March 2003

The corticothalamic system has an important role in synchronizing the activities of thalamic and cortical neurons. Numerically, its synapses dominate the inputs to relay cells and to the γ–amino butyric acid (GABA)ergic cells of the reticular nucleus (RTN). The capacity of relay neurons to operate in different voltage–dependent functional modes determines that the inputs from the cortex have the capacity directly to excite the relay cells, or indirectly to inhibit them via the RTN, serving to synchronize high– or low–frequency oscillatory activity respectively in the thalamocorticothalamic network. Differences in the α–amino–3–hydroxy–5–methyl–4–isoxazolepropionic acid (AMPA) subunit composition of receptors at synapses formed by branches of the same corticothalamic axon in the RTN and dorsal thalamus are an important element in the capacity of the cortex to synchronize low–frequency oscillations in the network. Interactions of focused corticothalamic axons arising from layer VI cortical cells and diffuse corticothalamic axons arising from layer V cortical cells, with the specifically projecting core relay cells and diffusely projecting matrix cells of the dorsal thalamus, form a substrate for synchronization of widespread populations of cortical and thalamic cells during high–frequency oscillations that underlie discrete conscious events.
Is there a thalamic component to experience–dependent cortical plasticity?
Authors: Fox Kevin , Wallace Helen , Glazewski Stanislaw
Published: March 2003

Sensory deprivation and injury to the peripheral nervous system both induce plasticity in the somatosensory system of adult animals, but in different places. While injury induces plasticity at several locations within the ascending somatosensory pathways, sensory deprivation appears only to affect the somatosensory cortex. Experiments have been performed to detect experience–dependent plasticity in thalamic receptive fields, thalamic domain sizes and convergence of thalamic receptive fields onto cortical cells. So far, plasticity has not been detected with sensory deprivation paradigms that cause substantial cortical plasticity. Part of the reason for the lack of thalamic plasticity may lie in the synaptic properties of afferent systems to the thalamus. A second factor may lie in the differences in the organization of cortical and thalamic circuits. Many deprivation paradigms induce plasticity by decreasing phasic lateral inhibition. Since lateral inhibition appears to be far weaker in the thalamus than the cortex, sensory deprivation may not cause large enough imbalances in thalamic activity to induce plasticity in the thalamus.
Corticothalamic interactions in the transfer of visual information
Authors: Sillito Adam M. , Jones Helen E.
Published: March 2003

Thalamic function does not stand apart, as a discrete processing step, from the cortical circuitry. The thalamus receives extensive feedback from the cortex and this influences the firing pattern, synchronization and sensory response mode of relay cells. A crucial question concerns the extent to which the feedback simply controls the state and transmission mode of relay cells and the extent to which the feedback participates in the specific processing of sensory information. Using examples from experiments examining the influence of feedback from the visual cortex to the lateral geniculate nucleus (LGN), we argue that thalamic mechanisms are selectively focused by visually driven feedback to optimize the thalamic contribution to segmentation and global integration. This involves effects on both the temporal and spatial parameters characterizing the responses of LGN cells and includes, for example, motion–driven feedback effects from MT (middle temporal visual area) relayed via layer 6 of V1 (primary visual cortex).
The thalamus as a monitor of motor outputs
Authors: Guillery R. W. , Sherman S. M.
Published: March 2003

Many of the ascending pathways to the thalamus have branches involved in movement control. In addition, the recently defined, rich innervation of ‘higher’ thalamic nuclei (such as the pulvinar) from pyramidal cells in layer five of the neocortex also comes from branches of long descending axons that supply motor structures. For many higher thalamic nuclei the clue to understanding the messages that are relayed to the cortex will depend on knowing the nature of these layer five motor outputs and on defining how messages from groups of functionally distinct output types are combined as inputs to higher cortical areas. Current evidence indicates that many and possibly all thalamic relays to the neocortex are about instructions that cortical and subcortical neurons are contributing to movement control. The perceptual functions of the cortex can thus be seen to represent abstractions from ongoing motor instructions.
Feed-forward synchronization: propagation of temporal patterns along the retinothalamocortical pathway
Authors: Neuenschwander Sergio , Castelo-Branco Miguel , Baron Jerome , Singer Wolf
Published: March 2003

Visual responses in the cortex and lateral geniculate nucleus (LGN) are often associated with synchronous oscillatory patterning. In this short review, we examine the possible relationships between subcortical and cortical synchronization mechanisms. Our results obtained from simultaneous multi–unit recordings show strong synchronization of oscillatory responses between retina, LGN and cortex, indicating that cortical neurons can be synchronized by oscillatory activity relayed through the LGN. This feed–forward synchronization mechanism operating in the 60 to 120 Hz frequency range was observed mostly for static stimuli. In response to moving stimuli, by contrast, cortical synchronization was independent of oscillatory inputs from the LGN, with oscillation frequency in the range of 30 to 60 Hz. The functional implications of synchronization of activity from parallel channels are discussed, in particular its significance for signal transmission and cortical integration processes.
Adaptation in the corticothalamic loop: computational prospects of tuning the senses
Authors: Hillenbrand Ulrich , Hemmen J. Leo van
Published: March 2003

The present article discusses computational hypotheses on corticothalamic feedback and modulation of cortical response properties. We have recently proposed that the two phenomena are related, hypothesizing that neuronal velocity preference in the visual cortex is altered by feedback to the lateral geniculate nucleus. We now contrast the common view that response adaptation to stimuli subserves a function of redundancy reduction with the idea that it may enhance cortical representation of objects. Our arguments lead to the concept that the corticothalamic loop is involved in reducing sensory input to behaviourally relevant aspects, a pre–attentive gating.
Dynamic shifting in thalamocortical processing during different behavioural states
Authors: Nicolelis Miguel A.L , Fanselow Erika E.
Published: March 2003

Recent experiments in our laboratory have indicated that as rats shift the behavioural strategy employed to explore their surrounding environment, there is a parallel change in the physiological properties of the neuronal ensembles that define the main thalamocortical loop of the trigeminal somatosensory system. Based on experimental evidence from several laboratories, we propose that this concurrent shift in behavioural strategy and thalamocortical physiological properties provides rats with an efficient way to optimize either the detection or analysis of complex tactile stimuli.
Phosphorylation of RNA polymerase II CTD regulates H3 methylation in yeast
Authors: Xiao Tiaojiang , Hall Hana , Kizer Kelby O. , Shibata Yoichiro , Hall Mark C. , Borchers Christoph H. , Strahl Brian D.
Published: March 2003

Histone methylation is now realized to be a pivotal regulator of gene transcription. Although recent studies have shed light on atrans-histone regulatory pathway that controls H3 Lys 4 and H3 Lys 79 methylation inSaccharomyces cerevisiae, the regulatory pathway that affects Set2-mediated H3 Lys 36 methylation is unknown. To determine the functions of Set2, and identify factors that regulate its site of methylation, we genomically tagged Set2 and identified its associated proteins. Here, we show that Set2 is associated with Rbp1 and Rbp2, the two largest subunits of RNA polymerase II (RNA pol II). Moreover, we find that this association is specific for the interaction of Set2 with the hyperphosphorylated form of RNA pol II. We further show that deletion of the RNA pol II C-terminal domain (CTD) kinase Ctk1, or partial deletion of the CTD, results in a selective abolishment of H3 Lys 36 methylation, implying a pathway of Set2 recruitment to chromatin and a role for H3 Lys 36 methylation in transcription elongation. In support, chromatin immunoprecipitation assays demonstrate the presence of Set2 methylation in the coding regions, as well as promoters, of genes regulated by Ctk1 or Set2. These data document a new link between histone methylation and the transcription apparatus and uncover a regulatory pathway that is selective for H3 Lys 36 methylation.
Inactivation of Exonuclease 1 in mice results in DNA mismatch repair defects, increased cancer susceptibility, and male and female sterility
Authors: Wei Kaichun , Clark Alan B. , Wong Edmund , Kane Michael F. , Mazur Dan J. , Parris Tchaiko , Kolas Nadine K. , Russell Robert , Hou Harry , Kneitz Burkhard , Yang Guohze , Kunkel Thomas A. , Kolodner Richard D. , Cohen Paula E. , Edelmann Winfried
Published: March 2003

Exonuclease 1 (Exo1) is a 5′–3′ exonuclease that interacts with MutS and MutL homologs and has been implicated in the excision step of DNA mismatch repair. To investigate the role of Exo1 in mammalian mismatch repair and assess its importance for tumorigenesis and meiosis, we generated anExo1mutant mouse line. Analysis ofExo1−/−cells for mismatch repair activity in vitro showed that Exo1 is required for the repair of base:base and single-base insertion/deletion mismatches in both 5′ and 3′ nick-directed repair. The repair defect inExo1−/−cells also caused elevated microsatellite instability at a mononucleotide repeat marker and a significant increase in mutation rate at theHprtlocus.Exo1−/−animals displayed reduced survival and increased susceptibility to the development of lymphomas. In addition,Exo1−/−male and female mice were sterile because of a meiotic defect. Meiosis inExo1−/−animals proceeded through prophase I; however, the chromosomes exhibited dynamic loss of chiasmata during metaphase I, resulting in meiotic failure and apoptosis. Our results show that mammalian Exo1 functions in mutation avoidance and is essential for male and female meiosis.
Suppression of Ras-stimulated transformation by the JNK signal transduction pathway
Authors: Kennedy Norman J. , Sluss Hayla K. , Jones Stephen N. , Bar-Sagi Dafna , Flavell Richard A. , Davis Roger J.
Published: March 2003

The c-Jun NH2-terminal kinase (JNK) phosphorylates and activates members of the activator protein-1 (AP-1) group of transcription factors and is implicated in oncogenic transformation. To examine the role of JNK, we investigated the effect of JNK deficiency on Ras-stimulated transformation. We demonstrate that although JNK does play a role in transformation in vitro, JNK is not required for tumor development in vivo. Importantly, the loss of JNK expression resulted in substantial increases in the number and growth of tumor nodules in vivo. Complementation assays demonstrated that this phenotype was caused by JNK deficiency. These data demonstrate that, in contrast to expectations, the normal function of JNK may be to suppress tumor development in vivo. This conclusion is consistent with the presence in human tumors of loss-of-function mutations in the JNK pathway.
Retraction of theDrosophilagerm band requires cell–matrix interaction
Authors: Schöck Frieder , Perrimon Norbert
Published: March 2003

Integrins and laminins are important mediators of cell–matrix interactions in both vertebrates and invertebrates. Here, we show that germ-band retraction in theDrosophilaembryo, during which the tail end of the embryo retracts to its final posterior position, allows the investigation of cell spreading and lamellipodia formation in real time in vivo. We demonstrate that α1, 2 laminin and αPS3βPS integrin are required for the spreading of a small group of cells of the amnioserosa epithelium over the tail end of the germ band. We further implicate a role for this spreading in the process of germ-band retraction.
Is Part‐Time Farming Really a Step in the Way Out of Agricultural?
Authors: Kimhi Ayal
Published: March 2003

One of the most important questions regarding the phenomenon of part‐time farming is whether it is a stable situation or just a step in the way out of agriculture. This article provides support for the first possibility in the context of Israeli family farms in the 1970s. A multinomial logit model of work choices is estimated, including estimated transition probabilities among the explanatory variables. The results indicate that a high objective probability of exit from farming within the next decade decreased the tendency to engage in off‐farm work, especially in a full‐time job, in 1971.
The Output Decision of a Risk‐Neutral Producer under Risk of Liquidation
Authors: Mahul Oliver
Published: March 2003

A risk‐neutral producer faces liquidity constraint that forces him to liquidate real assets in imperfect markets if he cannot meet his cash obligation, entailing a fixed liquidation cost. Under this risk of liquidation, the risk‐neutral producer is shown to refuse to undertake a risky project with positive expected excess return, exhibiting first‐order risk aversion. The optimal output level of the producer is piecewise linear in wealth; it decreases or increases depending on whether the expected excess return is positive or negative. The producer's apparent utility function exhibits an S‐shaped curve when the initial liquid assets are affected by a background risk that is normally distributed.
Does Market Timing Contribute to the Cattle Cycle?
Authors: Hamilton Stephen F. , Kastens Terry L.
Published: March 2003

Recent evidence suggests that cyclical cattle inventories are driven by exogenous shocks. This article examines a second possible contributing factor to the cattle cycle: a market timing effect that arises from individual attempts to maintain countercyclical inventories. The model uncovers an important conceptual point: to the extent that cycles are driven by exogenous shocks, a representative producer should outperform one who maintains a constant inventory; whereas, for cycles induced by market timing, a representative producer should underperform one with a constant inventory. Simulated net returns over 1974−98 reveal that a constant‐inventory manager significantly outperformed the representative U.S. producer, which indicates that market timing influences the cattle cycle.
The Economics of Livestock Waste and Its Regulation
Authors: Innes Robert
Published: March 2003

This article develops a spatial model of regional livestock production and three attendant environmental effects: spills from animal waste stores;nutrient runoff due to the application of manure to croplands; and direct ambient pollution, including odors, pests, and gases. Assuming that neither environmental outcomes nor operators' manure‐spreading practices can be monitored and regulated, constrained efficient production arrangements and waste‐handling practices are described. The efficiency effects of several regulatory policies are then explored, including (a) scale regulations that limit animal inventories, (b) chemical fertilizer taxes, and (c) waste storage and handling standards that affect storm protections and manure transport.
Hedonic Prices for Fish: Tuna Prices in Hawaii
Authors: McConnell Kenneth E. , Strand Ivar E.
Published: March 2003

Over the past two decades, the marketing of fish has recognized the importance of quality. Yet empirical analysis of market transactions could give us little insight into the value of different qualities of fish because market data are typically aggregated. We exploit a dataset on the auction price of tuna sold in Hawaii to estimate a hedonic model. The model provides empirical estimates of price increments due to species, quality of the fish such as size or fat content, method of handling, and market conditions. The empirical results are also used to estimate price flexibilities for landings in Hawaii.
Inventory Dynamics under Transaction Costs
Authors: Chavas Jean‐Paul , Despins Paula M. , Fortenbery T. Randy
Published: March 2003

A conceptual model of storage behavior is developed. Optimal intertemporal pricing is derived to analyze the effects of transaction costs on storage andarbitrage pricing. It is shown how transaction costs can rationalize the existence of an inverse carrying charge for inventory. The model is applied to U.S. soybeans stocks for the period 1960–95. The empirical results suggest that transaction costs have a significant influence on storage behavior andintertemporal arbitrage pricing.
An Analysis of Occupational Health in Pork Production
Authors: Hurley Terrance M. , Kliebenstein James B. , Orazem Peter F.
Published: March 2003

Data on attendees at the World Pork Expo from 1991–95 are used to evaluate the impact of farming generally, and hog farming and confinement operations more specifically, on the measured health outcomes of participants. Hog farming is found to increase risk of reduced hand strength and respiratory symptoms. To the extent these health risks are known, hog farmers will require a compensating differential to reward them for accepting a greater risk, an outcome consistent with higher long‐term returns for hog production relative to cash‐grain production.
Cumulative Effects and Optimal Targeting of Conservation Efforts: Steelhead Trout Habitat Enhancement in Oregon
Authors: Wu JunJie , Adams Richard M. , Boggess William G.
Published: March 2003

A major problem with some conservation programs is that they ignore potential cumulative (threshold) effects in environmental quality management. The objective of this study is to investigate the importance of cumulative effects in the targeting of conservation efforts. The empirical focus is on habitat investments to protect an important anadromous fish species in the Pacific Northwest, steelhead trout. Results of the analysis point to a substantial cumulative effect in the relationship between water quality and abundance in this fishery, which affects the efficiency of specific habitat investments.
Survey Response‐Related Biases in Contingent Valuation: Concepts, Remedies, and Empirical Application to Valuing Aquatic Plant Management
Authors: Messonnier Mark L. , Bergstrom John C. , Cornwell Christopher M. , Teasley R. Jeff , Cordell H. Ken
Published: March 2003

Sample nonresponse and selection biases that may occur in survey research such as contingent valuation applications are discussed and tested. Correction mechanisms for these types of biases are demonstrated. Results indicate the importance of testing and correcting for unit and item nonresponse bias in contingent valuation survey data. When sample nonresponse and selection bias go uncorrected, welfare measures may be overestimated or underestimated contributing to potential errors in resource policy and management decisions.
The Effect of Nesting Structure Specification on Welfare Estimation in a Random Utility Model of Recreation Demand: An Application to the Demand for Recreational Fishing
Authors: Hauber A. Brett , Parsons George R.
Published: March 2003

Nested logit has become common in estimating random utility models of recreation demand. Because welfare analysis is often the objective of estimating these models, it seems natural to ask, what effect does the choice of nesting structure have on the welfare estimates generated by these models? Therefore, we compare the results of nine nesting structures andfindthat the variation in welfare estimates across the models is not large. Our results are contrary to those of Kling and Thomson and Shaw and Ozog. The difference appears to originate with differences in the estimated dissimilarity coefficients in the nested models.
Insect Population Dynamics, Pesticide Use, and Farmworker Health
Authors: Sunding David , Zivin Joshua
Published: March 2003

We address the impacts of regulations designed to reduce pesticide poisoning of farmers and farm laborers. Attention is concentratedon pre‐harvest interval regulations that impose a time interval between pesticide application and harvest. The incidence of poisoning is determined by aggregate pesticide use, worker exposure, and toxicity. A dynamic, stochastic model of insect population growth is developed and used to measure the incentives for pesticide use. Increasing the pre‐harvest interval has an ambiguous effect on the number of harvest worker poisonings. Pesticide taxation unambiguously reduces the number of worker poisonings. Theoretical results are quantified in a case study of mevinphos application on leaf lettuce in California's Salinas Valley.