bluuebunny/crossref_metadata_2025 huggingface dataset:
High-altitude projectile motion
Authors: Mohazzabi P , Fields J C
Published: April 2004

It is known that the optimal projection angle corresponding to the maximum range of a projectile moving in air is less than 45° for low-power resistance laws such as the linear or quadratic law. We show that this result is valid only for a uniform atmosphere, in which the density of air is a constant. In a more realistic atmosphere, such as an exponential atmosphere, where the air density decreases with altitude, and specially for high-speed projectiles, the optimal projection angle is much greater than 45°.PACS No.: 45.50.Dd
Regulation of Auditory Responses in the Central Nucleus of the Inferior Colliculus by Tetraethylammonium-Sensitive Potassium Channels
Authors: Zhang Huiming , Wu Shu Hui , Kelly Jack B.
Published: April 2004

The role of potassium channels in regulating spontaneous firing and sound-evoked responses in the central nucleus of the inferior colliculus was studied by recording single-unit activity before and during iontophoretic application of a nonspecific potassium channel blocker, tetraethylammonium (TEA). Tone bursts and sinusoidal amplitude-modulated tones were used to evoke auditory responses. Our results show that release of TEA increased the width of spikes for all neurons tested. There was an increase in spontaneous firing for most of the neurons. There was also an increase in responses to tone bursts for most of the neurons, although in some cases there was a reduction in the evoked responses. TEA also increased the firing rate in responses to sinusoidal amplitude-modulated sounds in the majority of the neurons tested. For some neurons, the change in firing reduced the selectivity of responses for particular rates of modulation. There was also a reduction in the synchrony of action potentials to the modulation envelope in many cells. Our results show that potassium channels are important for regulating the strength of sound-evoked responses and the level of spontaneous activity, and determining the temporal properties of responses to amplitude-modulated sounds.
Modeling Interactions Between Electrical Activity and Second-Messenger Cascades inAplysiaNeuron R15
Authors: Yu Xintian , Byrne John H. , Baxter Douglas A.
Published: April 2004

The biophysical properties of neuron R15 in Aplysia endow it with the ability to express multiple modes of oscillatory electrical activity, such as beating and bursting. Previous modeling studies examined the ways in which membrane conductances contribute to the electrical activity of R15 and the ways in which extrinsic modulatory inputs alter the membrane conductances by biochemical cascades and influence the electrical activity. The goals of the present study were to examine the ways in which electrical activity influences the biochemical cascades and what dynamical properties emerge from the ongoing interactions between electrical activity and these cascades. The model proposed by Butera et al. in 1995 was extended to include equations for the binding of Ca2+to calmodulin (CaM) and the actions of Ca2+/CaM on both adenylyl cyclase and phosphodiesterase. Simulations indicated that levels of cAMP oscillated during bursting and that these oscillations were approximately antiphasic to the oscillations of Ca2+. In the presence of cAMP oscillations, brief perturbations could switch the electrical activity between bursting and beating (bistability). Compared with a constant-cAMP model, oscillations of cAMP substantially expanded the range of bistability. Moreover, the integrated electrical/biochemical model simulated some early experimental results such as activity-dependent inactivation of the anomalous rectifier. The results of the present study suggest that the endogenous activity of R15 depends, in part, on interactions between electrical activity and biochemical cascades.
Interpretation of a Discontinuity in the Sense of Verticality at Large Body Tilt
Authors: Kaptein Ronald G. , Van Gisbergen Jan A. M.
Published: April 2004

Results of earlier spatial-orientation studies focusing on the sense of verticality have emphasized an intriguing paradox. Despite evidence that nearly veridical signals for gravicentric head orientation and egocentric visual stimulus orientation are available, roll-tilted subjects err in the direction of the long body axis when adjusting a visual line to vertical in darkness (Aubert effect). This has led to the suggestion that a central egocentric bias signal with fixed strength and direction acts to pull the perceived vertical to the subjects' zenith (M-model). In the present study, the subjective visual vertical (SVV) was tested in six human subjects, across the entire 360° range. For comparison, body-tilt estimates from four subjects where collected in a separate series of experiments. For absolute tilts up to ∼135°, SVV responses showed a gradually increasing Aubert effect that could not be attributed to errors in perceived body tilt but was nicely in line with the M-model. At larger absolute tilts, SVV errors abruptly reversed sign, now showing a pattern concordant with errors in body-tilt estimates but incompatible with the M-model. These results suggest that, in the normal working range, the perception of external space and the perception of body posture are based on different processing of body-tilt signals. Beyond this range, both spatial-orientation tasks seem to rely mainly on a common tilt signal.
Trunk Position Influences Vestibular Responses of Fastigial Nucleus Neurons in the Alert Monkey
Authors: Kleine J. F. , Guan Y. , Kipiani E. , Glonti L. , Hoshi M. , Büttner U.
Published: April 2004

Vestibulospinal reflexes play an important role for body stabilization during locomotion and for postural control. For an appropriate distribution of vestibular signals to spinal motoneurons, the orientation of the body relative to the head needs to be taken into account. For different trunk positions, identical vestibular stimuli must activate different sets of muscles to ensure body stabilization. Because the cerebellar vermis and the underlying fastigial nucleus (FN) might be involved in this task, vestibular neurons in the rostral FN of alert rhesus monkeys were recorded during sinusoidal vestibular stimulation (0.1–1.0 Hz) in the roll and pitch planes at different trunk-re-head positions (center and ±45°). From the sensitivity and phase values measured in these planes, the response properties in the intermediate planes and the stimulus orientation eliciting the optimal response [response vector orientation (RVO)] were calculated. In most neurons, the RVOs rotated systematically with respect to the head, when trunk-re-head position was altered, so that they tended to maintain their orientation with respect to the trunk. Sensitivity and phase at the RVO were not affected. This pattern was the same for neurons in the right and left FN and independent of stimulus frequency. The average sensitivity of this partially compensatory RVO shift in response to trunk-re-head displacements, evaluated by linear regression analyses, was 0.59°/° ( n = 73 neurons). These data show that FN neurons may encode vestibular information in a coordinate system that is closer to a trunk-centered than to a head-centered reference frame. They indicate an important role of this nucleus in motor programs related to posture and gait control.
Equalization of Synaptic Efficacy by Activity- and Timing-Dependent Synaptic Plasticity
Authors: Rumsey Clifton C. , Abbott L. F.
Published: April 2004

In many neurons, synapses increase in strength as a function of distance from the soma in a manner that appears to compensate for dendritic attenuation. This phenomenon requires a cooperative interaction between local factors that control synaptic strength, such as receptor density and vesicle release probability, and global factors that affect synaptic efficacy, such as attenuation and boosting by active membrane conductances. Anti-spiketiming-dependent plasticity, in combination with nonassociative synaptic potentiation, can accomplish this feat even though it acts locally and independently at individual synapses. Analytic computations and computer simulations show that this combination of synaptic plasticity mechanisms equalizes the efficacy of synapses over an extended dendritic cable by adjusting local synaptic strengths to compensate for global attenuation.
Auditory Responses in Multiple Sensorimotor Song System Nuclei Are Co-Modulated by Behavioral State
Authors: Cardin Jessica A. , Schmidt Marc F.
Published: April 2004

Auditory responsiveness in nucleus HVC, a high-order sensorimotor area of the avian song system, is modulated by changes in behavioral state. Modulation is not observed in the primary thalamo-recipient auditory area Field L, the indirect source of auditory input to HVC. In this study, we show that auditory responsiveness in nucleus interfacialis (NIf), the immediate auditory afferent to HVC, is modulated by behavioral state. While auditory responsiveness is generally greater in NIf during wakefulness and in HVC during sedation, simultaneous recordings reveal a co-variation of auditory response magnitude. This co-variation is observed both in awake birds, where responses are spontaneously variable, and in sedated birds during manipulations of arousal levels. Auditory responses in NIf and HVC, which are selective for the bird's own song (BOS) during sedation, become predominantly unselective during wakefulness. This loss of selectivity is accompanied by a decrease in the similarity of NIf and HVC response patterns. To explore the role of NIf in shaping HVC auditory responses, we pharmacologically manipulated NIf while recording in HVC. Injection of the GABAAagonist muscimol into NIf eliminated most spontaneous activity and all auditory responses in the ipsilateral HVC, while injections of the GABAAantagonist bicuculline increased HVC auditory responsiveness and selectivity. These findings indicate that HVC is not the initial site of behavioral state–dependent modulation in the song system. Together with the suppression of HVC auditory responses by muscimol in NIf, these results suggest that NIf plays an important role in the flow of auditory information to HVC.
A Model of Reverse Spike Frequency Adaptation and Repetitive Firing of Subthalamic Nucleus Neurons
Authors: Wilson Charles J. , Weyrick Angela , Terman David , Hallworth Nicholas E. , Bevan Mark D.
Published: April 2004

Subthalamic nucleus neurons exhibit reverse spike-frequency adaptation. This occurs only at firing rates of 20–50 spikes/s and higher. Over this same frequency range, there is an increase in the steady-state frequency–intensity ( F– I) curve's slope (the secondary range). Specific blockade of high-voltage activated calcium currents reduced the F– I curve slope and reverse adaptation. Blockade of calcium-dependent potassium current enhanced secondary range firing. A simple model that exhibited these properties used spike-triggered conductances similar to those in subthalamic neurons. It showed: 1) Nonaccumulating spike afterhyperpolarizations produce positively accelerating F– I curves and spike-frequency adaptation that is complete after the second spike. 2) Combinations of accumulating aftercurrents result in a linear F– I curve, whose slope depends on the relative contributions of inward and outward currents. Spike-frequency adaptation can be gradual. 3) With both accumulating and nonaccumulating aftercurrents, primary and secondary ranges will be present in the F– I curve. The slope of the primary range is determined by the nonaccumulating conductance; the accumulating conductances govern the secondary range. The transition is determined by the relative strengths of accumulating and nonaccumulating currents. 4) Spike-threshold accommodation contributes to the secondary range, reducing its slope at high firing rates. Threshold accommodation can stabilize firing when inward aftercurrents exceed outward ones. 5) Steady-state reverse adaptation results when accumulated inward aftercurrents exceed outward ones. This requires spike-threshold accommodation. Transient speedup arises when inward currents are smaller than outward ones at steady state, but accumulate more rapidly. 6) The same mechanisms alter firing in response to irregular patterns of synaptic conductances, as cell excitability fluctuates with changes in firing rate.
Body Scheme Gates Visual Processing
Authors: Dohle Christian , Kleiser Raimund , Seitz Rüdiger J. , Freund Hans-Joachim
Published: April 2004

We used functional magnetic resonance imaging (fMRI) to explore how guidance of motor acts is influenced by the visually perceived body scheme. We found that when subjects view their hand as their opposite hand, i.e., the right hand is seen as the left hand and vice versa, activation in the visual cortex was lateralized opposite to the seen hand. This demonstrates for the first time that our body scheme to which vision relates our environment is already represented at the level of visual cortex.
Increased Neuronal Firing in Computer Simulations of Sodium Channel Mutations That Cause Generalized Epilepsy With Febrile Seizures Plus
Authors: Spampanato Jay , Aradi Ildiko , Soltesz Ivan , Goldin Alan L.
Published: April 2004

Generalized epilepsy with febrile seizures plus (GEFS+) is an autosomal dominant familial syndrome with a complex seizure phenotype. It is caused by mutations in one of 3 voltage-gated sodium channel subunit genes ( SCN1B, SCN1A, and SCN2A) and the GABAAreceptor γ2 subunit gene ( GBRG2). The biophysical characterization of 3 mutations (T875M, W1204R, and R1648H) in SCN1A, the gene encoding the CNS voltage-gated sodium channel α subunit Nav1.1, demonstrated a variety of functional effects. The T875M mutation enhanced slow inactivation, the W1204R mutation shifted the voltage dependency of activation and inactivation in the negative direction, and the R1648H mutation accelerated recovery from inactivation. To determine how these changes affect neuronal firing, we used the NEURON simulation software to design a computational model based on the experimentally determined properties of each GEFS+ mutant sodium channel and a delayed rectifier potassium channel. The model predicted that W1204R decreased the threshold, T875M increased the threshold, and R1648H did not affect the threshold for firing a single action potential. Despite the different effects on the threshold for firing a single action potential, all of the mutations resulted in an increased propensity to fire repetitive action potentials. In addition, each mutation was capable of driving repetitive firing in a mixed population of mutant and wild-type channels, consistent with the dominant nature of these mutations. These results suggest a common physiological mechanism for epileptogenesis resulting from sodium channel mutations that cause GEFS+.
Signal Processing in the Vestibular System During Active Versus Passive Head Movements
Authors: Cullen Kathleen E. , Roy Jefferson E.
Published: April 2004

In everyday life, vestibular receptors are activated by both self-generated and externally applied head movements. Traditionally, it has been assumed that the vestibular system reliably encodes head-in-space motion throughout our daily activities and that subsequent processing by upstream cerebellar and cortical pathways is required to transform this information into the reference frames required for voluntary behaviors. However, recent studies have radically changed the way we view the vestibular system. In particular, the results of recent single-unit studies in head-unrestrained monkeys have shown that the vestibular system provides the CNS with more than an estimate of head motion. This review first considers how head-in-space velocity is processed at the level of the vestibular afferents and vestibular nuclei during active versus passive head movements. While vestibular information appears to be similarly processed by vestibular afferents during passive and active motion, it is differentially processed at the level of the vestibular nuclei. For example, one class of neurons in vestibular nuclei, which receives direct inputs from semicircular canal afferents, is substantially less responsive to active head movements than to passively applied head rotations. The projection patterns of these neurons strongly suggest that they are involved in generating head-stabilization responses as well as shaping vestibular information for the computation of spatial orientation. In contrast, a second class of neurons in the vestibular nuclei that mediate the vestibuloocular reflex process vestibular information in a manner that depends principally on the subject's current gaze strategy rather than whether the head movement was self-generated or externally applied. The implications of these results are then discussed in relation to the status of vestibular reflexes (i.e., the vestibuloocular, vestibulocollic, and cervicoocular reflexes) and implications for higher-level processing of vestibular information during active head movements.
Eye Movements of the Murine P/Q Calcium Channel MutantRocker, and the Impact of Aging
Authors: Stahl John S.
Published: April 2004

Mutations in the gene encoding the ion pore of the P/Q voltage-activated calcium channel (CACNA1A) are predicted to alter synaptic transmission and dendritic excitability within cerebellar granule and Purkinje cells. Determining the relationships between these alterations, neuronal activity, and behavior may yield insight into the relationship between neuronal intrinsic properties and signal processing within the ocular motor system. Toward this end, we compared ocular motor performance in the CACNA1A mutant rocker and C57BL/6 controls. Average vertical eye position was abnormally elevated in the mutants, a finding that may be analogous to downbeat nystagmus seen in human cerebellar disorders. Fast phases of vestibular nystagmus were slowed by approximately 18% of control values. The angular vestibuloocular reflex (VOR) in darkness and light (visual VOR, or VVOR), assessed at 0.1–1.6 Hz, exhibited subnormal gains at the highest stimulus frequencies and increased phase leads at the lowest stimulus frequencies. Horizontal optokinetic responses to constant velocity drum rotation of ±2.5–40°/s exhibited minimally reduced gains. Attempts to increase VOR gain by concomitant optokinetic and vestibular stimulation were confounded by the tendency of the mice to habituate to repetitive vestibular stimulation, but attempts to induce coupling of vertical eye movements to horizontal vestibular stimulation (cross-axis adaptation) generated rapid plastic changes in controls and little effect in mutants. With the notable exceptions of the vertical elevation and optokinetic gains, the ocular motor abnormalities were stable over a broad range of animal age, a result compatible with the abnormalities arising as direct consequences of the inborn alteration in calcium channel biophysics.
Using Auditory and Visual Stimuli to Investigate the Behavioral and Neuronal Consequences of Reflexive Covert Orienting
Authors: Bell Andrew H. , Fecteau Jillian H. , Munoz Douglas P.
Published: April 2004

Reflexively orienting toward a peripheral cue can influence subsequent responses to a target, depending on when and where the cue and target appear relative to each other. At short delays between the cue and target [cue-target onset asynchrony (CTOA)], subjects are faster to respond when they appear at the same location, an effect referred to as reflexive attentional capture. At longer CTOAs, subjects are slower to respond when the two appear at the same location, an effect referred to as inhibition of return (IOR). Recent evidence suggests that these phenomena originate from sensory interactions between the cue- and target-related responses. The capture of attention originates from a strong target-related response, derived from the overlap of the cue- and target-related activities, whereas IOR corresponds to a weaker target-aligned response. If such interactions are responsible, then modifying their nature should impact the neuronal and behavioral outcome. Monkeys performed a cue-target saccade task featuring visual and auditory cues while neural activity was recorded from the superior colliculus (SC). Compared with visual stimuli, auditory responses are weaker and occur earlier, thereby decreasing the likelihood of interactions between these signals. Similar to previous studies, visual stimuli evoked reflexive attentional capture at a short CTOA (60 ms) and IOR at longer CTOAs (160 and 610 ms) with corresponding changes in the target-aligned activity in the SC. Auditory cues used in this study failed to elicit either a behavioral effect or modification of SC activity at any CTOA, supporting the hypothesis that reflexive orienting is mediated by sensory interactions between the cue and target stimuli.
Synaptic Interactions Between Thalamic and Cortical Inputs Onto Cortical Neurons In Vivo
Authors: Fuentealba Pablo , Crochet Sylvain , Timofeev Igor , Steriade Mircea
Published: April 2004

To study the interactions between thalamic and cortical inputs onto neocortical neurons, we used paired-pulse stimulation (PPS) of thalamic and cortical inputs as well as PPS of two cortical or two thalamic inputs that converged, at different time intervals, onto intracellularly recorded cortical and thalamocortical neurons in anesthetized cats. PPS of homosynaptic cortico-cortical pathways produced facilitation, depression, or no significant effects in cortical pathways, whereas cortical responses to thalamocortical inputs were mostly facilitated at both short and long intervals. By contrast, heterosynaptic interactions between either cortical and thalamic, or thalamic and cortical, inputs generally produced decreases in the peak amplitudes and depolarization area of evoked excitatory postsynaptic potentials (EPSPs), with maximal effect at ∼10 ms and lasting from 60 to 100 ms. All neurons tested with thalamic followed by cortical stimuli showed a decrease in the apparent input resistance ( Rin), the time course of which paralleled that of decreased responses, suggesting that shunting is the factor accounting for EPSP's decrease. Only half of neurons tested with cortical followed by thalamic stimuli displayed changes in Rin. Spike shunting in the thalamus may account for those cases in which decreased synaptic responsiveness of cortical neurons was not associated with decreased Rin because thalamocortical neurons showed decreased firing probability during cortical stimulation. These results suggest a short-lasting but strong shunting between thalamocortical and cortical inputs onto cortical neurons.
Electrophysiological and Morphological Characterization of Identified Motor Neurons in theDrosophilaThird Instar Larva Central Nervous System
Authors: Choi James C. , Park Demian , Griffith Leslie C.
Published: April 2004

We have used dye fills and electrophysiological recordings to identify and characterize a cluster of motor neurons in the third instar larval ventral ganglion. This cluster of neurons is similar in position to the well-studied embryonic RP neurons. Dye fills of larval dorsomedial neurons demonstrate that individual neurons within the cluster can be reproducibly identified by observing their muscle targets and bouton morphology. The terminal targets of these five neurons are body wall muscles 6/7, 1, 14, and 30 and the intersegmental nerve (ISN) terminal muscles (1, 2, 3, 4, 9, 10, 19, 20). All cells except the ISN neuron, which has a type Is ending, display type Ib boutons. Two of these neurons appear to be identical to the embryonic RP3 and aCC cells, which define the most proximal and distal innervations within a hemisegment. The targets of the other neurons in the larval dorsomedial cluster do not correspond to embryonic targets of the neurons in the RP cluster, suggesting rewiring of this circuit during early larval stages. Electrophysiological studies of the five neurons in current clamp revealed that type Is neurons have a longer delay in the appearance of the first spike compared with type Ib neurons. Genetic, biophysical, and pharmacological studies in current and voltage clamp show this delay is controlled by the kinetics and voltage sensitivity of inactivation of a current whose properties suggest that it may be the Shal IAcurrent. The combination of genetic identification and whole cell recording allows us to directly explore the cellular substrates of neural and locomotor behavior in an intact system.
Movement Reduces the Dynamic Response of Muscle Spindle Afferents and Motoneuron Synaptic Potentials in Rat
Authors: Haftel Valerie K. , Bichler Edyta K. , Nichols T. Richard , Pinter Martin J. , Cope Timothy C.
Published: April 2004

Among the mechanisms that may result in modulation of the stretch reflex by the recent history of muscle contraction is the history dependence observed under some conditions in the response properties of muscle spindles. The present study was designed to test one report that in successive trials of muscle stretch-release, spindle afferent firing during stretch, i.e., the dynamic response shows no history dependence beyond the initial burst of firing at stretch onset. Firing responses of spindle afferents were recorded during sets of three consecutive trials of triangular stretch-release applied to triceps surae muscles in barbiturate-anesthetized rats. All 69 spindle afferents fired more action potentials (spikes) during the dynamic response of the first trial, excluding the initial burst, than in the following two trials. The reduced dynamic response (RDR) was nearly complete after trial 1 and amounted to an average of ∼12 fewer spikes (16 pps slower firing rate) in trial 3 than in trial 1. RDR was sensitive to the interval between stretch sets but independent of stretch velocity (4–32 mm/s). RDR was reflected in the synaptic potentials recorded intracellularly from 16 triceps surae α-motoneurons: depolarization during muscle stretch was appreciably reduced after trial 1. These findings demonstrate history dependence of spindle afferent responses that extends throughout the dynamic response in successive muscle stretches and that is synaptically transmitted to motoneurons with the probable effect, unless otherwise compensated, of modulating the stretch reflex.
Modulation of Mechanosensory Responses by Motoneurons That Regulate Skin Surface Topology in the Leech
Authors: Rodriguez Mariano Julián , Iscla Irene Raquel , Szczupak Lidia
Published: April 2004

Central regulation of somatosensory signals has been extensively studied, but little is known about their regulation in the periphery. Given the widespread exposure of the skin sensory terminals to the environment, it is of interest to explore how somatosensory sensitivity is affected by changes in properties of the skin. In the leech, the annuli that subdivide the skin can be erected under the control of the annulus erector (AE) motoneurons. To analyze whether this surface change influences mechanosensory sensitivity, we studied the responses of low threshold mechanosensory T cells to mechanical stimulation of the skin as AE motoneurons were activated. In segments of the body wall connected to the corresponding ganglion and submerged in an aqueous environment, T cells responded to localized bubbling on the skin and to water flow parallel to its surface. Excitation of AE motoneurons diminished these responses in a way that depended on the motoneuron firing frequency. Video recordings established that the range of AE firing frequencies that produced effective annulus erection coincided with that influencing T cell responses. In isolated ganglia, AE firing had no effect on T cell excitability, suggesting that annulus erection diminished T cell responsiveness to mechanical input. Counteracting this effect, mechanosensory inputs inhibited AE motoneurons. However, because depolarization of AE cells caused a decrease in their input resistance, the more active the motoneuron, the less sensitive it became to inhibitory signals. Thus when brought to fire, AE motoneurons would stay “committed” to a high activity level, and this would limit sensory responsiveness to incoming mechanical signals.
Estimating the Temporal Interval Entropy of Neuronal Discharge
Authors: Reeke George N. , Coop Allan D.
Published: April 2004

To better understand the role of timing in the function of the nervous system, we have developed a methodology that allows the entropy of neuronal discharge activity to be estimated from a spike train record when it may be assumed that successive interspike intervals are temporally uncorrelated. The so-called interval entropy obtained by this methodology is based on an implicit enumeration of all possible spike trains that are statistically indistinguishable from a given spike train. The interval entropy is calculated from an analytic distribution whose parameters are obtained by maximum likelihood estimation from the interval probability distribution associated with a given spike train. We show that this approach reveals features of neuronal discharge not seen with two alternative methods of entropy estimation. The methodology allows for validation of the obtained data models by calculation of confidence intervals for the parameters of the analytic distribution and the testing of the significance of the fit between the observed and analytic interval distributions by means of Kolmogorov-Smirnov and Anderson-Darling statistics. The method is demonstrated by analysis of two different data sets: simulated spike trains evoked by either Poissonian or near-synchronous pulsed activation of a model cerebellar Purkinje neuron and spike trains obtained by extracellular recording from spontaneously discharging cultured rat hippocampal neurons.
Dynamic Analysis of Neural Encoding by Point Process Adaptive Filtering
Authors: Eden Uri T. , Frank Loren M. , Barbieri Riccardo , Solo Victor , Brown Emery N.
Published: April 2004

Neural receptive fields are dynamic in that with experience, neurons change their spiking responses to relevant stimuli. To understand how neural systems adapt the irrepresentations of biological information, analyses of receptive field plasticity from experimental measurements are crucial. Adaptive signal processing, the well-established engineering discipline for characterizing the temporal evolution of system parameters, suggests a framework for studying the plasticity of receptive fields. We use the Bayes' rule Chapman-Kolmogorov paradigm with a linear state equation and point process observation models to derive adaptive filters appropriate for estimation from neural spike trains. We derive point process filter analogues of the Kalman filter, recursive least squares, and steepest-descent algorithms and describe the properties of these new fil-ters. We illustrate our algorithms in two simulated data examples. The first is a study of slow and rapid evolution of spatial receptive fields in hippocampal neurons. The second is an adaptive decoding study in which a signal is decoded from ensemble neural spiking activity as the recep-tive fields of the neurons in the ensemble evolve. Our results provide a paradigm for adaptive estimation for point process observations and suggest a practical approach for constructing filtering algorithms to track neural receptive field dynamics on a millisecond timescale.
Neural Mechanisms for the Robust Representation of Junctions
Authors: Hansen Thorsten , Neumann Heiko
Published: April 2004

Junctions provide important cues in various perceptual tasks, such as the determination of occlusion relationships for figure-ground separation, transparency perception, and object recognition, among others. In computer vision, junctions are used in a number of tasks, like point matching for image tracking or correspondence analysis. We propose a biologically motivated approach to junction representation in which junctions are implicitly characterized by high activity for multiple orientations within a cortical hypercolumn. A local measure of circular variance is suggested to extract junction points from this distributed representation. Initial orientation measurements are often fragmented and noisy. A coherent contour representation can be generated by a model of V1 utilizing mechanisms of collinear long-range integration and recurrent interaction. In the model, local oriented contrast estimates that are consistent within a more global context are enhanced while inconsistent activities are suppressed. In a series of computational experiments, we compare junction detection based on the new recurrent model with a feedforward model of complex cells. We show that localization accuracy and positive correctness in the detection of generic junction configurations such as L- and T-junctions is improved by the recurrent long-range interaction. Further, receiver operating characteristics analysis is used to evaluate the detection performance on both synthetic and camera images, showing the superior performance of the new approach. Overall, we propose that nonlocal interactions implemented by known mechanisms within V1 play an important role in detecting higher-order features such as corners and junctions.
Greedy Learning of Multiple Objects in Images Using Robust Statistics and Factorial Learning
Authors: Williams Christopher K.I. , Titsias Michalis K.
Published: April 2004

We consider data that are images containing views of multiple objects. Our task is to learn about each of the objects present in the images. This task can be approached as a factorial learning problem, where each image must be explained by instantiating a model for each of the objects present with the correct instantiation parameters. A major problem with learning a factorial model is that as the number of objects increases, there is a combinatorial explosion of the number of configurations that need to be considered. We develop a method to extract object models sequentially from the data by making use of a robust statistical method, thus avoiding the combinatorial explosion, and present results showing successful extraction of objects from real images.
Are Loss Functions All the Same?
Authors: Rosasco Lorenzo , Vito Ernesto De , Caponnetto Andrea , Piana Michele , Verri Alessandro
Published: April 2004

In this letter, we investigate the impact of choosing different loss functions from the viewpoint of statistical learning theory. We introduce a convexity assumption, which is met by all loss functions commonly used in the literature, and study how the bound on the estimation error changes with the loss. We also derive a general result on the minimizer of the expected risk for a convex loss function in the case of classification. The main outcome of our analysis is that for classification, the hinge loss appears to be the loss of choice. Other things being equal, the hinge loss leads to a convergence rate practically indistinguishable from the logistic loss rate and much better than the square loss rate. Furthermore, if the hypothesis space is sufficiently rich, the bounds obtained for the hinge loss are not loosened by the thresholding stage.
Trading Variance Reduction with Unbiasedness: The Regularized Subspace Information Criterion for Robust Model Selection in Kernel Regression
Authors: Sugiyama Masashi , Kawanabe Motoaki , Müller Klaus-Robert
Published: April 2004

A well-known result by Stein (1956) shows that in particular situations, biased estimators can yield better parameter estimates than their generally preferred unbiased counterparts. This letter follows the same spirit, as we will stabilize the unbiased generalization error estimates by regularization and finally obtain more robust model selection criteria for learning. We trade a small bias against a larger variance reduction, which has the beneficial effect of being more precise on a single training set. We focus on the subspace information criterion (SIC), which is an unbiased estimator of the expected generalization error measured by the reproducing kernel Hilbert space norm. SIC can be applied to the kernel regression, and it was shown in earlier experiments that a small regularization of SIC has a stabilization effect. However, it remained open how to appropriately determine the degree of regularization in SIC. In this article, we derive an unbiased estimator of the expected squared error, between SIC and the expected generalization error and propose determining the degree of regularization of SIC such that the estimator of the expected squared error is minimized. Computer simulations with artificial and real data sets illustrate that the proposed method works effectively for improving the precision of SIC, especially in the high-noise-level cases. We furthermore compare the proposed method to the original SIC, the cross-validation, and anempirical Bayesian method in ridge parameter selection, withgood results.
Legal Protection of Economic Development Agreements
Authors:   Al-Saeed
Published: April 2004

No Abstract
L'Obligation Précontractuelle d'Information de l'Assureur
Authors:   Al Hendyani
Published: April 2004

No Abstract
Assignment of Contract Proceeds
Authors:   Fahl
Published: April 2004

No Abstract
Documentary Credits in the UAE
Authors:   Fahl
Published: April 2004

No Abstract
Execution of Foreign Judgments in the UAE
Authors:   Arab
Published: April 2004

No Abstract
Principles of Environmental Law in Islam
Authors:   Shah Haneef
Published: April 2004

No Abstract
Banking Confidentiality – A Comparative Analysis of Malaysian Banking Statutes
Authors:   Jawahitha
Published: April 2004

No Abstract
Le Pluralisme Politique – Vision Islamique
Authors:   El-Baz
Published: April 2004

No Abstract
Prosecutions of the Press in Yemen
Authors:   Ghanem
Published: April 2004

No Abstract
The Emergence of Citizenship in Islamdom
Authors: Salam
Published: April 2004

No Abstract
Reasoning in Islamic Law: Part One
Authors: Omar
Published: April 2004

No Abstract
The Federal Penal Code and the Aim of Unification
Authors: Al-Muhairi
Published: April 2004

No Abstract
The 1995 Noel Coulson Memorial Lecture: the Application of Islamic Law in the English Courts
Authors: Pearl
Published: April 2004

No Abstract
Commerial Suit 19/1995, The First Common Law Shipping Auction in the History of Hodeidah
Authors: Ghanem LL.B. M.A. FBIM. M.Phil Ph.D
Published: April 2004

No Abstract
Note on Recent Precedents in Yemen Applying New Legislation
Authors: Ghanem LL.B. M.A. FBIM. M.Phil Ph.D
Published: April 2004

No Abstract
Reasoning in Islamic Law: Part Two
Authors: Omar
Published: April 2004

No Abstract
Conclusion to the Series of Articles on the UAE Penal Law (with Glossary and Bibliography)
Authors: Al-Muhairi
Published: April 2004

No Abstract
Introduction
Authors: The Editor
Published: April 2004

No Abstract
Tax Reform Issues In The 1990s: A Case Study Of Lebanon And Its Prospects For Long-Term Prosperity
Authors: Azzi
Published: April 2004

No Abstract
The Concept Of Custody In Islamic Law
Authors: Zahraa  , Malek
Published: April 2004

No Abstract
Privatisation And Protection Of Investors In Kuwait: Reality And Ambition
Authors: Al-Melhem
Published: April 2004

No Abstract
Some Trends in Banking Law and their Relevance to the GCC States
Authors: Shihata
Published: April 2004

No Abstract
Piercing the Corporate Veil: A UAE Perspective
Authors: O'Connor
Published: April 2004

No Abstract
Frank E. Vogel and Samuel L. Hayes, Islamic Law and Finance: Religion, Risk and Return
Authors: Wilson
Published: April 2004

No Abstract
Limitation of Liability for Maritime Claims in the UAE
Authors: Kurtha
Published: April 2004

No Abstract
Richard Price and Essam Tamimi, United Arab Emirates Court of Cassation Judgments 1989-1997
Authors: Turck
Published: April 2004

No Abstract
The Structure of Public Administration in Jordan: A Constitutional and Administrative Law Perspective
Authors: Massadeh
Published: April 2004

No Abstract
The Kuwaiti National Assembly and the Targeted Legislation in the Privatisation Process
Authors: Al Moqatei
Published: April 2004

No Abstract
L'exploitation Petroliere a La Lumiere Du Droit Islamique: II
Authors: Faouk-El-Adé
Published: April 2004

No Abstract
Homicide in Islam: Major Legal Themes
Authors: Shah
Published: April 2004

No Abstract
Enforcement of Foreign Judgments in Some Arab Countries - Legal Provisions and Court Precedents: Focus on Bahrain
Authors: Ahmed
Published: April 2004

No Abstract
The Concept of Musharakah and its Application as an Islamic Method of Financing
Authors: Usmani
Published: April 2004

No Abstract
The Experience of the Islamic Republic of Iran in Musharakah Financing
Authors: Yasseri
Published: April 2004

No Abstract
The Role of Musharakah Financing in the Agricultural Bank of Iran
Authors: Sadr
Published: April 2004

No Abstract
Partnership (Musharakah): A New Option for Financing Small Enterprises?
Authors: Abdalla
Published: April 2004

No Abstract
Proposal for a Modified Model of Musharakah Financing, with Emphasis on Iran
Authors: Najmabadi
Published: April 2004

No Abstract
Le Cadre Juridique de la Production, du Transport et de la Distribution de L'eau a Djibouti
Authors: Blanc
Published: April 2004

No Abstract
Legal Aspects of Setting Up Business in Iraq and Iraqi Company Regulations
Authors: Al Tamimi  , Sinjakli
Published: April 2004

No Abstract
Insurance: the Islamic Perspective and its Development in Saudi Arabia
Authors: Al-Ghadyan
Published: April 2004

No Abstract
Quantum of Damages in Takaful (Islamic Insurance): a Reappraisal of the Possibility of Adopting the Doctrines of Al-Diyah and Al-Daman
Authors: Billah
Published: April 2004

No Abstract
Al-Mal: the Concept of Property in Islamic Legal Thought
Authors: Islam
Published: April 2004

No Abstract
Baudouin Dupret, Maurits Berger and Laila al-Zwaini (eds.), Legal Pluralism in the Arab World
Authors: Wilson
Published: April 2004

No Abstract
Validating a test of second language written lexical retrieval: a new measure of fluency in written language production
Authors: Snellings Patrick , van Gelderen Amos , de Glopper Kees
Published: April 2004

Lexical retrieval is an essential subprocess of language production and crucial in fluent writing and speaking. In this study we discuss a new measure of the speed of written lexical retrieval in a second language, the Written Productive Translation Task (WPTT). In contrast to Picture Naming tasks, the WPTT is not restricted to concrete nouns and verbs that can be depicted by pictures. The WPTT can also test combinations of words. Moreover, the WPTT can be used in a classroom setting because individual testing is not required. Using Messick’s 1989 framework, we investigated whether the WPTT is an appropriate measure of the construct of written lexical retrieval. In a multiple regression approach we examined the relationship between the WPTT and another test of the same construct, a written Picture Naming task. In addition, we looked at the relationship with other related constructs such as lexical access and orthographic encoding. Results suggest that the WPTT is both a reliable and valid measure of written lexical retrieval. When corrected for typing speed, the WPTT is related to the Picture Naming task and Orthographic Encoding. Finally, the results show that the relationship between the WPTT and the Lexical Decision Task is not very strong. This study makes clear that Lexical Decision Tasks do not constitute a valid substitute for measuring written lexical retrieval. At the same time it underlines the advantages of using a test specifically developed for measuring speed of written lexical retrieval like the WPTT.
Effect of mercury vapour exposure on urinary excretion of calcium, zinc and copper: relationship to alterations in functional and structural integrity of the kidney
Authors: EL-Safty Ibrahim AM , Gadallah Mohsen , Shafik Ahmed , Shouman Ahmed E
Published: April 2004

Background: The kidney has a remarkable capacity to concentrate mercury (Hg) and as such is a primary target organ when exposure to Hg occurs, and it is also an organ for Hg excretion. Objective: The present work aims to investigate the effect of occupational Hg vapour exposure on the urinary excretion of calcium (Ca), zinc (Zn) and copper (Cu), and the possible association of this excretion to work duration as well as renal alterations. Methods: 83 non-smoker participants (36 referents, age: 35.69 / 9.5 years; 27 Hg vapour-exposed workers with 5 / 10 years work duration, age: 33.09 / 5.1 years; and 20 Hg vapour-exposed workers with] / 11 years work duration, age: 39.509 / 8.50 years) were included in the present study. Urinary levels of microalbumin (U-Malb) and retinol-binding protein (U-RBP) as well as cytosolic glutathione S-transferase activity (U-GST) were measured to assess the glomerular and proximal tubular reabsorption functions as well as structural integrity of proximal tubules; respectively. In addition, blood Hg (B-Hg), serum levels of Hg (S-Hg) and Ca (S-Ca), and urinary levels of Hg (U-Hg), Ca (U-Ca), Zn (U-Zn), Cu (U-Cu) and creatinine (U-cr) were estimated. Results: In comparison to referents, all investigated parameters showed significant increase (except S-Ca and U-Zn/U-Cu ratio that significantly decreased among the workers as one group, S-Ca and U-Zn/U-Cu ratio that significantly and nonsignificantly decreased; respectively among workers with 5 / 10 years work duration, S-Ca and U-Zn/U-Cu ratio that significantly decreased among workers with] / 11 years work duration). In addition, B-Hg was nonsignificantly increased and S-Ca was significantly decreased; also, both U-Hg and U-Zn/U-Cu were nonsignificantly decreased among workers with] / 11 years work duration in comparison to those with 5 / 10 years work duration. Also, each of U-Hg, U-Ca, U-Zn and U-Cu was related to one another, while each of U-Ca, U-Zn and U-Cu was related to each of U-Malb, U-RBP and U-GST (except U-Zn was not related to U-GST). Conclusion: Hg vapour exposure leads to renal alterations which may parallel the change in proteinuria and enzymuria as well as the increased loss in urine of each of Ca, Zn and Cu. The urinary assessment of these metals may be used as a good indicator for renal dysfunction.
In vivo ingestion of heavy metal particles of Se, Hg and W by murine macrophages. A study using scanning electron microscopy coupled with X-ray microanalysis
Authors: Cherdwongcharoensuk Duangrudee , Cunha Elisabete M , Upatham Suchart , Pereira António Sousa , Oliveira Maria João R , Águas Artur P
Published: April 2004

Several heavy metals that are currently employed in industry may become polluters of work and natural environments. As particulate matter, heavy metals are suitable for entering the human body through the respiratory and digestive systems. They often end up inside phagocytes; the size of the microscopic particles modulates both their phagocytosis, and the physiology of macrophages. Here we have adopted an experimental model to investigate the ingestion of particles of three industrial heavy metals (Se, Hg, W) by murine peritoneal macrophages in vivo. The phagocytes were studied by scanning electron microscopy coupled with X-ray elemental microanalysis (SEM-XRM), a method that allows specific identification of Se, W and Hg in cells at high resolution. We found that Hg that was taken up by macrophages was organized into small, round particles (0.319 / 0.14 mm). This was in contrast with the larger size of intracellular particles of Se (2.379 / 1.84 mm) or W (1.759-1.34 mm). Ingested particles of Se and W, but not Hg, often caused bulging of the cell surface of macrophages. We conclude that particulate matters of Se, W and Hg are organized in particles of different size inside macrophages. This size difference is likely to be associated with distinct phlogistic activities of these heavy metals, Se and W causing a milder inflammatory reaction than Hg.