bluuebunny/crossref_metadata_2025 huggingface dataset:
Glutamatergic Propagation of GABAergic Seizure-Like Afterdischarge in the Hippocampus In Vitro
Authors: Isomura Yoshikazu , Fujiwara-Tsukamoto Yoko , Takada Masahiko
Published: May 2006

Previous investigations have suggested that GABA may act actively as an excitatory mediator in the generation of seizure-like (ictal) or interictal epileptiform activity in several experimental models of temporal lobe epilepsy. However, it remains to be known whether or not such GABAergic excitation may participate in seizure propagation into neighboring cortical regions. In our in vitro study using mature rat hippocampal slices, we examined the cellular mechanism underlying synchronous propagation of seizure-like afterdischarge in the CA1 region, which is driven by depolarizing GABAergic transmission, into the adjacent subiculum region. Tetanically induced seizure-like afterdischarge was always preceded by a GABAergic, slow posttetanic depolarization in the pyramidal cells of the original seizure-generating region. In contrast, the slow posttetanic depolarization was no longer observed in the subicular pyramidal cells when the afterdischarge was induced in the CA1 region. Surgical cutting of axonal pathways through the stratum oriens and the alveus between the CA1 and the subiculum region abolished the CA1-generated afterdischarge in the subicular pyramidal cells. Intracellular loading of fluoride ions, a GABAA receptor blocker, into single subicular pyramidal cells had no inhibitory effect on the CA1-generated afterdischarge in the pyramidal cells. Furthermore, the CA1-generated afterdischarge in the subicular pyramidal cells was largely depressed by local application of glutamate receptor antagonists to the subiculum region during afterdischarge generation. The present results indicate that the excitatory GABAergic generation of seizure-like activity seems to be restricted to epileptogenic foci of origin in the seizure-like epilepsy model in vitro.
Persistent Sodium Current, Membrane Properties and Bursting Behavior of Pre-Bötzinger Complex Inspiratory Neurons In Vitro
Authors: Del Negro Christopher A. , Koshiya Naohiro , Butera Robert J. , Smith Jeffrey C.
Published: May 2006

We measured persistent Na+current and membrane properties of bursting-pacemaker and nonbursting inspiratory neurons of the neonatal rat pre-Bötzinger complex (pre-BötC) in brain stem slice preparations with a rhythmically active respiratory network in vitro. In whole-cell recordings, slow voltage ramps (≤100 mV/s) inactivated the fast, spike-generating Na+ current and yielded N-shaped current-voltage relationships with nonmonotonic, negative-slope regions between −60 and −35 mV when the voltage-sensitive component was isolated. The underlying current was a TTX-sensitive persistent Na+ current ( I NaP) since the inward current was present at slow voltage ramp speeds (3.3–100 mV/s) and the current was blocked by 1 μM TTX. We measured the biophysical properties of I NaP after subtracting the voltage-insensitive “leak” current ( I Leak) in the presence of Cd2+ and in some cases tetraethylammonium (TEA). Peak I NaP ranged from −50 to −200 pA at a membrane potential of −30 mV. Decreasing the speed of the voltage ramp caused time-dependent I NaPinactivation, but this current was present at ramp speeds as low as 3.3 mV/s. I NaP activated at −60 mV and obtained half-maximal activation near −40 mV. The subthreshold voltage dependence and slow inactivation kinetics of I NaP, which closely resemble those of I NaP mathematically modeled as a burst-generation mechanism in pacemaker neurons of the pre-BötC, suggest that I NaP predominantly influences bursting dynamics of pre-BötC inspiratory pacemaker neurons in vitro. We also found that the ratio of persistent Na+conductance to leak conductance ( g NaP/ g Leak) can distinguish the phenotypic subpopulations of bursting pacemaker and nonbursting inspiratory neurons: pacemaker neurons showed g NaP/ g Leak> g NaP/ g Leakin nonpacemaker cells ( P < 0.0002). We conclude that I NaP is ubiquitously expressed by pre-BötC inspiratory neurons and that bursting pacemaker behavior within the heterogeneous population of inspiratory neurons is achieved with specific ratios of these two conductances, g NaP and g Leak.
Selectivity for Conspecific Song in the Zebra Finch Auditory Forebrain
Authors: Grace Julie A. , Amin Noopur , Singh Nandini C. , Theunissen Frédéric E.
Published: May 2006

The selectivity of neurons in the zebra finch auditory forebrain for natural sounds was investigated systematically. The principal auditory forebrain area in songbirds consists of the tonotopically organized field L complex, which, by its location in the auditory processing stream, can be compared with the auditory cortex of mammals. We also recorded from a secondary auditory area, cHV. Field L and cHV are auditory processing stages that are presynaptic to the specialized song system nuclei where auditory neurons show an extremely selective response for the bird's own song, but weak response to almost any other sounds, including conspecific songs. In our study, we found that neurons in field L and cHV had stronger responses to conspecific song than to synthetic sounds that were designed to match the lower order acoustical properties of song, such as their overall power spectra and AM spectra. Such preferential responses to natural sounds cannot be explained by linear frequency tuning or simple nonlinear intensity tuning and requires linear or nonlinear spectro-temporal neuronal transfer functions tuned to the acoustical properties of song. The selectivity for conspecific songs in field L and cHV might reflect an intermediate auditory processing stage for vocalizations that then contributes to the generation of the very specific selectivity for the bird's own song seen in the postsynaptic song system.
Differential Effects of Ethanol on GABAA and Glycine Receptor-Mediated Synaptic Currents in Brain Stem Motoneurons
Authors: Sebe Joy Y. , Eggers Erika D. , Berger Albert J.
Published: May 2006

Ethanol potentiates glycinergic synaptic transmission to hypoglossal motoneurons (HMs). This effect on glycinergic transmission changes with postnatal development in that juvenile HMs (P9–13) are more sensitive to ethanol than neonate HMs (P1–3). We have now extended our previous study to investigate ethanol modulation of synaptic GABAA receptors (GABAARs), because both GABA and glycine mediate inhibitory synaptic transmission to brain stem motoneurons. We tested the effects of ethanol on GABAergic and glycinergic miniature inhibitory postsynaptic currents (mIPSCs) recorded from neonate and juvenile rat HMs in an in vitro slice preparation. Bath application of 30 mM ethanol had no significant effect on the GABAergic mIPSC amplitude or frequency recorded at either age. At 100 mM, ethanol significantly decreased the GABAergic mIPSC amplitude recorded from neonate (6 ± 3%, P < 0.05) and juvenile (16 ± 3%, P < 0.01) HMs. The same concentration of ethanol increased the GABAergic mIPSC frequency recorded from neonate (64 ± 17%, P < 0.05) and juvenile (40 ± 15%, n.s.) HMs. In contrast, 100 mM ethanol robustly potentiated glycinergic mIPSC amplitude in neonate (31 ± 3%, P < 0.0001) and juvenile (41 ± 7%, P < 0.001) HMs. These results suggest that glycine receptors are more sensitive to modulation by ethanol than GABAA receptors and that 100 mM ethanol has the opposite effect on GABAAR-mediated currents in juvenile HMs, that is, inhibition rather than enhancement. Further, comparing ethanol's effects on GABAergic mIPSC amplitude and frequency, ethanol modulates GABAergic synaptic transmission to HMs differentially. Presynaptically, ethanol enhances mIPSC frequency while postsynaptically it decreases mIPSC amplitude.
The Roles Potassium Currents Play in Regulating the Electrical Activity of Ventral Cochlear Nucleus Neurons
Authors: Rothman Jason S. , Manis Paul B.
Published: May 2006

Using kinetic data from three different K+currents in acutely isolated neurons, a single electrical compartment representing the soma of a ventral cochlear nucleus (VCN) neuron was created. The K+currents include a fast transient current ( IA), a slow-inactivating low-threshold current ( ILT), and a noninactivating high-threshold current ( IHT). The model also includes a fast-inactivating Na+current, a hyperpolarization-activated cation current ( Ih), and 1–50 auditory nerve synapses. With this model, the role IA, ILT, and IHTplay in shaping the discharge patterns of VCN cells is explored. Simulation results indicate that IHTmainly functions to repolarize the membrane during an action potential, and IAfunctions to modulate the rate of repetitive firing. ILTis found to be responsible for the phasic discharge pattern observed in Type II cells (bushy cells). However, by adjusting the strength of ILT, both phasic and regular discharge patterns are observed, demonstrating that a critical level of ILTis necessary to produce the Type II response. Simulated Type II cells have a significantly faster membrane time constant in comparison to Type I cells (stellate cells) and are therefore better suited to preserve temporal information in their auditory nerve inputs by acting as precise coincidence detectors and having a short refractory period. Finally, we demonstrate that modulation of Ih, which changes the resting membrane potential, is a more effective means of modulating the activation level of ILTthan simply modulating ILTitself. This result may explain why ILTand Ihare often coexpressed throughout the nervous system.
Responses to Natural Scenes in Cat V1
Authors: Kayser Christoph , Salazar Rodrigo F. , König Peter
Published: May 2006

Studies on processing in primary visual areas often use artificial stimuli such as bars or gratings. As a result, little is known about the properties of activity patterns for the natural stimuli processed by the visual system on a daily basis. Furthermore, in the cat, a well-studied model system for visual processing, most results are obtained from anesthetized subjects and little is known about neuronal activations in the alert animal. Addressing these issues, we measure local field potentials (lfp) and multiunit spikes in the primary visual cortex of awake cats. We compare changes in the lfp power spectra and multiunit firing rates for natural movies, movies with modified spatio-temporal correlations as well as gratings. The activity patterns elicited by drifting gratings are qualitatively and quantitatively different from those elicited by natural stimuli and this difference arises from both spatial as well as temporal properties of the stimuli. Furthermore, both local field potentials and multiunit firing rates are most sensitive to the second-order statistics of the stimuli and not to their higher-order properties. Finally, responses to natural movies show a large variability over time because of activity fluctuations induced by rapid stimulus motion. We show that these fluctuations are not dependent on the detailed spatial properties of the stimuli but depend on their temporal jitter. These fluctuations are important characteristics of visual activity under natural conditions and impose limitations on the readout of possible differences in mean activity levels.
Compensatory and Orienting Eye Movements Induced By Off-Vertical Axis Rotation (OVAR) in Monkeys
Authors: Kushiro Keisuke , Dai Mingjia , Kunin Mikhail , Yakushin Sergei B. , Cohen Bernard , Raphan Theodore
Published: May 2006

Nystagmus induced by off-vertical axis rotation (OVAR) about a head yaw axis is composed of a yaw bias velocity and modulations in eye position and velocity as the head changes orientation relative to gravity. The bias velocity is dependent on the tilt of the rotational axis relative to gravity and angular head velocity. For axis tilts <15°, bias velocities increased monotonically with increases in the magnitude of the projected gravity vector onto the horizontal plane of the head. For tilts of 15–90°, bias velocity was independent of tilt angle, increasing linearly as a function of head velocity with gains of 0.7–0.8, up to the saturation level of velocity storage. Asymmetries in OVAR bias velocity and asymmetries in the dominant time constant of the angular vestibuloocular reflex (aVOR) covaried and both were reduced by administration of baclofen, a GABABagonist. Modulations in pitch and roll eye positions were in phase with nose-down and side-down head positions, respectively. Changes in roll eye position were produced mainly by slow movements, whereas vertical eye position changes were characterized by slow eye movements and saccades. Oscillations in vertical and roll eye velocities led their respective position changes by ≈90°, close to an ideal differentiation, suggesting that these modulations were due to activation of the orienting component of the linear vestibuloocular reflex (lVOR). The beating field of the horizontal nystagmus shifted the eyes 6.3°/ g toward gravity in side down position, similar to the deviations observed during static roll tilt (7.0°/ g). This demonstrates that the eyes also orient to gravity in yaw. Phases of horizontal eye velocity clustered ∼180° relative to the modulation in beating field and were not simply differentiations of changes in eye position. Contributions of orientating and compensatory components of the lVOR to the modulation of eye position and velocity were modeled using three components: a novel direct otolith-oculomotor orientation, orientation-based velocity modulation, and changes in velocity storage time constants with head position re gravity. Time constants were obtained from optokinetic after-nystagmus, a direct representation of velocity storage. When the orienting lVOR was combined with models of the compensatory lVOR and velocity estimator from sequential otolith activation to generate the bias component, the model accurately predicted eye position and velocity in three dimensions. These data support the postulates that OVAR generates compensatory eye velocity through activation of velocity storage and that oscillatory components arise predominantly through lVOR orientation mechanisms.
Neck Muscle Vibration and Spatial Orientation During Stepping in Place in Humans
Authors: Bove Marco , Courtine Gregoire , Schieppati Marco
Published: May 2006

Unilateral long-lasting vibration was applied to the sternomastoid muscle to assess the influence of asymmetric neck proprioceptive input on body orientation during stepping-in-place. Blindfolded subjects performed 3 sequences of 3 trials, each lasting 60 s: control, vibration applied during stepping (VDS), and vibration applied before stepping (VBS). VDS caused clear-cut whole body rotation toward the side opposite to vibration. The body rotated around a vertical axis placed at about arm's length from the body. The rotation did not begin immediately on switching on the vibrator. The delay varied from subject to subject from a few seconds to about 10 s. Once initiated, the angular velocity of rotation was remarkably constant (about 1°/s). In VBS, at the beginning of stepping, subjects rotated for a while as if their neck were still vibrated. At a variable delay, the direction of rotation reversed, and the effects were opposite to those observed during VDS. Under no condition did head rotation, head roll, or lateral body tilt accompany rotation. The results confirm and extend the notion that the neck proprioceptive input plays a major role in body orientation during locomotion. The body rotation does not seem to depend on the same mechanisms that modify the erect posture; rather, the asymmetric neck input would seem to modify the egocentric body-centered coordinate system.
The Mormyromast Region of the Mormyrid Electrosensory Lobe. II. Responses to Input From Central Sources
Authors: Mohr Claudia , Roberts Patrick D. , Bell Curtis C.
Published: May 2006

This is the second in a series of two papers on the mormyromast regions of the electrosensory lobe (ELL) of mormyrid electric fish. In this study, we examined the effects of artificial stimulation of two of the three major central inputs to ELL on different morphologically identified cell types of ELL. The three major central inputs to ELL are the eminentia granularis posterior, the juxtalobar nucleus, and the preeminential nucleus. We stimulated the juxtalobar and preeminential nuclei. We compared the effects of such stimulation with effects of the electric organ corollary discharge (EOCD) on the same cells to understand the origins of EOCD effects in ELL. Responses to juxtalobar stimulation were different in different cell types and remarkably similar to corollary discharge responses in the same cells. In addition, responses to juxtalobar stimulation were consistently depressed when the stimulus was delivered immediately after the naturally occurring EOCD response. These findings indicate that the juxtalobar nucleus is a major source of the EOCD responses of ELL cells. In contrast, preeminential stimulation evoked similar responses in medium ganglionic cells, and the two types of efferent cells that were quite different from the EOCD responses of these cells, suggesting that the preeminential nucleus is less important than the juxtalobar nucleus in determining the EOCD responses of ELL cells. Preeminential responses of medium ganglionic and efferent cells consisted of a short-latency excitatory postsynaptic potential (EPSP) followed by a long-lasting inhibitory postsynaptic potential (IPSP). Both the EPSPs and IPSPs were facilitated when brief bursts of closely spaced stimuli were delivered.
Noise and Coupling Affect Signal Detection and Bursting in a Simulated Physiological Neural Network
Authors: Stacey William C. , Durand Dominique M.
Published: May 2006

Signal detection in the CNS relies on a complex interaction between the numerous synaptic inputs to the detecting cells. Two effects, stochastic resonance (SR) and coherence resonance (CR) have been shown to affect signal detection in arrays of basic neuronal models. Here, an array of simulated hippocampal CA1 neurons was used to test the hypothesis that physiological noise and electrical coupling can interact to modulate signal detection in the CA1 region of the hippocampus. The array was tested using varying levels of coupling and noise with different input signals. Detection of a subthreshold signal in the network improved as the number of detecting cells increased and as coupling was increased as predicted by previous studies in SR; however, the response depended greatly on the noise characteristics present and varied from SR predictions at times. Careful evaluation of noise characteristics may be necessary to form conclusions about the role of SR in complex systems such as physiological neurons. The coupled array fired synchronous, periodic bursts when presented with noise alone. The synchrony of this firing changed as a function of noise and coupling as predicted by CR. The firing was very similar to certain models of epileptiform activity, leading to a discussion of CR as a possible simple model of epilepsy. A single neuron was unable to recruit its neighbors to a periodic signal unless the signal was very close to the synchronous bursting frequency. These findings, when viewed in comparison with physiological parameters in the hippocampus, suggest that both SR and CR can have significant effects on signal processing in vivo.
Adaptation and Temporal Decorrelation by Single Neurons in the Primary Visual Cortex
Authors: Wang Xiao-Jing , Liu Yinghui , Sanchez-Vives Maria V. , McCormick David A.
Published: May 2006

Limiting redundancy in the real-world sensory inputs is of obvious benefit for efficient neural coding, but little is known about how this may be accomplished by biophysical neural mechanisms. One possible cellular mechanism is through adaptation to relatively constant inputs. Recent investigations in primary visual (V1) cortical neurons have demonstrated that adaptation to prolonged changes in stimulus contrast is mediated in part through intrinsic ionic currents, a Ca2+-activated K+ current ( IKCa) and especially a Na+-activated K+ current ( IKNa). The present study was designed to test the hypothesis that the activation of adaptation ionic currents may provide a cellular mechanism for temporal decorrelation in V1. A conductance-based neuron model was simulated, which included an IKCa and an IKNa. We show that the model neuron reproduces the adaptive behavior of V1 neurons in response to high contrast inputs. When the stimulus is stochastic with 1/ f 2 or 1/ f-type temporal correlations, these autocorrelations are greatly reduced in the output spike train of the model neuron. The IKCa is effective at reducing positive temporal correlations at approximately 100-ms time scale, while a slower adaptation mediated by IKNa is effective in reducing temporal correlations over the range of 1–20 s. Intracellular injection of stochastic currents into layer 2/3 and 4 (pyramidal and stellate) neurons in ferret primary visual cortical slices revealed neuronal responses that exhibited temporal decorrelation in similarity with the model. Enhancing the slow afterhyperpolarization resulted in a strengthening of the decorrelation effect. These results demonstrate the intrinsic membrane properties of neocortical neurons provide a mechanism for decorrelation of sensory inputs.
Effects of Altering Initial Position on Movement Direction and Extent
Authors: Sainburg Robert L. , Lateiner Jordan E. , Latash Mark L. , Bagesteiro Leia B.
Published: May 2006

The purpose of this study was to examine the relative influence of initial hand location on the direction and extent of planar reaching movements. Subjects performed a horizontal-plane reaching task with the dominant arm supported above a table top by a frictionless air-jet system. A start circle and a target were reflected from a horizontal projection screen onto a horizontally positioned mirror, which blocked the subject's view of the arm. A cursor, representing either actual or virtual finger location, was only displayed between each trial to allow subjects to position the cursor in the start circle. Prior to occasional “probe trials,” we changed the start location of the finger relative to the cursor. Subjects reported being unaware of the discrepancy between cursor and finger. Our results indicate that regardless of initial hand location, subjects did not alter the direction of movement. However, movement distance was systematically adjusted in accord with the baseline target position. Thus when the hand start position was perpendicularly displaced relative to the target direction, neither the direction nor the extent of movement varied relative to that of baseline. However, when the hand was displaced along the target direction, either anterior or posterior, movements were made in the same direction as baseline trials but were shortened or lengthened, respectively. This effect was asymmetrical such that movements from anterior displaced positions showed greater distance adjustment than those from posterior displaced positions. Inverse dynamic analysis revealed substantial changes in elbow and shoulder muscle torque strategies for both right/left and anterior/posterior pairs of displacements. In the case of right/left displacements, such changes in muscle torque compensated changes in limb configuration such that movements were made in the same direction and to the same extent as baseline trials. Our results support the hypothesis that movement direction is specified relative to an origin at the current location of the hand. Movement extent, on the other hand, appears to be affected by the workspace learned during baseline movement experience.
Effects of Amplitude Modulation on the Coding of Interaural Time Differences of Low-Frequency Sounds in the Inferior Colliculus. II. Neural Mechanisms
Authors: D'Angelo W. R. , Sterbing S. J. , Ostapoff E.-M. , Kuwada S.
Published: May 2006

In our companion paper, we reported on interaural time difference (ITD)-sensitive neurons that enhanced, suppressed, or did not change their response when identical AM was added to both ears. Here, we first examined physical factors such as the difference in the interaural correlation, spectrum, or energy between the modulated and unmodulated signals. These were insufficient to explain the observed enhancement and suppression. We then examined neural mechanisms by selectively modulating the signal to each ear, varying modulation depth, and adding background noise to the unmodulated signal. These experiments implicated excitatory and inhibitory monaural inputs to the inferior colliculus (IC). These monaural inputs are postulated to adapt to an unmodulated signal and adapt less to a modulated signal. Thus enhancement or suppression is created by the convergence of these excitatory or inhibitory inputs with the inputs from the binaural comparators. Under modulation, the role of the monaural input is to shift the threshold of the IC neuron. Consistent with this role, background noise mimicked the effect of modulation. Functionally, enhancement and suppression may serve in detecting the degree of modulation in a sound source while preserving ITD information.
Dendritic Initiation and Propagation of Spikes and Spike Bursts in a Multimodal Sensory Interneuron: The Crustacean Parasol Cell
Authors: Mellon DeForest
Published: May 2006

Invasion of dendrites by spikes and spike bursts can play a critical role in regulating the output of central neurons by modifying their dynamic input-output relationships. Back-propagating bursts can modulate voltage-gated channels in the short term and can also modify long-term responses to synaptic input. Determining the morphological site of spike initiation and the mode of propagation through the dendritic arbor is therefore crucial to an understanding of a neuron's functional properties. I used electrophysiological methods to study parasol cells in isolated, perfused head preparations of the freshwater crayfish Procambarus clarkii to determine the compartment of origin of orthodromically activated action potentials and bursts that propagate within the dendritic arbor and to examine the identity of low-amplitude, electrotonically recorded spike events that are present in more than one-half of the intracellular recordings obtained from dendrites in these neurons. Experiments using antidromic activation of parasol cell axons indicated that electrotonically recorded spikes probably are generated in neighboring parasol cells, to which the impaled neurons are electrically coupled. Both paired intracellular recordings and extracellular field potential measurements were used to compare arrival times of antidromic and orthodromic spikes at loci in the vicinity of the trunk and the basal branch compartments of parasol cell dendrites. These methods provided consistent results, indicating that synaptically evoked action potentials are initiated at a site on the trunk, from which point they back-propagate into the basal branches within the hemiellipsoid body, and presumably, also orthodromically to the axon. Data are presented suggesting that bursts also arise at a trunk locus, but one that is different from the initiation point of single spikes evoked by excitatory postsynaptic potentials (EPSPs). Morphological specializations between the dendritic trunk and basal branches may facilitate back-propagation of spikes and spike bursts into the basal branches.
Contribution of Head Movement to Gaze Command Coding in Monkey Frontal Cortex and Superior Colliculus
Authors: Martinez-Trujillo Julio C. , Klier Eliana M. , Wang Hongying , Crawford J. Douglas
Published: May 2006

Most of what we know about the neural control of gaze comes from experiments in head-fixed animals, but several “head-free” studies have suggested that fixing the head dramatically alters the apparent gaze command. We directly investigated this issue by quantitatively comparing head-fixed and head-free gaze trajectories evoked by electrically stimulating 52 sites in the superior colliculus (SC) of two monkeys and 23 sites in the supplementary eye fields (SEF) of two other monkeys. We found that head movements made a significant contribution to gaze shifts evoked from both neural structures. In the majority of the stimulated sites, average gaze amplitude was significantly larger and individual gaze trajectories were significantly less convergent in space with the head free to move. Our results are consistent with the hypothesis that head-fixed stimulation only reveals the oculomotor component of the gaze shift, not the true, planned goal of the movement. One implication of this finding is that when comparing stimulation data against popular gaze control models, freeing the head shifts the apparent coding of gaze away from a “spatial code” toward a simpler visual model in the SC and toward an eye-centered or fixed-vector model representation in the SEF.
Quantitative Analysis of Attention and Detection Signals During Visual Search
Authors: Shulman Gordon L. , McAvoy Mark P. , Cowan Melanie C. , Astafiev Serguei V. , Tansy Aaron P. , d'Avossa Giovanni , Corbetta Maurizio
Published: May 2006

Prior work has distinguished regions in the intraparietal sulcus (IPs) and frontal eye field (FEF) involved in the voluntary control of attention, from more ventral regions in the temporoparietal junction (TPJ) involved in target detection. The present results show that when subjects search for and detect a visual target stimulus among nontargets, these regions show sensory-, search-, and detection-related signals that both confirm and refine these functional distinctions. The different signals were isolated by an additive model that accounted for a large fraction of BOLD (blood oxygenation level-dependent) signal modulation over the brain. Both IPs and FEF were activated during search through nontargets, consistent with a role in maintaining attention-related signals during search. However, unlike FEF, IPs also showed stimulus-related activations, and may combine signals related to sensory and task-dependent components of salience. Although IPs-FEF showed search-related activations, the TPJ was deactivated during search. TPJ activations were confined to detection-related signals. These results provide a much stronger dissociation between the TPJ and IPs-FEF than previous work, while indicating functional differences between frontal and parietal regions that are often coactivated in studies of attention. Finally, continuous flow models of information processing predict that during search, signals from missed targets should be fed from sensory to associative regions rather than being gated by the decision criterion. Correspondingly, missed targets significantly activated parietal (e.g., right TPJ) and frontal (e.g., anterior insula, anterior cingulate) regions, although with a smaller magnitude than detected targets. Surprisingly, many cortical regions showed equivalent signals from detected targets and the completion of target-absent trials, reflecting a widespread signal unrelated to motor execution.
Functional Segregation of ITD Sensitivity in the Inferior Colliculus of Decerebrate Cats
Authors: Ramachandran Ramnarayan , May Bradford J.
Published: May 2006

Decerebration allows single-unit responses in the central nucleus of the inferior colliculus (ICC) to be studied in the absence of anesthesia and descending efferent influences. When this procedure is applied to cats, three neural response types (V, I, and O) can be identified by distinct patterns of excitation and inhibition in pure-tone frequency-response maps. Similarities of the definitive response map features with those of projection neurons in the auditory brain stem have led to the proposal that the ICC response types are derived from different sources of ascending input that remain functionally segregated within the midbrain. Additional evidence for the existence of these hypothesized parallel processing pathways has been obtained in our previous investigations of the effects of interaural level differences, brain stem lesions, and pharmacological manipulations on physiologically classified units. This study extends our characterization of the functional segregation of single-unit activity in the ICC by investigating how sensitivity to interaural time differences (ITDs) is related to the response types that are observed in decerebrate cats. The results of these experiments support our parallel-processing model of the ICC by linking the ITD sensitivity of type V and I units to putative inputs from the medial superior olive and lateral superior olive and by showing that most type O units lack a systematic sensitivity to binaural temporal information presumably because their dominant ascending inputs arise from weakly binaural neurons in the dorsal cochlear nucleus.
Development of Reorganization of the Auditory Cortex Caused by Fear Conditioning: Effect of Atropine
Authors: Ji Weiqing , Suga Nobuo
Published: May 2006

Reorganization of the frequency map in the central auditory system is based on shifts in the best frequencies (BFs; hereafter, BF shifts), together with the frequency-response curves, of auditory neurons. In the big brown bat, conditioning with acoustic stimulation followed by electric leg-stimulation causes BF shifts of collicular and cortical neurons. The collicular BF shift develops quickly and is short term, whereas the cortical BF shift develops slowly and is long term. The acetycholine level in the auditory cortex must be high during conditioning to develop these BF shifts. We studied the effect of atropine (an antagonist of muscarinic acetylcholine receptors) applied to the auditory cortex on the development of the long-term cortical BF shift in the awake bat caused by a 30-min conditioning session. We found 1) the cortical BF shift starts to develop ∼15 min after the onset of the conditioning, gradually increases over 60 min, and reaches a plateau, 2) the cortical BF shift changes from short to long term ∼45 min after the onset of the conditioning, 3) the cortical BF shift can plateau at different frequencies between the BF of a given neuron in the control condition and the frequency of the conditioning tone, 4) the maximum BF shift is determined ∼70 min after the onset of the conditioning, and 5) acetylcholine plays an important role in the development of the cortical BF shift. Its role ends ∼180 min after the onset of the conditioning.
Coding of Sound Intensity in the Chick Cochlear Nerve
Authors: Saunders James C. , Ventetuolo Corey E. , Plontke Stefan K.-R. , Weiss Burton A.
Published: May 2006

Tuning curves, spontaneous activity, and rate-intensity (RI) functions were obtained from units in the chick cochlear nerve. The characteristic frequency (CF) was determined from each tuning curve. The shape of each RI function was subjectively evaluated and assigned to one of four RI types. The breakpoint, discharge rate at the highest SPLs, and slopes of the primary and secondary segments were quantified for each function. The CF and RI type were then related to these variables. A new RI function was observed in which the discharge activity in the secondary segment diminished as stimulus level increased above the breakpoint. This function was called a “sloping-down” type. In 959 units, saturating, sloping-up, sloping-down, and straight RI types were identified in 39.2, 35.5, 12.6, and 12.7% of the sample, respectively. The slope of the primary segment was nearly the same in each of the four types and averaged 5.48 S · s−1· dB−1 across all units. The slopes of the secondary segments formed four groupings when segregated by RI type based on the subjective assignments and averaged 0.03, 1.22, −0.90, and 3.95 S · s−1 · dB−1 in the saturating, sloping-up, sloping-down, and straight types, respectively. The data describing the secondary segments of all units were fit with a multi-compartment polynomial and showed a continuous distribution that segregated, with some overlap, into the different RI categories. The proportion of RI types, as well as the secondary and primary slopes were approximately constant across CFs. In addition, it would appear that the other parameters that define the four types were, for the most part, homogeneously distributed across the frequency axis of the chick inner ear. Finally, a comparison of RI functions having a common CF suggested that the compressive nonlinearity that determines RI type may be a phenomenon localized to individual hair cells in the bird ear.
Ionic Currents Underlying Fast Action Potentials in the Obliquely Striated Muscle Cells of the Octopus Arm
Authors: Rokni Dan , Hochner Binyamin
Published: May 2006

The octopus arm provides a unique model for neuromuscular systems of flexible appendages. We previously reported the electrical compactness of the arm muscle cells and their rich excitable properties ranging from fast oscillations to overshooting action potentials. Here we characterize the voltage-activated ionic currents in the muscle cell membrane. We found three depolarization-activated ionic currents: 1) a high-voltage-activated L-type Ca2+current, which began activating at approximately −35 mV, was eliminated when Ca2+was substituted by Mg2+, was blocked by nifedipine, and showed Ca2+-dependent inactivation. This current had very rapid activation kinetics (peaked within milliseconds) and slow inactivation kinetics (τ in the order of 50 ms). 2) A delayed rectifier K+current that was totally blocked by 10 mM TEA and partially blocked by 10 mM 4-aminopyridine (4AP). This current exhibited relatively slow activation kinetics (τ in the order of 15 ms) and inactivated only partially with a time constant of ∼150 ms. And 3) a transient A-type K+current that was totally blocked by 10 mM 4AP and was partially blocked by 10 mM TEA. This current exhibited very fast activation kinetics (peaked within milliseconds) and inactivated with a time constant in the order of 60 ms. Inactivation of the A-type current was almost complete at −40 mV. No voltage-dependent Na+current was found in these cells. The octopus arm muscle cells generate fast (∼3 ms) overshooting spikes in physiological conditions that are carried by a slowly inactivating L-type Ca2+current.
Neural Basis for the Processes That Underlie Visually Guided and Internally Guided Force Control in Humans
Authors: Vaillancourt David E. , Thulborn Keith R. , Corcos Daniel M.
Published: May 2006

Despite an intricate understanding of the neural mechanisms underlying visual and motor systems, it is not completely understood in which brain regions humans transfer visual information into motor commands. Furthermore, in the absence of visual information, the retrieval process for motor memory information remains unclear. We report an investigation where visuomotor and motor memory processes were separated from only visual and only motor activation. Subjects produced precision grip force during a functional MRI (fMRI) study that included four conditions: rest, grip force with visual feedback, grip force without visual feedback, and visual feedback only. Statistical and subtractive logic analyses segregated the functional process maps. There were three important observations. First, along with the well-established parietal and premotor cortical network, the anterior prefrontal cortex, putamen, ventral thalamus, lateral cerebellum, intermediate cerebellum, and the dentate nucleus were directly involved in the visuomotor transformation process. This activation occurred despite controlling for the visual input and motor output. Second, a detailed topographic orientation of visuomotor to motor/sensory activity was mapped for the premotor cortex, parietal cortex, and the cerebellum. Third, the retrieval of motor memory information was isolated in the dorsolateral prefrontal cortex, ventral prefrontal cortex, and anterior cingulate. The motor memory process did not extend to the supplementary motor area (SMA) and the basal ganglia. These findings provide evidence in humans for a model where a distributed network extends over cortical and subcortical regions to control the visuomotor transformation process used during visually guided tasks. In contrast, a localized network in the prefrontal cortex retrieves force output from memory during internally guided actions.
Infants Adapt Their Stepping to Repeated Trip-Inducing Stimuli
Authors: Pang Marco Y. C. , Lam Tania , Yang Jaynie F.
Published: May 2006

This study examined whether human infants under the age of 12 mo learn to modify their stepping pattern after repeated trip-inducing stimuli. Thirty three infants aged from 5 to 11 mo were studied. The infants were held over a moving treadmill belt to induce stepping. Occasionally, a mechanical tap was applied to the dorsum of the left foot during the early swing phase to elicit a high step. In some trials, the stimulus was applied for only one step. In other trials, the foot was stimulated for a few consecutive steps. We determined whether the infants continued to show high stepping immediately after the removal of the stimuli. The results showed that after the foot was touched for two or more consecutive steps, some infants continued to demonstrate high stepping for a few steps after the removal of the stimuli (i.e., aftereffect). Such adaptation was achieved by an increase in hip and knee flexor muscle torque, which led to greater hip and knee flexion during the early swing phase. Aftereffects were more commonly seen in older infants (9 mo or older). The results indicated that before the onset of independent walking, the locomotor circuitry in human infants is capable of adaptive locomotor plasticity. The increased incidence of aftereffect in older infants also suggests that the ability to adapt to repeated trip-inducing stimuli may be related to other factors such as experience in stepping and maturation of the nervous system.
Neural Control of Rhythmic Human Arm Movement: Phase Dependence and Task Modulation of Hoffmann Reflexes in Forearm Muscles
Authors: Zehr E. Paul , Collins David F. , Frigon Alain , Hoogenboom Nienke
Published: May 2006

Although we move our arms rhythmically during walking, running, and swimming, we know little about the neural control of such movements. Our working hypothesis is that neural mechanisms controlling rhythmic movements are similar in the human lumbar and cervical spinal cord. Thus reflex modulation during rhythmic arm movement should be similar to that seen during leg movement. Our main experimental hypotheses were that the amplitude of H-reflexes in the forearm muscles would be modulated during arm movement (i.e., phase-dependent) and would be inhibited during cycling compared with static contraction (i.e., task-dependent). Furthermore, to determine the locus of any modulation, we tested the effect that active and passive movement of the ipsilateral (relative to stimulated arm) and contralateral arm had on H-reflex amplitude. Subjects performed rhythmic arm cycling on a custom-made hydraulic ergometer in which the two arms could be constrained to move together (180° out of phase) or could rotate independently. Position of the stimulated limb in the movement cycle is described with respect to the clock face. H-reflexes were evoked at 12, 3, 6, and 9 o'clock positions during static contraction as well as during rhythmic arm movements. Reflex amplitudes were compared between tasks at equal M wave amplitudes and similar levels of electromyographic (EMG) activity in the target muscle. Surface EMG recordings were obtained bilaterally from flexor carpi radialis as well as from other muscles controlling the wrist, elbow, and shoulder. Compared with reflexes evoked during static contractions, movement of the stimulated limb attenuated H-reflexes by 50.8% ( P < 0.005), 65.3% ( P < 0.001), and 52.6% ( P < 0.001) for bilateral, active ipsilateral, and passive ipsilateral movements, respectively. In contrast, movement of the contralateral limb did not significantly alter H-reflex amplitude. H-reflexes were also modulated by limb position ( P < 0.005). Thus task- and phase-dependent modulation were observed in the arm as previously demonstrated in the leg. The data support the hypothesis that neural mechanisms regulating reflex pathways in the moving limb are similar in the human upper and lower limbs. However, the inhibition of H-reflex amplitude induced by contralateral leg movement is absent in the arms. This may reflect the greater extent to which the arms can be used independently.
Effects of High-Frequency Stimulation in the Internal Globus Pallidus on the Activity of Thalamic Neurons in the Awake Monkey
Authors: Anderson Marjorie E. , Postupna Nadia , Ruffo Mark
Published: May 2006

The reduction in symptoms of Parkinson's disease produced by high-frequency stimulation (HFS) in the internal globus pallidus (GPi) has been proposed to be due to stimulus-induced inactivation of pallidal neurons and resulting disinhibition of thalamic neurons. We tested this in awake Macaca fascicularis by stimulating between pairs of electrodes inserted into GPi under electrophysiological control and recording the responses evoked in thalamic neurons. HFS produced a reduction, not an increase, in discharge frequency during the stimulus train in 77% of the responsive thalamic neurons. Only 16% of the responsive cells showed an increase in discharge during stimulation and, for some of these, stimulation at a similar intensity produced contralateral muscle contraction, a probable sign of current spread to the internal capsule. The few thalamic neurons studied during bursting had a reduction in burst frequency and duration during HFS. We conclude that high-frequency stimulation within GPi does not necessarily facilitate thalamic discharge, and it may act, instead, to interrupt abnormal patterns of thalamic discharge associated with parkinsonian symptoms.
Contribution of Cutaneous Inputs From the Hindpaw to the Control of Locomotion. I. Intact Cats
Authors: Bouyer L.J.G. , Rossignol S.
Published: May 2006

The goal of this study was to evaluate the role of hindpaw cutaneous feedback in the control of locomotion, by cutting some (in one cat) or all (in 2 cats) cutaneous nerves bilaterally at ankle level. Kinematic and electromyographic (EMG) recordings were obtained before and for several weeks after denervation during level and incline (15° up and down) treadmill walking. Ladder walking and ground reaction forces were also documented sporadically. Early after the denervation (1–3 days), cats could not walk across a ladder, although deficits were small during level treadmill walking. Increased knee flexion velocity caused a 14% reduction in swing phase duration. EMG activity was consistently increased in knee, ankle, and toe flexors, and in at least one knee or ankle extensor. The adaptive changes during walking on the incline were much reduced after denervation. Ladder walking gradually recovered within 3–7 wk. By this time, level treadmill walking kinematics had completely returned to normal, but EMG activity in flexors remained above control. Incline walking improved but did not return to normal. Mediolateral ground reaction forces during overground walking were increased by 200%. It is concluded that in intact cats, cutaneous inputs contribute more to demanding situations such as walking on a ladder or on inclines than to level walking. Active adaptive mechanisms are likely involved given that the EMG locomotor pattern never returned to control level. The companion paper shows on the other hand that when the same cats are spinalized, these cutaneous inputs become critical for foot placement during locomotion.