Biomedical subjects

C I Moore

Publications and source records attributed to C I Moore.

15 recordsLinked to original sources

PubMed2006

Analysis of LFP phase predicts sensory response of barrel cortex.

Several previous studies have shown the existence of Up and Down states and have linked their magnitude (e.g., depolarization level) to the size of sensory-evoked responses. Here, we studied how the temporal dynamics of such states influence the sensory-evoked response to vibrissa deflection. Under alpha-chloralose anesthesia, barrel cortex exhibits strong quasi-periodic approximately 1-Hz local field potential (LFP) oscillations generated by the synchronized fluctuation of large populations of neurons between depolarized (Up) and hyperpolarized (Down) states. Using a linear depth electrode array, we recorded the LFP and multiunit activity (MUA) simultaneously across multiple layers of the barrel column and used the LFP to approximate the subthreshold Up-Down fluctuations. Our central finding is that the MUA response is a strong function of the LFP oscillation's phase. When only ongoing LFP magnitude was considered, the response was largest in the Down state, in agreement with previous studies. However, consideration of the LFP phase revealed that the MUA response varied smoothly as a function of LFP phase in a manner that was not monotonically dependent on LFP magnitude. The LFP phase is therefore a better predictor of the MUA response than the LFP magnitude is. Our results suggest that, in the presence of ongoing oscillations, there can be a continuum of response properties and that each phase may, at times, need to be considered a distinct cortical state.

Animals↗

PubMed2001

Wakefield generation and GeV acceleration in tapered plasma channels.

To achieve multi-GeV electron energies in the laser wakefield accelerator (LWFA), it is necessary to propagate an intense laser pulse long distances in a plasma without disruption. One of the purposes of this paper is to evaluate the stability properties of intense laser pulses propagating extended distances (many tens of Rayleigh ranges) in plasma channels. A three-dimensional envelope equation for the laser field is derived that includes nonparaxial effects such as group velocity dispersion, as well as wakefield and relativistic nonlinearities. It is shown that in the broad beam, short pulse limit the nonlinear terms in the wave equation that lead to Raman and modulation instabilities cancel. This cancellation can result in pulse propagation over extended distances, limited only by dispersion. Since relativistic focusing is not effective for short pulses, the plasma channel provides the guiding necessary for long distance propagation. Long pulses (greater than several plasma wavelengths), on the other hand, experience substantial modification due to Raman and modulation instabilities. For both short and long pulses the seed for instability growth is inherently determined by the pulse shape and not by background noise. These results would indicate that the self-modulated LWFA is not the optimal configuration for achieving high energies. The standard LWFA, although having smaller accelerating fields, can provide acceleration for longer distances. It is shown that by increasing the plasma density as a function of distance, the phase velocity of the accelerating field behind the laser pulse can be made equal to the speed of light. Thus electron dephasing in the accelerating wakefield can be avoided and energy gain increased by spatially tapering the plasma channel. Depending on the tapering gradient, this luminous wakefield phase velocity is obtained several plasma wavelengths behind the laser pulse. Simulations of laser pulses propagating in a tapered plasma channel are presented. Experimental techniques for generating a tapered density in a capillary discharge are described and an example of a GeV channel guided standard LWFA is presented.

Journal Article↗

PubMed2001

Simulation and design of stable channel-guided laser wakefield accelerators.

Most laser wakefield accelerator (LWFA) experiments to date have operated in the self-modulated (SM) regime and have been self-guided. A channel-guided LWFA operating in the standard or resonant regime is expected to offer the possibility of high electron energy gain and high accelerating gradients without the instabilities and poor electron beam quality associated with the SM regime. Plasma channels such as those produced by a capillary discharge have demonstrated guiding of intense laser pulses over distances of several centimeters. Optimizing the performance in a resonant LWFA constrains the on-axis plasma density in the channel to a relatively narrow range. A scaling model is presented that quantifies resonant LFWA performance in terms of the maximum accelerating gradient, dephasing length, and dephasing-limited energy gain. These performance quantities are expressed in terms of laser and channel experimental parameters, clearly illustrating some of the tradeoffs in the choice of parameters. The predicted energy gain in this model is generally lower than that indicated by simpler scaling models. Simulations agree well with the scaling model in both low and high plasma density regimes. Simulations of a channel-guided, self-modulated LWFA are also presented. Compared with the resonant LWFA regime, the requirements on laser and channel parameters in the SM regime are easier to achieve, and a channel-guided SM-LWFA is likely to be less unstable than a self-guided SM-LWFA.

Journal Article↗

PubMed2000

Referred phantom sensations and cortical reorganization after spinal cord injury in humans.

To test the hypothesis that cortical remapping supports phantom sensations, we examined referred phantom sensations and cortical activation in humans after spinal-cord injury (SCI) at the thoracic level (T3-T12). Of 12 SCI subjects, 9 reported phantom sensations, and 2 reported referred phantom sensations. In both of these subjects, referred phantom sensations were evoked by contact in reference zones (RZ) that were not adjacent in the periphery and were not predicted to be adjacent in the postcentral gyrus (PoCG), suggesting that representations separated by centimeters of cortical space were simultaneously engaged. This finding was supported by functional MRI (fMRI). In a subject with a T6-level complete SCI, contact in RZ on the left or right forearm projected referred phantom sensations to the ipsilateral chest. During fMRI, contact in either forearm RZ evoked activity in the central PoCG (the position of the forearm representation) and the medial PoCG (the position of the chest representation) with >/=1.6 cm of nonresponsive cortex intervening. In contrast, stimulation in non-RZ forearm and palm regions in this subject and in lesion-matched SCI subjects evoked central but not medial PoCG activation. Our findings support a relation between PoCG activation and the percept of referred phantom sensations. These results, however, present an alternative to somatotopic cortical reorganization, namely, cortical plasticity expressed in coactivation of nonadjacent representations. The observed pattern suggests that somatotopic subcortical remapping, projected to the cortex, can support perceptual and cortical reorganization after deafferentation in humans.

Adult↗

PubMed2000

Acupuncture modulates the limbic system and subcortical gray structures of the human brain: evidence from fMRI studies in normal subjects.

Acupuncture, an ancient therapeutic technique, is emerging as an important modality of complementary medicine in the United States. The use and efficacy of acupuncture treatment are not yet widely accepted in Western scientific and medical communities. Demonstration of regionally specific, quantifiable acupuncture effects on relevant structures of the human brain would facilitate acceptance and integration of this therapeutic modality into the practice of modern medicine. Research with animal models of acupuncture indicates that many of the beneficial effects may be mediated at the subcortical level in the brain. We used functional magnetic resonance imaging (fMRI) to investigate the effects of acupuncture in normal subjects and to provide a foundation for future studies on mechanisms of acupuncture action in therapeutic interventions. Acupuncture needle manipulation was performed at Large Intestine 4 (LI 4, Hegu) on the hand in 13 subjects [Stux, 1997]. Needle manipulation on either hand produced prominent decreases of fMRI signals in the nucleus accumbens, amygdala, hippocampus, parahippocampus, hypothalamus, ventral tegmental area, anterior cingulate gyrus (BA 24), caudate, putamen, temporal pole, and insula in all 11 subjects who experienced acupuncture sensation. In marked contrast, signal increases were observed primarily in the somatosensory cortex. The two subjects who experienced pain instead of acupuncture sensation exhibited signal increases instead of decreases in the anterior cingulate gyrus (BA 24), caudate, putamen, anterior thalamus, and posterior insula. Superficial tactile stimulation to the same area elicited signal increases in the somatosensory cortex as expected, but no signal decreases in the deep structures. These preliminary results suggest that acupuncture needle manipulation modulates the activity of the limbic system and subcortical structures. We hypothesize that modulation of subcortical structures may be an important mechanism by which acupuncture exerts its complex multisystem effects.

Acupuncture Therapy↗

PubMed2000

Segregation of somatosensory activation in the human rolandic cortex using fMRI.

The segregation of sensory information into distinct cortical areas is an important organizational feature of mammalian sensory systems. Here, we provide functional magnetic resonance imaging (fMRI) evidence for the functional delineation of somatosensory representations in the human central sulcus region. Data were collected with a 3-Tesla scanner during two stimulation protocols, a punctate tactile condition without a kinesthetic/motor component, and a kinesthetic/motor condition without a punctate tactile component. With three-dimensional (3-D) anatomical reconstruction techniques, we analyzed data in individual subjects, using the pattern of activation and the anatomical position of specific cortical areas to guide the analysis. As a complimentary analysis, we used a brain averaging technique that emphasized the similarity of cortical features in the morphing of individual subjects and thereby minimized the distortion of the location of cortical activation sites across individuals. A primary finding of this study was differential activation of the cortex on the fundus of the central sulcus, the position of area 3a, during the two tasks. Punctate tactile stimulation of the palm, administered at 3 Hz with a 5.88(log10.mg) von Frey filament, activated discrete regions within the precentral (PreCG) and postcentral (PoCG) gyri, corresponding to areas 6, 3b, 1, and 2, but did not activate area 3a. Conversely, kinesthetic/motor stimulation, 3-Hz flexion and extension of the digits, activated area 3a, the PreCG (areas 6 and 4), and the PoCG (areas 3b, 1, and 2). These activation patterns were observed in individual subjects and in the averaged data, providing strong evidence for the existence of a distinct representation within area 3a in humans. The percentage signal changes in the PreCG and PoCG regions activated by tactile stimulation, and in the intervening gap region, support this functional dissociation. In addition to this distinction within the fundus of the central sulcus, the combination of high-resolution imaging and 3-D analysis techniques permitted localization of activation within areas 6, 4, 3a, 3b, 1, and 2 in the human. With the exception of area 4, which showed inconsistent activation during punctate tactile stimulation, activation in these areas in the human consistently paralleled the pattern of activity observed in previous studies of monkey cortex.

Adult↗

PubMed2000

A pilot study of somatotopic mapping after cortical infarct.

BACKGROUND AND PURPOSE: Animal studies have described remodeling of sensory and motor representational maps after cortical infarct. These changes may contribute to return of function after stroke. METHODS: Functional MRI was used to compare sensory and motor maps obtained in 35 normal control subjects with results from 2 patients with good recovery 6 months after a cortical stroke. RESULTS: During finger tapping in controls, precentral gyrus activation exceeded or matched postcentral gyrus activation in 40 of 42 cases. Patient 1 had a small infarct limited to precentral gyrus. Finger tapping activated only postcentral gyrus, a pattern not seen in any control subject. During tactile stimulation of a finger or hand in controls, postcentral gyrus activation exceeded or matched precentral gyrus activation in 11 of 14 cases. Patient 2 had a small infarct limited to postcentral gyrus and superior parietal lobule. Tactile stimulation of the finger activated only precentral gyrus, a pattern not seen in any control. In both patients, activation during pectoralis contraction was medial to the site activated during finger tapping. CONCLUSIONS: Results during finger tapping (patient 1) and finger stimulation (patient 2) may reflect amplification of a preserved component of normal sensorimotor function, a shift in the cortical site of finger representation, or both. Cortical map reorganization along the infarct rim may be an important contributor to recovery of motor and sensory function after stroke. Functional MRI is useful for assessing motor and sensory representational maps.

Adult↗

PubMed1999

Dynamics of neuronal processing in rat somatosensory cortex.

Recently, the study of sensory cortex has focused on the context-dependent evolution of receptive fields and cortical maps over millisecond to second time-scales. This article reviews advances in our understanding of these processes in the rat primary somatosensory cortex (SI). Subthreshold input to individual rat SI neurons is extensive, spanning several vibrissae from the center of the receptive field, and arrives within 25 ms of vibrissa deflection. These large subthreshold receptive fields provide a broad substrate for rapid excitatory and inhibitory multi-vibrissa interactions. The 'whisking' behavior, an approximately 8 Hz ellipsoid movement of the vibrissae, introduces a context-dependent change in the pattern of vibrissa movement during tactile exploration. Stimulation of vibrissae over this frequency range modulates the pattern of activity in thalamic and cortical neurons, and, at the level of the cortical map, focuses the extent of the vibrissa representation relative to lower frequency stimulation (1 Hz). These findings suggest that one function of whisking is to reset cortical organization to improve tactile discrimination. Recent discoveries in primary visual cortex (VI) demonstrate parallel non-linearities in center-surround interactions in rat SI and VI, and provide a model for the rapid integration of multi-vibrissa input. The studies discussed in this article suggest that, despite its original conception as a uniquely segregated cortex, rat SI has a wide array of dynamic interactions, and that the study of this region will provide insight into the general mechanisms of cortical dynamics engaged by sensory systems.

Animals↗

PubMed1999

High efficiency guiding of terawatt subpicosecond laser pulses in a capillary discharge plasma channel.

Transmission efficiencies in excess of 75% were obtained in the optical guiding of subpicosecond, terawatt laser pulses in a 2-cm-long capillary discharge plasma channel at the Naval Research Laboratory. The guided laser beam size at the exit of the channel was measured using far field imaging and Thomson scattering techniques. The guided laser intensity was >1 x 10(17) W/cm(2) at a guided beam diameter of 35 microm for a propagation length of 22 Rayleigh ranges. There is evidence that the plasma channel extends beyond the ends of the capillary and affects the far field beam structure of the transmitted laser pulse.

Journal Article↗

PubMed1999

Analysis of Gaussian beam and Bessel beam driven laser accelerators.

This paper presents a comparison of Gaussian and Bessel beam driven laser accelerators. The emphasis is on the vacuum beat wave accelerator (VBWA), employing two laser beams of differing wavelengths to impart a net acceleration to particles. Generation of Bessel beams by means of circular slits, holographic optical elements, and axicons is outlined and the image space fields are determined by making use of Huygens' principle. Bessel beams-like Gaussian beams-experience a Guoy phase shift in the vicinity of a focal region, resulting in a phase velocity that exceeds c, the speed of light in vacuo. In the VBWA, by appropriate choice of parameters, the Guoy phases of the laser beams cancel out and the beat wave phase velocity equals c. The particle energy gain and beam quality are determined by making use of an analytical model as well as simulations. The analytical model--including the v x B interaction--predicts that for equal laser powers Gaussian and Bessel beams lead to identical energy gains. However, three-dimensional, finite-emittance simulations, allowing for detuning, transverse displacements, and including all the electromagnetic field components, show that the energy gain of a Gaussian beam driven VBWA exceeds that of a Bessel beam driven VBWA by a factor of 2-3. The particle beam emerging from the interaction is azimuthally symmetric and collimated, with a relatively small angular divergence. A table summarizing the ratios of final energies, acceleration lengths, and gradients for a number of acceleration mechanisms is given.

Journal Article↗

PubMed1998

Temporal modulation of spatial borders in rat barrel cortex.

We examined the effects of varying vibrissa stimulation frequency on intrinsic signal and neuronal responses in rat barrel cortex. Optical imaging of intrinsic signals demonstrated that the region of cortex activated by deflection of a single vibrissa at 1 Hz is more diffuse and more widespread than the territory activated at 5 or 10 Hz. With the use of two different paradigms, constant time of stimulation and constant number of vibrissa deflections, we showed that the optically imaged spread of activity is more discrete at higher stimulation frequencies. We combined optical imaging with multiple electrode recording and confirmed that the neuronal response to individual vibrissa stimulation at the optically imaged center of activity is greater than the response away from the imaged center. Consistent with the imaging data, these recordings also showed no response to a second vibrissa deflection at 5 Hz at a peripheral recording site, though there was a significant response to a second vibrissa deflection at 1 Hz at the same peripheral site. These findings demonstrate that vibrissa stimulation at higher frequencies leads to more focused physiological responses in cortex. Thus the spread of activation in rat barrel cortex is modulated in a dynamic fashion by the frequency of vibrissa stimulation.

Animals↗

PubMed1998

Spatio-temporal subthreshold receptive fields in the vibrissa representation of rat primary somatosensory cortex.

Spatio-temporal subthreshold receptive fields in the vibrissa representation of rat primary somatosensory cortex. J. Neurophysiol. 80: 2882-2892, 1998. Whole cell recordings of synaptic responses evoked by deflection of individual vibrissa were obtained from neurons within adult rat primary somatosensory cortex. To define the spatial and temporal properties of subthreshold receptive fields, the spread, amplitude, latency to onset, rise time to half peak amplitude, and the balance of excitation and inhibition of subthreshold input were quantified. The convergence of information onto single neurons was found to be extensive: inputs were consistently evoked by vibrissa one- and two-away from the vibrissa that evoked the largest response (the "primary vibrissa"). Latency to onset, rise time, and the incidence and strength of inhibitory postsynaptic potentials (IPSPs) varied as a function of position within the receptive field and the strength of evoked excitatory input. Nonprimary vibrissae evoked smaller amplitude subthreshold responses [primary vibrissa, 9.1 +/- 0.84 (SE) mV, n = 14; 1-away, 5. 1 +/- 0.5 mV, n = 38; 2-away, 3.7 +/- 0.59 mV, n = 22; 3-away, 1.3 +/- 0.70 mV, n = 8] with longer latencies (primary vibrissa, 10.8 +/- 0.80 ms; 1-away, 15.0 +/- 1.2 ms; 2-away, 15.7 +/- 2.0 ms). Rise times were significantly faster for inputs that could evoke action potential responses (suprathreshold, 4.1 +/- 1.3 ms, n = 8; subthreshold, 12.4 +/- 1.5 ms, n = 61). In a subset of cells, sensory evoked IPSPs were examined by deflecting vibrissa during injection of hyperpolarizing and depolarizing current. The strongest IPSPs were evoked by the primary vibrissa (n = 5/5), but smaller IPSPs also were evoked by nonprimary vibrissae (n = 8/13). Inhibition peaked by 10-20 ms after the onset of the fastest excitatory input to the cortex. This pattern of inhibitory activity led to a functional reversal of the center of the receptive field and to suppression of later-arriving and slower-rising nonprimary inputs. Together, these data demonstrate that subthreshold receptive fields are on average large, and the spatio-temporal dynamics of these receptive fields vary as a function of position within the receptive field and strength of excitatory input. These findings constrain models of suprathreshold receptive field generation, multivibrissa interactions, and cortical plasticity.

Animals↗

PubMed1996

Ca2+/calmodulin-dependent protein kinase II phosphorylation of the presynaptic protein synapsin I is persistently increased during long-term potentiation.

Long-term potentiation (LTP) is an increase in synaptic responsiveness thought to be involved in mammalian learning and memory. The localization (presynaptic and/or postsynaptic) of changes underlying LTP has been difficult to resolve with current electrophysiological techniques. Using a biochemical approach, we have addressed this issue and attempted to identify specific molecular mechanisms that may underlie LTP. We utilized a novel multiple-electrode stimulator to produce LTP in a substantial portion of the synapses in a hippocampal CA1 minislice and tested the effects of such stimulation on the presynaptic protein synapsin I. LTP-inducing stimulation produced a long-lasting 6-fold increase in the phosphorylation of synapsin I at its Ca2+/calmodulin-dependent protein kinase II (CaM kinase II) sites without affecting synapsin I levels. This effect was fully blocked by either the N-methyl-D-aspartate receptor antagonist D(-)-2-amino-5-phosphonopentanoic acid (APV) or the CaM kinase II inhibitor KN-62. Our results indicate that LTP expression is accompanied by persistent changes in presynaptic phosphorylation, and specifically that presynaptic CaM kinase II activity and synapsin I phosphorylation may be involved in LTP expression.

Animals↗

PubMed1995

Transient expression of NADPH-diaphorase in the lateral geniculate nucleus of the ferret during early postnatal development.

Retinogeniculate projections in the ferret are refined during postnatal development so that inputs from the two eyes become segregated into eye-specific laminae, and each eye-specific lamina is further divided into sublaminae containing inputs from on-center or off-center afferents. Segregation into eye-specific laminae and on/off sublaminae is dependent on neuronal activity; sublamination depends on activation of N-methyl-d-aspartate (NMDA) receptors. By analogy with the suggested role of nitric oxide in NMDA-mediated long-term potentiation in the hippocampus, we investigated a possible role for nitric oxide in ferret retinogeniculate development. The expression of NADPH-diaphorase, a nitric oxide synthase, was examined histologically in the lateral geniculate nucleus of ferrets at several postnatal ages. At birth, neuropil is labeled in the nucleus, although no cell bodies are visible. After the first postnatal week, some labeled cells appear, predominantly in the C laminae. By three postnatal weeks, cell bodies are clearly labeled in all geniculate laminae. Staining reaches a peak in density at about four postnatal weeks, then declines such that by six postnatal weeks labeled cells are no longer visible. This transient expression of NADPH-diaphorase activity is consistent with a role for nitric oxide in the development of mature connections within the ferret lateral geniculate nucleus.

Animals↗

PubMed1993

Hippocampal plasticity induced by primed burst, but not long-term potentiation, stimulation is impaired in area CA1 of aged Fischer 344 rats.

The effect of two types of electrical stimulation designed to induce long-lasting plasticity of the Schaffer/commissural inputs to CA1 pyramidal neurons was investigated using in vitro hippocampal slices made from young (3-6 month) and old (24-27 month) Fischer 344 rats. The first stimulation paradigm, primed burst (PB) stimulation, consisted of a total of five physiologically patterned stimuli: a single priming pulse followed 170 ms later by a burst of four pulses at 200 Hz. The second stimulation paradigm, long-term potentiation (LTP) stimulation, consisted of a 200 Hz/1 second train (a total of 200 stimuli). Primed burst and LTP stimulation were equally effective at inducing a lasting increase in the population spike recorded from slices made from young rats. However, the enhancement of population spike amplitude produced by PB, but not LTP, stimulation was significantly less in slices made from old rats. These results suggest that the capacity of the hippocampus to demonstrate long-lasting synaptic plasticity is not altered with age, but that engaging plasticity-inducing mechanisms becomes more difficult. Furthermore, these data suggest that physiologically patterned paradigms for inducing long-lasting synaptic plasticity may more accurately assess the functional status of hippocampal memory encoding mechanisms than does conventional LTP stimulation.

Action Potentials↗