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At least 613 records · Page 34Linked to original sources
PubMed1998

Studies of neuroplasticity with transcranial magnetic stimulation.

In recent years, there has been increasing interest in studies of brain plasticity. Although still loosely defined, this term describes the ability of the brain to change. Cortical plasticity encompasses a wide variety of phenomena and mechanisms, including modifications in cortical properties such as strength of internal connections, representational patterns, or neuronal modifications, either morphological or functional (Donoghue et al., 1996). We focus on the description of different ways in which transcranial magnetic stimulation (TMS) can be used to study patterns of reorganization and some of the mechanisms involved in these changes. Correlation between TMS and neuroimaging studies in humans and animal studies addressing similar questions is discussed. It is important to identify in each situation whether plasticity plays a beneficial role or is maladaptive in terms of functional compensation. The understanding of patterns, mechanisms, and functional relevance of cortical plasticity will hopefully lead to the design of effective strategies to enhance plasticity when it is beneficial and to down-regulate it when it is maladaptive. An example of a possible strategy, using TMS, is discussed.

Blindness↗
PubMed1998

Transcranial magnetic stimulation: language function.

Studies of language using transcranial magnetic stimulation (TMS) have focused both on identification of language areas and on elucidation of function. TMS may result in either inhibition or facilitation of language processes and may operate directly at a presumptive site of language cortex or indirectly through intracortical networks. TMS has been used to create reversible "temporary lesions," similar to those produced by Wada tests and direct cortical electrical stimulation, in cerebral cortical areas subserving language function. Rapid-rate TMS over the left inferior frontal region blocks speech output in most subjects. However, the results are not those predicted from classic models of language organization. Speech arrest is obtained most easily over facial motor cortex, and true aphasia is rare, whereas right hemisphere or bilateral lateralization is unexpectedly prominent. A clinical role for these techniques is not yet fully established. Interfering with language comprehension and verbal memory is currently more difficult than blocking speech output, but numerous TMS studies have demonstrated facilitation of language-related tasks, including oral word association, story recall, digit span, and picture naming. Conversely, speech output also facilitates motor responses to TMS in the dominant hemisphere. Such new and often-unexpected findings may provide important insights into the organization of language.

Cerebral Cortex↗
PubMed1998

Study and modulation of human cortical excitability with transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) can be applied in different paradigms to obtain a measure of various aspects of cortical excitability. These different TMS paradigms provide information about different neurotransmitter systems, enhance our understanding about the pathophysiology of neuropsychiatric conditions, and in the future may be helpful as a guide for pharmacological interventions. In addition, repetitive TMS (rTMS) modulates cortical excitability beyond the duration of the rTMS trains themselves. Depending on rTMS parameters, a lasting inhibition or facilitation of cortical excitability can be induced. These effects can be demonstrated neurophysiologically or by combining rTMS with neuroimaging techniques. The effects do not remain limited to the cortical area directly targeted by rTMS, but affect a wider neural network transynaptically. Modulation of cortical excitability by rTMS may in the future be useful not only as a research tool but also as a therapeutic intervention in neurology, psychiatry, and neurorehabilitation.

Cerebral Cortex↗
PubMed1980

Clinical use of vibratory stimuli to evaluate peripheral nerve injury and compression neuropathy.

The classical approach to evaluating the hand with impaired sensibility is unsatisfactory and frequently incorrect. Since the perception of vibratory stimuli is mediated through the same neural pathways as moving touch, we investigated the clinical use of the tuning fork in 101 patients with peripheral nerve injury and compression neuropathy. In potential nerve lacerations, the preoperative tuning fork evaluations accurately predicted the intraoperative findings: diminished perception of vibratory stimuli correlated with a neural conduction block, due usually to complete nerve division. Furthermore, this study suggests that diminished vibratory perception may be the earliest indication for surgical intervention in the acute compartment syndrome. We concluded that the clinical use of vibratory stimuli to evaluate peripheral nerve injury and compression neuropathy is valid, reliable, expeditious, and a test with high patient acceptability.

Adolescent↗
PubMed1997

Viscoelastic relaxation and regional blood flow response to spinal cord compression and decompression.

STUDY DESIGN: To better understand the relationships between primary mechanical factors of spinal cord trauma and secondary mechanisms of injury, this study evaluated regional blood flow and somatosensory evoked potential function in an in vivo canine model with controlled velocity spinal cord displacement and real-time piston-spinal cord interface pressure feedback. OBJECTIVES: To determine the effect of regional spinal cord blood flow and viscoelastic cord relaxation on recovery of neural conduction, with and without spinal cord decompression. SUMMARY OF BACKGROUND DATA: The relative contribution of mechanical and vascular factors on spinal cord injury remains undefined. METHODS: Twelve beagles were anesthetized and underwent T13 laminectomy. A constant velocity spinal cord compression was applied using a hydraulic loading piston with a subminiature pressure transducer rigidly attached to the spinal column. Spinal cord displacement was stopped when somatosensory evoked potential amplitudes decreased by 50% (maximum compression). Six animals were decompressed 5 minutes after maximum compression and were compared with six animals who had spinal cord displacement maintained for 3 hours and were not decompressed. Regional spinal cord blood flow was measured with a fluorescent microsphere technique. RESULTS: At maximum compression, regional spinal cord blood flow at the injury site fell from 19.0 +/- 1.3 mL/100 g/min to 12.6 +/- 1.0 mL/100 g/min, whereas piston-spinal cord interface pressure was 30.5 +/- 1.8 kPa, and cord displacement measured 2.1 +/- 0.1 mm (mean +/- SE). Five minutes after the piston translation was stopped, the spinal cord interface pressure had dissipated 51%, whereas the somatosensory evoked potential amplitudes continued to decrease to 16% of baseline. In the sustained compression group, cord interface pressure relaxed to 13% of maximum within 90 minutes; however, no recovery of somatosensory evoked potential function occurred, and regional spinal cord blood flow remained significantly lower than baseline at 30 and 180 minutes after maximum compression. In the six animals that underwent spinal cord decompression, somatosensory evoked potential function and regional spinal cord blood flow recovered to baseline 30 minutes after maximum compression. CONCLUSIONS: Despite rapid cord relaxation of more than 50% within 5 minutes after maximum compression, somatosensory evoked potential conduction recovered only with early decompression. Spinal cord decompression was associated with an early recovery of regional spinal cord blood flow and somatosensory evoked potential recovery. By 3 hours, spinal cord blood flow was similar in both the compressed and decompressed groups, despite that somatosensory evoked potential recovery occurred only in the decompressed group.

Animals↗
PubMed2005

Nitrous oxide with propofol reduces somatosensory-evoked potential amplitude in children and adolescents.

STUDY DESIGN: A repeat measures design. OBJECTIVES: Nitrous oxide significantly reduces cortical somatosensory-evoked potential amplitude in adults; however, its effect on children and adolescents is unknown. This study evaluates the effect of nitrous oxide combined with propofol on the amplitude of the cortical response in children and adolescents. SUMMARY OF BACKGROUND DATA: Somatosensory-evoked potential amplitude measurements are used to determine spinal cord function during spine surgery. A significant decrease in amplitude of the cortical response may indicate either a compromise of neural conduction in the spinothalamic tracts of the dorsal column medial lemniscus system or the effect of anesthesia, an innocuous event. METHODS: Ten consecutive participants were evaluated. The study group comprised 4 boys and 6 girls, ages 10 to 18 years, undergoing corrective spinal surgery at an orthopedic children's hospital in northwestern Pennsylvania. The tibial nerve was stimulated and responses recorded from the somatosensory cortex when anesthesia was administered with and without nitrous oxide. Results were compared using a paired t test (alpha = 0.05) RESULTS: A significant decrease of 42 +/- 17% (P < 0.05) occurred in the cortically recorded somatosensory-evoked potential amplitude from 1.63 +/- 0.59 uV without nitrous oxide to 0.92 +/- 0.47 uV with nitrous oxide. Responses were similar in right and left extremities. CONCLUSIONS: In our study, nitrous oxide use during low-dose isoflurane anesthesia supplemented with titrated propofol infusion caused a significant reduction in cortically recorded somatosensory-evoked potentials that was comparable with the 50% decrease observed in adults.

Adjuvants, Anesthesia↗
PubMed1984

The pattern of monosynaptic Ia-connections to hindlimb motor nuclei in the baboon: a comparison with the cat.

The pattern of Ia-connections to motor nuclei of 17 hindlimb muscles (or groups of muscles) has been investigated in baboons by intracellular recording of Ia-e.ps.p.s evoked in motoneurons from different muscle nerves. The amplitudes are normalized to 70 mV resting potential and compared with similarly normalized Ia-e.ps.p.s in cats. As in the cat, Ia-excitation is drawn from a restricted number of muscles and the homonymous effect is usually dominating. Heteronymous connections to many motor nuclei are different in the two species. For example, hip extensors are generally more Ia-isolated from each other in baboons than in cats, and also knee flexors have fewer Ia-interconnections than in cats. A unidirectional Ia-synergism between some hip extensors and knee flexors in cats has changed to a bidirectional one in baboons, with a tendency to lateralization of the connections. Among ankle extensors, soleus has smaller heteronymous Ia-connections from its synergic ankle extensors than in cats. In baboons, plantaris is heteronymously Ia-excited from gastrocnemius-soleus but not from the intrinsic plantar muscles; whereas in cats there exists a considerable Ia-projection from the intrinsic plantar muscles but not from gastrocnemius-soleus. There is a corresponding difference in the insertion of the plantaris tendon, which shows that this muscle acts as toe extensor in cats but as ankle extensor in baboons. For most of the motor nuclei, the homonymous as well as the total aggregate of Ia-e.ps.p.s is smaller in the baboon than in the cat; but the amplitude range between different motor nuclei is larger in the baboon. Reciprocal Ia-i.ps.p.s are evoked only after spinal transection or when brain function is depressed. It is postulated that baboons, contrary to cats, have descending tonic inhibition of transmission in the reciprocal Ia-inhibitory pathway. The phylogenetic flexibility of Ia-connections is discussed and contrasted with their ontogenetic stability.

Action Potentials↗
PubMed2004

Olfactory fear conditioning induces field potential potentiation in rat olfactory cortex and amygdala.

The widely used Pavlovian fear-conditioning paradigms used for studying the neurobiology of learning and memory have mainly used auditory cues as conditioned stimuli (CS). The present work assessed the neural network involved in olfactory fear conditioning, using olfactory bulb stimulation-induced field potential signal (EFP) as a marker of plasticity in the olfactory pathway. Training consisted of a single training session including six pairings of an odor CS with a mild foot-shock unconditioned stimulus (US). Twenty-four hours later, the animals were tested for retention of the CS as assessed by the amount of freezing exhibited in the presence of the learned odor. Behavioral data showed that trained animals exhibited a significantly higher level of freezing in response to the CS than control animals. In the same animals, EFPs were recorded in parallel in the anterior piriform cortex (aPC), posterior piriform cortex (pPC), cortical nucleus of the amygdala (CoA), and basolateral nucleus of the amygdala (BLA) following electrical stimulation of the olfactory bulb. Specifically, EFPs recorded before (baseline) and after (during the retention test) training revealed that trained animals exhibited a lasting increase (present before and during presentation of the CS) in EFP amplitude in CoA, which is the first amygdaloid target of olfactory information. In addition, a transient increase was observed in pPC and BLA during presentation of the CS. These data indicate that the olfactory and auditory fear-conditioning neural networks have both similarities and differences, and suggest that the fear-related behaviors in each sensory system may have at least some distinct characteristics.

Amygdala↗
PubMed2004

Finite-size dynamics of inhibitory and excitatory interacting spiking neurons.

The dynamic mean-field approach we recently developed is extended to study the dynamics of population emission rates nu (t) for a finite network of coupled excitatory (E) and inhibitory (I) integrate-and-fire (IF) neurons. The power spectrum of nu (t) in an asynchronous state is computed and compared to simulations. We calculate the interpopulations transfer functions and show how synaptic interaction modulates the otherwise low-pass filter with resonances which go well beyond the filter's cut (omega approximately nu) , allowing efficient information transmission on very short time scales determined by spike transmission delays. The saddle-node instability of the asynchronous state is studied and a simple exact dependence of the stability condition on the current-to-rate gain functions is derived, by which self-couplings (EE and II) decrease stability while mutual interaction (EI and IE) favor stability.

Action Potentials↗
PubMed2003

Formation of electrically active clusterized neural networks.

Ordinarily, in vitro neurons self-organize into homogeneous networks of single neurons linked by dendrites and axons. We show that under special conditions they can also self-organize into neuronal clusters, which are linked by bundles of axons. Multielectrode array measurement reveals that the clusterized networks are also electrically active and exhibit synchronized bursting events similar to those observed in the homogeneous networks. From time-lapse recording, we deduced the features required for the neuronal clusterized versus homogeneous self-organization and developed a simple model for testing their validity.

Animals↗
PubMed1997

Parameter estimation by reduced-order linear associative memory (ROLAM).

In a series of papers we have shown that nonlinear parameter estimation by linear association provides accurate estimates of the parameters in complex systems described by nonlinear differential equations even in the presence of additive white noise of considerable power. The technique is based on linearly associating the system's output with a set of parameter values spanning the region of interest. When an actual output is measured, the system's unknown parameters could be estimated by a matrix inversion. The size of the inverted matrix, being equal to the length of the output vector, poses a limiting factor upon the generalization of the technique. In this paper we propose a modification which requires the inversion of a matrix whose dimension equals the number of model parameters. The modified version is called reduced-order associative memory (ROLAM). The technique is applied to two complex lumped-parameter nonlinear models: the Van der Pol relaxation oscillator and the passive neuron model of the granule cells. Results validate ROLAM as a parameter-estimation tool which is especially suited in cases where the number of parameters is large, the number of samples in the observation signal is high, or when on-line parameter estimation is required. It is also shown that ROLAM provides an optimal parameter estimate in the special case of single-parameter nonlinear models.

Algorithms↗
PubMed1999

Endoneural selective stimulating using wire-microelectrode arrays.

In acute experiments eight 5- to 24-wire-microelectrode arrays were inserted into the common peroneal nerve of the rat, to investigate whether the electrodes could selectively stimulate motor units of the extensor digitorum longus (EDL) muscle. Twitch-force-recruitment curves were measured from the EDL for each array electrode. The curves were plotted on a double-logarithmic scale and parameterized by the low-force slope (which represents the power p in the power-law relationship of force F versus stimulus current I, or F approximately I(p)) and the threshold current. The slopes and threshold currents measured with array electrodes did not differ significantly from those obtained with randomly inserted single wire-microelectrodes. This indicates that, although involving a more invasive insertion procedure, electrode arrays provide neural contacts with low-force recruitment properties similar to those of single wires. Array results revealed partial blocking of neural conduction, similar to that reported with microneurographic insertion with single needles. The efficiency of the array was defined as the fraction of array electrodes selectively contacting a motor unit and evoking the corresponding threshold force. Efficiency thus expresses the practical value of the used electrode array in terms of the total number of distinct threshold forces that can be stimulated by selecting the appropriate electrodes. The eight arrays were capable of evoking threshold forces selectively with an average efficiency of 0.81 (or 81%).

Animals↗
PubMed2004

Stimulus-induced change in long-range temporal correlations and scaling behaviour of sensorimotor oscillations.

The human brain spontaneously generates large-scale network oscillations at around 10 and 20 Hz. The amplitude envelope of these oscillations fluctuates intermittently and was recently reported to exhibit power-law decay of the autocorrelation for hundreds of seconds. This indicates that the underlying networks are in a dynamic state resembling the self-organized critical state known to exist in many complex systems. Based on the mechanism of how correlations emerge in these systems, we hypothesized that the physiological basis of long-range power-law correlations is the buildup of a memory of past activity by a continuous modification of the network's functional connectivity by the ongoing oscillations. In this framework, exogenous perturbations of ongoing oscillations would degrade or abolish this dynamic network memory. We investigated the sensitivity of the temporal correlations in sensorimotor 10- and 20-Hz oscillations to median nerve stimulation that is known to have immediate effects on ongoing oscillations. Our results show that the amplitude fluctuations of these oscillations were effectively modulated by the somatosensory stimuli but still exhibited long-range temporal correlations and power-law scaling behaviour. The magnitude of the temporal correlations was, however, attenuated and the power-law exponents were decreased. This implies that the stimuli indeed degraded the network's memory of its past.

Adult↗