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Biomedical subjects

H R Siebner

Publications and source records attributed to H R Siebner.

At least 19 recordsLinked to original sources
PubMed2004

Inhibitory interactions between pairs of subthreshold conditioning stimuli in the human motor cortex.

OBJECTIVE: These experiments examined short interval paired-pulse paradigms for intracortical inhibition (ICI) and facilitation (ICF). We tested whether pairs of subthreshold conditioning stimuli interact, and whether they showed rapid periodicity similar to that observed in subthreshold I-wave interaction. METHODS: Transcranial magnetic stimulation (TMS) was given over left M1 to evoke a motor-evoked potential (MEP) of approximately 1 mV peak-to-peak amplitude in the contralateral first dorsal interosseous (FDI) muscle. Each test shock (TS) was preceded by single or paired subthreshold conditioning stimuli (CS(1) and CS(2)) at short interstimulus intervals (ISIs 1-15 ms). Intensities of CS were set just below thresholds for intracortical inhibition (ICI) or intracortical facilitation (ICF). RESULTS: Each CS(single) alone had no effect on the test MEP, but with two CS, clear inhibition was elicited at certain intervals. With a CS(2)-TS interval of 2 ms, maximum suppression occurred if CS(1) was applied 1-2.5 ms before CS(2). This inhibitory effect tapered off gradually as the CS(2)-CS(1) interval was increased up to 13 ms. When facilitation was present with a CS(single)-TS interval of 10 ms, a small but non-significant extra-facilitation occurred at ISIs between CS(2) and CS(1) of 6-15 ms. CONCLUSIONS: Two subthreshold conditioning stimuli facilitate inhibition that lacks the rapid periodicity typical of I-wave interaction. The data would be compatible with a model in which synaptic inputs converge on a common inhibitory interneurone.

Adult↗
PubMed2002

Parietal magnetic stimulation delays visuomotor mental rotation at increased processing demands.

Visuomotor rotation (VMR) is a variant of the classic mental rotation paradigm. Subjects perform a center-out arm reaching movement, with the instruction to point clockwise or anticlockwise away from the direction of a reaction signal by a prespecified amount. Like classic mental rotation (MR) tasks, there is a linear relationship between reaction time (RT) and required angle of rotation (angular disparity). Although functional imaging studies have consistently demonstrated parietal activations centered around the intraparietal sulcus during MR tasks, the involvement of parietal cortex in VMR has not been investigated. The aim of the present experiments was to test in human subjects whether VMR also involves activity in parietal areas. We used short trains of transcranial magnetic stimulation (TMS) to produce a temporary "virtual lesion" of the posterior parietal cortex (PPC) around the intraparietal sulcus during the reaction period of a VMR task. Four pulses of 20-Hz rTMS were applied to the left PPC, right PPC, or vertex (control condition) 100 ms after the presentation of an instruction cue. Reaction times (RTs) were evenly prolonged by right or left parietal TMS compared with vertex stimulation, but only for large angles of rotation, and without affecting the spatial accuracy of the final response. A control experiment showed that parietal rTMS did not impair visual perception or the ability to judge the size of visual angles. The data thus provide evidence for bilateral involvement of the PPC in VMR that increases with processing demands.

Adult↗
PubMed2001

Functional MRI of cortical activations induced by transcranial magnetic stimulation (TMS).

The effects of repetitive transcranial magnetic stimulation (rTMS) on human brain activity and associated hemodynamics were investigated by blood-oxygenation-level-dependent (BOLD) MRI using echo-planar imaging at 2.0 T. Apart from bilateral activation of the auditory cortex by the audible rTMS discharges (23 bursts, 1 s duration, 10 Hz, 10-20 s interstimulus intervals), BOLD responses were restricted to cortical representations of actual finger movements performed either voluntarily or evoked by suprathreshold rTMS of the motor cortex. Neither subthreshold rTMS of the motor cortex nor suprathreshold rTMS of the lateral premotor cortex induced a detectable BOLD response. These findings suggest that neuronal depolarization as induced by rTMS modulates the spiking output of a brain area but does not automatically alter cerebral blood flow and oxygenation. The observation of BOLD MRI activations probably reflects the afferent intracortical processing of real movements.

Adult↗
PubMed2001

Age-related decrease in paired-pulse intracortical inhibition in the human primary motor cortex.

Using biphasic magnetic stimuli, paired-pulse transcranial magnetic stimulation (TMS) at short interstimulus intervals (ISIs) was employed to investigate age-related changes in the balance between intracortical inhibition and facilitation. In 26 right-handed healthy individuals, motor evoked potentials were recorded from the relaxed right first dorsal interosseus muscle after paired-pulse TMS of the left primary motor hand area. The magnitude of intracortical paired-pulse inhibition at ISIs of 1-5 ms was markedly reduced in elderly individuals, whereas no age effect was observed for intracortical paired-pulse facilitation at ISIs of 11-15 ms. This finding demonstrates that normal aging is associated with a relative decrease in the excitability of intracortical inhibitory circuits. In conclusion, paired-pulse TMS provides a non-invasive means of studying age-related functional changes in the motor cortex.

Adult↗
PubMed2001

Decreased corticospinal excitability after subthreshold 1 Hz rTMS over lateral premotor cortex.

OBJECTIVE: To study whether trains of subthreshold 1 Hz repetitive transcranial magnetic stimulation (rTMS) over premotor, prefrontal, or parietal cortex can produce changes in excitability of motor cortex that outlast the application of the train. BACKGROUND: Prolonged 1 Hz rTMS over the motor cortex can suppress the amplitude of motor-evoked potentials (MEP) for several minutes after the end of the train. Because TMS can produce effects not only at the site of stimulation but also at distant sites to which it projects, the authors asked whether prolonged stimulation of sites distant but connected to motor cortex can also lead to lasting changes in MEP. METHODS: Eight subjects received 1500 magnetic stimuli given at 1 Hz over the left lateral frontal cortex, the left lateral premotor cortex, the hand area of the left motor cortex, and the left anterior parietal cortex on four separate days. Stimulus intensity was set at 90% active motor threshold. Corticospinal excitability was probed by measuring the amplitude of MEP evoked in the right first dorsal interosseous muscle by single suprathreshold stimuli over the left motor hand area before, during, and after the conditioning trains. RESULTS: rTMS over the left premotor cortex suppressed the amplitude of MEP in the right first dorsal interosseous muscle. The effect was maximized (approximately 50% suppression) after 900 pulses and outlasted the full train of 1500 stimuli for at least 15 minutes. Conditioning rTMS over the other sites did not modify the size of MEP. A control experiment showed that left premotor cortex conditioning had no effect on MEP evoked in the left first dorsal interosseous muscle. CONCLUSIONS: Subthreshold 1 Hz rTMS of the left premotor cortex induces a short-lasting inhibition of corticospinal excitability in the hand area of the ipsilateral motor cortex. This may provide a model for studying the functional interaction between premotor and motor cortex in healthy subjects and patients with movement disorders.

Adult↗
PubMed2001

Continuous transcranial magnetic stimulation during positron emission tomography: a suitable tool for imaging regional excitability of the human cortex.

In six healthy volunteers, H(2)(15)O positron emission tomography (PET) was employed to evaluate rate-dependent functional activation of the left primary sensorimotor hand area (SM1(HAND)) during subthreshold repetitive transcranial magnetic stimulation (rTMS). Using an eight-shaped coil, continuous trains of rTMS were delivered during nine 50-s H(2)(15)O PET scans. Nine different stimulation frequencies were used, ranging from 1 to 5 Hz. Stimulus intensity was set at 10% below active motor threshold. During three additional PET scans, an ineffective rTMS was applied via another eight-shaped coil, which was held 10 cm above the vertex. Statistical parametric mapping was employed to assess relative differences in normalized regional cerebral blood flow (rCBF) across conditions. Compared with ineffective rTMS, subthreshold rTMS increased normalized rCBF in the stimulated SM1(HAND). Moreover, the increase in rCBF in the left SM1(HAND) showed a linear positive relationship with the rate of rTMS, indicating a rate-dependent functional activation of the stimulated SM1(HAND). These data demonstrate that, by varying the variables of rTMS across scans, continuous rTMS during H(2)(15)O PET provides a noninvasive tool to study the regional excitability profile of a distinct cortical area.

Adult↗
PubMed2001

[New insights into brain function by combination of transcranial magnetic stimulation and functional brain mapping].

The present paper aims to summarize potential applications of transcranial magnetic stimulation (TMS) combined with functional brain imaging. Transcranial magnetic stimulation is a well-established noninvasive tool for stimulating circumscribed areas of the human cortex. Functional imaging techniques such as positron emission tomography, functional magnetic resonance imaging, and electroencephalographic mapping enable assessment of TMS-related functional brain activation. A combination of TMS and functional imaging can be useful in three principal ways. (1) Brain imaging before TMS is helpful in defining the accurate coil position over a distinct cortical area which is targeted by TMS. Since TMS can be used to interfere with regional cortical function during a given task, the effects of focal TMS on task performance can help to clarify the task-specific functional contribution of a given cortical area which has previously shown task-related activation in a functional imaging study. (2) Imaging the brain during TMS is a promising approach for assessing cortical excitability and intracerebral functional connectivity. (3) By evaluating lasting effects of TMS, brain imaging after TMS can be employed to study the plasticity of the human cortex. Moreover, this approach will help to advance our understanding of therapeutical effects related to TMS.

Brain↗
PubMed2001

Brain correlates of fast and slow handwriting in humans: a PET-performance correlation analysis.

The present study examined the cerebral control of velocity during handwriting. We employed H215O positron emission tomography (PET) to measure the regional cerebral blood flow (rCBF) in 10 healthy subjects. Participants were required to write the German verb 'bellen' ('to bark') either at their normal speed (i.e. fast open-loop handwriting) or to write at approximately half of their normal speed without visual feedback. The second task required a continuous modification of the motor output according to the kinaesthetic feedback from the hand (i.e. slow closed-loop handwriting). Pencil movements were recorded during PET scanning and analysed off-line using a stroke-based analysing program. The mean number of inversions in velocity (NIV) per stroke was used to quantify the mode of motor control during each PET scan. A NIV of 1 indicates fast open-loop processing, whereas an increase in NIV reflects a shift towards slow closed-loop processing of handwriting. Foci in the left primary sensorimotor cortex, the right lateral premotor cortex, the left anterior parietal cortex, the left anterior putamen, the left rostral supplementary motor area and the right precuneus showed a graded increase in functional activation with the mean NIV per stroke, suggesting that this set of brain regions is particularly involved in the processing of slow closed-loop writing movements. No area showed a negative relationship between rCBF and the mean NIV per stroke, suggesting that fast open-loop handwriting is achieved by an optimized cooperation of the manual sensorimotor network rather than by a selective activation of a distinct network component.

Adult↗
PubMed2000

Subthreshold 5-Hz repetitive transcranial magnetic stimulation of the human primary motor cortex reduces intracortical paired-pulse inhibition.

Paired-pulse transcranial magnetic stimulation (TMS) at short interstimulus intervals was employed to investigate short-term effects of 5-Hz repetitive TMS (rTMS) over the primary motor hand area (M1(HAND)) on intracortical excitability. In ten healthy individuals, 1250 pulses of 5-Hz rTMS were applied at 90% of motor resting threshold over the left M1(HAND). Ten minutes after 5-Hz rTMS, paired-pulse inhibition was significantly reduced, whereas paired-pulse facilitation was not modified. Sham-rTMS had no lasting effect on intracortical excitability. These findings suggest that subthreshold 5-Hz rTMS causes a short-term modulation of the excitability of intracortical circuitry in the stimulated M1(HAND). The lasting effect of subthreshold 5-Hz rTMS on intracortical inhibition provides a useful probe for studying short-term plasticity of the human M1(HAND).

Adult↗
PubMed2000

Short-term motor improvement after sub-threshold 5-Hz repetitive transcranial magnetic stimulation of the primary motor hand area in Parkinson's disease.

Ten unmedicated patients with Parkinson's disease received sub-threshold 5-Hz repetitive transcranial magnetic stimulation (rTMS) over the primary motor hand area (M1(HAND)) contralaterally to the more affected upper limb. Compared to a midfrontal sham-rTMS, real-rTMS over M1(HAND) was associated with a significant decrease in the global motor UPDRS score 1 h after the cessation of rTMS. This was mainly due to a clinical improvement in the arm contralaterally to the stimulated M1(HAND). These preliminary data suggest that focal rTMS of M1(HAND) may have a therapeutic potential in PD.

Electromagnetic Fields↗
PubMed2000

Repetitive transcranial magnetic stimulation causes a short-term increase in the duration of the cortical silent period in patients with Parkinson's disease.

In ten patients with Parkinson's disease (PD) and ten age-matched healthy controls, we applied 15 30-s trains of subthreshold 5-Hz repetitive transcranial magnetic stimulation (rTMS) over the primary motor hand area. Ten minutes after rTMS, PD patients showed a significant prolongation of the transcranially evoked silent period (SP) in the contralateral first dorsal interosseus muscle, whereas the SP remained unchanged in healthy subjects. Since the duration of the transcranially evoked SP is a well-established measure of intracortical inhibition, this finding demonstrates that rTMS is capable of inducing a short-term increase in intracortical inhibition in PD. The lack of a prolongation of the SP in healthy controls suggests that PD patients may be particularly susceptible to modulatory effects of rTMS on motocortical inhibition.

Adult↗
PubMed2000

Lasting cortical activation after repetitive TMS of the motor cortex: a glucose metabolic study.

OBJECTIVE: Cerebral [18F]fluorodeoxy-D-glucose PET ([18F]FDG-PET) was used to visualize the lasting neuronal activation after repetitive transcranial magnetic stimulation (rTMS) over the left hand area of the primary motor cortex (M1HAND). BACKGROUND: Applied over M1HAND, rTMS has been shown to produce a modulation of corticomotor excitability beyond the time of stimulation itself. METHODS: Eight right-handed subjects underwent nonquantitative [18F]FDG-PET measurements during two experimental conditions: at rest and after focal subthreshold 5-Hz rTMS over the left M1HAND. In the post-rTMS condition, [18F]FDG was injected immediately after the administration of 1,800 magnetic pulses over the left M1HAND. Relative differences in normalized regional cerebral metabolic rate of glucose (normalized rCMRglc) between conditions were determined using a voxel-by-voxel Student's t-test and volume-of-interest (VOI) analysis. Analysis was a priori restricted to the M1HAND, the supplementary motor area (SMA), and the primary auditory cortex of both hemispheres. RESULTS: A 5-Hz rTMS of the left M1HAND caused a lasting relative increase in normalized rCMRglc within the M1HAND bilaterally and the SMA. The magnitude and the topographic pattern of persisting relative rCMRglc increases within these motor cortical areas demonstrated considerable interindividual variations. CONCLUSIONS: Subthreshold 5-Hz repetitive transcranial magnetic stimulation (rTMS) over the hand area of the primary motor cortex is associated with a persisting neuronal activation in a distinct set of motor cortical areas beyond the time of stimulation. The current findings demonstrate that [18F]FDG-PET can localize and quantify regional net changes in synaptic cortical activity after rTMS and thus might elucidate the mechanisms underlying rTMS-associated therapeutic effects.

Adult↗
PubMed1999

Imaging functional activation of the auditory cortex during focal repetitive transcranial magnetic stimulation of the primary motor cortex in normal subjects.

Positron emission tomography (PET) during focal repetitive transcranial magnetic stimulation (rTMS) has emerged as a promising approach to study cortical connectivity in awake humans. However, the noise caused by the discharging magnetic coil might have confounding effects on the rTMS-related cortical activation pattern. In twelve healthy volunteers, 18-fluoro-2-deoxy-D-glucose (18FDG) PET was employed to visualize the functional activation of the primary auditory cortex (PAC) during 2 Hz rTMS of the left primary sensorimotor hand area. Magnetic stimuli (1800) were applied at an intensity of 140% of motor resting threshold during the uptake period of 18FDG. Though all subjects wore earplugs, rTMS-related noise induced a consistent bilateral increase of regional glucose utilization in the PAC (P < 0.05, corrected). Thus, rTMS-related acoustic input needs to be taken into account in combined rTMS/PET studies.

Adult↗
PubMed1999

Abnormal increase in the corticomotor output to the affected hand during repetitive transcranial magnetic stimulation of the primary motor cortex in patients with writer's cramp.

In fourteen right-handed patients with writer's cramp and 10 right-handed controls, 10 trains of suprathreshold 1 Hz repetitive transcranial magnetic stimulation (rTMS) were applied over the left primary motor hand area. Each rTMS train lasted for a minute with an intertrain interval of 10 s. The motor evoked potentials (MEPs) were recorded from the relaxed contralateral first dorsal interosseus muscle. The mean MEP area of each rTMS train was taken as a measure of the corticospinal motor output to the affected hand. While the mean MEP area decreased significantly in controls, patients showed a significant increase in mean MEP area during rTMS. The differential effect on MEP size was highly significant. The abnormal rTMS-associated facilitation of the corticomotor output to the affected hand demonstrates an increased excitability of the corticospinal motor system in writer's cramp.

Adult↗
PubMed1999

Repetitive transcranial magnetic stimulation has a beneficial effect on bradykinesia in Parkinson's disease.

The effect of focal 5 Hz repetitive transcranial magnetic stimulation (rTMS) of the motor hand area (M1) on bradykinesia was studied in 12 unmedicated patients with Parkinson's disease (PD). On 2 separate days, a real-rTMS was applied to M1 contralaterally to the more severely affected limb or a frontal sham-rTMS was applied 3 cm anteriorly to Fz in a random order. Stimulus intensity was 10% below resting motor threshold. Before and 20 min after rTMS, patients performed 15 consecutive pointing movements with the index finger contralaterally to the stimulated M1. Compared with sham-rTMS, real-rTMS over the contralateral M1 caused a significant decrease in total movement time without affecting end-point accuracy. Our data provide evidence that 5 Hz rTMS over M1 can improve bradykinesia in PD patients beyond the time of magnetic stimulation.

Adult↗
PubMed1999

Changes in handwriting resulting from bilateral high-frequency stimulation of the subthalamic nucleus in Parkinson's disease.

High-frequency stimulation of the subthalamic nucleus (STN) is a promising therapeutic approach in patients with severely disabling Parkinson's disease (PD). Whereas STN stimulation improves the cardinal signs of PD, little is known about the effects of STN stimulation on fine manual skills like handwriting. Therefore, the present study investigated the changes in handwriting during bilateral STN stimulation in 12 patients with advanced PD. Dopaminergic medication was discontinued at least 12 hours before the study. The patients were asked to write a standardized sentence repetitively. Five samples of the patient's script were recorded during effective bilateral STN stimulation and 1 hour after both stimulators had been switched off. The movements of the tip of the pencil were recorded using a digitizing tablet. Handwriting movements were segmented into subsequent up- and down-strokes, and a stroke-based kinematic analysis of handwriting was performed. During high-frequency STN stimulation, handwriting movements became faster and smoother indicating a partial restoration of an "open-loop" automatic performance. In addition, STN stimulation gave rise to a significant increase in the mean vertical stroke length demonstrating a stimulation-related reduction in micrographia. The present data underscores the importance of the STN in "open-loop" performance of highly skilled sequential hand movements.

Aged↗