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Biomedical subjects

L G Cohen

Publications and source records attributed to L G Cohen.

At least 91 records · Page 5Linked to original sources
PubMed1995

Role of intracortical mechanisms in the late part of the silent period to transcranial stimulation of the human motor cortex.

Transcranial magnetic stimulation (TMS) and transcranial electrical stimulation (TES) of the human motor cortex produce a silent period (SP) following motor evoked potentials (MEPs). The early part of the SP can be explained by decreased alpha motor neuron excitability, whereas the late part is presumably due to suprasegmental mechanisms. In order to determine the level of the suprasegmental contribution of the generation of SPs, we recorded excitatory and inhibitory responses to TMS, TES and percutaneous electrical brainstem stimulation (PBS) in the voluntarily activated first dorsal interosseous muscle of the hand. Stimulus intensities were set so that PBS and TES induced MEPs with areas equal to or larger than those of MEPs obtained with TMS. This procedure revealed that SPs were 49% and 83% shorter with TES and PBS, respectively, than with TMS. As TMS is more effective than TES or PBS in activating cortical interneurons, these findings support the idea that a significant component of the SP arises from intracortical mechanisms.

Adult↗
PubMed1995

Modulation of muscle responses evoked by transcranial magnetic stimulation during the acquisition of new fine motor skills.

1. We used transcranial magnetic stimulation (TMS) to study the role of plastic changes of the human motor system in the acquisition of new fine motor skills. We mapped the cortical motor areas targeting the contralateral long finger flexor and extensor muscles in subjects learning a one-handed, five-finger exercise on the piano. In a second experiment, we studied the different effects of mental and physical practice of the same five-finger exercise on the modulation of the cortical motor areas targeting muscles involved in the task. 2. Over the course of 5 days, as subjects learned the one-handed, five-finger exercise through daily 2-h manual practice sessions, the cortical motor areas targeting the long finger flexor and extensor muscles enlarged, and their activation threshold decreased. Such changes were limited to the cortical representation of the hand used in the exercise. No changes of cortical motor outputs occurred in control subjects who underwent daily TMS mapping but did not practice on the piano at all (control group 1). 3. We studied the effect of increased hand use without specific skill learning in subjects who played the piano at will for 2 h each day using only the right hand but who were not taught the five-finger exercise (control group 2) and who did not practice any specific task. In these control subjects, the changes in cortical motor outputs were similar but significantly less prominent than in those occurring in the test subjects, who learned the new skill.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Central fatigue as revealed by postexercise decrement of motor evoked potentials.

We have previously shown that the amplitudes of motor evoked potentials (MEPs) elicited by transcranial magnetic stimulation (TMS) were transiently decreased after exercise, indicating fatigue of motor pathways in the central nervous system. The responsible mechanism is apparently decreased efficiency in the generation of the descending volleys in the motor cortex. We also noted a progressive decrement in amplitude from the first to the fourth MEP. To further clarify the mechanism of this phenomenon, 5 subjects were studied with TMS delivered at the rates of 0.1, 0.15, 0.3, 1, 3, and 6 Hz. The effect was best demonstrated at 0.3 Hz, and occurred after both isometric and isotonic exercise. Three of the subjects also had 0.3-Hz percutaneous electrical stimulation of the brainstem, and a decrement in MEP amplitude did not occur. Further, the delivery of TMS during muscle contraction after muscle fatigue failed to produce a decrement. The results are similar to those found at the neuromuscular junction in myasthenia gravis and are consistent with a reduced safety factor of cortical synaptic transmission in central nervous system fatigue.

Adult↗
PubMed1994

Differentiation of sensorimotor neuronal structures responsible for induction of motor evoked potentials, attenuation in detection of somatosensory stimuli, and induction of sensation of movement by mapping of optimal current directions.

Transcranial magnetic stimulation (TMS) of the sensorimotor cortex can evoke motor evoked potentials (MEPs), attenuation in detection of somatosensory stimuli (ADSS), and sensation of movement (SOM) referred to the same body part. In this study we tried to differentiate the substrates responsible for these effects. In 6 normal volunteers, TMS was applied with a nearly monopolar Dantec stimulator and a butterfly coil. Optimal scalp location and current direction were determined for induction of MEPs in abductor pollicis brevis (APB), first dorsal interosseous (FDI), and adductor digiti minimi (ADM); SOM in digits 2 and 5 in an ischemically paralyzed hand; and ADSS applied to digits 2 and 5. All 3 muscles' MEPs and SOM and ADSS in both digits were optimally activated from a single scalp position. In all subjects, optimal current directions for MEPs pointed anteriorly; those for ADSS and SOM pointed posteriorly. Optimal current directions showed the same progression in all subjects for MEPs (ADM, FDI, and APB from antero-lateral to antero-medial), ADSS (digit 5 postero-medial, 2 postero-lateral), and SOM (digit 1 through 5 postero-lateral to postero-medial). We conclude that neuronal networks targeting corticospinal neurons responsible for MEPs are different from those leading to SOM and ADSS (which could not be differentiated).

Adult↗
PubMed1994

Non-invasive differentiation of motor cortical representation of hand muscles by mapping of optimal current directions.

Non-invasive mapping of human motor cortex by stimulating different scalp positions with a magnetic coil held at a constant orientation allows differentiation of proximal and distal arm muscles. This study describes a technique for more precise mapping of closely represented muscles using different orientations of a coil that delivers nearly monopolar current pulses. EMG was recorded from abductor pollicis brevis (APB), first dorsal interosseous (FDI), abductor digiti minimi (ADM), and flexor carpi radialis (FCR) of 9 normal volunteers. Stimuli were delivered from a Dantec stimulator through an 8-shaped coil. The center of the coil was kept flat on the scalp on a given position, and the coil rotated at different angles. The amplitudes of the motor evoked potentials were used for calculation of optimal current directions in the brain for activation of each muscle in each position. The optimal current direction for FCR activation pointed antero-medially. ADM, FDI and APB mapped progressively more antero-laterally. The relationship between current directions was constant across subjects and did not change in different scalp positions. This technique improves the spatial resolution of non-invasive cortical mapping and may express the differences in orientations of interneuronal nets in the precentral gyrus.

Adult↗
PubMed1994

Limitations of electromyography and magnetic stimulation for assessing laryngeal muscle control.

The development of new phonosurgical techniques has increased the level of interest in the field of neurolaryngology. This field requires valid techniques for determining if muscle activation is normal. Laryngeal electromyography is being used more frequently to assess muscle innervation and synkinesis. Further, magnetic stimulation has been introduced as a noninvasive technique for nerve stimulation. Technical limitations that affect the clinical utility of both these techniques are reviewed: 1) difficulties obtaining selective and accurate electromyographic laryngeal muscle recordings, 2) normal variation in movement and muscle activation patterns within and between normal individuals when producing the same speech syllables, and 3) variation in laryngeal muscle response latencies between and within normal subjects during peripheral magnetic stimulation. Given the normal variation in laryngeal electromyography and magnetic stimulation response latencies, these techniques may not yet be reliable or accurate for assessing reinnervation or synkinesis following recurrent laryngeal nerve injury.

Adult↗
PubMed1994

Long-term botulinum toxin treatment of focal hand dystonia.

We treated focal hand dystonia in 53 patients with botulinum toxin injections for up to 6 years. Eighty-one percent of the patients improved with at least one injection session. Sixty-five percent of the injections produced transient weakness. We followed 37 of the patients for at least 2 years from the start of treatment, 24 of whom discontinued treatment because of inadequate response, loss of response, inaccessibility of a treatment provider, or the expense of the toxin. Women, who had a greater extent and longer duration of benefit than men, were more likely to continue treatment. The mean interval between injection sessions was 6 months. In most patients, we injected the toxin into the same combination of muscles at each session. The dose of toxin generally fluctuated within a range of 20 units. Side effects were mild and transient and unrelated to the long-term use of botulinum toxin. Botulinum toxin injection is safe and effective for the long-term management of focal hand dystonia.

Adult↗
PubMed1994

Akinesia in Parkinson's disease. I. Shortening of simple reaction time with focal, single-pulse transcranial magnetic stimulation.

We studied the effects of transcranial magnetic stimulation (TMS) of the motor cortex on simple reaction time (RT) in 10 patients with Parkinson's disease compared with 10 age-matched normal controls. The subjects flexed their right elbow rapidly in response to a visual go-signal. In random trials, TMS was applied to the left motor cortex at varying delays after the go-signal. In trials without TMS, RT was longer in the patients. However, in the trials with subthreshold TMS, RT in the patients became as fast as RT in trials without TMS in the controls. This shortening was associated with normalization of the voluntary triphasic EMG pattern and the pre-movement cortical excitability increase.

Aged↗
PubMed1993

Modulation of motor cortical outputs to the reading hand of braille readers.

We used focal transcranial magnetic stimulation to map the motor cortical areas targeting the first dorsal interosseous and the abductor digiti minimi muscles bilaterally in 10 proficient braille readers and 10 blind controls who were matched for age (mean, 50.6 yr) and age at time of blindness (mean, 7.5 yr). The proficient braille readers had learned braille at age 8 to 14 years and used it daily for 5 to 10 hours. Controls had not learned braille until age 17 to 21 years and used it daily for < 1 hour. In the controls, motor representations of the right and left first dorsal interosseous and abductor digiti minimi muscles were not significantly different. However, in the proficient braille readers, the representation of the first dorsal interosseous muscle in the reading hand was significantly larger than that in the nonreading hand or in either hand of the controls. Conversely, the representation of the abductor digiti minimi muscle in the reading hand was significantly smaller than that in the nonreading hand or in either hand of the controls. These differences were not due to differences in motor thresholds. Our results suggest that the cortical representation of the reading finger in proficient braille readers is enlarged at the expense of the representation of other fingers.

Adolescent↗
PubMed1993

Postexercise depression of motor evoked potentials: a measure of central nervous system fatigue.

Fatigue of voluntary muscular effort is a complex and multifaceted phenomenon. Fatigue of peripheral nervous system components, including the contractile apparatus and the neuromuscular junction, has been well studied. Central nervous system components also fatigue, but studies have lagged for want of objective methods. Transcranial magnetic stimulation is a relatively new technique that can be used to assess central nervous system excitability from the motor cortex to the alpha-motoneuron. In six normal volunteers, including four of the investigators, the amplitudes of motor evoked potentials elicited by transcranial magnetic stimulation were transiently decreased after exercise, indicating fatigue of motor pathways in the central nervous system. The decrease in amplitude was associated with a feeling of fatigue. The mechanism of this phenomenon is apparently decreased efficiency in the generation of the motor command in the motor cortex.

Central Nervous System↗
PubMed1993

Safety of rapid-rate transcranial magnetic stimulation in normal volunteers.

In 9 normal volunteers, we studied the safety of rapid-rate transcranial magnetic stimulation (rTMS) applied to different scalp positions at various frequencies and intensities. Pure tone threshold audiometry showed temporary threshold shifts in 3 subjects. In the subject stimulated at the highest intensity, rTMS induced a focal, secondarily generalized seizure despite the absence of definite risk factors for seizures. Rapid-rate TMS did not result in any important changes in the neurological examination findings, cognitive performance, electroencephalogram, electrocardiogram, and hormone levels (prolactin, adrenocorticotropic hormone, thyroid-stimulating hormone, luteinizing hormone, and follicle-stimulating hormone). In 10 additional subjects, the electromyographic activity in several contralateral muscles showed that trains of rTMS applied to the motor cortex induced a spread of cortical excitability. The spread of excitability depended on the intensity and frequency of the stimuli and probably constituted an early epileptogenic effect of rTMS. Guidelines for preventing the undesirable side effects of rTMS are offered.

Adult↗
PubMed1993

The effect of transcranial magnetic stimulation on median nerve somatosensory evoked potentials.

Somatosensory evoked potentials (SEPs) evoked by electrical stimulation of the median nerve at the wrist were conditioned by a preceding magnetic stimulus given over the contralateral central scalp using interstimulus intervals of 0-50 msec. The conditioning stimulus had no effect on the size of sub-cortical or N20 components of the response, but caused a large increase in the P25 parietal wave. Later components (P40/N65) of the SEP were reduced in size. The optimum position for producing the effect was with the magnetic stimulator held over the best point on the scalp for evoking muscle responses in the contralateral hand. The threshold for affecting the SEP was approximately the same as the threshold for evoking EMG responses in active hand muscles. We conclude that the magnetic conditioning shock changed the excitability of somatosensory cortex neurones involved in producing the P25. The subsequent decrease in the later components of the SEP may be related to the increase in somatosensory detection thresholds produced by a conditioning magnetic shock.

Adult↗
PubMed1993

Topography of the inhibitory and excitatory responses to transcranial magnetic stimulation in a hand muscle.

We studied the excitatory motor evoked potentials (MEPs) and the inhibitory (silent period) responses to focal transcranial magnetic stimulation (TMS) in the abductor pollicis brevis (APB) of 5 normal subjects to learn whether the scalp topography of the two responses differed. At the scalp location where stimulation produced the highest-amplitude MEP in the voluntarily activated APB, stimulus intensities below the MEP threshold produced silent periods with little or no preceding facilitation. The silent periods had a mean duration of 26.8 +/- 6.8 msec and a mean onset latency of 27.6 +/- 3.6 msec, which was 7.2 +/- 2.3 msec longer than the latency of MEPs produced in the APB by higher stimulus intensities. A period of excitation, with an onset latency of 50-80 msec, often followed the silent period. On averaged trials, a stimulus intensity just above the threshold of the MEP at its optimal position produced MEPs followed by silent periods at a cluster of scalp locations 1 cm apart on the central scalp (medial area) and silent periods with very slight or no preceding facilitation in 3-9 locations lateral to the MEP area (lateral area). This finding was confirmed in 3 subjects with maps constructed from statistical analysis of multiple trials. These maps also showed that MEPs produced from the medial area occurred 4-6 msec earlier than those produced from the lateral area. The integral of the silent period tended to be larger in the lateral area. The motor representation of APB, as defined by TMS, is not homogeneous but rather contains at least two components that differ physiologically and topographically.

Adult↗
PubMed1993

Rapid modulation of human cortical motor outputs following ischaemic nerve block.

The amplitudes of motor evoked potentials to transcranial magnetic stimulation from muscles immediately proximal to a temporarily anaesthetized (Bier's block) human forearm increase in minutes after the onset of anaesthesia and return to control values after the anaesthesia subsides. In order to determine the level at which the early modulation of human motor outputs takes place, we recorded maximal H reflexes, peripheral M responses, motor evoked potentials to transcranial magnetic stimulation, and motor evoked potentials to transcranial electrical stimulation and spinal electrical stimulation from a muscle immediately proximal to a limb segment made ischaemic by a pneumatic tourniquet. The amplitudes of motor evoked potentials to transcranial magnetic stimulation, but not to transcranial electrical stimulation and spinal electrical stimulation, were larger during ischaemia, implying that the site of change was in the motor cortex. The maximal H/M ratios were unaffected by ischaemia, indicating that alpha-motor neuron excitability to segmental Ia inputs remained unchanged. The map of cortical representation areas for this muscle obtained with transcranial magnetic stimulation was also enlarged. Taken together, our findings suggest that the temporary removal by ischaemic nerve block of myelinated afferent inputs reduces inhibition at the motor cortical level and that this disinhibition is responsible for the increased excitability of the corticospinal system.

Adult↗
PubMed1993

Pharmacokinetics and bioavailability of fluconazole in patients with AIDS.

Fluconazole pharmacokinetics were evaluated for 10 volunteers with AIDS who had no clinical evidence of gastroenteritis. Single 100-mg intravenous (i.v.) and oral (p.o.) doses were administered in a randomized, crossover design. i.v. doses were delivered by a constant-rate infusion over 30 min. Serum fluconazole concentrations were measured by gas-liquid chromatography. The i.v. and p.o. studies were modelled simultaneously by iterative two-stage analysis, which provided individual parameter estimates and a population pharmacokinetic model. Median areas under the concentration-time curves for i.v. and p.o. studies did not differ (90.6 and 99.3 micrograms/ml.h, respectively). Consistent with this finding, the median fractional bioavailability was 1.1 (range, 0.45 to 1.3), comparable to those in healthy subjects. Serum pharmacokinetics in these AIDS patients were generally similar to published data for healthy volunteers. However, following p.o. dosing, we observed a slightly delayed and highly variable time to maximum concentration in serum (median, 2 h; range, 15 min to 8 h). Data were well described by a linear, two-compartment pharmacokinetic model with first-order absorption and elimination. Repeated-measures analysis of variance found no significant differences among any of the pharmacokinetic parameters between i.v. and p.o. studies. On the basis of our findings, we suggest no change in dosage of p.o. fluconazole in patients with AIDS who show no clinical signs of enteropathy.

Acquired Immunodeficiency Syndrome↗