The silent period threshold as a measure of corticospinal inhibition

Corticomotor threshold is a clinically useful concept for defining motor cortex excitability using transcranial magnetic stimulation. Although the silent period duration can be a quantitative measure of corticospinal inhibitory function, its threshold value has not been formally determined. The authors studied 20 right-handed normal subjects and found that silent period threshold was a reproducible parameter that did not vary with age and sex. Its value was significantly lower with left-sided transcranial magnetic stimulation and right-hand recordings. The silent period threshold may be a novel parameter useful for future studies of cortical inhibitory function.

H-reflex and motor responses to acute ischemia in apparently healthy individuals

The authors examined the effect of acute ischemia on peripheral nerve function in healthy subjects. It was hypothesized that acute ischemia would interfere with the ability of sensory and motor nerves to propagate an impulse. Twelve young, apparently healthy adults participated in the study. Soleus H-reflex and motor recruitment curves were determined for subjects during a control condition, after 5 minutes of ischemia by femoral artery occlusion, and after a 5-minute recovery. During ischemia, the stimulus intensity required to evoke an H-reflex or M-wave was reduced by 18.3% and 18.4%, respectively. Hmax/Mmax ratios were significantly reduced with acute ischemia (mean +/- standard error) 66.29% +/- 5.4% and 58.81% +/- 6.7% for control and ischemia, respectively, owing to a decrease in Hmax during acute ischemia with no change in Mmax. After ischemia, the Hmax/Mmax returned to control values, as did the M-threshold. However, although the H-threshold slightly recovered, it failed to return to control threshold after 5 minutes of recovery. The results suggest that acute ischemia decreases motor and H-reflex thresholds in healthy individuals with a longer lasting effect for the H-reflex. In addition, a decrease in Hmax/Mmax ratio was observed, suggesting that acute ischemia has differential effects on sensory nerve propagation and synapse transmission.

Shortening of distal motor latency in anode distal stimulation

OBJECTIVE

Distal motor latency (DML) is shortened when the anode is held distally instead of the cathode by increasing the stimulus intensity. The objective of this study was to clarify the mechanism responsible for this shortening.

METHODS

In seven healthy subjects, compound muscle action potential (CMAP) was obtained from the thenar muscle by bipolar stimulation of the median nerve at the wrist, and the intensity at which the first motor units were stimulated was defined as the threshold. Bipolar stimulation with extended interpole distance was employed to identify the generating site of the CMAP and F-wave.

RESULTS

The shortening of DML was dependent on the stimulus intensity and threshold. For the low threshold condition, the CMAP generating site was replaced from the proximal cathodal pole to the distal anodal pole by increasing the stimulus intensity. The generating site of the F-wave remained at the proximal cathodal pole irrespective of stimulus intensity.

CONCLUSIONS

Replacement of the generating site results in the shortening of DML. When the F-wave is recorded after being induced by anode distal stimulation, CMAP should not be simultaneously evaluated.

SIGNIFICANCE

This study clarified the generation sites of CMAP and the F-wave when induced by anode distal stimulation.

Endogenous ATP involvement in mustard-oil-induced central sensitization in trigeminal subnucleus caudalis (medullary dorsal horn)

Central sensitization represents a sustained hypersensitive state of dorsal horn nociceptive neurons that can be evoked by peripheral inflammation or injury to nerves and tissues. It reflects neuroplastic changes such as increases in neuronal spontaneous activity, receptive field size, and responses to suprathreshold stimuli and a decrease in activation threshold. We recently demonstrated that purinergic receptor mechanisms in trigeminal subnucleus caudalis (Vc; medullary dorsal horn) are also involved in the initiation and maintenance of central sensitization in brain stem nociceptive neurons of trigeminal subnucleus oralis. The aim of the present study was to investigate whether endogenous ATP is involved in the development of central sensitization in Vc itself. The experiments were carried out on urethan/alpha-chloralose anesthetized and immobilized rats. Single neurons were recorded and identified as nociceptive-specific (NS) in the deep laminae of Vc. During continuous saline superfusion (0.6 ml/h it) over the caudal medulla, Vc neuronal central sensitization was readily induced by mustard oil application to the tooth pulp. However, this mustard-oil-induced central sensitization could be completely blocked by continuous intrathecal superfusion of the wide-spectrum P2X receptor antagonist pyridoxal-phosphate-6-azophenyl-2, 4-disulphonic acid tetra-sodium (33-100 microM) and by apyrase (an ectonucleotidase enzyme, 30 units/ml). Superfusion of the selective P2X1, P2X3 and P2X(2/3) receptor antagonist 2',3'-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (6-638 microM) partially blocked the Vc central sensitization. The two P2X receptor antagonists did not significantly affect the baseline nociceptive properties of the Vc neurons. These findings implicate endogenous ATP as an important mediator contributing to the development of central sensitization in nociceptive neurons of the deep laminae of the dorsal horn.

Aversive effects elicited by electrical stimulation of the inferior colliculus in normal and audiogenic seizure susceptible rats

Trains of electrical stimulations were applied to the dorsal or ventral part of the inferior colliculus (IC) of audiogenic seizure susceptible rats from the AGSR strain. Threshold and duration of wild running (WR), were evaluated in the first experiment. All stimulation sites elicited WR, even in normal control rats. Stimulation of the IC of AGSR rats required a lower quantity of current, i.e., such brain sites were more sensitive to the current, than normal controls. The duration of post-stimulus WR was shorter in AGSR rats. Lower quantities of current applied to the ventral IC were needed to elicit WR than to the dorsal IC in AGSR rats. In a second experiment, using the same stimulations sites in the same rats, the emotional effect of the stimulation was tested through an instrumental learning procedure (switch-off paradigm) in which the rat was trained to press a bar to put an end to the stimulation. Both dorsal and ventral IC stimulation sites sustained switch-off behavior in AGSR rats, but only ventral IC stimulation sites sustained switch-off learning in control rats.

Irradiation impairment of umami taste in patients with head and neck cancer

BACKGROUND

Previous studies have reported the alteration of the four basic tastes (sweet, salty, sour and bitter) in patients with head and neck cancer (HNC) during radiotherapy. However, there has been no investigation on the function of umami taste, a novel basic taste recognized recently, during head and neck irradiation.

OBJECTIVE

To investigate the recognition threshold of umami and the four basic tastes at different irradiation dose intervals during radiotherapy for head and neck cancer.

METHODS

In 30 patients with malignant neoplasm of head and neck, objective taste thresholds were examined by use of a whole-mouth method at pre-radiotherapy, irradiation doses at 15, 30, 45, and 60 Gy, respectively. Subjective taste loss and some distresses were recorded simultaneously.

RESULTS

Sweet, sour, salty, and bitter tastes showed temporarily and slightly increased thresholds during the treatment, but no statistical difference was found between the threshold at pre-radiotherapy and that at 15, 30, 45, and 60 Gy in any taste quality (all P > 0.05). Significantly impaired threshold of umami taste was revealed at 30 Gy (P < 0.05) and remained throughout the following treatment (at 45 and 60 Gy, both P < 0.01). Subjective taste impairment, appetite loss and satisfaction with the current state tended to deteriorate significantly during the irradiation. Mean body weight of the patients experienced a continuous loss, decreasing from an average of 60.4 kg before treatment to 57.3 kg at 60 Gy (P < 0.01). Scores of satisfaction with current state showed a significant correlation with umami taste thresholds (P = 0.035).

CONCLUSION

The clinical impairment pattern of umami taste is different from that of the other four basic tastes in HNC patients during radiotherapy. Impaired umami taste acuity plays an important role in impacting the quality of life of the patients irradiated to the head and neck.

Control of locomotor cycle durations

In intact animals and humans, increases in locomotor speed are usually associated with decreases in step cycle duration. Most data indicate that the locomotor central pattern generator (CPG) shortens cycle duration mainly by shortening the durations of extensor rather than flexor phases of the step cycle. Here we report that in fictive locomotion elicited by electrical stimulation of the midbrain locomotor region (MLR) in the cat, spontaneous variations in cycle duration were due more to changes in flexor rather than extensor phase durations in 22 of 31 experiments. The locomotor CPG is therefore not inherently extensor- or flexor-biased. We coined the term "dominant" to designate the phase (flexion or extension) showing the larger variation. In a simple half-center oscillator model, experimental phase duration plots were fitted well by adjusting two parameters that corresponded to background drive ("bias") and sensitivity ("gain") of the oscillator's timing elements. By analogy we argue that variations in background drive to the neural timing elements of the CPG could produce larger variations in phase duration in the half-center receiving the lower background drive, i.e., background drive may determine which half-center is dominant. The fact that data from normal cats were also fitted well by the model indicates that sensory input and central drive combine to determine locomotor phase durations. We conclude that there is a considerable flexibility in the control of phase durations in MLR-induced fictive locomotion. We posit that this may be explained by changes in background excitation of neural timing elements in the locomotor CPG.

Reduction of common synaptic drive to ankle dorsiflexor motoneurons during walking in patients with spinal cord lesion

It is possible to obtain information about the synaptic drive to motoneurons during walking by analyzing motor-unit coupling in the time and frequency domains. The purpose of the present study was to compare motor-unit coupling during walking in healthy subjects and patients with incomplete spinal cord lesion to obtain evidence of differences in the motoneuronal drive that result from the lesion. Such information is of importance for development of new strategies for gait restoration. Twenty patients with incomplete spinal cord lesion (SCL) participated in the study. Control experiments were performed in 11 healthy subjects. In all healthy subjects, short-term synchronization was evident in the discharge of tibialis anterior (TA) motor units during the swing phase of treadmill walking. This was identified from the presence of a narrow central peak in cumulant densities constructed from paired EMG recordings and from the presence of significant coherence between these signals in the 10- to 20-Hz band. Such indicators of short-term synchrony were either absent or very small in the patient group. The relationship between the amount of short-term synchrony and the magnitude of the 10- to 20-Hz coherence in the patients is discussed in relation to gait ability. It is suggested that supraspinal drive to the spinal cord is responsible for short-term synchrony and coherence in the 10- to 20-Hz frequency band during walking in healthy subjects. Absence or reduction of these features may serve as physiological markers of impaired supraspinal control of gait in SCL patients. Such markers could have diagnostic and prognostic value in relation to the recovery of locomotion in patients with central motor lesions.

Fully tuneable stochastic resonance in cutaneous receptors

Stochastic resonance describes a phenomenon whereby the addition of "noise" to the input of a nonlinear system can improve sensitivity. "Fully tuneable stochastic resonance" is a particular form of the phenomenon that requires the matching of two time scales: one being that of the subthreshold periodic stimulus of the system and the other being the noise-induced response of the system. First proposed in 1981, stochastic resonance has been reported in a wide range of biological systems; however, conclusive experimental evidence for fully tuneable stochastic resonance in biological systems is limited. Evidence of fully tuneable stochastic resonance in the response of slowly adapting type I mechanoreceptors in the toad is presented. The results are extended to include the first evidence supporting fully tuneable stochastic resonance in psychophysical experiments, namely tactile detection thresholds, indicating that the human CNS is capable of accessing the improved information available via fully tuneable stochastic resonance.

Transcranial magnetic stimulation in lower motor neuron diseases

OBJECTIVE

To study the diagnostic value of transcranial magnetic stimulation (TMS) in a group of patients with lower motor neuron disease (LMND). Among LMND, several chronic immune mediate motor neuropathies may simulate amyotrophic lateral sclerosis (ALS).

METHODS

Forty patients with LMND were included TMS was performed at the first visit. The patients were seen prospectively every 3 months for a period of 1-4 years.

RESULTS

Three different groups were distinguished at the end of follow-up: (1) ALS group with 7 patients, (2) Pure motor neuropathy with 14 patients and (3) Other LMND including 12 patients with hereditary spinal amyotrophy, 3 patients with Kennedy's disease and 4 patients with post-poliomyelitis. On the basis of the results of TMS variables, 6 out of 7 ALS patients had abnormality of silent period (SP) associated or not with abnormality of excitatory threshold or amplitude ratio. Patients with pure motor neuropathy had normal SP and amplitude ratio. Four out of 14 patients had increased central motor conduction time (CMCT), one had increased CMCT and excitatory threshold, and one patient had a slightly increased excitatory threshold. Considering the abnormality of TMS variables in the groups, SP, excitatory threshold, and amplitude ratio were chosen in a post-hoc attempt to select variables yielding high sensitivity and specificity. The overall sensitivity of TMS for diagnosis of ALS among LMND was 85.7%, its specificity was 93.9%. When only the abnormality of SP was taken into account, the sensitivity was unchanged. But the specificity was improved to 100%.

CONCLUSIONS

TMS helped to distinguish suspected ALS from pure motor neuropathy.

Follow-up of advanced diabetic neuropathy

BACKGROUND

The course of advanced diabetic neuropathy (DN) is largely unknown.

AIM

To find variables allowing the follow-up of late stages of DN.

METHODS

Thirty diabetic patients with DN were observed. Patients were examined at intervals of 6 months over a period of 2 years. The compound muscle action potentials (CMAPs) were recorded in extensor digitorum brevis (EDB) and flexor hallucis brevis (FHB) muscles. Clinical severity of DN, nerve conduction studies (NCS), quantitative sensory testing (QST) and heart rate variability (HRV) were evaluated. The data were compared with age- and sex-matched controls.

RESULTS

All measures were sensitive to the detection of DN. Significant deterioration during follow-up was exclusively found in CMAP analysis of the EDB (p<0.05) and FHB muscles (p<0.03). NCS, QST and HRV remained unchanged within the 2 years of observation. Coincidental changes might occur, if only two time points are chosen for followup.

CONCLUSION

Our results indicate that ongoing axonopathy predominates in advanced DN. Repeated testing helps to minimize the impact of coincidental or chance changes in DN follow-up studies.

Ontogeny of feline temporal lobe epilepsy in amygdala-kindled kittens: an update

This report describes amygdala-kindled seizure development and the post-kindling course in 58 cats (29 males and 29 females), including 40 preadolescents between 2.5 and 6.5 months of age and 18 adults >1 year of age at the beginning of kindling. The results extend previous findings, as follows: (1) Youngest animals (<5.0 months, n=30) are far more likely than older kittens and adults to exhibit spontaneous epilepsy, indexed by seizures that occur >1 h after stimulus-evoked seizures. (2) The youngest animals also exhibit accelerated kindling rates and rapid post-kindling onset of multifocal spontaneous epilepsy with a catastrophic clinical course. The profile includes a variety of EEG and/or clinical seizure manifestations and a progressive increase in the number and density of convulsive seizure clusters. Behavioral sequelae accompany seizure clusters and can range from sensory or motor deficits (visual agnosia, sensory hypersensitivity, atonic episodes, restricted mobility) to social isolation and placidity. (3) Onset of spontaneous epilepsy with developmental deterioration is substantially enhanced by recurrent evoked seizures early in the post-kindling course. The post-kindling progression can be stopped or minimized by suspension of evoked seizure trials and/or by management of frequent spontaneous convulsions (>1 per hour) with anticonvulsants. (4) In older cats, many more evoked seizures are required to generate fewer spontaneous seizures with relatively innocuous behavioral disorders. The findings suggest a 'critical period' in kindled kittens for onset of spontaneous temporal lobe epilepsy with severe behavioral consequences and a favorable prognosis for the young following early detection and intervention.

Calcineurin activation contributes to noise-induced hearing loss

Acoustic overstimulation increases Ca(2+) concentration in auditory hair cells. Because calcineurin is known to activate cell death pathways and is controlled by Ca(2+) and calmodulin, this study assessed the role of calcineurin in auditory hair cell death in guinea pigs after intense noise exposure. Immediately after noise exposure (4-kHz octave band, 120 dB, for 5 hr), a population of hair cells exhibited calcineurin immunoreactivity at the cuticular plate, with a decreasing number of positive-stained cells on Days 1-3. By Day 7, the levels of calcineurin immunoreactivity had diminished to near control, non-noise exposed values, concomitant with an increasing loss of hair cells. Staining of hair cell nuclei with propidium iodide (PI), restricted to calcineurin-immunopositive cells, indicated breakdown of cell membranes symptomatic of incipient cell death. The local application of the calcineurin inhibitors, FK506 and cyclosporin A, reduced the level of noise-induced auditory brain stem response threshold shift and hair cell death, indicating that calcineurin is a factor in noise-induced hearing loss. The results suggest that calcineurin inhibitors are of potential therapeutic value for long-term protection of the morphologic integrity and function of the organ of Corti against noise trauma.

Acute tetrodotoxin-induced neurotoxicity after ingestion of puffer fish

This study documents the effects of puffer-fish poisoning on peripheral nerve. Excitability measurements investigated membrane properties of sensory and motor axons in four patients. The median nerve was stimulated at the wrist, with compound muscle potentials recorded from abductor pollicis brevis and compound sensory potentials from digit 2. Stimulus-responses, strength-duration time constant (tau(SD)), threshold electrotonus, and current-threshold relations were recorded. The urine of each patient tested positive for tetrodotoxin. Compared with controls, axons were of higher threshold, compound muscle action potentials and compound sensory nerve action potentials were reduced in amplitude, latency was prolonged, and tau(SD) was reduced. In recovery cycles, refractoriness, superexcitability, and late subexcitability were decreased. Threshold electrotonus of motor axons exhibited distinctive abnormalities with less threshold decline than normal on depolarization and greater threshold increase on hyperpolarization (p < 0.0005 for each patient). The changes in excitability were reproduced in a mathematical model by reducing sodium (Na(+)) permeabilities by a factor of two. This study confirms that the neurotoxic effects of puffer-fish poisoning can be explained by tetrodotoxin blockade of Na(+) channels. It demonstrates the ability of noninvasive nerve excitability studies to detect Na(+) channel blockade in vivo and also the utility of mathematical modeling to aid interpretation of altered excitability properties in disease.

Corticospinal physiology in patients with Prader-Willi syndrome: a transcranial magnetic stimulation study

BACKGROUND

Prader-Willi syndrome (PWS) is a genetic developmental disorder, mostly caused by a deletion on the paternal chromosome 15 or by a maternal uniparental disomy 15. Some PWS clinical and neurochemical features suggest an involvement of the corticospinal motor structures.

OBJECTIVE

To explore the corticospinal physiology of PWS by transcranial magnetic stimulation.

SETTING

A community-based hospital.

METHODS

We studied motor evoked potentials in the first dorsal interosseous muscle of 21 young-adult patients with PWS. Thirteen patients had a deletion at chromosome 15; 8 had a uniparental disomy. We measured the following variables: relaxed motor threshold, central motor conduction time, duration of the central silent period, and short-interval intracortical inhibition and facilitation. We also recorded F waves in the first dorsal interosseous muscle. We had 11 normal controls.

RESULTS

In the whole PWS group, motor threshold was higher as compared with controls (P<.05). The central motor conduction time, central silent period, and F waves were normal. Intracortical facilitation was reduced significantly (P<.001). Patients with PWS and a deletion had a weaker intracortical inhibition as compared with patients with PWS and a uniparental disomy (P<.05).

CONCLUSIONS

Transcranial magnetic stimulation changes in patients with PWS suggested a hypo-excitability of the motor cortical areas. Defective neurogenesis of the cortical tissue and multiple transmitter alterations are the putative causes. Impaired intracortical inhibition might represent an electrical marker for a deletion defect.

Cortical and limbic excitability in rats with absence epilepsy

The classical cortico-reticular theory on absence epilepsy suggests that a hyperexcitable cortex is a precondition for the occurrence of absence seizures. In the present experiment seizure thresholds and characteristics of cortical and limbic epileptic afterdischarges (AD) were determined in a comparative cortical stimulation study in young and old adult genetically epileptic WAG/Rij, congenic ACI and Wistar rats. Fifteen-second series of 8Hz stimulation of the sensory-motor cortex were applied in 80- and 180-day-old rats with implanted electrodes. Strain differences were found for the threshold for movements directly induced by stimulation, low frequency spike-and-wave AD, maximal clonic intensity of seizures accompanying direct stimulation, and frequency characteristics of low frequency AD. None of these results agreed with a higher cortical excitability exclusively in WAG/Rij rats. However, WAG/Rij rats had the longest duration of the low frequency AD, and the lowest threshold for the transition to the limbic type of AD. The decrease of this threshold correlated with the increase of the incidence and total duration of spontaneous SWDs in WAG/Rij rats. It is concluded that the elevated excitability of the limbic system or pathways mediating the spread of the epileptic activity into this system can be attributed to the development of genetic epileptic phenotype in WAG/Rij rats.

A reliable method for the preferential activation of C- or A-fibre heat nociceptors

There is strong evidence that A- and C-fibre nociceptors evoke significantly different sensory experiences, are differentially sensitive to pharmacological intervention, and play different roles in pain pathology. It is therefore of considerable interest to be able to selectively activate one fibre type or the other in studies of nociceptive processing. Here, we report significant modifications to a non-invasive technique, first described by Yeomans et al. [Pain 59 (1994) 85; Pain 68 (1996) 141; Pain 68 (1996) 133], which uses different rates of skin heating to preferentially activate A- or C-nociceptors. A copper disk (diameter: 4mm) was used to transfer heat evenly across the dorsal surface of the rat hindpaw. Initial experiments established the relationship between the temperature at the skin surface and the sub-epidermal temperature. Subsequently, the vanilloid capsaicin, which sensitises unmyelinated C-mechanoheat nociceptors, was shown to decrease the thresholds of reflex responses evoked by slow rates of heating. In contrast thresholds of responses to fast rates of skin heating were unchanged, indicating that nociceptors activated by this stimulus were capsaicin-insensitive A-fibre heat nociceptors.

AM representation in green treefrog auditory nerve fibers: neuroethological implications for pattern recognition and sound localization

In addition to spectral call components, temporal patterns in the advertisement-call envelope of green treefrog males ( Hyla cinerea) provide important cues for female mate choice. Rapid amplitude modulation (AM) with rates of 250-300 Hz is typical for this species' advertisement calls. Here we report data on the encoding of these rapid call modulations by studying the responses of single auditory nerve fibers to two-tone stimuli with envelope periodicities close to those of the natural call. The free-field response properties of 86 nerve fibers were studied from 32 anesthetized males. The accuracy of stimulus envelope coding was quantified using both a Gaussian function fit to the interspike interval histograms derived from the first seven 20-ms stimulus segments, and the vector-strength metric applied to the phase-locked responses. Often, AM encoding in the initial stimulus segment was more faithful than that in its second half. This result may explain why conspecific females prefer calls in which the initial segment is unmasked rather than masked. Both the questions of pattern recognition and localization are discussed, and the data are related to behavioral observations of female choice and localization performance in this species.

Motor cortical thresholds and cortical silent periods in epilepsy

We studied motor cortical thresholds (TIs) and cortical silent periods (SPs) evoked by transcranial magnetic stimulation (TMS) in 110 epileptic patients. Sixty-two had primary generalised, 48 had partial type seizures. Fifteen out 110 patients were analysed both before and after anticonvulsant medication. Our aims were to evaluate the TI levels and the duration of SPs in patients with epilepsy and to determine the reliability of TMS in patients with epilepsy. There was no negative effect of TMS on the clinical status and EEG findings in patients with epilepsy. TIs obtained from patients with partial epilepsy were higher than those obtained from both controls and primary epileptics. The duration of SP in patients with primary epileptics was more prolonged than those obtained from controls. There was no correlation between EEG lateralisation and both SP duration and TI values. In de novo patient group, SP duration was significantly prolonged after anticonvulsant medication. We concluded that TMS is a reliable electrophysiological investigation in patients with epilepsy. The analysis of SP duration may be an appropriate investigation in monitoring the effect of anticonvulsant medication on the cortical inhibitory activity.

Transcranial direct current stimulation disrupts tactile perception

The excitability of the cerebral cortex can be modulated by various transcranial stimulation techniques. Transcranial direct current stimulation (tDCS) offers the advantage of portable equipment and could, therefore, be used for ambulatory modulation of brain excitability. However, modulation of cortical excitability by tDCS has so far mostly been shown by indirect measures. Therefore, we examined whether tDCS has a direct behavioral/perceptional effect. We compared tactile discrimination of vibratory stimuli to the left ring finger prior to, during and after tDCS applied for 7 min at 1-mA current intensity in 13 subjects. Stimulation was pseudorandomized into cathodal, anodal and sham conditions in a within-subject design. The active electrode was placed over the corresponding somatosensory cortex at C4 according to the 10-20 EEG system and the reference electrode at the forehead above the contralateral orbita. Cathodal stimulation compared with sham induced a prolonged decrease of tactile discrimination, while anodal and sham stimulation did not. Thus, cortical processing can be modulated in a behaviorally/perceptually meaningful way by weak transcranial current stimulation applied through portable technology. This finding offers a new perspective for the treatment of conditions characterized by alterations of cortical excitability.

Facilitative effect of high frequency subthreshold repetitive transcranial magnetic stimulation on complex sequential motor learning in humans

We investigated the effect of repetitive transcranial magnetic stimulation (rTMS) applied to the motor cortex, on the motor learning of sequential finger movements. Fifteen healthy subjects were trained to perform seven sequential finger movements of the left hand. Ten Hertz or sham rTMS with a resting motor threshold of 80% was applied to each subject during the task period. Stimulation with 10Hz rTMS produced a better learning performance in terms of target score and execution time than sham stimulation. We conclude that high-frequency rTMS may modulate the excitability of the motor cortex and facilitate the sequential motor learning process in normal subjects. These findings may provide a basis for the development of therapeutic applications of rTMS in patients with impaired motor skill.

Changes to cold detection and pain thresholds following low and high frequency transcranial magnetic stimulation of the motor cortex

There is some evidence that repetitive transcranial magnetic stimulation (rTMS) can alleviate the experience of chronic pain. The mechanisms by which rTMS may induce pain relief, however, are unknown. The present study examined whether a session of rTMS would produce sensory threshold changes in healthy individuals. Detection and pain thresholds for cold sensations were compared following low frequency (1 Hz) (Experiment 1) and high frequency (20 Hz) (Experiment 2) repetitive TMS. While cold detection threshold was significantly lowered by both rTMS rates, only high frequency rTMS produced a significant change in cold pain threshold. In contrast, sham rTMS did not alter thresholds for cold stimuli. These findings provide evidence that sensory thresholds can be influenced by repetitive transcranial magnetic stimulation.

Corticofugal shaping of frequency tuning curves in the central nucleus of the inferior colliculus of mice

Plasticity of the auditory cortex can be induced by conditioning or focal cortical stimulation. The latter was used here to measure how stimulation in the tonotopy of the mouse primary auditory cortex influences frequency tuning in the midbrain central nucleus of the inferior colliculus (ICC). Shapes of collicular frequency tuning curves (FTCs) were quantified before and after cortical activation by measuring best frequencies, FTC bandwidths at various sound levels, level tolerance, Q-values, steepness of low- and high-frequency slopes, and asymmetries. We show here that all of these measures were significantly changed by focal cortical activation. The changes were dependent not only on the relationship of physiological properties between the stimulated cortical neurons and recorded collicular neurons but also on the tuning curve class of the collicular neuron. Cortical activation assimilated collicular FTC shapes; sharp and broad FTCs were changed to the shapes comparable to those of auditory nerve fibers. Plasticity in the ICC was organized in a center (excitatory)-surround (inhibitory) way with regard to the stimulated location (i.e., the frequency) of cortical tonotopy. This ensures, together with the spatial gradients of distribution of collicular FTC shapes, a sharp spectral filtering at the core of collicular frequency-band laminae and an increase in frequency selectivity at the periphery of the laminae. Mechanisms of FTC plasticity were suggested to comprise both corticofugal and local ICC components of excitatory and inhibitory modulation leading to a temporary change of the balance between excitation and inhibition in the ICC.

Human exposure to a specific pulsed magnetic field: effects on thermal sensory and pain thresholds

Exposure to pulsed magnetic fields (MF) has been shown to have a therapeutic benefit in both animals (e.g. mice, snails) and humans. The current study investigated the potential analgesic benefit of MF exposure on sensory and pain thresholds following experimentally induced warm and hot sensations. Thirty-nine subjects (Study 1) and 31 subjects (Study 2) were randomly and double-blindly assigned to 30 min of MF or sham exposure between two sets of tests of sensory and pain thresholds and latencies at, 1 degrees C above, and 2 degrees C above pain thresholds. Results indicated that MF exposure does not affect sensory thresholds [e.g. [F(1,31) = 0.073, NS]. Pain thresholds were significantly increased following MF exposure [F(1,6) = 9.45, P < 0.01] but not following sham exposure [F (1,4) = 4.22, NS]. A significant condition by gender interaction existed for post-exposure pain thresholds [F(1,27) = 5.188, P < 0.05]. Taken together, these results indicate that MF exposure does not affect basic human perception, but can increase pain thresholds in a manner indicative of an analgesic response. The potential involvement of the placebo effect is discussed.

Diseases of modern living: neurological changes associated with mobile phones and radiofrequency radiation in humans

Health effects of radiofrequency radiations (RFR) including mobile phone technology and the adequacy of their safety standards remain uncertain. Case reports of peripheral neurological effects of RFR describe mainly disturbances of noxious sensation (dysaesthesia). Cases associated with other RFR sources as well as mobile phone technology are examined seeking insights into neurophysiological mechanisms and safety levels. Cases have arisen after exposure to much of the frequency range (low MHz to GHz). In some instances symptoms are transitory, but may be lasting in others. After very high intensity exposures nerves may be grossly injured. However, after lower intensity exposures which may result in dysaesthesia, ordinary nerve conduction studies demonstrate no abnormality although current perception threshold studies may. Only a small proportion of similarly exposed persons develop symptoms. The role of modulations (e.g. pulses) needs clarification. Some of these observations are not consistent with the prevailing hypothesis that all health effects of RFR arise from thermal mechanisms. It is concluded that RFR from mobile phones can cause peripheral neurophysiological changes in some persons. The effects occur at exposure levels below the present safety levels for RFR. Possible non-thermal mechanisms are discussed and may point to future directions of research.