A clinical study of motor evoked potentials using a triple stimulation technique.
Amplitudes of motor evoked potentials (MEPs) are usually much smaller than those of motor responses to maximal peripheral nerve stimulation, and show marked variation between normal subjects and from one stimulus to another. Consequently, amplitude measurements have low sensitivity to detect central motor conduction failures due to the broad range of normal values. Since these characteristics are mostly due to varying desynchronization of the descending action potentials, causing different degrees of phase cancellation, we applied the recently developed triple stimulation technique (TST) to study corticospinal conduction to 489 abductor digiti minimi muscles of 271 unselected patients referred for possible corticospinal dysfunction. The TST allows resynchronization of the MEP, and thereby a quantification of the proportion of motor units activated by the transcranial stimulus. TST results were compared with those of conventional MEPs. In 212 of 489 sides, abnormal TST responses suggested conduction failure of various degrees. By contrast, conventional MEPs detected conduction failures in only 77 of 489 sides. The TST was therefore 2.75 times more sensitive than conventional MEPs in disclosing corticospinal conduction failures. When the results of the TST and conventional MEPs were combined, 225 sides were abnormal: 145 sides showed central conduction failure, 13 sides central conduction slowing and 67 sides both conduction failure and slowing. It is concluded that the TST is a valuable addition to the study of MEPs, since it improves detection and gives quantitative information on central conduction failure, an abnormality which appears to be much more frequent than conduction slowing. This new technique will be useful in following the natural course and the benefit of treatments in disorders affecting central motor conduction. (+info)
The effects of posteroventral pallidotomy on the preparation and execution of voluntary hand and arm movements in Parkinson's disease.
We studied the effect of posteroventral pallidotomy on movement preparation and execution in 27 parkinsonian patients using various motor tasks. Patients were evaluated after overnight withdrawal of medication before and 3 months after unilateral pallidotomy. Surgery had no effect on initiation time in unwarned simple and choice reaction time tasks, whereas movement time measured during the same tasks was improved for the contralesional hand. Movement times also improved for isometric and isotonic ballistic movements. In contrast, repetitive, distal and fine movements measured in finger-tapping and pegboard tasks were not improved after pallidotomy. Preparatory processes were investigated using both behavioural and electrophysiological measures. A precued choice reaction time task suggested an enhancement of motor preparation for the contralesional hand. Similarly, movement-related cortical potentials showed an increase in the slope of the late component (NS2) when the patients performed joystick movements with the contralesional hand. However, no significant change was found for the early component (NS1) or when the patient moved the ipsilesional hand. The amplitude of the long-latency stretch reflex of the contralesional hand decreased after surgery. In summary, the data suggest that pallidotomy improved mainly the later stages of movement preparation and the execution of proximal movements with the contralesional limb. These results provide detailed quantitative data on the impact of posteroventral pallidotomy on previously described measures of upper limb akinesia in Parkinson's disease. (+info)
Coherent cortical and muscle discharge in cortical myoclonus.
There is increasing evidence in man that the cortical drive to motor neurons is rhythmic. This oscillatory drive may be exaggerated in patients with cortical myoclonus. Spectral analysis of surface bipolar EEG and EMG activity was performed in eight such patients. Only three cases had evidence of giant cortical evoked potentials or a cortical correlate on back-averaging at the time of study. In six subjects, significant coherence between contralateral and vertex EEG and EMG was observed in ranges similar to that previously reported for normal subjects (15-30 and 30-60 Hz). Three out of these six subjects also had significant coherence at higher frequencies (up to 175 Hz). All eight patients had a correlate in the cumulant density estimate between EEG and contralateral EMG. EMG lagged EEG by about 14, 25 and 35 ms for the muscles of the forearm, hand and foot, respectively. These delays were estimated from the slope of the phase curves and the timing of the peaks in the cumulant density estimates, and are appropriate for conduction in fast pyramidal pathways. The results provide clear evidence of a cortical drive synchronizing muscle discharge over a broad range of frequencies in patients with cortical myoclonus. Fourier analysis is a promising technique in the diagnosis and investigation of such patients. (+info)
A theory of geometric constraints on neural activity for natural three-dimensional movement.
Although the orientation of an arm in space or the static view of an object may be represented by a population of neurons in complex ways, how these variables change with movement often follows simple linear rules, reflecting the underlying geometric constraints in the physical world. A theoretical analysis is presented for how such constraints affect the average firing rates of sensory and motor neurons during natural movements with low degrees of freedom, such as a limb movement and rigid object motion. When applied to nonrigid reaching arm movements, the linear theory accounts for cosine directional tuning with linear speed modulation, predicts a curl-free spatial distribution of preferred directions, and also explains why the instantaneous motion of the hand can be recovered from the neural population activity. For three-dimensional motion of a rigid object, the theory predicts that, to a first approximation, the response of a sensory neuron should have a preferred translational direction and a preferred rotation axis in space, both with cosine tuning functions modulated multiplicatively by speed and angular speed, respectively. Some known tuning properties of motion-sensitive neurons follow as special cases. Acceleration tuning and nonlinear speed modulation are considered in an extension of the linear theory. This general approach provides a principled method to derive mechanism-insensitive neuronal properties by exploiting the inherently low dimensionality of natural movements. (+info)
Corticospinal excitability modulation to hand muscles during movement imagery.
Motor evoked potentials (MEPs) to magnetic transcranial stimulation (TCS) were recorded from right abductor digiti minimi (ADM) and first dorsal interosseous (FDI) muscles, sharing the same peripheral innervation but engaged in two different motor demands. In seven healthy and trained subjects, the latencies, amplitudes and variability of MEPs were investigated under the following, randomly intermingled, conditions: full muscular and mental relaxation; mental simulation of selective index finger or little finger abduction; mental non-motor activity (arithmetical calculation); and real motor task (little and index finger abduction). The whole procedure was performed by continuous audiovisual monitoring of electromyographic 'silence' in the tested muscles. The maximal facilitatory effects (= latency shortening and amplitude increase) on MEPs were induced by the real motor task. An amplitude potentiation of MEPs in both tested muscles was present during non-motor mental activity, in comparison to basal values. A further amplitude potentiation, without latency shifts, was confined to the muscle acting as 'prime mover' for the mentally simulated movement, according to the motor program dispatched but not executed by the subject. Similar results were also found in the F-wave, showing that mental simulation affects spinal motoneuronal excitability as well, although -- due to the lack of MEP and F-wave latency shift -- the main effect takes place at cortical level. The study shows that movement imagery can focus specific facilitation on the prime-mover muscle for the mentally simulated movement. This is mainly evident on FDI muscle, which controls fingers (i.e. the index) with highly corticalized motor representation. (+info)
Cortical control of spinal pathways mediating group II excitation to human thigh motoneurones.
1. The possibility was investigated that cortical excitation to human thigh motoneurones is relayed via lumbar premotoneurones. 2. Test responses were evoked by transcranial magnetic stimulation (TMS) in voluntarily contracting quadriceps (Q) and semitendinosus (ST) muscles: either a motor evoked potential (MEP) in surface recordings or a peak of cortical excitation in the post-stimulus time histogram (PSTH) of single motor units was used. These test responses were conditioned by stimuli to the common peroneal (CP) or gastrocnemius medialis (GM) nerves. 3. CP stimulation evoked a large biphasic facilitation of the Q MEP, with early, short-lasting, low-threshold (0.6-0.8 x motor threshold (MT)) and late, longer lasting and higher threshold (1.2-1.5 x MT) peaks separated by a period of depression. GM nerve stimulation evoked a similar early depression and late facilitation in the ST MEP. 4. CP-induced effects in the Q H reflex were different (smaller late facilitation not preceded by any depression), suggesting that CP and cortical volleys interact at a premotoneuronal level to modify the Q MEP. 5. Peaks of cortical excitation evoked by TMS in single motor unit PSTHs were modulated by the conditioning volley like the MEPs with, in Q motor units, early and late CP-induced facilitations separated by a depression, and in ST motor units a late GM-induced facilitation. Facilitations on combined stimulation (i) were greater than the sum of effects by separate stimuli and (ii) never affected the initial part of the cortical peak. 6. It is concluded that the features of the reported facilitatory interactions between cortical and peripheral volleys are consistent with interactions in a population of lumbar excitatory premotoneurones co-activated by group I and group II afferents. The potency of the effects suggests that a significant part of the cortical excitation to motoneurones of thigh muscles is relayed via these interneurones. 7. It is argued that the early depression in ST motoneurones and the separation of the two peaks of facilitation in Q motoneurones reflect a cortical facilitation of spinal inhibitory interneurones projecting on excitatory premotoneurones. (+info)
Successful resection of arteriovenous malformations in eloquent areas diagnosed by surface anatomy scanning and motor evoked potential.
Successful resection of cerebral arteriovenous malformations (AVMs) involving the sensorimotor cortex was achieved in 17 cases. The theoretical basis for performing resection of AVMs in eloquent areas is the fact that the brain in and around the nidus about 1 mm in thickness is considered not to be functioning. It is also considered that any center of important function, when an AVM is involved, shifts to the near-by cortex from the original site. Nevertheless, it is critically important to recognize the cortex functioning as sensorimotor centers before and during operation. For this purpose, we have used surface anatomy scanning (SAS) in combination with magnetic resonance angiography. SAS is found to be very useful for the recognition of the topographical relationship between the surface anatomy and AVM. During operation, the motor cortex is identified with motor evoked potential. We have found that, in some cases, the motor center has shifted to the accessory motor cortex. With these information, it is possible to start resection of the lesion from dissection of the main feeders and dissection of the nidus from a silent cortex toward the critical area. Apparent neurological improvements were achieved in 15 of 17 patients treated surgically (88%). With this result, we think that AVMs in eloquent areas can be treated successfully when the surgery is well-designed and well-oriented with the combined use of diagnostic imaging and monitoring. As for control of intraoperative bleeding, careful attention to small but important surgical techniques avoids troublesome bleeding during AVM surgery. (+info)
Differential effects on motorcortical inhibition induced by blockade of GABA uptake in humans.
1. Blockade of uptake carriers of gamma-aminobutyric acid (GABA) has been shown to modulate inhibition in cortical slices of experimental animals, although little is known about this mechanism in vivo and, in particular, in humans. 2. The effects of blockade of GABA uptake were studied using transcranial magnetic stimulation (TMS) in humans. In eight healthy volunteers several measures of cortical excitation and inhibition were obtained before and approximately 2 h after ingestion of 5-15 mg of tiagabine (TGB). 3. After TGB ingestion, the duration of the TMS-induced silent period observable in the electromyogram of the voluntarily contracted target muscle was prolonged. Similarly, paired-pulse inhibition of the motor-evoked potential (MEP), as tested by delivering two magnetic shocks of equal suprathreshold intensities at 160 ms interstimulus interval (ISI), was more pronounced. In apparent contradistinction, paired-pulse inhibition of the MEPs produced by a subthreshold conditioning stimulus delivered 3 ms prior to a suprathreshold stimulus was reduced. Paired-pulse facilitation elicited by the same double-shock protocol at an ISI of 10 ms was increased. 4. The prolongation of the GABAB receptor-mediated component of the inhibitory postsynaptic potential observed with TGB in in vitro studies probably underlies the increase in cortical silent period duration. The reduction of the paired-pulse inhibition at 3 ms, in turn, probably reflects inhibition of GABAA receptor-mediated inhibition via presynaptic GABAB receptors. 5. These data provide in vivo evidence of differential modulation of cortical inhibition by blockade of GABA uptake. Presynaptic GABA autoreceptors may be involved in modulating cortical inhibition in the human motor cortex. (+info)