Johannes Netz

Hemispheric asymmetry of transcallosalinhibition in man

The transcallosal connecting fibres linking corresponding projection areas of the same muscles of the right and left primary motor cortex may play an important role in control of unilateral movements. It appears that they have mainly inhibitory effects. This was further evaluated by transcranial magnetic stimulation using two focal coils placed on the optimal positions, i.e. the positions with the lowest thresholds at the motor representation areas of the first dorsal interosseous muscle of the left and right sides. A conditioning stimulus was given to one hemisphere 10 ms prior to the test stimulus at the opposite hemisphere. The inhibition was evaluated as relative amplitude reduction. Eleven normal right-handed subjects and 11 normal left-handed subjects participated in this study. Handedness was evaluated by the Oldfield inventory. It was found that in right-handers the inhibition after stimulation of the “dominant” left hemisphere was more marked than after stimulation of the “non-dominant” right hemisphere. In contrast, the group of left-handed subjects showed inhomogeneous findings with either right- or left-side predominant inhibition. It is concluded that not handedness but hemispheric dominance contributes to the laterality of inhibition. The results point to a superior role of the language-dominant hemisphere in governing inter-hemispheric control of motor cortical connections, supporting the view that the “language-dominant” hemisphere is also “motor dominant”.

Spinal and supraspinal mechanisms contribute to the silent period in the contracting soleus muscle after transcranial magnetic stimulation of human motor cortex.

In the voluntarily activated muscle, transcranial magnetic stimulation (TMS) of motor cortex produces subsequently to the motor evoked potential (MEP) a silent period (SP) in the electromyogram. We studied the time course of soleus motoneuron (MN) pool excitability after conditioning TMS by Hoffmann reflex (HR) testing, to determine whether inaccessibility of MNs after corticospinal input contributes to the SP. Coincidently with the early part of the SP, and only in the contracting soleus, MN depression was obtained that covaried with the degree of preinnervation, and with the size of the preceding MN discharge. However, MN excitability recovered significantly prior to the end of the SP. It is concluded that in the contracting soleus spinal mechanisms (most likely Renshaw inhibition and MN afterhyperpolarization) contribute to the early part of the SP, while the late part of the SP is supraspinal (probably cortical) in origin.

Relationship Between Interhemispheric Inhibition and Motor Cortex Excitability in Subacute Stroke Patients

Background. Studies of stroke patients using functional imaging and transcranial magnetic stimulation (TMS) of the primary motor cortex (M1) demonstrated increased recruitment and abnormally decreased short interval cortical inhibition (SICI) of the M1 contralateral to the lesioned hemisphere (contralesional M1) within the first month after infarction of the M1 or its corticospinal projections. Objective. The authors sought to identify mechanisms underlying decreased SICI of the contralesional M1. Methods. In patients within 6 weeks of their first ever infarction of the M1 or its corticospinal projections, SICI in the M1 of the lesioned and nonlesioned hemisphere was studied using paired-pulse TMS. Interhemispheric inhibition (IHI) was measured by applying TMS to the M1 of the lesioned hemisphere and a second pulse to the homotopic M1 of the nonlesioned hemisphere and vice versa with the patient at rest. The results were compared to M1 stimulation of age-matched healthy controls. Results. SICI was decreased in the M1 of lesioned and nonlesioned hemispheres regardless of cortical or subcortical infarct location. IHI was abnormally decreased from the M1 of the lesioned on nonlesioned hemisphere. In contrast, IHI was normal from the M1 of the nonlesioned on the lesioned hemisphere. Abnormal IHI and SICI were correlated in patients with cortical but not with subcortical lesions. Conclusions. In subacute stroke patients, abnormally decreased SICI of a contralesional M1 can only partially be explained by loss of IHI from the lesioned on nonlesioned hemisphere. As decreased SICI of the contralesional M1 did not result in excessive IHI from the nonlesioned on lesioned hemisphere with subsequent suppression of ipsilesional M1 excitability and all patients showed excellent recovery of motor function, decreased SICI of the contralesional M1 may represent an adaptive process supporting recovery.

Delay in simple reaction time after focal transcranial magnetic stimulation of the human brain occurs at the final motor output stage

It is known that the execution of the motor response in a simple reaction time (RT) task can be delayed by transcranial magnetic stimulation (TMS). This paper is aimed at determining the site of action where the delay in RT occurs. A delay in RT was obtained only at those TMS sites over the motor cortex contralateral to the responding hand, which produced also a muscle twitch in the responding hand. The delay in RT covaried with the TMS intensity and increased the closer the time of TMS approached the expected time of reaction onset. Visual and auditory go-signals yielded similar delays in RT, but only when TMS was applied about 40 ms later for the visual go-signal, corresponding to the modality specific difference in RT control values. TMS of the supplementary motor area (SMA) immediately prior to the expected time of reaction onset produced no delay in RT. Spinal excitability as tested by F waves showed a pre-movement facilitation in the control trials which continued seemingly undisturbed during the period of RT delay after TMS. It can be concluded that the delay in RT is not due to SMA stimulation or spinal inhibition but depends on effective stimulation of neural elements in the motor cortex which are active very late in the process of movement release from the final motor output stage.

Functional retrograde amnesia: A multiple case study

Functional retrograde amnesia (RA) is a rare pathology and has been rarely studied in detail across different patients. We extensively examined five functional RA patients and compared their neuropsychological profile including anterograde and retrograde memory performance, executive functions, emotional processing, and formally assessed psychiatric symptoms. Across patients, neuropsychological deficits beyond RA were most consistently seen in executive functions and attention suggesting that these dysfunctions contribute to the remote memory deficit. In a majority of the patients, problems in social cognition and emotional behaviour were reflected in Theory of Mind deficits and accompanying psychiatric symptoms. Aberrances in a measure of social desirability were detected, pointing to repressive tendencies in three out of the five patients. Future studies of functional RA patients may investigate more specifically which frontal-lobe associated (dys-) functions contribute to the memory retrieval deficit. Moreover, studying more closely the interaction between social cognition, repressive personality style and memory inhibition in this disease seems worthwhile pursuing.

Cerebral potentials during smooth goal-directed hand movements in right-handed and left-handed subjects.

During smooth goal-directed hand movements a negative potential shift can be recorded from the human scalp. This goal-directed movement potential (GDMP) is preponderant over central areas with a maximum at the vertex and, over the motor cortex, contralaterally larger than ipsilterally to the moving hand. In 11 right-handed and 6 left-handed subjects, the morphology and distribution of these potential shifts were studied in relation to task differences, moving hand and handedness. The results show a functional differentiation of two components of the GDMP: (1), a lateralized, slow negative wave, restricted to the precentral area and selectively varying with the hand used, and (2), a widespread, bilateral symmetrical component, selectively influenced by taskspecific and individual factors. Two effects reflect an influence of the subjectss handedness on the GDMP: (a) averaged for both hand conditions, there are larger amplitudes over the hemisphere contralateral to the dominant hand, and (b) averaged for both hemispheres, larger amplitudes result from using the nondominant hand.ZusammenfassungWährend langsamer, zielgerichteter Handbewegungen kann ein negatives Hirnpotential vom menschlichen Schädel abgeleitet werden. Dieses «Zielbewegungspotential» (goal-directed movement potential) ist über zentralen Arealen am stärksten ausgeprägt, zeigt ein Maximum am Vertex und ist über dem motorischen Cortex kontralateral zur bewegungsausführenden Hand größer als ípsilateral. An 11 Rechtshändern und 6 Linkshändern wurden Form und Verteilung dieser Potentiale in Abhängigkeit von der Bewegungsaufgabe, der benutzten Hand und der Händigkeit untersucht. Die Ergebnisse zeigen eine funktionelle Differenzierung von zwei Komponenten des Zielbewegungspotentials: 1. Eine lateralisierte langsame negative Welle, die auf das präzentrale Areal beschränkt ist und selektiv mit der benutzten Hand variiert. 2 Eine breitverteilte bilateral symmetrische Komponente, die selektiv durch aufgabenspezifische und individuelle Faktoren beeinflußt wird. Zwei Effekte spiegeln den Einfluß der Händigkeit des Probanden auf das Zielbewegungspotential wieder: a) gemittelt über beide Handbedingungen sind die Amplituden größer über der Hemisphäre kontralateral zur dominanten Hand. b) Im Mittel über beide Hemisphären sind die Potentiale größer bei Benutzung der nichtdominanten Hand.

Hebbian-type stimulation during robot-assisted training in patients with stroke.

Background. Training-related improvements in motor function are associated with changes in movement representation of the primary motor cortex (M1). In healthy individuals, transcranial magnetic stimulation (TMS) of M1 delivered in a strict temporal relationship (Hebbian-type stimulation) during execution of movements enhances these effects and is superior to random stimulation. Objective. The authors tested whether training combined with Hebbian-type M1 stimulation enhances M1 reorganization in patients with stroke. Methods. Six patients with chronic stroke participated in the study. Patients executed robot-assisted wrist extension movements at 0.2 Hz frequency while subthreshold repetitive TMS was applied over M1 in a strict temporal relationship to the training movements. TMS was applied to either the affected hemisphere (contralateral M1) or the nonaffected hemisphere (ipsilateral M1) at 0.1 Hz. Intervention-related changes in motor maps and intracortical excitability were measured using TMS. Results. Training alone or combined Hebbian-type stimulation of either M1 resulted in differential effects on motor maps and intracortical inhibition. Shifts in motor maps were associated with increases in intracortical excitability. In contrast to previous results for healthy participants, the inhibitory effect of ipsilateral M1 Hebbian-type stimulation was not present, and the facilitatory effect of contralateral M1 stimulation was more subtle. Conclusions. Hebbian-type stimulation is feasible in patients poststroke and induces map reorganization and associated decreases in GABAergic inhibition. However, because TMS protocols have a different effect on motor reorganization in the injured brain and may depend on location of the lesion, protocols need to be tailored to the patient’s pathology.

Category Scaling of the Interest Value of Complex Visual Stimuli and Late Positive Components of the Evoked Potential

Amplitude changes of a late positive component (P300) of evoked potentials have been related to the subjective importance or value of the eliciting stimuli as an explanatory dimension (Sutton et al., 1978; Sutton, 1979). This interpretation is based on the behavioral context of the stimuli (“task relevance”: for a review see Donchin et al., 1978) or on assumptions about their connotative meaning (e.g., monetary payoff: Sutton et al., 1978; Johnston, 1979; Johnston and Holcomb, 1980; Homberg et al., 1981). The present study examines this issue in terms of the subject’s personal rating of the stimulus value. It investigates late positive components of visual evoked potentials in relation to concurrently obtained categorical judgments of the perceived degree of interest in the stimuli.‘

Intact conduction of fastest corticospinal efferents is not sufficient for normal voluntary muscle strength: transcranial motor cortex stimulation in patients with tetraplegia.

In six patients with chronic severe tetraparesis, caused by closed head injury (3 cases), basilar thrombosis (2 cases) or global hypoxia (1 case), responses of suprathreshold transcranial magnetic stimulation of motor cortex to thenar and abductor hallucis muscles on both sides were studied. Results showed in most patients normal thresholds and latencies (17 out of 24 latencies within normal +/- 2 S.D., 20 thresholds within normal +/- 1 S.D.) and amplitudes of even higher than normal values. Latencies or thresholds did not correlate with severity of pareses and were in marked contrast to the severe pareses of the muscles under investigation. Despite different etiology CT-scans showed diffuse atrophy of cerebral white matter as a common finding. It is concluded that intact descending corticospinal motor pathways are a necessary but not a sufficient condition for voluntary motor function.

Event-related changes of fast rhythmic EEG activity in a positioning movement task.

Various EEG and ECoG studies in man (Jasper and Andrews, 1938; Jasper and Penfield, 1949; Gastaut, 1952; Pfurtscheller, 1981) have demonstrated that the central beta rhythm is attenuated or blocked in preparation for and especially during phasic sensorimotor activity. Therefore, having studied slow negative potential shifts during ramplike voluntary positioning movements (Grunewald-Zuberbier et al., 198 I ; Griinewald and Griinewald-Zuber6ier, 1982), we were interested in analyzing concomitant changes of the beta rhythm in the same experimental paradigm in the same subjects. In the following, we give a preliminary report on these studies.