Volker Hömberg

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”.

Auditory sustained attention is a marker of unilateral spatial neglect

The relationships between performance on a non-spatially-lateralized measure of sustained attention and spatial bias on tests sensitive to unilateral neglect were considered in a group of 44 patients with right hemisphere lesions following stroke. As predicted from earlier studies showing a strong association between unilateral spatial neglect and sustained attention, performance on a brief and monotonous tone-counting measure formed a significant predictor of spatial bias across a variety of measures of unilateral visual neglect. This study provides further evidence for a very close link between two attentional systems hitherto regarded as being quite separate, namely a spatial attention system implicated in unilateral neglect and a sustained attention system. A close connection between these two systems was predicted by Posner, who argued that the right hemisphere-dominant sustained attention system provides a strong modulatory influence on the functioning of the lateralized posterior attention system.

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.

Magnetic stimulation of motor cortex and nerve roots in children. Maturation of cortico-motoneuronal projections

Magnetoelectrical stimulation of motor cortex and peripheral nerve roots was used to establish the maturational profiles of central and peripheral nervous system conduction times in normal children in upper and lower extremity muscles. To obtain true central conduction times, not contaminated by variation due to different preinnervational levels, a protocol using stimulation under full muscular relaxation was used. With this protocol clear responses of the upper extremity could be obtained after the first year of life, whereas in the lower extremity they could not be obtained before the fourth year of life. The developmental profile of central conduction times to upper and lower extremity muscles showed an age-dependent acceleration with adult values not being reached before the age of about 10 years. In contrast, peripheral conduction times obtained after cervical or lumbar root stimulation to upper and lower extremities showed a much faster maturational profile, with adult values being reached at about the age of 3 years. The data are discussed in relation to the available literature on neuromorphological development and provide normative values for the understanding of normal development of motor control in children as well as for application of this technique in children with motor disturbances.

Representations of graphomotor trajectories in the human parietal cortex: Evidence for controlled processing and automatic performance

The aim of this study was to identify the cerebral areas activated during kinematic processing of movement trajectories. We measured regional cerebral blood flow (rCBF) during learning, performance and imagery of right‐hand writing in eight right‐handed volunteers. Compared with viewing the writing space, increases in rCBF were observed in the left motor, premotor and frontomesial cortex, and in the right anterior cerebellum in all movement conditions, and the increases were related to mean tangential writing velocity. No rCBF increases occurred in these areas during imagery. Early learning of new ideomotor trajectories and deliberately exact writing of letters both induced rCBF increases in the cortex lining the right intraparietal sulcus. In contrast, during fast writing of overlearned trajectories and in the later phase of learning new ideograms the rCBF increased bilaterally in the posterior parietal cortex. Imagery of ideograms that had not been practised previously activated the anterior and posterior parietal areas simultaneously. Our results provide evidence suggesting that the kinematic representations of graphomotor trajectories are multiply represented in the human parietal cortex. It is concluded that different parietal subsystems may subserve attentive sensory movement control and whole‐field visuospatial processing during automatic performance.

Conscious and subconscious sensorimotor synchronization--prefrontal cortex and the influence of awareness.

One of the most compelling challenges for modern neuroscience is the influence of awareness on behavior. We studied prefrontal correlates of conscious and subconscious motor adjustments to changing auditory rhythms using regional cerebral blood flow measurements. At a subconscious level, movement adjustments were performed employing bilateral ventral mediofrontal cortex. Awareness of change without explicit knowledge of the nature of change led to additional ventral prefrontal and premotor but not dorsolateral prefrontal activations. Only fully conscious motor adaptations to a changing rhythmic pattern showed prominent involvement of anterior cingulate and dorsolateral prefrontal cortex. These results demonstrate that while ventral prefrontal areas may be engaged in motor adaptations performed subconsciously, only fully conscious motor control which includes motor planning will involve dorsolateral prefrontal cortex.

Development of speed of repetitive movements in children is determined by structural changes in corticospinal efferents

This study was aimed to determine the relationship between the maturation of corticospinal efferents, determined by transcranial stimulation of motor cortex, and the development of fastest repetitive voluntary motor activity in children. The development of fastest repetitive voluntary motor activity was assessed for 3 different types of movements including fastest repetitive tapping movements, aiming movements and a pegboard transportation task. These 3 motor activities were chosen as they were different as to their dependence on detailed sensory guidance. Despite these differences the speed of all 3 movements showed a very similar developmental profile, which was matched, however, by the developmental slope of the fastest cortico-motoneuronal efferents. Hence the development of central conduction times determines the speed of repetitive movements in children. In contrast, we could not observe significant effects of repetitive training on speed of these movements. We show for the first time that the development of fastest voluntary movements is a structure-bound phenomenon, being independent from learning.

Transcranial stimulation of motor cortex in upper motor neurone syndrome: its relation to the motor deficit.

The purpose of this investigation was to clarify the functional significance of the fastest cortico-motoneuronal connections in chronic upper motor neurone syndromes. Using magneto-electrical stimulation of motor cortex the intactness of cortico-motoneuronal connections was assessed in 51 patients presenting with variable degrees of impairment. There was a gross correlation between clinical impairment of the patient and the degree of pathology of cortico-motoneuronal efferents. Covariation of clinical data with transcranial stimulation was better than covariation with the size of lesion on CT scans. In some patients, however, definite clinical impairment, especially affecting distal fractionated movements, was associated with completely normal responses. There was no evidence of response abnormality in distal muscles ipsilateral to the hemispheric lesion. The data indicate that motor deficit can exist in the presence of normal cortico-motoneuronal conduction times, showing that intactness of these connections is not a sufficient condition for preservation of voluntary motor activities. This underlines the importance of other pathways for the pathogenesis of upper motor neurone syndromes.

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.