Georg Kerkhoff

Electrified minds: Transcranial direct current stimulation (tDCS) and Galvanic Vestibular Stimulation (GVS) as methods of non-invasive brain stimulation in neuropsychology—A review of current data and future implications

Transcranial direct current stimulation (tDCS) is a noninvasive, low-cost and easy-to-use technique that can be applied to modify cerebral excitability. This is achieved by weak direct currents to shift the resting potential of cortical neurons. These currents are applied by attaching two electrodes (usually one anode and one cathode) to distinct areas of the skull. Galvanic Vestibular Stimulation (GVS) is a variant of tDCS where the electrodes are attached to the mastoids behind the ears in order to stimulate the vestibular system. tDCS and GVS are safe when standard procedures are used. We describe the basic physiological mechanisms and application of these procedures. We also review current data on the effects of tDCS and GVS in healthy subjects as well as clinical populations. Significant effects of such stimulation have been reported for motor, visual, somatosensory, attentional, vestibular and cognitive/emotional function as well as for a range of neurological and psychiatric disorders. Moreover, both techniques may induce neuroplastic changes which make them promising techniques in the field of neurorehabilitation. A number of open research questions that could be addressed with tDCS or GVS are formulated in the domains of sensory and motor processing, spatial and nonspatial attention including neglect, spatial cognition and body cognition disorders, as well as novel treatments for various neuropsychological disorders. We conclude that the literature suggests that tDCS and GVS are exciting and easily applicable research tools for neuropsychological as well as clinical-therapeutic investigations.

Spatial hemineglect in humans

The paper reviews the main findings of studies of hemispatial neglect after acquired brain lesions in people. The behavioral consequences of experimentally induced lesions in animals and electrophysiological studies, which shed light on the nature of the disorder, are briefly considered. Neglect is behaviorally defined as a deficit in processing or responding to sensory stimuli in the contralateral hemispace, a part of the own body, the part of an imagined scene, or may include the failure to act with the contralesional limbs despite intact motor functions. Neglect in humans is frequently encountered after right parieto-temporal lesions and leads to a multicomponent syndrome of sensory, motor and representational deficits. Relevant findings relating to neglect, extinction and unawareness are reviewed and include the following topics: etiological and anatomical basis, recovery; allocentric, egocentric, object-centered and representational neglect; motor neglect and directional hypokinesia; elementary sensorimotor and associated disorders; subdivisions of space and frames of reference; extinction versus neglect; covert processing of information; unawareness of deficits; human and animal models; effects of sensory stimulation and rehabilitation techniques.

Neck muscle vibration induces lasting recovery in spatial neglect

Objectives: To evaluate whether neck muscle vibration is an effective technique for neglect rehabilitation, with lasting beneficial effects. Methods: The effects of differential treatment of visual exploration training alone or in combination with neck muscle vibration were evaluated in a crossover study of two matched groups of 10 patients suffering from left sided neglect. Each group received a sequence of 15 consecutive sessions of exploration training and combined treatment. The effects of treatment were assessed with respect to different aspects of the neglect disorder such as impaired perception of the egocentric midline, exploration deficits in visual and tactile modes, and visual size distortion. The transfer of treatment effects to activities of daily living was examined by a reading test and a questionnaire of neglect related everyday problems. All variables were measured six times: three baseline measurements, two post-treatment measurements, and one follow up after two months. Results: The results showed superior effects of combination treatment. A specific and lasting reduction in the symptoms of neglect was achieved in the visual mode, which transferred to the tactile mode with a concomitant improvement in activities of daily living. The improvement was evident two months after the completion of treatment. In contrast, isolated exploration training resulted in only minor therapeutic benefits in visual exploration without any significant transfer effects to other tasks. Conclusions: Neck muscle vibration is a decisive factor in the rehabilitation of spatial neglect and induces lasting recovery when given as a supplement to conventional exploration training.

Rehabilitation of neglect: An update

Spatial neglect is a characteristic sign of damage to the right hemisphere and is typically characterized by a failure to respond to stimuli on the left side. With about a third of stroke victims showing initial signs of neglect, it is a frequent but also one of the most disabling neurological syndromes. Despite partial recovery in the first months after stroke one third of these patients remain severely disabled in all daily activities, have a poor rehabilitation outcome and therefore require specific treatment. The last decades have seen an intensive search for novel, more effective treatments for this debilitating disorder. An impressive range of techniques to treat neglect has been developed in recent years. Here, we describe those techniques, review their efficacy and identify gaps in the current research on neglect therapy.

Rehabilitation of homonymous scotomata in patients with postgeniculate damage of the visual system: saccadic compensation training

A systematic training procedure for patients with disturbed visual search was evaluated in 92 patients with postchiasmatic visual field disorders (VFD) and 30 VFD patients with additional leftsided visual neglect (VFD +). Visual fields and areas of visual search via saccadic eye movements in the scotomatous field (search field) were mapped perimetrically in all patients before and after training and after a follow-up interval (mean follow-up interval: 22 months). A significant increase of more than 20° of visual angle was found in visual search field in the scotomatous field during training. Search field remained stable at follow-up in both patient groups. In contrast, only minor, though significant increases in visual field size were obtained during search field training in some patients. Multiple baseline designs in 5 patients revealed that search field enlargements were training dependent and not related to spontaneous recovery, adaptation to test procedures or measurement variability. VFD + Neglect patients needed some 25% more therapy sessions than VFD patients to achieve the same amount of increase in visual search field. Frequent head movements had a deleterious effect on training progress. It is concluded that the presented training procedure can lead to a significant and stable improvement in visual search within 15 to 25 training sessions and hence to a better compensation of visual deficits in daily living in most patients with VFD and/or visual neglect.

Modulation and rehabilitation of spatial neglect by sensory stimulation.

After unilateral cortical or subcortical, often parieto-temporal lesions, patients exhibit a marked neglect of their contralateral space and/or body side. These patients are severely disabled in all daily activities, have a poor rehabilitation outcome and therefore require professional treatment. Unfortunately, effective treatments for neglect are just in the process of development. The present chapter reviews three aspects related to the rehabilitation of neglect. The first part summarizes findings about spontaneous recovery in patients and experimental animals with neglect. The second part deals with techniques and studies evaluating short-term sensory modulation effects in neglect. In contrast to many other neurological syndromes spatial neglect may be modulated transiently but dramatically in its severity by sensory (optokinetic, neck proprioceptive, vestibular, attentional, somatosensory-magnetic) stimulation. In part three, current treatment approaches are summarized, with a focus on three novel techniques: repetitive optokinetic stimulation, neck vibration training and peripheral somatosensory-magnetic stimulation. Recent studies of repetitive optokinetic as well as neck vibratory treatment both indicate significantly greater as well as multimodal improvements in neglect symptomatology as compared to the standard treatment of neglect. This clear superiority might result from the partial (re)activation of a distributed, multisensory vestibular network in the lesioned hemisphere. Somatosensory-magnetic stimulation of the neglected or extinguishing hand provides another feasible, non-invasive stimulation technique. It may be particularly suited for the rehabilitation of somatosensory extinction and unawareness of the contralesional body side. Finally, pharmacological approaches for the treatment of neglect are shortly addressed. Isolated drug treatment of neglect is currently no successful rehabilitation strategy due to inconsistent results as well as possible side effects. However, combined behavioural and drug treatments might yield better results. This has to be tested empirically in patient studies. In conclusion, the findings obtained in short-term sensory stimulation studies led to the development of effective techniques for the long-term rehabilitation of neglect. Future rehabilitation studies should evaluate effective treatment combinations considering all possible techniques and devices (behavioural, pharmacological, prosthetic or physiological).

Effects of unilateral brain damage on grip selection, coordination, and kinematics of ipsilesional prehension

Abstract To determine whether the left and right hemispheres play specific roles in goal-directed movements, prehension with the ipsilesional hand was tested in patients with unilateral brain damage. The task required that subjects rotate the hand while reaching for a bar that was presented in different orientations in the frontal plane, thus making high demands on visuospatial processing. The grasped bar had to be put into a hole: under one task condition the placement of the bar was specified, while under another it was not. The constrained task required that the subject anticipate the placing action when planning the initial prehensile movement. Grip selection, reaction times, kinematics of the transport movement, and coordination of hand rotation during transport were assessed in ipsilesional movements of 22 patients with either left or right brain damage (LBD and RBD) and in control subjects. Patients in both groups exhibited performance deficits; however, impairment characteristics differed profoundly between the groups. RBD patients showed prolonged reaction time and degraded kinematics in the unconstrained task, whereas LBD patients performed relatively well when only the orientation of the bar varied, but slowly and frequently incoordinated when the subsequent action was specified. Our findings emphasize the dominant role of the right hemisphere in processing visuospatial aspects of goal-directed movements, whereas the left hemisphere subserves non-spatial aspects of preplanning under increased task demands. Correlations of the patient’s performance with results from clinical tests showed that neither deficits in visuospatial perception of RBD patients nor apraxia of LBD patients could account for the observed abnormalities in the use of the ipsilesional hand.

Rehabilitation of hemianopic alexia in patients with postgeniculate visual field disorders

Abstract A new systematic training procedure for patients with hemianopic alexia resulting from postchiasmatic visual field defects (VFD) was evaluated in 56 patients. Reading performance was measured with standardised reading tests before and after treatment and at a follow-up after treatment (mean follow-up interval, 22 months). In addition, visual fields were mapped perimetrically in all patients before and after training and at follow-up. Significant improvements in reading time and reading errors were obtained during the treatment phase, which remained stable at long-term follow-up. In contrast, only minor, though significant, increases in visual field size were obtained during training. Multiple baseline designs in four patients revealed that improvements in reading were training-dependent and not related to spontaneous recovery, measurement variability or other therapies performed partially in parallel to reading training. A significant improvement in reading could be reached within about 14 (range...

Repetitive peripheral magnetic stimulation alleviates tactile extinction.

Despite its frequency in right brain damaged patients crucial mechanisms of tactile extinction are still obscure and treatments are unavailable. Recent PET observations suggest a hypometabolism in the primary and secondary somatosensory cortex of the lesioned hemisphere in patients with tactile extinction. Functional and morphological investigations have shown that the sensorimotor cortex has a remarkable capability of reorganization when the sensory inflow is changed. Repetitive peripheral magnetic stimulation (RPMS) applied in patients suffering from central paresis alleviates sensorimotor as well as cognitive deficits by the induction of proprioceptive inflow, thereby activating plasticity in the CNS. Based on the observation of reduced metabolic activity in patients suffering from tactile extinction we applied RPMS to explore the effects of peripheral sensory stimulation on tactile extinction. Fourteen right-hemisphere lesioned patients with tactile extinction were randomly allocated to an experimental and a control group. The experimental group received one single RPMS treatment of the left forearm as well as a condition of attentional cueing known to improve visual extinction. The control group, with comparable tactile extinction scores, neither received RPMS nor verbal cueing, but was tested twice to evaluate possible learning or test repetition effects. In the experimental group RPMS led to a significant reduction of left-sided extinctions in the recognition of different tactual surfaces, but had no effect on ipsilesional errors. In contrast, attentional cueing had no significant effect on left-sided extinction errors but unexpectedly increased right-hand extinction errors slightly but significantly. The control group showed stable extinction scores of the left- and right-hand stimulus across two measurements, thus ruling out learning or test repetition effects. These results show that sensory inflow is an important modulatory factor in tactile extinction. Furthermore, multiple RPMS may prove a promising way for the rehabilitation of patients with this disorder.

Head and trunk orientation modulate visual neglect

FIVE patients with left visual neglect after right hemisphere lesions were examined with a line bisection and a reading task under five different conditions: head and trunk straight ahead, head or trunk oriented 20° to the left and head or trunk oriented 20° to the right. Fixation was always straight ahead. Five patients with right hemisphere lesions but without neglect and five normal subjects served as controls. In all neglect patients, turning the head or trunk to the left reduced line bisection and reading errors significantly as compared with the other three conditions and with the control groups. The modulation of neglect behaviour by trunk as well as head position supports the hypothesis of a disturbed egocentric coordinate system leading to neglect.