Transcranial magnetic stimulation and neuroimaging

Ebmeier KP. Transcranial magnetic stimulation and neuroimaging. Bipolar Disord 2002: 4(Suppl. 1): 96–97. © Blackwell Munksgaard, 2002     

Transcranial magnetic stimulation and neuroplasticity

We review past results and present novel data to illustrate different ways in which TMS can be used to study neural plasticity. Procedural learning during the serial reaction time task (SRTT) is used as a model of neural plasticity to illustrate the applications of TMS. These different applications of TMS represent principles of use that we believe are applicable to studies of cognitive neuroscience in general and exemplify the great potential of TMS in the study of brain and behavior. We review the use of TMS for (1) cortical output mapping using focal, single-pulse TMS; (2) identification of the mechanisms underlying neuroplasticity using paired-pulse TMS techniques; (3) enhancement of the information of other neuroimaging techniques by transient disruption of cortical function using repetitive TMS; and finally (4) modulation of cortical function with repetitive TMS to influence behavior and guide plasticity.     

Transcranial magnetic stimulation in depression

Among the new therapeutic techniques in psychiatry, transcranial magnetic stimulation (TMS) seems to bring a profit in the treatment of depressions. It uses the principle of inductance to generate a magnetic current, which in turn activates cortical neurons. Stimulation is highly focused and interests specific regions of the cerebral cortex. This therapeutic technique is generally well tolerated. Side effects are rare, the most hampering one is epileptic seizures. It is favored by high frequencies (above 5 Hz) and arises mainly with patients having a history of personal or family epileptic seizures. The first open trials, quickly confirmed by controlled studies showed the efficiency of TMS in depression. With depression, double blind randomized trials, using high frequencies, stimulation of the left dorsolateral prefrontal cortex give positive results with significant decrease of scores on depressive scales applied to resistant and non resistant depressions. Some studies have stimulated the right dorsolateral prefrontal cortex using low frequencies. The decrease of scores is also significant on depressive scales. The modulating effect of rTMS on cortical excitability of the brain justifies this distinction between high and low frequencies, high frequencies having a facilitating effect whereas low frequencies have an inhibitory effect.     

Transcranial magnetic stimulation in neurology

Transcranial magnetic stimulation (TMS) is a non-invasive tool for the electrical stimulation of neural tissue, including cerebral cortex, spinal roots, and cranial and peripheral nerves. TMS can be applied as single pulses of stimulation, pairs of stimuli separated by variable intervals to the same or different brain areas, or as trains of repetitive stimuli at various frequencies. Single stimuli can depolarise neurons and evoke measurable effects. Trains of stimuli (repetitive TMS) can modify excitability of the cerebral cortex at the stimulated site and also at remote areas along functional anatomical connections. TMS might provide novel insights into the pathophysiology of the neural circuitry underlying neurological and psychiatric disorders, be developed into clinically useful diagnostic and prognostic tests, and have therapeutic uses in various diseases. This potential is supported by the available studies, but more work is needed to establish the role of TMS in clinical neurology.     

Transcranial magnetic stimulation in cognitive rehabilitation

Repetitive transcranial magnetic stimulation (rTMS) can generate an increase or a decrease of neuronal excitability, which can modulate cognition and behaviour. Transcranial magnetic stimulation-induced cortical changes have been shown to result in neural plasticity. Thus, TMS provides an important opportunity to gain more insight into the mechanisms responsible for the remarkable flexibility of the central nervous system. The aim of this review was to cover the topics that could be useful when using TMS in the cognitive rehabilitation field after brain damage. The basic TMS principles are introduced, together with the clinical application for diagnosis and prognosis, the biological aspects, and the use in cognitive neuroscience studies. Finally, several hypotheses are discussed to explain the likely mechanisms induced by TMS that favour the recovery of a function after brain damage and cause the adult brain to undergo plasticity. The possibility of non-invasively interacting with the functioning of the brain and its plasticity mechanisms - a possibility that may eventually lead to cognitive and behavioural modifications - opens new and exciting scenarios in the cognitive neurorehabilitation field.     

Transcranial magnetic stimulation and epilepsy.

Transcranial magnetic stimulation has been used to study generalized and focal epilepsies for more than a decade. The technique appears safe and has yielded important information about the mechanisms underlying epilepsy. Transcranial magnetic stimulation findings differ depending on the epilepsy syndrome, lending support to the concept that there are distinct pathophysiologies underlying each condition. In most studies of generalized epilepsies, transcranial magnetic stimulation has indicated a state of relative hyperexcitability of excitatory cortical interneurons and possibly inhibitory interneurons as well, which can be reversed through the actions of anticonvulsant medications. Transcranial magnetic stimulation studies in patients with a seizure focus in the motor cortex indicate increased cortical excitability and reduced inhibition, but in patients with seizure foci located elsewhere the findings are similar to those in generalized epilepsies. Transcranial magnetic stimulation has also been used to study the mode of action of anticonvulsants and may prove to be a useful means of testing the potential for new drugs to act as anticonvulsants. Repetitive transcranial magnetic stimulation may prove to have a therapeutic role by producing long-lasting cortical inhibition after a train of impulses.     

Transcranial magnetic stimulation for nicotine dependence

70-80% of regular smokers fulfill the ICD-10-criteria of dependence. In Germany, approximately 120,000 deaths per year are caused by tobacco-associated diseases. In contrast, therapeutic interventions, such as nicotine substitution or bupropione, yield poor abstinence rates of 30% after 12 months, at best. In animal experiments, repetitive transcranial magnetic stimulation (rTMS) exhibited modulatory effects on dopaminergic neurotransmission in regions of the so-called reward system. This pilot study should evaluate, if rTMS could modulate subjective craving for tobacco, which quite often leads to relapse to smoking. Therefore, 11 tobacco-dependent cigarette smokers were randomly assigned to a course of verum- and placebo-rTMS on consecutive days. Craving, as measured by a visual analogue scale, is significantly decreased after Verumstimulation compared to placebo-stimulation intra-individually. This encourages further studies to clarify, if rTMS might be helpful in achieving higher tobacco abstinence rates in smokers willing to quits.     

Transcranial magnetic stimulation in children.

Developmental disabilities (e.g. attention deficit disorder; cerebral palsy) are frequently associated with deviations of the typical pattern of motor skill maturation. Neurophysiologic tools, such as transcranial magnetic stimulation (TMS), which probe motor cortex function, can potentially provide insights into both typical neuromotor maturation and the mechanisms underlying the motor skill deficits in children with developmental disabilities. These insights may set the stage for finding effective interventions for these disorders. We review the literature pertaining to the use of TMS in pediatrics. Most TMS-evoked parameters show age-related changes in typically developing children and some of these are abnormal in a number of childhood-onset neurological disorders. Although no TMS-evoked parameters are diagnostic for any disorder, changes in certain parameters appear to reflect disease burden or may provide a measure of treatment-related improvement. Furthermore, TMS may be especially useful when combined with other neurophysiologic modalities (e.g. fMRI). However, much work remains to be done to determine if TMS-evoked parameters can be used as valid and reliable biomarkers for disease burden, the natural history of neurological injury and repair, and the efficacy of pharmacological and rehabilitation interventions.