Jari Karhu

Ipsi- and contralateral EEG reactions to transcranial magnetic stimulation

OBJECTIVES Transcranial magnetic stimulation (TMS) and high-resolution electroencephalography (EEG) were used to study the spreading of cortical activation in 6 healthy volunteers. METHODS Five locations in the left sensorimotor cortex (within 3cm(2)) were stimulated magnetically, while EEG was recorded with 60 scalp electrodes. A frameless stereotactic method was applied to determine the anatomic locus of stimulation and to superimpose the results on magnetic resonance images. Scalp potential and cortical current-density distributions were derived from averaged electroencephalographic (EEG) data. RESULTS The maxima of the ipsilateral activation were detected at the gyrus precentralis, gyrus supramarginalis, and lobulus parietalis superior, depending on the subject. Activation over the contralateral cortex was observed in all subjects, appearing at 22plus minus2ms (range 17--28); the maxima were located at the gyrus precentralis, gyrus frontalis superior, and the lobulus parietalis inferior. Contralateral EEG waveforms showed consistent changes when different sites were stimulated: stimulation of the two most medial points evoked the smallest responses fronto-parietally. CONCLUSIONS With the combination of TMS, EEG, and magnetic resonance imaging, an adequate spatiotemporal resolution may be achieved for tracing the intra- and interhemispheric spread of activation in the cortex caused by a magnetic pulse.

Ethanol modulates cortical activity: Direct evidence with combined TMS and EEG

Abstract The motor cortex of 10 healthy subjects was stimulated by transcranial magnetic stimulation (TMS) before and after ethanol challenge (0.8 g/kg resulting in blood concentration of 0.77 ± 0.14 ml/liter). The electrical brain activity resulting from the brief electromagnetic pulse was recorded with high-resolution electroencephalography (EEG) and located using inversion algorithms. Focal magnetic pulses to the left motor cortex were delivered with a figure-of-eight coil at the random interstimulus interval of 1.5–2.5 s. The stimulation intensity was adjusted to the motor threshold of abductor digiti minimi. Two conditions before and after ethanol ingestion (30 min) were applied: (1) real TMS, with the coil pressed against the scalp; and (2) control condition, with the coil separated from the scalp by a 2-cm-thick piece of plastic. A separate EMG control recording of one subject during TMS was made with two bipolar platinum needle electrodes inserted to the left temporal muscle. In each condition, 120 pulses were delivered. The EEG was recorded from 60 scalp electrodes. A peak in the EEG signals was observed at 43 ms after the TMS pulse in the real-TMS condition but not in the control condition or in the control scalp EMG. Potential maps before and after ethanol ingestion were significantly different from each other ( P = 0.01), but no differences were found in the control condition. Ethanol changed the TMS-evoked potentials over right frontal and left parietal areas, the underlying effect appearing to be largest in the right prefrontal area. Our findings suggest that ethanol may have changed the functional connectivity between prefrontal and motor cortices. This new noninvasive method provides direct evidence about the modulation of cortical connectivity after ethanol challenge.

Transcranial magnetic stimulation as a tool for cognitive studies

Transcranial Magnetic Stimulation (TMS) is a tool for the non-invasive stimulation of the human brain. It allows the activation of arbitrary sites of the superficial cortex and, combined with other brain-imaging techniques such as EEG, PET, and fMRI, it can be used to evaluate cortical excitability and connectivity. This is of major importance in, for example, the study of cognitive processes such as language, learning, memory and self-representation, which are thought to be represented in multiple brain areas. The mechanisms of action of TMS are known on a basic level, but its effect on the activation state of brain tissue is still poorly understood. Clinical applications of TMS have also been proposed and guidelines for its safe use drafted.

Navigated Transcranial Magnetic Stimulation: Principles and Protocol for Mapping the Motor Cortex

Transcranial magnetic stimulation (TMS) is a unique method for studying the human brain. Whereas the majority of imaging tools detect and map all the brain areas that participate during a given task (both primary and secondary network activations), TMS, when used to evoke a measurable physiological response, maps only those areas that are mandatory for the observed reaction. As such, TMS is particularly suitable for mapping cortical motor areas and for assessing the functional status of the motor tracts, both in normal subjects and in patients. In this chapter, we explore the physical and mechanistic background of using TMS to map the motor cortex. In addition, we outline a detailed protocol for mapping the cortical representation of various muscles—a protocol which can be used in basic research or as part of a clinical diagnostic or treatment procedure.