Marko Ollikainen

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.

Alcohol Reduces Prefrontal Cortical Excitability in Humans: A Combined TMS and EEG Study

The effects of alcohol (0.8 g/kg) on the prefrontal cortex were studied in nine healthy subjects using the technique of transcranial magnetic stimulation (TMS) combined with electroencephalography (EEG). A total of 120 magnetic pulses were delivered with a figure-of-eight coil to the left prefrontal cortex at the rate of 0.4–0.7 Hz. The EEG was recorded simultaneously with 60 scalp electrodes (41 electrodes were used for analysis); the TMS-evoked activation was estimated by the area under the global mean field amplitude (GMFA) time curve. TMS caused changes in EEG activity lasting up to 270 ms poststimulus. Alcohol decreased GMFA at 30–270 ms poststimulus (713±303 vs 478±142 μV ms; p=0.007). Alcohol-induced differences were most pronounced at anterior electrodes. These results suggest that alcohol reduces the excitability in the prefrontal cortex.

Coil design for real and sham transcranial magnetic stimulation

Transcranial magnetic stimulation (TMS) can be used to excite the human cortex noninvasively. TMS also activates scalp muscles and sensory receptors; additionally, the loud sound from the stimulating coil activates auditory pathways. These side effects complicate the interpretation of the results of TMS studies. For control experiments, the authors have designed a coil that can produce both real and sham stimulation without moving the coil. The sham TMS is similar to the real TMS, except for the different relative direction of the currents in the two loops of the figure of eight coil. While the real TMS elicited activation of hand muscles, sham TMS had no such effect; however, the auditory-evoked potentials were similar.

Frequency-related effects in the optimization of coils for the magnetic stimulation of the nervous system

Magnetic stimulation of the nervous system is a noninvasive technique with a large number of applications in neurological diagnosis, brain research, and, recently, therapy. New applications require engineering modifications in order to decrease power consumption and coil heating. This can be accomplished by optimized coils with minimized resistance. In this study the influence of some frequency-related effects (skin and proximity effect) on the coil resistance are discussed, together with the role played by wire shape, wire section, and twisting effect. The results show that the coil resistance increases with frequency. As an example, for a 20-mm/sup 2/ circular wire section, the skin effect in the typical frequency range of magnetic stimulator devices (2-4 kHz) increases the coil resistance up to about 45% with respect to its dc value. Moreover, the influence of the frequency is lower for flat wire sections and reasonably small helix twist angle of the coil.

Transcallosal connectivity revealed by transcranial magnetic stimulation and high-resolution EEG

Transcranial magnetic stimulation (TMS) combined with high-resolution electmencephalography (HR-EEG) enables the noninvasive study of cortical reactivity and connectivity. The TMS-EEG method can be utilized to study directly the transcallosal connections between brain hemispheres. In TMS, strong magnetic field pulses induce an electric field in the brain, This causes membrane depolarization and, above a certain threshold, neuronal discharge. The spread of activation can be detected with EEG electrodes on the scalp. The effect of TMS is strongest on neurons in the superficial gray matter. Cortico-cortical information transfer is contributed by pyramidal cells of the cortical layers II and III. The majority of the fibers forming a conduction pathway between hemispheres cross via the corpus callosmn. The present work was carried out in order to define transcallosally mediated conduction velocities and times between frontal and anterior parietal regions, i.e., the connectivity of these brain areas, in normals. Nine healthy, right-handed volunteers (23-75 years of age; mean, 44) took part in the study. Five target points of the left frontal and anterior parietal cortex were stimulated magnetically, and EEG was continuously recorded with 60 electrodes from the scalp. For three subjects, the study was repeated after a 2-5 minutes’ break. The positions of the head, electrodes, and the stimulating coil were digitized (Polhemus, USA) and transferred to the same co-ordinate system as the subject’s SD-MRI image set. Potential maps were interpolated from the measured electrode signals. For the calculation of current-density estimates, representing the brain activity that caused the observed electrode potentials, the inverse problem was solved using the minimum-norm estimate [l]. Potential and current-density distributions were visualized above the actual cortical structure of the subjects. The lower limit for the axonal conduction velocity from the primary hand motor area to the opposite hemisphere was determined by measuring the average distance from the cortical area likely representing the small right hand muscles to the genu of the corpus callosum and by dividing that by half of the average time of the emergence of the contralateral activation in the current-density maps. The contralateral activation was first observed at 21 + 4 ms post-stimulus, and it reached its maximum at 24 + 4 ms. When stimulating the primary hand motor area, the contralateral activation times were the same within 1 ms in the three subjects studied. The conduction velocity was calculated at 7 -t I m/s. The transcallosal conduction times are in agreement with earlier smdies [2,3]. Our results, in agreement with the study of Ilmoniemi et al. [2], suggest the use of the combined TMS and EEG method in the direct investigation of defective transcallosal conduction in patients with neurodegenerative diseases such as stroke. Lesions in the cerebral tissue can hinder the transcallosal conduction or change the conduction pathways between cortical areas. Earlier, transcallosal conduction times have been evaluated indirectly using TMS and, e.g., EMG recording from extremities [4,5]. Recording from a muscle is, however, useful only when peripheral activity can be measured, which in many patients is not the case. The TMS-EEG method also enables the study of other than motor-associated functions. Transcallosally mediated conduction times from frontal and anterior parietal regions to the opposite hemisphere were determined using combined TMS and EEG. The standard deviation in conduction times between normals was 5 4 ms. Transcallosal conduction times can be determined with an accuracy of 1 ms with the combination of TMS and HR-EEG. Based on these conclusions, we suggest the use of this method in the evaluation of interhemispheric conduction times and the reorganization of conduction pathways during the recovery and treatment of stroke patients.