EEG Artifact Removal in TMS Studies of Cortical Speech Areas.

The combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) is commonly applied for studying the effective connectivity of neuronal circuits. The stimulation excites neurons, and the resulting TMS-evoked potentials (TEPs) are recorded with EEG. A serious obstacle in this method is the generation of large muscle artifacts from scalp muscles, especially when frontolateral and temporoparietal, such as speech, areas are stimulated. Here, TMS-EEG data were processed with the signal-space projection and source-informed reconstruction (SSP-SIR) artifact-removal methods to suppress these artifacts. SSP-SIR suppressed muscle artifacts according to the difference in frequency contents of neuronal signals and muscle activity. The effectiveness of SSP-SIR in rejecting muscle artifacts and the degree of excessive attenuation of brain EEG signals were investigated by comparing the processed versions of the recorded TMS-EEG data with simulated data. The calculated individual lead-field matrix describing how the brain signals spread on the cortex were used as simulated data. We conclude that SSP-SIR was effective in suppressing artifacts also when frontolateral and temporoparietal cortical sites were stimulated, but it may have suppressed also the brain signals near the stimulation site. Effective connectivity originating from the speech-related areas may be studied even when speech areas are stimulated at least on the contralateral hemisphere where the signals were not suppressed that much.

Transcranial magnetic stimulation-evoked potentials after the stimulation of the right-hemispheric homologue of Broca's area.

The combination of transcranial magnetic stimulation and electroencephalography can be applied to probe effective connectivity. Neurons are excited by magnetic pulses, which produce transcranial magnetic stimulation-evoked potentials that can be monitored with electroencephalography. Effective connectivity refers to causal connections in the brain; it describes how different brain areas communicate with each other. Broca's area is crucial for all phases of speech processing and is located in the frontotemporal region of the cortex. Only a few studies have investigated this region using transcranial magnetic stimulation-electroencephalography because of the large cranial muscles that are located over these areas, resulting in large artifacts covering the transcranial magnetic stimulation-evoked potentials. However, it is shown that this obstacle can be overcome with new artifact-removal tools. We used minimum-norm estimation to locate the sources of the neuronal signals in electroencephalography data after stimulating the right-hemispheric homologue of Broca's area in three right-handed subjects; it was shown that the spreading of brain activity might be different for different individuals and that the brain activity spread fast to the contralateral hemisphere.

The effect of experimental pain on short-interval intracortical inhibition with multi-locus transcranial magnetic stimulation.

Chronic neuropathic pain is known to alter the primary motor cortex (M1) function. Less is known about the normal, physiological effects of experimental neurogenic pain on M1. The objective of this study is to determine how short-interval intracortical inhibition (SICI) is altered in the M1 representation area of a muscle exposed to experimental pain compared to SICI of another muscle not exposed to pain. The cortical representation areas of the right abductor pollicis brevis (APB) and biceps brachii (BB) muscles of 11 subjects were stimulated with a multi-locus transcranial magnetic stimulation device while the resulting motor-evoked potentials (MEPs) were recorded with electromyography. Single- and paired-pulse TMS was administered in seven conditions, including one with the right hand placed in cold water. The stimulation intensity for the conditioning pulses in the paired-pulse examination was 80% of the resting motor threshold (RMT) of the stimulated site and 120% of RMT for both the test and single pulses. The paired-pulse MEP amplitudes were normalized with the mean amplitude of the single-pulse MEPs of the same condition and muscle. SICI was compared between conditions. After the cold pain, the normalized paired-pulse MEP amplitudes decreased in APB, but not in BB, indicating that SICI was potentially increased only in the cortical area of the muscle subjected to pain. These data suggest that SICI is increased in the M1 representation area of a hand muscle shortly after exposure to pain has ended, which implies that short-lasting pain can alter the inhibitory balance in M1.

Individual Activation Patterns After the Stimulation of Different Motor Areas: A Transcranial Magnetic Stimulation-Electroencephalography Study.

The combination of transcranial magnetic stimulation (TMS) and electroencephalography (EEG) enables one to study effective connectivity and activation order in neuronal networks. To characterize effective connectivity originating from the primary motor cortex (M1), dorsal premotor area (PMd), and supplementary motor area (SMA). Three right-handed volunteers (two men, aged 25-30 years) participated in a navigated TMS-EEG experiment. M1, PMd, and SMA over the nondominant hemisphere were stimulated with 150 TMS pulses. Minimum-norm estimates were derived from the EEG data to estimate the spatial spreading of TMS-elicited neuronal activation on an individual level. The activation order of the cortical areas varied depending on the stimulated area. There were similarities and differences in the spatial distribution of the TMS-evoked potentials between subjects. Similarities in cortical activation patterns were seen at short poststimulus latencies and the differences at long latencies. This pilot study suggests that cortical activation patterns and the activation order of motor areas differ interindividually and depend on the stimulated motor area. It further indicates that TMS-activated effective connections or underlying structural connections vary between subjects. The spatial patterns of TMS-evoked potentials differ between subjects especially at long latencies, when probably more complex neuronal networks are active.