www.brainproducts.com Brain Products Press Release August 2009, Volume 32 page 5 of 12 different types of Magstim stimulator (monophasic, biphasic with four boosters, and biphasic with single power supply module), four figure- of-eight coils (standard 50-70 mm, custom 25-70 mm), intensities (ranging from 10% to 100% of the stimulator output) and frequencies (single pulse, 5 Hz and 20 Hz) was compared. Most of the sessions were recorded from TMS-compatible sintered Ag/AgCl electrodes (EasyCap GmbH, Herrsching, Germany) i.e., rings of 2 mm thickness, with inner and outer diameters of 6 mm and 12 mm, respectively. Moreover to verify whether the electrodes shape is able to modify the artifacts features, some electrodes had a 2 mm slit in the ring or the slit closed by means of silicone. Additional re- cordings were done with small sintered Ag/AgCl disks that were 1 mm thick and 3 mm in diameter, mounted in an elastic cap (EasyCap GmbH, Herrsching, Germany). Our main result indicates that, regardless to the above cited para- meters, TMS induced artifact always lasted about 5 ms (5–5.6 ms). When the knee was stimulated, we found an induced artifact comparable in length to that evoked by the stimulation of the phantom. Finally, when cortical stimulation was compared to the other models, a similar time- course was found up to 5 ms. Interestingly, differences just appeared at about 5 ms after the TMS pulse when the EEG signal went back to baseline for all conditions with exception of the cortical stimulation in which two additional deflections appeared at 6 and 8 ms. Besides the above described TMS-artifact, several milliseconds after TMS pulses, the signal was contaminated by a coil-recharge- artifact that was present with biphasic stimulators, but not with the monophasic one. Its amplitude was constant (±12 μV), while its latency increased with the increase of the power strength, i.e., from 8 to 70 ms. A great advantage of EEG is the ability to acquire simultaneous measurements of activity in the entire brain, thus providing a broader picture of the cortical responses during a task execution or a given state of the subject (i.e., physiological or pathological). Nevertheless, as all neuroimaging techniques, EEG has its limitations. It only identifies correlational links between brain activity and behaviour/state. Combining two different methods, such as transcranial magnetic stimulation (TMS) and EEG, has the advantage of overcoming this limitation, thereby supplementing the information provided by correlational analysis with a technique that can establish a causal link between brain function and behaviour. The combination of TMS with EEG provides unique information on cortical reactivity and connectivity and is a powerful tool to directly investigate the effects induced by TMS on brain activity (1, 2). Co- registration also allows to study the TMS evoked activity from silent brain areas, so it theoretically extends our possibilities to spatially and functionally characterize complex brain networks (2, 3). Finally, TMS EEG co-registration can be used to infer the role of specific brain activity (4). Nevertheless, even after the introduction of recording systems which can work in high magnetic field, preventing saturation of the amplifiers, TMS-EEG co-registration may be technically challenging (5). We still miss crucial information about what the best technical conditions to record such a signal are and, above all, how long the TMS-induced artifact lasts. In this vein, we conducted a study (6) to provide experimental data about the artifact duration and to investigate the influence of some para- meters on TMS-EEG co-registration. To better characterize the artifacts and to exclude any cortical responses, a phantom ‘head’ was employed and then compared to the results obtained from a knee stimulation (a model with skin properties similar to the scalp but without cortical responses) and a cortical stimulation. EEG signal was acquired with BrainAmp 32 MR plus or BrainAmp DC, with a resolution of 0.1, band-pass filtered at 0.01–1000 Hz and sampled at 5000 Hz. The artifact shape and duration induced by Domenica Veniero Figure 1. Effect of stimulus intensity (from 10 to 100% of MSO) on the artifact length. Each line represents the average of 100 stimuli. Figure 2. Amplitude and latency of the later artifact in the EEG signal as a function of stimulus intensity (% of MSO). User Research On the combination of EEG transcranial magnetic stimulation by Domenica Veniero & Carlo Miniussi University of Brescia & IRCCS San Giovanni di Dio Fatebenefratelli, Brescia, Italy msec