15,17 Given the strong representation of synaptic activity in the EEG, the question arises as to whether fMRI signals are related to similar or different aspects of neuronal activity, such as neuronal spiking. In fact, in many experimental situations, synaptic activity is highly correlated with the firing rate of the neuron to which the synapses

under consideration belong. Accordingly, it is not surprising that in many cases the fMRI signal correlates equally well with LFPs and spiking activity. However, in a few studies, there has been successful differentiation Inhibitors,research,lifescience,medical between synaptic activity and spiking activity with regard to the related hemodynamic changes.18 These studies have provided evidence for a much closer relationship between the blood oxygenation level-dependent (BOLD) contrast mechanism Inhibitors,research,lifescience,medical and presynaptic and postsynaptic processing of incoming afferents to a region and only to a lesser degree the activity of its output efferents.19-22 In summary, although obviously very different in terms of the signal that is Inhibitors,research,lifescience,medical actually measured, both EEG and fMRI have a considerable overlap concerning the neuronal activity that they represent, which is mainly synaptic activity. Technique: safety and quality issues From the very early days of simultaneous EEG-fMRI, safety issues have been an important aspect. Radiofrequency-related heating of electrodes or brain tissue has to be

considered, and there are several factors that are relevant such as the scanning sequence, the number of EEG electrodes, or the field strength of the MRI scanner.23 Taking all aspects into account, simultaneous EEG-fMRI recordings have been Inhibitors,research,lifescience,medical safely performed at many different MRI centers, and simultaneous EEG-fMRI is considered as a standard imaging technique. Since simultaneous EEG-fMRI recordings today are typically Inhibitors,research,lifescience,medical performed with commercially available MR-compatible EEG equipment, specific safety instructions are provided by the companies. Another issue in simultaneous EEG-fMRI is the quality of the EEG recorded in mafosfamide the scanner. Here, two main artefacts have to be considered: the cardiac pulse-related

artefact and the image acquisition artefact. For both types of artefacts, today there are post-processing artefact removal strategies available with sufficient efficacy. The pulse-related artefact, which is often also referred to as a ballistocardiogram, or BCG artefact, is complex in its origin with a role of pulsatile movements of scalp vessels on adjacent electrodes and head rotation. Post-processing strategies are based buy trans-isomer either on waveform removal approaches such as the average artefact subtraction algorithm24 or on pattern removal approaches such as independent component analysis.25 In addition to a priori avoiding of BCG artefacts,26 new strategies include the application of optical motion tracking systems.