Improving the quality of combined EEG-TMS neural recordings: Introducing the coil spacer.

BACKGROUND: In the last decade, interest in combined transcranial magnetic stimulation (TMS) and electroencephalography (EEG) approaches has grown substantially. Aside from the obvious artifacts induced by the magnetic pulses themselves, separate and more sinister signal disturbances arise as a result of contact between the TMS coil and EEG electrodes. NEW METHOD: Here we profile the characteristics of these artifacts and introduce a simple device - the coil spacer - to provide a platform allowing physical separation between the coil and electrodes during stimulation. RESULTS: EEG data revealed high amplitude signal disturbances when the TMS coil was in direct contact with the EEG electrodes, well within the physiological range of viable EEG signals. The largest artifacts were located in the Delta and Theta frequency range, and standard data cleanup using independent components analysis (ICA) was ineffective due to the artifact's similarity to real brain oscillations. COMPARISON WITH EXISTING METHOD: While the current best practice is to use a large coil holding apparatus to fixate the coil 'hovering' over the head with an air gap, the spacer provides a simpler solution that ensures this distance is kept constant throughout testing. CONCLUSIONS: The results strongly suggest that data collected from combined TMS-EEG studies with the coil in direct contact with the EEG cap are polluted with low frequency artifacts that are indiscernible from physiological brain signals. The coil spacer provides a cheap and simple solution to this problem and is recommended for use in future simultaneous TMS-EEG recordings.

The anterior cingulate cortex: monitoring the outcomes of others' decisions.

The ability to attribute mental states to others and understand the basis of their decisions is essential for human social interaction. A controversial theory states that this is achieved by simulating another's information processing in one's own neural circuits. The anterior cingulate cortex (ACC) is known to play an important role in the registration of discrepancies between the predicted and actual outcomes of decisions (prediction errors).When positive and negative feedback fails altogether, the failure may also signal errors in the prediction that the outcome of that decision would be informative and guide future decisions. Does the ACC signal that an outcome is unexpectedly uninformative? When an outcome directed to others is uninformative, do we understand their mental states by simulating them in the circuits of the ACC in our own brain? The aim of our study was to test for these two possibilities in the human brain with event-related fMRI. We tested whether the ACC processes errors in the prediction of informative feedback and whether the ACC is also activated when scanned subjects process the same outcomes of another's decisions. We show that each is processed by a separate subregion of the ACC.

Evolution of the cerebellar cortex: the selective expansion of prefrontal-projecting cerebellar lobules.

It has been suggested that interconnected brain areas evolve in tandem because evolutionary pressures act on complete functional systems rather than on individual brain areas. The cerebellar cortex has reciprocal connections with both the prefrontal cortex and motor cortex, forming independent loops with each. Specifically, in capuchin monkeys cerebellar cortical lobules Crus I and Crus II connect with prefrontal cortex, whereas the primary motor cortex connects with cerebellar lobules V, VI, VIIb, and VIIIa. Comparisons of extant primate species suggest that the prefrontal cortex has expanded more than cortical motor areas in human evolution. Given the enlargement of the prefrontal cortex relative to motor cortex in humans, our hypothesis would predict corresponding volumetric increases in the parts of the cerebellum connected to the prefrontal cortex, relative to cerebellar lobules connected to the motor cortex. We tested the hypothesis by comparing the volumes of cerebellar lobules in structural MRI scans in capuchins, chimpanzees and humans. The fractions of cerebellar volume occupied by Crus I and Crus II were significantly larger in humans compared to chimpanzees and capuchins. Our results therefore support the hypothesis that in the cortico-cerebellar system, functionally related structures evolve in concert with each other. The evolutionary expansion of these prefrontal-projecting cerebellar territories might contribute to the evolution of the higher cognitive functions of humans.

Symbolic representations of action in the human cerebellum.

Cerebellar cortical areas connected to the neocortical motor system process information important for the sensory guidance of action. Converging evidence also supports the view that cerebellar cortical areas connected with the prefrontal cortex process information similarly in the cognitive domain. Here, we test the hypothesis that the prefrontal-projecting zones in the human cerebellum process the abstract content of information embedded within sensory cues. Specifically, we use event-related fMRI to determine whether symbolic visual instructions activate the prefrontal-projecting zones of the cerebellum. On the basis of connectional anatomy, we predicted that such activity would be found in lobule HVIIA and adjacent vermal territories in the same lobule. Our experimental design enabled us to investigate activity time-locked specifically to instructions foraction that were either purely symbolic, or specified actions directly. Such activity was independent of action. Activity specifically time-locked to symbolic cues (compared with non-symbolic control cues) activated cerebellar cortical lobule HVIIA (Crus I and Crus II). Our results provide support for the view that prefrontal-projecting areas of the cerebellar cortex process information that is of a purely abstract nature.