## 4 Discussion
This increased interhemispheric synchrony between sensorimotor cortices has been evidenced previously in experimental animal models of PD [35] and also associated with cognitive dysfunctions in MEG studies [29]. In our study, Levodopa intake modulates the changes seen in some of our analyses. It restrains the correlation coefficient increase produced with motor activation in the left hand. Although the values are still altered, they are closer to those seen in controls. This asymmetric effect deserves to be explored in a future study because asymmetrical changes in PD produced by levodopa administration have been previously reported [36]. When pre- and post- levodopa EEG coherence results are compared, the drug increases the coherence in all frequencies in resting-state, as reported in previous studies [37]. Levodopa intake has been associated mainly with power increases in the alpha and beta rhythms in central-parietal areas [38], but some other studies are rather contradictory showing that resting-state EEG connectivity among regions of the motor network after levodopa administration did not change significantly [39]. During motor activation, the drug seems to prevent the power increase in PD subjects, remaining very similar to resting-state. Levodopa effects over connectivity could be caused by widespread dopaminergic projections ascending from the ventral tegmental area and SNc, which may be important in determining cortico-cortical coupling during executive tasks [37,40]. Finally, when interhemispheric \"independence\" is analyzed through KLD, levodopa intake does not change the independence of information processing on both sides in the resting-state of PD. However, during motor activation, it diminishes the assortativity increase described previously in PD subjects. Comparison of connectivity results across studies may be a difficult task because of the type of functional data, connectivity definitions, and computational algorithms used. Nevertheless, our results are coherent with those demonstrated using resting-state fMRI, one of the most used techniques to study connectivity. PD subjects off medication compared to controls show large-scale network reorganization in resting-state fMRI with increased connectivity in cortical sensorimotor areas [30]. Dopaminergic therapy normalized this increased connectivity [41]. SpEn allows us to define a frequency dependent entropy and thus makes this information more comprehensible and closer to clinical neurophysiological studies. The present study was not without limitations. First, the sample size was relatively small. Notwithstanding, our sample was adequate to detect a number of network assortativity differences between the two study groups. A small sample size lessens the likelihood of detecting a statistically significant difference, but in our study, not only the magnitude of the correlation coefficients but also the significance of some analysis were very high. Second, EEG has its own known limitations in terms of spatial resolution, impossibility to detect neuronal activity far from the scalp surface and signal to noise ratio. This last issue has made us eliminate several electrodes in the periphery of the setting. This has been made systematically in all the subjects in order to reduce the influence on the results. In third place, in our study, differences in assortativity between PD patients and control subjects were significant, but not large enough to distinguish PD patients on an individual subject basis. Therefore, further refinements in signal processing techniques are needed to be able to utilize this technique diagnostically. A larger sample and a thorough register of cognitive variables would allow making significant contributions to the search of biomarkers of progression of the disease.