# Perspectives and Challenges in Robotic Neurorehabilitation
## 3 Current Trends of Robot-Aided Neurorehabilitation
### Multimodal Assessment of Recovery
#### 3.2.3 Neural and Muscular Correlates of the Sensorimotor Performance  
Sensorimotor performance can also be assessed through brain and/or muscle activity recordings. In fact, given the complexity of the brain's functional reorganization [25] in response to central nervous system diseases, it is pivotal to study brain activity at high temporal (in the order of milliseconds) and good spatial (in the order of 1 cm or less) scales [198]. Thus, combining these two scales might increase the chance of understanding the recovery process. Achieving both these resolutions is nowadays possible thanks to the combination of high density electroencephalography (hdeEEG) recordings and source imaging techniques that recently allowed reliable reconstruction of the brain's resting-state networks [199] and even measurement of electrophysiological sub-cortical activity [200]. Furthermore, hdeEEG (or EEG) is attractive because it is inexpensive and portable compared to other non-invasive neuroimaging techniques, such as magnetoencephalography (MEG) or functional and structural magnetic resonance imaging (MRI). In addition, it is possible to use EEG during task performance in an ecological environment. This framework allows exploration of the brain's functional changes and monitoring of brain activity in response to rehabilitation tasks [201; 202; 203], potentially shedding light on specific brain-behavior relationships. In addition to the investigation of task-related activity, it is interesting to estimate the functional connectivity (FC) both during tasks and/or the resting state. FC represents a significant statistical relationship between separate brain areas and any imbalance occurring in FC properties, as modeled by graph-based metrics, might underlie neuropathological processes and insults occurring in the cerebral organization [204; 205; 206; 207]. Moreover, FC during spontaneous oscillatory activity, as measured by other techniques, such as resting state functional MRI, changes after proprioceptive training with a robotic manipulandum [208]. Furthermore, FC predicts the behavioral outcomes of rehabilitation protocols [209] as well as the recovery of motor function [168; 207] in stroke subjects and also correlates with the level of clinical disability in early relapsing-remitting multiple sclerosis subjects [210]. As for the EEG, FC measurements follow spontaneous recovery [204], are related to motor deficits in stroke [211] and multiple sclerosis [206], and predict learning rate [212; 213] in healthy subjects. Taken together, all these evidences suggest that an evaluation of the FC pre- and post-rehabilitation is a valuable tool to evaluate the effectiveness of the rehabilitation protocol administered with a newly developed robotic device [168]. There are other reasons supporting the use of the resting state paradigm. First, the resting state is compliant even for those subjects who suffer from very high degrees of physical impairment [214] that may cause a dramatic decrease of the signal to noise ratio. Second, it is not sensitive to task-related parameters and it is very well-suited to exploration of the potential long-term retention of the protocol [208]. Third, movement-related artifacts occurring during task performance might be very detrimental, also in healthy and minimally impaired subjects. The amount of sensorimotor recovery can also be assessed by making use of other electrophysiological measurements, such as sEMG. Indeed, many diseases affecting the human sensorimotor system are associated with abnormal patterns of muscle activation [215; 216], and thus motor recovery could be directly measured by EMG patterns [217]. In this context, muscular activity is usually decomposed into invariant modules of spatial and/or temporal activation [218]. Abnormal muscle synergies with respect to their healthy counterparts can, for example, be informative of compensatory strategies, which typically reflect the nature of the deficit and the level of impairment [219].