More Than Cerebral Cortex: 7T Anatomo-functional Characterization of Cortical, Subcortical, and Cerebellar Structures Involved in the Network of Action Observation

In healthy humans, brain functional representation of actions, either executed or observed, relies on the human action observation network (AON). Several studies demonstrated that AON activation is crucial for action understanding and for subserving imitation by observation of new motor skills. The demonstration of the activation of this circuit in patients with motor disorders have opened the way to new rehabilitation protocols based on the observation of meaningful actions followed by their execution (Observation to Imitate), often referred as Action Observation Therapy (AOT). In the last decade, AOT was widely used in adult stroke patients as well as in children with cerebral palsy (CP). Overall, these studies demonstrate a significant motor improvement after therapy, that is maintained at follow-up.

Functional Magnetic Resonance Imaging (fMRI) studies on the effects of AOT found a positive correlation between the functional improvements of manual abilities of these patients and the enlargement of movement-related brain regions, suggesting that AOT can promote the activation of mechanisms of brain plasticity and functional reorganization of the cortical areas responsible for movements. Recently, fMRI studies have demonstrated that also other brain regions, such as the cerebellum, the thalamus and the basal ganglia, belong to the AON.

Despite numerous studies on AON, the fine-grained details of this complex network, especially at subcortical level, and the reciprocal interactions between different brain regions are largely unknown. Moreover, no studies are available at submillimeter level concerning the involvement of subcortical structures. Compared with lower magnetic field systems, Ultra-High Field Magnetic Resonance (UHF-MR) permits a remarkable gain in terms of signal to noise ratio (SNR). This significant increase in SNR produces a great improvement of all imaging parameters, and can be exploited increasing spatial and/or temporal resolution. Moreover, at 7T, we assist also to an empowered sensitivity of the signal to modifications of the composition of tissue that can be translated in new or improved contrasts, such as susceptibility-weighted imaging.

fMRI technique is an advanced MR method that benefits from the use of UHF thanks to the positive combination of increased SNR and increased sensitivity to the effect generating the contrast (BOLD effect).

It has been demonstrated that an increase in the spatial resolution of UHF fMRI allows to describe the functional architecture of the cerebral cortex at mesoscopic (sub-millimeter) scale, hence revealing cortical columns-laminar fMRI profiles, and to segment subcortical structures, to explore their functional selectivity to external stimuli and topographic organization, and to study the cortex. Moreover, the increase in the sensitivity of fMRI at 7T allows to obtain statistically significant functional maps of brain activation not only in groups of subjects but also in individual subjects and individual events, opening new perspectives in the use of fMRI for clinical purposes.