Face muscle system: motor control and expressiveness

Face expressions have a crucial role in human non-verbal communication and social interactions and can be either voluntarily or emotionally controlled. Due to the diversity of face muscles’ functions, they are different in both anatomical and physiological features from other muscles of the body, especially from hand muscles. Non-invasive study of face motor system in humans is possible through neuroimaging and neurophysiological procedures, such as transcranial magnetic stimulation. While the hand motor system has been widely studied, only few studies have investigated the face motor system in humans due the inherent technical difficulties to assess it non-invasively. For instance, studies investigating the effect of learning within the face primary motor cortex (M1) are lacking and it is still debated which cortical or subcortical areas play a role in the face expression recognition process and whether this process may be influenced by aging. Thus, a first study was performed to better understand the physiological strategies adopted by the face motor system to adapt to the long-term learning and practice of a complex and fine motor task, such as paying a musical instrument. Secondarily, the involvement of the cerebellum was investigated during the passive viewing of faces expressing emotions, with particular regard to cerebellar circuits underpinning the learning and retention processes. Indeed, the cerebellum has a role in both motor and emotional processing, but no studies investigated how face expressions may affect cerebellar learning processes. Finally, we focused on the influence exerted by the viewing of faces expressing emotions on M1 excitability in aged adults. Aging affects the capability to recognize faces, but most of the studies focused on the perceptive aspects of this cognitive decline, while no studies investigated whether face motor responses to the observation of face expressions are impaired by aging. The results of the present thesis demonstrated for the first time that the face motor system can be modified by the process of learning to play a musical instrument and by the execution of this complex task in the long term. In particular, wind professional musicians, through constant work since infancy, developed the ability to differently modulate face movements, compared to non-musicians and string musicians, changing the functional aspects of face muscles. Moreover, results of the second study demonstrated that the cerebellar circuits underpinning conditioning learning processes are influenced only by the view of sad face expressions. Finally, the third study highlighted that, although aging impairs the perception and the recognition of faces expressing emotions, the face M1 excitability in response to these expressions is preserved. This may suggest that aging does not affect face emotional processing in cortical motor areas modulating face mimicry.