Frontoparietal network activity during a combined action observation and proprioceptive stimulation protocol reveals long-term plasticity in the primary motor cortex

The aim of this study was to investigate changes in cortical hemodynamic activity within a frontoparietal network during the administration of an innovative action observation (AO) and proprioceptive stimulation (PS) protocol, and to examine whether this activity could predict the efficacy of the protocol in evoking M1 plasticity, reflected in significant long-term changes in M1 excitability. AO-PS was composed of 50 bursts of combined stimuli. Each burst consisted of five couples of AO and PS during which participants observed a video showing thumb opposition movements and simultaneously received a mechanical vibration on the extensor pollicis brevis muscle (stimulation frequency 80 Hz). During AO-PS, the hemodynamic activity was measured by means of functional Near-Infrared Spectroscopy. Recruitment curves were assessed using transcranial magnetic stimulation before, immediately, 30 and 60 min after AO-PS, to evaluate changes in M1 excitability. During AO-PS, a significant increase in oxyhemoglobin (HbO) concentration changes was found in the following Brodmann Areas (BA): left and right BA6, BA44, and BA43, left BA3, BA4, BA40 and BA7. The highest increment was found in the left BA4. In left BA7 and BA40 the time-to-peak in HbO concentration changes were reached significantly later than in the other BAs. On average, no significant changes were observed after AO-PS administration in M1 excitability, but HbO concentration changes in the left BA7 correlated with plasticity index. These findings highlight the involvement of sensorimotor and associative fronto-parietal regions during AO-PS. Additionally, the activity of the left BA7 revealed the plasticity induced by AO-PS in M1.