Functional organizational principle evaluated by microstimulation in the cortical facial motor areas

Facial gestures are crucial for social interactions among all primates. Since these actions are performed in the absence of direct visual feedback, their effectiveness relies on their stereotypical, almost reflexive patterns. One efficient mechanism for controlling ethological actions could involve motor cortical action maps (Graziano et al., 2002; 2016). Here, we studied whether stereotypical socio-communicative facial expressions are organized into discrete cortical zones by applying long-duration, supra-threshold microstimulation to fMRI-identified cortical areas of the facial motor system. Our findings revealed that these stimulation parameters produced complex facial responses by engaging multiple facial regions but did not elicit social-communicative facial behavior. The effects of long-duration stimulation appeared to be an amplified (both spatially and temporally) version of the responses seen with short-duration stimulation. The stimulation-evoked neural activity extended across the facial motor system, suggesting that these cortical regions function as part of a large-scale network designed to generate coherent, context-specific, and socially relevant motor outputs.