Through video analysis, researchers focused on three specific facial gestures in macaques: the lipsmack, which signals receptivity or submission; the threat face used to confront an opponent; and chewing, a voluntary movement not related to social interactions. They then utilized fMRI scans to identify important brain regions responsible for these gestures. After this initial step, Ianni and her colleagues delved even deeper—literally.
Under the hood
“We targeted these brain areas with sub-millimeter precision for the implantation of micro-electrode arrays,” Ianni notes. This approach enabled the team to record activity from numerous neurons across the regions where facial gestures are generated. The electrodes were positioned in the primary motor cortex, ventral premotor cortex, primary somatosensory cortex, and cingulate motor cortex. After implantation, the macaques were exposed again to the same social stimuli to observe neural activity related to the three facial gestures. What they discovered was unexpected.
The researchers anticipated a clear separation of functions, with the cingulate cortex managing social signals and the motor cortex focusing on chewing. However, they found that all four brain areas activated in unison, regardless of whether the macaques were displaying a threat or just enjoying food.
This prompted the researchers to explore how the brain differentiates between social gestures and chewing, revealing that it wasn’t about specific brain regions but rather different neural codes—distinct ways neurons convey and represent information over time.
The hierarchy of timing
By examining neural dynamics, the team pinpointed a temporal hierarchy in the macaque cortex. The cingulate cortex utilized a static neural code. “Static means that the firing pattern of neurons remains consistent across multiple instances of the same facial gesture and over time,” Ianni explains. This firing pattern persisted up to 0.8 seconds afterward. “A single decoder that learns this pattern could be applied at any moment or trial to interpret the facial expression,” she adds.
