Simple visual stimuli are sufficient to drive responses in action observation and execution neurons in the macaque ventral premotor cortex

Neurons responding during action execution and action observation were discovered in the ventral premotor cortex three decades ago. However, the visual features that drive the responses of Action Observation/Execution Neurons (AOENs) have not been revealed at present. We investigated the neural responses of AOENs in ventral premotor area F5c of four macaques during the observation of action videos and crucial control stimuli. The large majority of AOENs showed highly phasic responses during the action videos, with a preference for the moment that the hand made contact with the object. They also responded to an abstract shape moving towards but not interacting with an object, even when the shape moved on a scrambled background, implying that most AOENs in F5c do not require the perception of causality or a meaningful action. Additionally, the majority of AOENs responded to static frames of the videos. Our findings show that very elementary stimuli, even without a grasping context, are sufficient to drive responses in F5c AOENs. 

Methods
We recorded single unit and multi-unit activity in four macaques that were implanted with a 96-channel Utah array (Blackrock Neurotech, UT, USA) in ventral premotor area F5c. The neural signals were high pass filtered (750Hz) and sampled at 30kHz. The data were subsequently sorted offline with a refractory period of 1 ms to isolate single units, using the Offline Spike sorter software (Plexon, Inc., Dallas, TX, USA). Neural data was recorded while the monkeys performed an action execution task and an action observation task to assess the visual selectivity of action observation/execution neurons (AOENs) in area F5c.