Infant Brain Responses to Object Weight: Exploring Goal-Directed Actions and Self-Experience.

Recent work has suggested the value of electroencephalographic (EEG) measures in the study of infants' processing of human action. Studies in this area have investigated desynchronization of the sensorimotor mu rhythm during action execution and action observation in infancy. Untested but critical to theory is whether the mu rhythm shows a differential response to actions which share similar goals but have different motor requirements or sensory outcomes. By varying the invisible property of object weight, we controlled for the abstract goal (reach, grasp, and lift the object), while allowing other aspects of the action to vary. The mu response during 14-month-old infants' own executed actions showed a differential hemispheric response between acting on heavier and lighter objects. EEG responses also showed sensitivity to "expected object weight" when infants simply observed an experimenter reach for objects that the infants' prior experience indicated were heavier versus lighter. Crucially, this neural reactivity was predictive - during the observation of the other reaching toward the object, before lifting occurred. This suggests that infants' own self-experience with a particular object's weight influences their processing of others' actions on the object, with implications for developmental social-cognitive neuroscience.