The role of the parietal cortex in sensorimotor transformations and action coding.

The picture of the human cortical motor system has fully changed in the last two decades. In the light of new data, the notion of a motor system devoted solely to action execution, strictly isolated from the sensory system, is not sustainable. There is evidence that parietal areas are strictly connected to frontal areas and these connections build up sensorimotor circuits aimed at interacting with objects in the environment, and at understanding actions. They are known as the canonic neuron system and mirror neuron system, respectively. These circuits are part of the classic dorsal stream. Recently, the dorsal stream has been further divided into a dorsodorsal and a ventrodorsal stream. The ventrodorsal stream is regarded as functionally linked to object awareness for action recognition/organization. The dorsodorsal stream is proposed to subserve online control of actions. Affordances indicate "action possibilities" as characterized by object properties the environment provides to interacting organisms. Affordances may be divided into stable and variable ones. According to this distinction, stable affordances emerge from slow offline processing of visual information based on object knowledge as well as previous experiences in object interaction. In contrast, variable affordances emerge from fast online processing of visual information during actual object interaction and refer to changing or temporary object characteristics, such as orientation in space, size changes, including the update of hand shape for grasping, defining overall the current state of the object. It has been proposed that the dorsodorsal stream codes for variable affordances, while the dorsoventral stream codes for stable affordances.