Vision Vision is not a single process, far more complex than it seems. Area of visual space in which a stimulus actives the neuron. Hence a second system.
Some aspects remain constant, regardless of orientation of perspective on object.
In identifying a pen, the sides of a pen are parallel. Recognise objects with parts missing, or occluded. If sufficient information is conveyed it might suggest only a single object. Identify parts and assemble them. We need some relationship that allows variability, but still specifies to some degree.