This is an illustrative drawing.
The right side image shows the prevalent anatomy wherein the multiple perforator branches of the basilar artery the medial thalami and the ventral midbrain, corresponding to a particular peculiar zone.
The left side image shows an Artery of Percheron (AOP), wherein the perforator arteries are either absent altogether or are severely hypoplastic and the above-said areas of the thalamus and midbrain are supplied by small perforator branches which arise from a solitary artery which usually arises from the P1 segment of the Posterior Cerebral Artery (PCA).
It has been reported that a right AOP is more common than the left side origin.