The term area 13a (macaque) refers to a portion of the orbital gyri defined on the basis of internal structure in the macaque. It is one of four components of area 13 (macaque) in the orbitomedial prefrontal cortex. The other components are area 13m (macaque), area 13l (macaque), and area 13b (macaque). Significant features of area 13a are generally dark staining for acetylcholinesterase, particularly of a fiber plexus in the internal pyramidal layer (V) and a lighter plexus in the molecular layer of the cerebral cortex (I). Other features that distinguish area 13a from some adjacent areas by Nissl stain are agranularity and aggregates of pyramidal cells in a bilaminate internal pyramidal layer (V) ( Carmichael-1994 ). Its human equivalent is area 13a ( Ongur-2003 ).