The perirhinal cortex is composed of two regions: areas 36 and 35. Area 36 is sometimes divided into three subdivisions: 36d is the most rostral and dorsal, 36r ventral and caudal, and 36c the most caudal. Area 35 can be divided in the same manner, into 35d and 35v (for dorsal and ventral, respectively).
The perirhinal cortex is involved in both visual perception and memory; it facilitates the recognition and identification of environmental stimuli. Lesions to the perirhinal cortex in both monkeys and rats lead to the impairment of visual recognition memory, disrupting stimulus-stimulus associations and object-recognition abilities.
The perirhinal cortex is also involved in item memory, especially in coding familiarity or recency of items. Rats with a damaged perirhinal cortex seemed unable to tell novel objects from familiar ones—they were still more interested in exploring when novel objects were present, but examined the novel and familiar objects equally, unlike undamaged rats. Thus, other brain regions are capable of noticing unfamiliarity, but the perirhinal cortex is needed to associate the feeling with a specific source.
The perirhinal cortex's role in the formation and retrieval of stimulus-stimulus associations (and in virtue of its unique anatomical position in the medial temporal lobe) suggest that it is part of a larger semantic system that is crucial for endowing objects with meaning.