What Is the Perirhinal Cortex?

The perirhinal cortex is an area in the mammalian brain that lies adjacent to the hippocampus, flanked on one side by the entorhinal cortex and on the other by the parahippocampal cortex. It is located in the medial temporal lobe and is commonly associated with memory and recognition. For anatomists who delineate areas within the brain according to Brodmann areas, the region covers the Brodmann areas 35 and 36.

Information from all senses is, in part, routed to the the perirhinal cortex, often in proportion to the senses on which the subject relies on most. For example, in primates it will receive a lot of visual information, but in the rodent, the information received by the same region is largely olfactory and auditory. Primates rely more heavily on vision for sensory and environmental information, while rodents rely more on smell and hearing. This shift in input ratios is caused by some sensory systems overriding others and how this imbalance is likely to influence how the subject remembers.

While the perirhinal cortex receives information from all cortical sensory areas, it is also noteworthy that this part of the brain also receives information that has been pre-processed. Early on in the cortical sensory areas within the brain, lateral inhibition circuits clean up incoming information signals, strengthening repeated, reliable information, and weakening firing patterns that many scientists consider "noise." All sensory areas within the brain have some form of information modification at an early stage of neural signal processing.

The perirhinal cortex sends projections to the thalamus, which acts as an information hub, further routing signals to other parts of the brain. The amygdala, which is associated with arousal and the fear response, is another brain region that receives projections from the perirhinal cortex. This is intuitive for neuroanatomists because memory processing is known to be more efficacious with an increase in adrenaline and arousal responses. The perirhinal cortex also has both reciprocal and direct connections with some cells in the CA1 region of the hippocampus.

There are different types of memory processing, and scientists have teased apart the role of the perirhinal cortex from the role of the hippocampus and other brain areas associated with memory formation, encoding and retrieval. The perirhinal cortex is involved in visual perception and environment identification. It is also considered a region associated with sentimentality, like imbuing objects with emotional meaning. While the hippocampus is more associated with recollection, the perirhinal cortex is, in contrast, more associated with familiarity and a sense of recognition rather than explicit memory retrieval, though both structures are active during memory encoding and learning processes.