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The perirhinal cortex is a cortical region in the
medial temporal lobe The temporal lobe is one of the four major lobes of the cerebral cortex in the brain of mammals. The temporal lobe is located beneath the lateral fissure on both cerebral hemispheres of the mammalian brain. The temporal lobe is involved in p ...
that is made up of Brodmann areas 35 and 36. It receives highly processed sensory information from all sensory regions, and is generally accepted to be an important region for
memory Memory is the faculty of the mind by which data or information is encoded, stored, and retrieved when needed. It is the retention of information over time for the purpose of influencing future action. If past events could not be remember ...
. It is bordered caudally by postrhinal cortex or parahippocampal cortex (homologous regions in rodents and primates, respectively) and ventrally and medially by
entorhinal cortex The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time.Integrating time from experience in th ...
.


Structure

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). Area 36 is six-layered, dysgranular, meaning that its layer IV is relatively sparse. Area 35 is
agranular cortex Agranular cortex is a cytoarchitecturally defined term denoting the type of heterotypic cortex that is distinguished by its relative thickness and lack of granule cells. Examples are Brodmann area 30, the agranular insula, and the precentral gyr ...
(lacking any cells in layer IV).


Function

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 also receives a large dopaminergic input and signals the rewards that are associated with visual stimuli Damage to the perirhinal cortex has been shown to cause impairment in discriminating among object concepts when there is a high degree of visual semantic overlap among choices, such as between a hairdryer and a gun. A growing body of evidence suggests that the perirhinal cortex protects against interference from low-level visual features.Graham, K. S., Barense, M. D., & Lee, A. C. H. (2010). Going beyond LTM in the MTL: a synthesis of neuropsychological and neuroimaging findings on the role of the medial temporal lobe in memory and perception. ''Neuropsychologia'', ''48''(4), 831–853. https://doi.org/10.1016/j.neuropsychologia.2010.01.001
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.


Other animals


Primates

The monkey perirhinal cortex receives a majority of its input from high-level
visual The visual system comprises the sensory organ (the eye) and parts of the central nervous system (the retina containing photoreceptor cells, the optic nerve, the optic tract and the visual cortex) which gives organisms the sense of sight ...
areas, whereas, in the rat, its inputs are primarily olfactory and, to a lesser extent, auditory. Outputs to orbitofrontal cortex and medial
prefrontal cortex In mammalian brain anatomy, the prefrontal cortex (PFC) covers the front part of the frontal lobe of the cerebral cortex. The PFC contains the Brodmann areas BA8, BA9, BA10, BA11, BA12, BA13, BA14, BA24, BA25, BA32, BA44, BA45, BA ...
regions (such as prelimbic and infralimbic) have been described. Perirhinal cortex also sends output to a number of subcortical structures, including the basal ganglia, the thalamus, the basal forebrain, and the
amygdala The amygdala (; plural: amygdalae or amygdalas; also '; Latin from Greek, , ', 'almond', 'tonsil') is one of two almond-shaped clusters of nuclei located deep and medially within the temporal lobes of the brain's cerebrum in complex ver ...
. It also has direct connections with
hippocampus The hippocampus (via Latin from Greek , 'seahorse') is a major component of the brain of humans and other vertebrates. Humans and other mammals have two hippocampi, one in each side of the brain. The hippocampus is part of the limbic syste ...
region CA1 and the subiculum. Perirhinal cortex projects to distal CA1 pyramidal cells, overlapping the projections from
entorhinal cortex The entorhinal cortex (EC) is an area of the brain's allocortex, located in the medial temporal lobe, whose functions include being a widespread network hub for memory, navigation, and the perception of time.Integrating time from experience in th ...
. The same CA1 cells send return projections back to perirhinal cortex. Inputs from subiculum terminate in both superficial and deep layers. Visual areas TE and TEO send and receive a significant reciprocal connection with perirhinal cortex. Weaker, but still significant, projections come from other parahippocampal regions and from the superior temporal sulcus. Other inputs include
anterior cingulate In the human brain, the anterior cingulate cortex (ACC) is the frontal part of the cingulate cortex that resembles a "collar" surrounding the frontal part of the corpus callosum. It consists of Brodmann areas 24, 32, and 33. It is involved i ...
and insular regions, in addition to prefrontal projections.


Rodents

Auditory inputs from temporal cortical regions are the primary inputs to rat 36d, with visual inputs becoming more prominent closer to the postrhinal cortical border. Area 36d projects to 36v and then to 35, which forms the primary output region of perirhinal cortex. Inputs to area 35 more strongly reflect olfactory and gustatory inputs from piriform and insular cortices, in addition to inputs from entorhinal cortex and frontal regions.


References

*Witter MP and Wouterlood F. 2002. The parahippocampal region: organization and role in cognitive function. Oxford University Press: New York. *Murray, E.A., & Bussey, T.J. (1999). Perceptual-mnemonic functions of the perirhinal cortex. Trends in Cognitive Sciences, 3(4), 142-151. *Winters, B.D., Forwood, S.E., Cowell, R., Saksida, L.M., & Bussey, T.J. (2004). Double dissociation between the effects of peri-postrhinal cortex and hippocampal lesions on tests of object recognition and spatial memory: heterogeneity of function within the temporal lobe. Journal of Neuroscience, 24, 5901-5908. {{Prosencephalon Cerebral cortex Limbic system