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Perirhinal cortex

The perirhinal cortex (PRC) is a polymodal association area located in the anteromedial portion of the temporal lobe, serving as a transitional region between sensory neocortex and the medial temporal lobe memory structures, and it plays a pivotal role in integrating sensory information for recognition memory and perceptual processing.^[1]^[2] Anatomically, the PRC encompasses Brodmann areas 35 and 36 in primates, extending laterally to the rhinal sulcus and including parts of the temporopolar cortex and anterior fusiform gyrus in humans; it features a juxtallocortical structure with reduced or absent layer IV in area 35, distinguishing it from typical neocortex.^[2]^[3] This region receives dense convergent inputs from higher-order sensory cortices, including the inferior temporal gyrus for visual object features, auditory association areas, olfactory regions like the piriform cortex, and somatosensory areas, while sending efferents to the entorhinal cortex, hippocampus (via the lateral entorhinal pathway and direct projections to CA1 and subiculum), amygdala, thalamus, and basal forebrain, positioning it as a hub for multisensory integration.^[1]^[3]^[2] Functionally, the PRC is essential for object recognition memory, particularly familiarity-based discrimination, where lesions impair performance on delayed nonmatching-to-sample tasks with long intervals (>10–40 minutes) or complex stimuli requiring resolution of feature ambiguity, such as overlapping visual patterns.^[3]^[1] It supports perceptual processing of complex, unitized representations by binding features across modalities—evident in cross-modal recognition deficits following ablation—and contributes to associative learning, including paired-associate tasks and reinforcement-based decisions.^[2]^[3] Electrophysiological recordings reveal PRC neurons exhibit repetition suppression for familiar objects and task-dependent firing for spatial contexts, such as in maze navigation where up to 72% of units respond to spatial segments, challenging strict dichotomies between object ("what") and spatial ("where") processing.^[3] In the broader medial temporal lobe system, the PRC facilitates memory consolidation and emotional processing by relaying sensory details to the hippocampus for episodic encoding and to the amygdala for affective associations, with early involvement in neurodegenerative conditions like Alzheimer's disease due to its vulnerability to tau pathology.^[1]^[2] Human neuroimaging corroborates these roles, showing PRC activation during semantic object processing, landmark-based navigation, and familiarity judgments, independent of hippocampal recollection.^[3] Debates on its perceptual versus mnemonic functions are reconciled by models emphasizing unitization, wherein the PRC flexibly merges object features, spatial contexts, and semantic elements into cohesive representations based on task demands, underscoring its adaptive contributions to cognition.^[3]

Anatomy

Location and boundaries

The perirhinal cortex is situated in the medial temporal lobe of the brain, forming a key component of the parahippocampal region. In both primates and rodents, it occupies a position along the rhinal sulcus, serving as a transitional zone between neocortical and allocortical areas. In humans and nonhuman primates, it corresponds to Brodmann areas 35 and 36, with area 35 located on the medial bank of the collateral sulcus and area 36 extending laterally across the fusiform and occipitotemporal gyri.^[4]^[5]^[6] Its boundaries are precisely defined relative to adjacent structures, varying slightly between species. Medially and ventrally, the perirhinal cortex borders the entorhinal cortex (Brodmann area 28) along the rhinal sulcus. Caudally, it adjoins the postrhinal cortex in rodents or the parahippocampal cortex in primates. Rostrally, it interfaces with area TE of the inferior temporal gyrus, marking the transition to higher-order visual association areas. These borders are delineated through cytoarchitectonic and connectional analyses in seminal studies of primate and rodent brains.^[5]^[7]^[6] The perirhinal cortex extends rostrocaudally along the collateral sulcus for approximately 1-2 cm in primates, encompassing the anterior two-thirds of the fusiform/occipitotemporal gyrus and the ventromedial temporopolar region. It overlies the uncus and rhinal sulcus, positioning it in close proximity to the hippocampus via the entorhinal cortex and the amygdala. This strategic location facilitates its integration into broader memory and perceptual networks, though detailed functional roles are beyond its anatomical demarcation here.^[4]^[8]^[6]

Cytoarchitecture

The perirhinal cortex exhibits a distinct cytoarchitectonic organization as a transitional periallocortical region, comprising two primary areas: area 35 and area 36. Area 35 is classified as agranular cortex, characterized by the absence of a prominent layer IV and a general lack of clear lamination in the superficial layers, with layers II and III merging without distinct boundaries and layer V appearing disorganized compared to adjacent regions. In contrast, area 36 is dysgranular, featuring a weakly developed layer IV with sparse granule cells, while maintaining a more defined six-layered structure overall. These features position the perirhinal cortex intermediate between the agranular entorhinal cortex medially and the fully granular neocortex laterally.^[9]^[10] Further subdivisions refine this organization, particularly within area 36, which is parceled into dorsal/rostral (36d), rostral/ventral (36r), and caudal (36c) subregions based on variations in laminar density and cell morphology; for instance, 36c displays heightened lamination and radial organization, while 36d appears less structured. Area 35 is similarly divided into dorsal (35d) and ventral (35v) portions, distinguished by subtle differences in cell packing and layer thickness along the rhinal sulcus. Layer-specific traits include dense populations of small to medium pyramidal cells in layers II and III across both areas, with heavier staining in deeper layers V and VI, where larger, darkly staining neurons predominate in area 35. Granule cells remain notably sparse in the rudimentary layer IV of area 36, underscoring its dysgranular nature.^[10]^[11] Histologically, the perirhinal cortex differs from neighboring structures in its degree of granularity and lamination. It is more laminated and granular than the entorhinal cortex, which lacks even a rudimentary layer IV and shows broader fusion of superficial layers, but less so than the adjacent neocortical area TE, where layer IV is fully developed with abundant granule cells and layers V and VI are more distinctly separated by columnar arrangements. These cytoarchitectonic properties highlight the perirhinal cortex's role as a bridge between allocortical and neocortical processing domains.^[10]^[9]

Connectivity

The perirhinal cortex (PRC) is characterized by dense reciprocal connections that position it as a critical hub for integrating sensory and associative information, with distinct patterns observed in primates and rodents. In primates, the PRC receives prominent afferent projections from higher-order visual processing areas, including area TE (inferior temporal cortex) and area TEO, which provide up to 62% of the total cortical inputs to the PRC, facilitating the processing of complex object features.^[12] Additional afferents arrive from multimodal regions such as the orbitofrontal cortex and prefrontal cortex, as well as from auditory areas in the superior temporal sulcus and somatosensory inputs from the insula.^[12] In rodents, afferents to the PRC include substantial olfactory projections from the piriform cortex, auditory inputs from temporal association areas, and visual information relayed through lateral entorhinal pathways, reflecting adaptations to species-specific sensory dominance.^[13] These afferent pathways are predominantly glutamatergic, enabling excitatory transmission of sensory signals. Efferent projections from the PRC further underscore its role in relaying integrated information to limbic and executive networks. In both primates and rodents, the PRC sends strong outputs to the entorhinal cortex, serving as a primary gateway to the hippocampus for memory-related processing, with projections originating from layers II and III of PRC areas 35 and 36. Additional efferents target the amygdala, particularly the basolateral nucleus, to incorporate emotional valence into sensory representations, and extend to the prefrontal cortex, including prelimbic and infralimbic regions in rodents and orbital prefrontal areas in primates, supporting decision-making and reward evaluation.^[13]^[15] Dopaminergic afferents from the ventral tegmental area (VTA) provide modulatory input to the PRC, though these constitute a minor proportion (0.2-0.7% of subcortical afferents in rodents), influencing plasticity and salience attribution.^[13] Intrinsic connections within the PRC are robust and reciprocal, particularly between areas 35 and 36, forming feedback loops that enhance local processing of feature conjunctions, with projections primarily glutamatergic in nature. These internal pathways, combined with extrinsic links to inferior temporal cortex, create hierarchical loops that underpin the PRC's contribution to object recognition by binding multisensory attributes into unified percepts.

Functions

Perceptual processing

The perirhinal cortex plays a crucial role in object perception by processing high-level visual features to enable the discrimination of similar or complex objects. This function is particularly important for resolving perceptual ambiguities arising from overlapping sensory attributes, such as distinguishing objects that share many visual elements. For instance, lesions to the perirhinal cortex in nonhuman primates impair performance on visual discrimination tasks involving high feature overlap, like identifying correct stimuli in sets where positive and negative exemplars share multiple components (e.g., configurations with maximum feature ambiguity, such as AB+ , CD+ , BC- , AD- ).^[16] This perceptual contribution extends beyond simple feature detection, supporting the integration of sensory inputs to form coherent object representations.^[17] A key aspect of perirhinal processing involves the representation of non-spatial object information, including attributes like shape, texture, and color, independent of their spatial location. Electrophysiological recordings in rats foraging among objects reveal that perirhinal neurons robustly encode object identity and novelty without exhibiting place-like or spatially stable firing patterns, contrasting with adjacent regions like the lateral entorhinal cortex that incorporate both object and spatial signals.^[18] This non-spatial encoding allows the perirhinal cortex to maintain representations of object features across different contexts, facilitating perceptual discrimination even when objects are viewed from varying angles or positions.^[19] The perirhinal cortex achieves feature integration by combining multisensory inputs into unitary object percepts, which is essential for resolving ambiguity in configural tasks. In perceptual oddity discrimination, where subjects identify differing elements among highly similar items, perirhinal activity supports the binding of features to prevent errors from partial matches.^[16] Neuroimaging evidence from human fMRI studies corroborates this, showing significant right perirhinal activation during event-related tasks requiring detection of object identity changes in arrays, particularly for fine-grained visual discriminations, independent of spatial rearrangements. Such findings indicate that perirhinal involvement in perception provides a foundation for subsequent familiarity-based memory signals.^[20]

Memory encoding and recognition

The perirhinal cortex plays a critical role in item recognition memory, particularly in supporting familiarity-based judgments, such as determining whether an object has been encountered previously without recalling specific contextual details.^[21] This function is evident in tasks like delayed non-matching-to-sample, where perirhinal lesions in primates impair performance on object recognition over delays, indicating its necessity for processing and storing representations of individual items.^[20] Human neuroimaging studies further corroborate this, showing perirhinal activation during familiarity assessments for faces and objects, distinct from hippocampal involvement in recollection. During memory encoding, the perirhinal cortex exhibits greater activation in response to novel stimuli compared to familiar ones, facilitating the formation of declarative memories for objects. Functional MRI evidence demonstrates that encoding novel picture pairs, but not their retrieval, selectively engages the perirhinal cortex bilaterally, suggesting it integrates complex object features into durable representations.^[22] This novelty signal supports the consolidation of item-specific information, enabling subsequent recognition without reliance on associative context. The perirhinal cortex interacts with the hippocampus to contribute item-level information essential for episodic memory formation, while lesions to this region selectively impair familiarity-based recognition but preserve recollection of contextual details mediated by the hippocampus. In patients with anterior temporal lobe resections including perirhinal areas, familiarity judgments for words and faces are significantly reduced, yet source memory (recollection) remains intact, highlighting a complementary division of labor within the medial temporal lobe.^[23] This interaction allows the perirhinal cortex to supply object representations that the hippocampus binds with spatial and temporal contexts. Neurophysiological recordings provide direct evidence of perirhinal involvement through single-unit responses that exhibit view-invariant selectivity to objects, underpinning recognition across different presentations. In rats, perirhinal neurons decrease firing rates to repeated objects over long delays, reflecting familiarity signals, while maintaining invariant responses to object identity regardless of viewpoint. Similar findings in primates show perirhinal cells responding robustly to complex objects in a manner independent of size, position, or angle, supporting the encoding of stable item representations for memory.

Associative learning

The perirhinal cortex plays a crucial role in associative learning by facilitating the integration of disparate stimuli into coherent representations, particularly through stimulus-stimulus associations. In paired associate tasks, where animals must learn connections between unrelated visual objects, perirhinal lesions in nonhuman primates lead to significant impairments, indicating its necessity for forming and retrieving these links.^[24] Neuronal recordings reveal that perirhinal cells signal newly learned associations, such as between scenes and locations, with firing rate changes occurring before, during, or after behavioral acquisition, often peaking during delay periods in tasks requiring sustained memory.^[25] Beyond basic pairings, the perirhinal cortex contributes to reward-value encoding by integrating sensory inputs with motivational signals, including dopaminergic inputs from the ventral tegmental area. This enables the assignment of value to specific stimuli, as evidenced by perirhinal neurons that respond to reward outcomes during conditional association tasks, where visual cues predict rewards; a majority of these cells show selectivity for negative feedback, modulating responses to guide future choices.^[26] Such encoding supports flexible representations beyond simple stimulus-reward pairing, allowing adaptation to changing contingencies. In semantic associations, the perirhinal cortex supports conceptual knowledge by linking related items across domains, such as object features or crossmodal attributes. Functional imaging in humans demonstrates object-specific semantic coding in this region, with increasing engagement for novel associations that build broader knowledge networks.^[27] Lesion studies further confirm its involvement in declarative associations, both within-domain (e.g., visual object pairs) and between-domain (e.g., visual-auditory links), essential for accumulating factual relationships over repeated experiences.^[28] Behavioral paradigms underscore these functions, particularly in delayed non-matching to sample tasks adapted for associations, where perirhinal damage disrupts performance by impairing the integration of sample cues with novel targets.^[29] In location-scene association variants, perirhinal neurons exhibit learning-related signals without habituation, correlating with successful task completion and highlighting its role in bridging perceptual familiarity with associative outcomes.^[25]

Clinical significance

Lesion effects

Lesions to the perirhinal cortex consistently impair object recognition memory across species, leading to deficits in discriminating objects and judging their familiarity. In nonhuman primates, bilateral perirhinal lesions disrupt performance on delayed nonmatching-to-sample tasks, where monkeys fail to preferentially select novel objects over familiar ones in visual paired comparison tests, even at short delays.^[30] Similarly, in rodents, perirhinal damage impairs spontaneous object recognition in the novel object preference paradigm, reducing exploration of novel items and indicating a loss of familiarity-based discrimination.^[31] These recognition deficits are particularly evident when stimuli share overlapping features, highlighting the perirhinal cortex's role in resolving such ambiguities for memory encoding.^[32] Perceptual impairments following perirhinal lesions manifest as difficulties in processing complex or confusable visual stimuli, while basic visual functions remain intact. Patients with medial temporal lobe lesions encompassing the perirhinal cortex exhibit selective deficits in naming and discriminations involving objects with high feature overlap, akin to integrative agnosia, but perform normally on simpler perceptual tasks without conjunction demands.^[33] In monkeys, perirhinal ablation impairs the identification of degraded or cluttered objects, suggesting a failure in feature integration rather than low-level vision loss.^[34] Rodent studies corroborate this, showing that perirhinal lesions spare elemental discriminations but hinder those requiring configural processing of overlapping cues.^[35] Animal models further reveal modality-specific effects, including olfactory discrimination deficits in rodents. Rats with perirhinal lesions show impaired recognition memory for social odors, failing to discriminate familiar from novel conspecific scents, though non-social odor recognition remains relatively preserved.^[36] In monkeys, the visual paired comparison deficits extend to associative tasks, where lesioned animals struggle with object pairings involving similar features.^[37] Human studies using lesion mapping and fMRI demonstrate correlations between perirhinal damage and object recognition impairments resembling prosopagnosia but for non-face stimuli. Voxel-based morphometry in patients with temporal lobe lesions links perirhinal volume loss to deficits in discriminating complex objects, such as quasi-realistic faces or novel shapes with shared attributes, independent of memory demands.^[38] Functional imaging confirms perirhinal activation during perceptual tasks with high feature ambiguity, supporting its role in resolving such confusions, with lesion overlap predicting symptom severity.^[39] These findings underscore a perceptual-memory continuum disrupted by perirhinal damage, often tied to disrupted connectivity with ventral visual areas.^[40]

Role in neurological disorders

The perirhinal cortex exhibits early atrophy in Alzheimer's disease (AD), which correlates strongly with episodic and semantic memory decline, often preceding hippocampal changes.^[41] This atrophy is particularly pronounced in Brodmann area 35 of the perirhinal cortex, where volumetric reductions measured via MRI are associated with impaired object recognition and familiarity-based memory tasks in early-stage patients.^[42] Additionally, tau pathology, including neurofibrillary tangles, accumulates prominently in the perirhinal cortex during the initial Braak stages (I-II) of AD, contributing to synaptic dysfunction and neurodegeneration that drive cognitive impairment.^[43] These pathological changes in the perirhinal cortex are detectable via PET imaging with tau tracers like 18F-AV-1451, showing uptake that predicts future atrophy rates in medial temporal regions.^[44] In temporal lobe epilepsy (TLE), the perirhinal cortex serves as a common epileptogenic zone, where seizures frequently originate due to its hyperexcitability and dense connectivity with limbic structures.^[45] Electrical stimulation studies and intracranial EEG recordings confirm that perirhinal activation can propagate seizures to the hippocampus and amygdala, leading to complex partial seizures characteristic of TLE.^[46] Surgical interventions, such as anterior temporal lobectomy or selective amygdalohippocampectomy that include perirhinal resection, achieve seizure freedom in up to 70% of refractory TLE cases, though they carry risks of memory deficits due to the region's role in recognition processes.^[47] Semantic dementia, a subtype of frontotemporal lobar degeneration, features hypometabolism in the perirhinal cortex as revealed by FDG-PET imaging, which correlates with profound loss of conceptual knowledge for objects and entities.^[48] This hypometabolism, often bilateral but left-predominant, disrupts the perirhinal cortex's contribution to semantic processing of visual and multimodal features, resulting in anomia, surface dyslexia, and impaired object-use comprehension while sparing episodic memory.^[49] Atrophy in adjacent anterior temporal regions exacerbates these deficits, but perirhinal involvement specifically impairs fine-grained semantic discriminations, such as distinguishing subordinate categories of objects.^[50] Post-2020 volumetric MRI studies have demonstrated that perirhinal cortex thinning is a potential early imaging marker for AD pathology in MCI, with the medial perirhinal cortex proposed as a sensitive preclinical biomarker.^[51] These findings underscore the perirhinal cortex's sensitivity to preclinical AD pathology, enabling targeted monitoring in at-risk populations.^[51]

Comparative neuroanatomy

In primates

The perirhinal cortex in primates, encompassing both non-human species such as macaques and humans, exhibits conserved cytoarchitectonic features, notably the presence of areas 35 and 36 located dorsal to the rhinal sulcus and caudal to the orbitofrontal cortex.^[52] These areas receive predominantly visual-dominant inputs from the inferior temporal (IT) cortex, particularly area TE, which conveys representations of complex feature conjunctions essential for object processing.^[53] This structural organization underscores evolutionary conservations, with area 35 characterized by agranular layers and area 36 by more differentiated granular layers across primate species.^[54] Functionally, the perirhinal cortex in primates emphasizes visual object recognition, where it resolves ambiguities arising from overlapping features in complex stimuli, facilitating accurate perceptual discrimination.^[55] Outputs from these areas project to prefrontal regions via the entorhinal cortex, supporting executive control over recognition processes, such as decision-making in visual tasks.^[52] This visual bias distinguishes primate perirhinal function from other mammals, highlighting adaptations for advanced object-based cognition. Experimental evidence from lesion studies in macaque monkeys demonstrates that ablation of the perirhinal cortex impairs performance on complex visual discrimination tasks involving high feature overlap, such as distinguishing objects with similar textures or patterns, while sparing simpler discriminations. For instance, monkeys with perirhinal lesions exhibit deficits in learning associations between overlapping visual cues, underscoring the region's role in perceptual-mnemonic integration rather than basic sensory processing.^[56] In humans, functional magnetic resonance imaging (fMRI) reveals homologous activation patterns in the perirhinal cortex during object processing tasks, paralleling non-human primate findings by showing increased activity for feature-ambiguous stimuli that demand conjunction resolution.^[57] These parallels suggest conserved mechanisms for visual recognition across primates, with human perirhinal engagement evident in tasks requiring discrimination of complex scenes or objects akin to those tested in monkeys.

In rodents

In rodents, particularly rats and mice, the perirhinal cortex (PRC) is a component of the parahippocampal region, located along the rhinal sulcus on the ventral surface of the temporal lobe, lateral to the hippocampus. It encompasses Brodmann's areas 35 and 36, forming a transitional zone between neocortex and allocortex.^[58] This region spans approximately the third quarter of the rhinal sulcus rostrocaudally, with area 35 occupying the ventral bank and area 36 the dorsal bank.^[59] The rostral boundary of the PRC adjoins the posterior agranular insular cortex and visceral association area, while caudally it borders the postrhinal cortex. Dorsally, it interfaces with the ventral temporal association cortex (areas Te2 and Te3), and ventrally with the lateral entorhinal cortex.^[58] These boundaries have been delineated using cytoarchitectonic criteria, connectional patterns, and histochemical markers such as acetylcholinesterase staining.^[60] Cytoarchitectonically, the PRC exhibits a six-layered structure with variations between its subdivisions. Area 35 is agranular, lacking a distinct layer IV; it features a broad layer I, radially organized cells in layers II and III forming clusters, and large pyramidal neurons in layer V.^[58] In contrast, area 36 is dysgranular, with a weakly defined layer IV, prominent layer II containing clustered round cells, and well-differentiated layers III and V.^[59] Further subdivisions include dorsal and ventral portions of area 35, and dorsal, ventral, and posterior regions of area 36, based on laminar differences and connectivity gradients.^[60] These features distinguish the PRC from adjacent areas like the entorhinal cortex, which has more pronounced laminar organization.^[58] The PRC receives extensive afferent projections, integrating multimodal sensory information. Cortical inputs arise from primary and association areas, including visual (areas 17, 18a), auditory (temporal), somatosensory (parietal), and olfactory (piriform) cortices, as well as prefrontal and cingulate regions.^[59] Subcortical afferents include the amygdala (basolateral and lateral nuclei), mediodorsal thalamus, and claustrum, while hippocampal inputs come from the entorhinal cortex and subiculum.^[58] Efferent projections reciprocate many of these connections, with strong outputs to the lateral entorhinal cortex, amygdala, medial prefrontal cortex (prelimbic and infralimbic areas), midline thalamic nuclei, and striatum (caudate-putamen and nucleus accumbens).^[60] These connections position the PRC as a hub for relaying sensory data to the hippocampal formation, with area 36 showing denser neocortical ties and area 35 more allocortical links.^[59] Functionally, the rodent PRC supports object recognition memory and perceptual discrimination of complex stimuli, similar to primates, with lesions impairing familiarity-based tasks and resolution of feature ambiguity in visual and multimodal contexts. For example, rat PRC lesions disrupt performance on delayed nonmatching-to-sample tasks with complex objects, highlighting its role in integrating sensory features for mnemonic processes.^[61] These functions underscore conserved roles across mammals, though with less visual dominance compared to primates.

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Perirhinal and parahippocampal cortices of the macaque monkey
Dec 22, 1994 · We found that the macaque monkey perirhinal and parahippocampal cortices receive different complements of cortical inputs.
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Where are the perirhinal and parahippocampal cortices? a historical ...
Strong evidence has emerged over the last 15 years showing that the perirhinal and parahippocampal cortices play an important role in normal memory function ...
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Selective Perceptual Impairments After Perirhinal Cortex Ablation
Dec 15, 2001 · We found clear support for our hypothesis that perirhinal lesions impair making perceptual discriminations between stimuli when the ...
[54]
Recognition Memory for Complex Visual Discriminations Is ...
Impairments in visual discrimination after perirhinal cortex lesions: Testing `declarative' vs. `perceptual-mnemonic' views of perirhinal cortex function.
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The rat perirhinal cortex: A review of anatomy, physiology ... - PubMed
Thirdly, we review the main functions of the perirhinal cortex; its roles in perception, recognition memory, spatial and contextual memory and fear conditioning ...
[56]
Perirhinal and Postrhinal Cortices of the Rat - PubMed - NIH
We examined the connectivity among the rat perirhinal (areas 35 and 36), postrhinal, and entorhinal cortices by placing anterograde and retrograde tracers in ...
[57]
Functional neuroanatomy of the parahippocampal region in the rat
The parahippocampal region in the rodent brain includes the perirhinal, postrhinal, and entorhinal cortices, the presubiculum, and the parasubiculum.