Abstract
Social recognition memory reflects the ability of the social animals to recognize and remember familiar individuals of the same species. The unique ability for mammals to recognize conspecifics is essential and beneficial when animals conduct daily social activities. This brief review summarizes a brain network underlying social recognition memory and explores the possible relationships between social isolation and social recognition memory. Finally, we introduce some possible related molecular mechanisms underlying social recognition memory. These findings help us explore potential targeting brain areas or circuits of social communication disorder.
Social Recognition Memory
For animals living in social groups, only individuals with social recognition memory can correctly respond to the visitors in cooperative or competitive social situations, such as remembering their mating partner (; ) and recognizing threatening visitors to the animal’s habitats (). If social animals cannot remember familiar conspecifics, the stability of the social group will be destroyed. We focus on the ability of the animal to recognize individual partners during the interaction between the two parties. Behavioral paradigms which can quantitatively test the recognition ability are described and discussed.
Individual Recognition
Individual recognition refers to a subset of recognition that occurs when one organism recognizes another based on its unique characteristics (). Almost all social behaviors require individual recognition. In individual recognition, the recognizer is regarded as “receiver,” and the sending individual is recognized as “signaler” (). Most studies focus on “receiver,” and relatively few have explored “signaler.” In fact, individual recognition is a task that requires cooperation between the “receiver” and “signaler.” The two parties can form a reciprocal feedback loop between them to interact with each other (). The information from acute sensory inputs detected from the “signaler” is transformed into the behavioral output of the “receiver.” In turn, the behavioral output of the “receiver” also provides sensory cues to the “signaler” (Figure 1).
FIGURE 1
Social Recognition Memory Paradigms
In rodents, mice and rats tend to investigate the novel social stimulus with longer investigation compared to familiar social stimulus. This innate tendency reflects a marked difference in individual recognition for these two social stimuli. Social recognition memory can be studied by detecting the investigation time of different social stimuli.
Three well-established behavioral paradigm methods are used to evaluate social recognition memory (
FIGURE 2

Three behavioral paradigms for evaluating social recognition memory. In the first two of the three paradigms, a familiar mouse means the social stimulus the subject mouse has met in the previous trials. In the third paradigm, a familiar mouse refers to a cage mate of the subject mouse. (A) The direct interaction paradigm includes two encounters. The social stimulus the subject mouse meets in the second encounter can be familiar or novel. (B) The social habituation-dishabituation paradigm includes five trials of encounters. In the fifth trial, the subject mouse meets a novel social stimulus. (C) The social discrimination paradigm includes one encounter trial. The subject mouse meets two social stimuli at the same time. One is a familiar cage mate, and the other is a novel stimulus.
The second paradigm is the social habituation-dishabituation paradigm (Figure 2B). This paradigm consists of five trials of encounters (
The third one is the social discrimination paradigm (Figure 2C). This paradigm includes an encounter in the situation that the familiar stimulus and the novel stimulus are presented to the receiver simultaneously (
Brain Network Underlying Social Recognition Memory
Social cognitive neuroscience studies mental, behavioral, and neural mechanisms underlying social behavior in socially organized individuals (
FIGURE 3

Brain networks underlying social recognition memory. OB: olfactory bulb; dHPC: dorsal hippocampus; dCA2: dorsal CA2; LS: lateral srptum; MS: medial septum; SuM: supramammillary nucleus; vCA1: ventral CA1; NAc: nucleus accumbens; IL: infralimbic area; PL: prelimbic area; mPFC: medial prefrontal cortex.
Hippocampus
Early lesion studies have shown that lesioning of the entire hippocampus can affect social memory, as detected by the direct social recognition memory paradigm described in Figure 1A (Kogan et al., 2000). Bilateral microinjection of ibotenic acid in the hippocampus disrupted the social recognition memory when the second trial was performed 30 min after the first encounter trial. However, if the second trial was performed immediately after the first trial, the social recognition memory was not affected. This experiment demonstrates that lesioning the hippocampus can cause social memory deficit but social recognition between two close encounters of familiar and novel stimuli may not require the hippocampus. Using protein synthesis inhibitor anisomycin, blocking the protein synthesis in the hippocampus did not impair the 24-h interval test of social recognition memory. However, the protein inhibitor did not affect the social recognition memory using a 30 min interval. The cyclic AMP responsive element-binding protein (CREB) mutant mice, lacking isoforms of CREB, could not form long-term social recognition memory but could form short-term social recognition memory. The results with the protein inhibitor and the CREB mutant mice demonstrate that intact hippocampal functions are required for long-term social recognition memory.
Hippocampal Subregions
Hippocampal lesioning studies point to a role of the hippocampus in the social recognition memory, however, this approach can not disclose which subregions of the hippocampus contribute to the social recognition memory. It has been found that the mRNA level of vasopressin 1b receptor is higher in the CA2 subregion (
Taking advantage of transgenic mouse line Amigo2-cre in which cre is selectively expressed in the pyramidal cells of the dorsal CA2 region, it was found that inactivation of the pyramidal neurons in this region impaired social recognition memory in three-chamber social recognition and 5-trial habituation-dishabituation social memory tests (
The ventral hippocampus region is regarded to have a role in emotional processing such as anxiety and fear memory (
Afferent and Efferent Areas of the Hippocampus
The afferent and efferent connectivity patterns of the hippocampus change along the longitudinal axis (
The afferent and efferent pathways of the ventral hippocampus also contributed to the process of social recognition. Optogenetic activation of the neural circuit of ventral CA1 to nucleus accumbens (NAc) shell reduced the time for the receiver subject to explore the novel stimulus in the social discrimination task (
Other Brain Areas Underlying Social Recognition Memory
In addition to the hippocampal circuits mentioned above,
Lateral septum has been implicated in the social behavior (
TABLE 1
| Behavior | Manipulation in cited articles | Social recognition | Object recognition | Olfaction test | |||||
| Article | Two-trial | Three Chamber | Five-trial | One-trial | Resident-intruder | Novel object recognition | Olfactory habituation/dishabituation | Buried food | |
| Whole HPC lesion | + | − | − | − | − | − | − | − | |
| dCA2 inhibition | + | + | + | − | − | + (*) | + | + | |
| CA2 silence | + | − | + | − | − | − | + | + | |
| dCA2, dCA2-vCA1 inhibition | + | + | − | − | − | − | − | − | |
| vCA1, vCA1-NAc inhibition | − | − | − | + | + | +(*) | − | − | |
| PVIs inhibition in vCA1 PVIs activation in vCA1 | − | + | − | + | − | + (Ns) | − | − | |
| vHPC-mPFC inhibition in WT vHPC-mPFC inhibition in Mecp2 | − | + | − | + | − | +(*) | − | − | |
| SuM-dCA2 activation | + | − | + | − | − | − | − | − | |
| MS, MS- dCA2 inhibition | + | + | + | − | − | +(*) | − | − | |
| Protein synthesis inhibition of mPFC, ACC, BLA | + | − | − | − | − | − | − | − | |
| PVIs activation in mPFC | − | − | − | + | − | +(*) | − | − | |
| OB, dHPC inhibition | + | − | − | − | − | − | − | − | |
| NAc-IL inhibition NAc-IL activation | − | + | − | − | − | +(*) | − | − | |
| Oxytocin knockout mice | + | − | + | − | − | − | + | − | |
| OXTRs knockout mice | + | − | + | − | − | − | − | − | |
| VAPRs knockout mice | + | − | + | − | − | − | + | − | |
| Oxytocin and vasopressin treatment | + | − | + | − | − | − | − | − | |
| Viral recombination of OXTRs in anterior DG, CA2, CA3 | − | − | − | + | − | +(*) | − | − | |
| Conditional deletion of the OXTRs in CA2/CA3 | − | + | + | − | − | − | − | − | |
| vCA3 inhibition | + | + | − | − | − | +(*) | − | − | |
Behavioral outcomes after manipulation of neural circuits using different social and non-social behavioral paradigms.
+Behavior performed –Behavior not performed. Two-trial: Two-trial social recognition paradigm in Figure 2A. Five-trial: Social habituation-dishabituation paradigm in Figure 2B. One-trial: Social discrimination paradigm in Figure 2C.
*Represents that both experimental and control groups have a significant difference between the investigation time of the novel object and the familiar object, indicating the intact object recognition memory. Ns means that experimental groups have no difference between the investigation of the novel object and the familiar object while the object recognition memory in the control groups remained unaffected, indicating the impaired object recognition memory. PVIs: Parvalbumin (PV) interneurons; Oxytocin receptors: OXTRs; Vasopressin 1a receptors: VAPRs.
Possible Neuron Types Regulating the Social Recognition Memory
Different neuronal types in the subregions of the hippocampus may have differential roles in social recognition memory. Single-unit recordings from the dorsal hippocampus in rats showed that the spiking activities did not distinguish between different individuals, but they could distinguish between rats and inanimate objects (
Parvalbumin (PV) interneurons in the ventral hippocampus also played a role in social recognition. Using fiber photometry, the calcium activity of PV neurons in the ventral CA1 was higher when the subject mice approached novel mice compared to the calcium activity when the subject mice approached familiar mice (
In addition to the neuron type in the hippocampus which can regulate the social recognition memory, different neuron types in other areas that have connections with the hippocampus also play important roles. Increased synaptic strength of the ventral hippocampal input to the fifth layer of mPFC pyramidal neurons was determined to be a factor of social memory deficits in Mecp2 mice (
The Effect of Social Isolation on Social Recognition Memory
The absence of companions may endanger the mental health of social animals. Thor and Holloway reported that the rats that experienced social isolation had impaired social recognition memory (
Interestingly, the social recognition memory deficit induced by the social isolation of the adult mice can be recovered by group housing for more than a few weeks (
The Molecular Mechanism Underlying Social Recognition Memory
The molecular components within the neural circuits provide insights into how the neurotransmitters or neuropeotide can contribute to the modulation of social recognition memory. It has been reported that oxytocin knockout mice had social recognition memory deficits. These mice investigated the familiar stimulus and the novel one without a clear preference, however, they did not have damaged novelty recognition ability (
It has been hypothesized that oxytocin and vasopressin play differential roles in social recognition memory (
Given the involvement of the dorsal CA2 in the social recognition memory, oxytocin pathways in this subregion of the hippocampus appear to be important for social recognition memory. Oxytocin receptors were expressed in hippocampal CA2 and CA3 pyramidal neurons (
Conclusion
Social recognition memory is critical for distinguishing between familiar and novel individuals and it forms an essential and basic part of social behavior. This brief review first summarizes the brain regions and neural circuits related to social recognition memory, mainly dorsal CA2 and ventral CA1, and their upstream and downstream circuits, including medium septum (MS), supramammillary nucleus (SuM), lateral septum (LS), prefrontal cortex and nuculeus accumbens (NAc). Then we discuss the neuron types within the hippocampal subregions underlying social recognition memory. Although both the dorsal and ventral hippocampus regions are involved in the social recognition memory, these two regions have very different input and output brain areas. It remains unknown whether there are functionally dissociable neurons that have connections with the dorsal and ventral hippocampus to regulate social recognition memory. These are the open hypotheses that will be explored further. Social isolation can lead to impaired social recognition memory in rodents. Consequently, the neural circuit mechanism underlying social isolation may be instructive to the understanding of the neural circuits that regulate social recognition memory. Re-socialization by group-housing conditions could ameliorate the social recognition memory deficits in the socially isolated adult mice. Finally, we investigated the contribution of oxytocin pathways in social recognition memory, with a particular focus on the oxytocin transmissions in the hippocampus. Still many unknown neural mechanisms at multiple levels about why and how animals recognize individuals. Future work needs to employ experimental methods and tools at behavioral, circuitry, and molecular levels to underpin our understandings of the social recognition memory.
Publisher’s Note
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.
Statements
Author contributions
Both authors discussed the ideas, wrote the manuscript and edited the manuscript.
Funding
This work was supported by National Natural Science Foundation of China Nos. 31671101 and 32070985; Shenzhen Key Laboratory of Translational Research for Brain Diseases No. ZDSYS2020082815480 0001.
Conflict of interest
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
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Summary
Keywords
social recognition memory, dorsal CA2, ventral CA1, social isolation, oxytocin
Citation
Wang X and Zhan Y (2022) Regulation of Social Recognition Memory in the Hippocampal Circuits. Front. Neural Circuits 16:839931. doi: 10.3389/fncir.2022.839931
Received
20 December 2021
Accepted
14 February 2022
Published
30 March 2022
Volume
16 - 2022
Edited by
Han Xu, Zhejiang University, China
Reviewed by
Dong Song, University of Southern California, United States; Sungchil Yang, City University of Hong Kong, Hong Kong SAR, China
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© 2022 Wang and Zhan.
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*Correspondence: Yang Zhan, yang.zhan@siat.ac.cn
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.