Is There a ‘Social’ Brain? Implementations and Algorithms
Authors: Patricia L. Lockwood, Matthew A.J. Apps, Steve W.C. Chang
Journal: Trends in Cognitive Sciences (2020)

A central question in psychology and neuroscience is the extent to which social behaviour is subserved by dedicated processes or systems that are 'socially specific' or shared with other 'non-social' cognitive, perceptual, and motor faculties.We suggest that a process can be socially specific at different levels of explanation. This approach could help to clarify the role of mirror neurons in social contexts and whether social learning is uniquely 'social'. Experimental design should be guided by an appreciation that social specificity is possible at different levels.Examining social behaviour across species can give unique clues about different implementations and algorithms. For example, converging evidence highlights the anterior cingulate gyrus as crucial for processing social implementations, and that 'theory of mind' is a putative social algorithm. A fundamental question in psychology and neuroscience is the extent to which cognitive and neural processes are specialised for social behaviour, or are shared with other 'non-social' cognitive, perceptual, and motor faculties. Here we apply the influential framework of Marr (1982) across research in humans, monkeys, and rodents to propose that information processing can be understood as 'social' or 'non-social' at different levels. We argue that processes can be socially specialised at the implementational and/or the algorithmic level, and that changing the goal of social behaviour can also change social specificity. This framework could provide important new insights into the nature of social behaviour across species, facilitate greater integration, and inspire novel theoretical and empirical approaches. A fundamental question in psychology and neuroscience is the extent to which cognitive and neural processes are specialised for social behaviour, or are shared with other 'non-social' cognitive, perceptual, and motor faculties. Here we apply the influential framework of Marr (1982) across research in humans, monkeys, and rodents to propose that information processing can be understood as 'social' or 'non-social' at different levels. We argue that processes can be socially specialised at the implementational and/or the algorithmic level, and that changing the goal of social behaviour can also change social specificity. This framework could provide important new insights into the nature of social behaviour across species, facilitate greater integration, and inspire novel theoretical and empirical approaches. Many behaviours occur in a social context. Social behaviours, in some form, are exhibited across a surprisingly broad array of species from single-celled microorganisms [1.Crespi B.J. The evolution of social behavior in microorganisms.Trends Ecol. Evol. 2001; 16: 178-183Abstract Full Text Full Text PDF PubMed Scopus (387) Google Scholar] to rodents [2.Chen P. Hong W. Neural circuit mechanisms of social behavior.Neuron. 2018; 98: 16-30Abstract Full Text Full Text PDF PubMed Scopus (174) Google Scholar], fish [3.Bshary R. et al.Social cognition in fishes.Trends Cogn. Sci. 2014; 18: 465-471Abstract Full Text Full Text PDF PubMed Scopus (92) Google Scholar], and primates [4.Wittmann M.K. et al.Neural mechanisms of social cognition in primates.Annu. Rev. Neurosci. 2018; 41: 99-118Crossref PubMed Scopus (58) Google Scholar]. However, a core question for psychology and neuroscience is whether there are cognitive processes, brain areas, circuits, or cells that process information in a manner that is socially specific. That is, are there processes that come online only in social situations in a way that is somehow different to what is required for 'non-social' cognitive, motor, and perceptual abilities? We draw on the pioneering idea of Marr's levels (see Glossary) [5.Marr D. Vision. MIT Press, 1982Google Scholar] to provide a framework to understand and test whether cognitive and neural processes are socially specialised or not. We argue that a process may be considered 'social' at the algorithmic level – it encodes a specific algorithm or rule that is different from what is being processed in a non-social domain – and/or at the implementational level, the same algorithm is used, but it is processed in a different brain area, circuit, or cell. Moreover, we suggest that changing the social goal of the information-processing system (computational level), such as during cooperation or competition, can change social specialisation at the other levels. These levels of description are often overlooked when studying information processing in social contexts. We contend that this can lead to inaccurate conclusions about whether cognitive or neural processes are specialised, and call for a more nuanced approach to the phrase 'the social brain' beyond its simple connotation. Marr's framework [5.Marr D. Vision. MIT Press, 1982Google Scholar] argued that, to understand an information-processing system, it is crucial to consider multiple levels of explanation – computational, algorithmic, and implementational (Figure 1) [6.Krakauer J.W. et al.Neuroscience needs behavior: correcting a reductionist bias.Neuron. 2017; 93: 480-490Abstract Full Text Full Text PDF PubMed Scopus (595) Google Scholar]. The highest level of description, computational, describes the 'why' of a system or the goal that it intends to perform. For example, if we want to understand bird flight we cannot do so 'by only studying the feathers' [5.Marr D. Vision. MIT Press, 1982Google Scholar]. We first need to know that the goal of the bird is to fly. The second level is the algorithmic – 'what' rules does the brain apply for a particular operation? This would be the bird's flapping of its wings. The final level, implementational, is 'how' the brain achieves a particular operation. For a bird, this would be its feathers. How can this apply to social behaviour? The computational goal of social interactions is dictated by the nature and the intentions of the agent, such as cooperating, affiliating, or competing with conspecifics. The algorithmic level would be a particular, formalised, model of a social or cognitive process that is deployed only when engaging in a social interaction. Lastly, the implementational level would be the specific brain region, circuit, or cell in which the social process is realised. Although the number of levels and their independence are debated [6.Krakauer J.W. et al.Neuroscience needs behavior: correcting a reductionist bias.Neuron. 2017; 93: 480-490Abstract Full Text Full Text PDF PubMed Scopus (595) Google Scholar,7.Bickle J. Marr and reductionism.Top. Cogn. Sci. 2015; 7: 299-311Crossref PubMed Scopus (25) Google Scholar], Marr's theory provides an important organising framework suggesting that social specificity can be delineated at different levels. The most crucial point is that, for a process to be considered socially specific, there must be a dissociation between social and non-social processing at either the algorithmic or implemental levels, and alternative, similar, domain-general processes must be ruled out. Marr's framework offers new insights into several debates in social neuroscience. We consider two notable examples. Mirror neurons or 'common currency' accounts suggest that social information is encoded based on an overlap in implementation (the same neuron fires similarly, or there is the same fMRI blood oxygen level-dependent, BOLD, response) to first- and third-person events. Examples include pain to self and another [8.Lamm C. et al.Meta-analytic evidence for common and distinct neural networks associated with directly experienced pain and empathy for pain.Neuroimage. 2011; 54: 2492-2502Crossref PubMed Scopus (1295) Google Scholar,9.Fan Y. et al.Is there a core neural network in empathy? An fMRI based quantitative meta-analysis.Neurosci. Biobehav. Rev. 2011; 35: 903-911Crossref PubMed Scopus (628) Google Scholar], monetary and social reward [10.Ruff C.C. Fehr E. The neurobiology of rewards and values in social decision making.Nat. Rev. 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However, such conclusions about mirroring are drawn with reference only to the implementational level. What if a different algorithm is being used to understand the states of another? Without manipulating or controlling the algorithmic level, mirroring could reflect either a total absence of social specificity, an absence of social specificity only at implementation, or only in one particular circuit or cell. Indeed, it is clear that the goal level of the system is different when monitoring another's actions. We are not necessarily aiming to reproduce the actions ourselves, and therefore even if there is overlap at the implementation, there is likely to be functional dissociation. However, the other levels need to be measured using carefully controlled designs to manipulate them to understand what – if anything – the overlap means for social cognition and behaviour (Box 1).Box 1The Importance of Non-Social Control ConditionsWhen designing an experiment to test whether a cognitive or neural process has a selective role in social behaviour, it is important to think about appropriate control conditions. This idea stems from basic principles in philosophy of science on the necessity of falsification [91.Popper K.R. The logic of scientific discovery. Basic Books, 1959Google Scholar]. For example, if we find that a particular brain area responds to both smiling faces and to monetary reward, can we conclude that this area is encoding something about how rewarding the stimulus is? We have not shown that this brain area is not involved in any other process, only that it is involved in two processes that share a common feature. Many studies in rodents and monkeys have included explicit 'non-social' control conditions [21.Apps M.A.J. et al.The anterior cingulate gyrus and social cognition: tracking the motivation of others.Neuron. 2016; 90: 692-707Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar,42.Lockwood P.L. The anatomy of empathy: vicarious experience and disorders of social cognition.Behav. Brain Res. 2016; 311: 255-266Crossref PubMed Scopus (124) Google Scholar,45.Chang S.W.C. et al.Neuronal reference frames for social decisions in primate frontal cortex.Nat. Neurosci. 2013; 16: 243-250Crossref PubMed Scopus (203) Google Scholar,47.Rudebeck P.H. et al.A role for the macaque anterior cingulate gyrus in social valuation.Science. 2006; 313: 1310-1312Crossref PubMed Scopus (242) Google Scholar,50.Carrillo M. et al.Emotional mirror neurons in the rat's anterior cingulate cortex.Curr. Biol. 2019; 29: 1301-1312Abstract Full Text Full Text PDF PubMed Scopus (85) Google Scholar,51.Hernandez-Lallement J. et al.Harm to others acts as a negative reinforcer in rats.Curr. Biol. 2020; 30: 949-961Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar]. In studies of self and other reward processing in monkeys, a 'neither' reward condition has been added, where a reward is seen as being delivered to neither the monkey nor their conspecific [45.Chang S.W.C. et al.Neuronal reference frames for social decisions in primate frontal cortex.Nat. Neurosci. 2013; 16: 243-250Crossref PubMed Scopus (203) Google Scholar,63.Dal Monte O. et al.Specialized medial prefrontal–amygdala coordination in other-regarding decision preference.Nat. Neurosci. 2020; 23: 565-574Crossref PubMed Scopus (46) Google Scholar]. In rodent studies of observational fear conditioning, a more typical classical conditioning condition, without any social context, has been employed to try to rule out a domain-general response to aversive events. In some studies of theory of mind processing, a 'computer' condition or a physical object condition has been introduced to try to show specificity for theory of mind processing [65.Balsters J.H. et al.Disrupted prediction errors index social deficits in autism spectrum disorder.Brain. 2016; 140: 235-246Crossref Scopus (48) Google Scholar,78.Rilling J.K. et al.The neural correlates of theory of mind within interpersonal interactions.Neuroimage. 2004; 22: 1694-1703Crossref PubMed Scopus (432) Google Scholar, 79.Gallagher H.L. Frith C.D. Functional imaging of 'theory of mind'.Trends Cogn. Sci. 2003; 7: 77-83Abstract Full Text Full Text PDF PubMed Scopus (1597) Google Scholar, 80.Apps M.A.J. et al.Reinforcement learning signals in the anterior cingulate cortex code for others' false beliefs.NeuroImage. 2013; 64: 1-9Crossref PubMed Scopus (35) Google Scholar].However, these control conditions are not always part of the experimental design. Sometimes it can be very difficult to create an equally matched non-social control that shares all or most attributes of a social stimulus except for its sociality – that is, a stimulus that is about or for another person or group. In the example of the computer condition, it may be that people anthropomorphise the computer and therefore still associate the stimulus with a social context, and it can be worth checking how participants perceive the condition. It is also well known that simple geometric shapes moving in a way that implies social interaction can be interpreted as social [92.Castelli F. et al.Movement and mind: a functional imaging study of perception and interpretation of complex intentional movement patterns.Neuroimage. 2000; 12: 314-325Crossref PubMed Scopus (1006) Google Scholar]. Therefore, a central factor in creating a social versus non-social condition appears to be the beliefs that the person has about whether the stimulus is social or non-social, rather than necessarily the observed behaviour. This is clearly shown in a study by Stanley and colleagues [93.Stanley J. et al.Effects of agency on movement interference during observation of a moving dot stimulus.J. Exp. Psychol. Hum. Percept. Perform. 2007; 33: 915-926Crossref PubMed Scopus (115) Google Scholar] who used a 2 × 2 design to probe the role of beliefs and behaviour in perceiving stimuli as social. Participants observed dot-motion animations and were instructed that they were either from prerecorded human movement or that they were computer-generated. They also manipulated whether the dot display followed biologically plausible or implausible velocity profiles. Participants experienced interference from the display when they were told that it reflected human movement, regardless of whether the velocity profiles were biologically plausible or not. This study therefore supports the idea that inducing beliefs that a stimulus is social versus non-social is crucial for creating social and non-social experimental conditions. When designing an experiment to test whether a cognitive or neural process has a selective role in social behaviour, it is important to think about appropriate control conditions. This idea stems from basic principles in philosophy of science on the necessity of falsification [91.Popper K.R. The logic of scientific discovery. Basic Books, 1959Google Scholar]. For example, if we find that a particular brain area responds to both smiling faces and to monetary reward, can we conclude that this area is encoding something about how rewarding the stimulus is? We have not shown that this brain area is not involved in any other process, only that it is involved in two processes that share a common feature. Many studies in rodents and monkeys have included explicit 'non-social' control conditions [21.Apps M.A.J. et al.The anterior cingulate gyrus and social cognition: tracking the motivation of others.Neuron. 2016; 90: 692-707Abstract Full Text Full Text PDF PubMed Scopus (255) Google Scholar,42.Lockwood P.L. The anatomy of empathy: vicarious experience and disorders of social cognition.Behav. Brain Res. 2016; 311: 255-266Crossref PubMed Scopus (124) Google Scholar,45.Chang S.W.C. et al.Neuronal reference frames for social decisions in primate frontal cortex.Nat. Neurosci. 2013; 16: 243-250Crossref PubMed Scopus (203) Google Scholar,47.Rudebeck P.H. et al.A role for the macaque anterior cingulate gyrus in social valuation.Science. 2006; 313: 1310-1312Crossref PubMed Scopus (242) Google Scholar,50.Carrillo M. et al.Emotional mirror neurons in the rat's anterior cingulate cortex.Curr. Biol. 2019; 29: 1301-1312Abstract Full Text Full Text PDF PubMed Scopus (85) Google Scholar,51.Hernandez-Lallement J. et al.Harm to others acts as a negative reinforcer in rats.Curr. Biol. 2020; 30: 949-961Abstract Full Text Full Text PDF PubMed Scopus (33) Google Scholar]. In studies of self and other reward processing in monkeys, a 'neither' reward condition has been added, where a reward is seen as being delivered to neither the monkey nor their conspecific [45.Chang S.W.C. et al.Neuronal reference frames for social decisions in primate frontal cortex.Nat. Neurosci. 2013; 16: 243-250Crossref PubMed Scopus (203) Google Scholar,63.Dal Monte O. et al.Specialized medial prefrontal–amygdala coordination in other-regarding decision preference.Nat. Neurosci. 2020; 23: 565-574Crossref PubMed Scopus (46) Google Scholar]. In rodent studies of observational fear conditioning, a more typical classical conditioning condition, without any social context, has been employed to try to rule out a domain-general response to aversive events. In some studies of theory of mind processing, a 'computer' condition or a physical object condition has been introduced to try to show specificity for theory of mind processing [65.Balsters J.H. et al.Disrupted prediction errors index social deficits in autism spectrum disorder.Brain. 2016; 140: 235-246Crossref Scopus (48) Google Scholar,78.Rilling J.K. et al.The neural correlates of theory of mind within interpersonal interactions.Neuroimage. 2004; 22: 1694-1703Crossref PubMed Scopus (432) Google Scholar, 79.Gallagher H.L. Frith C.D. Functional imaging of 'theory of mind'.Trends Cogn. Sci. 2003; 7: 77-83Abstract Full Text Full Text PDF PubMed Scopus (1597) Google Scholar, 80.Apps M.A.J. et al.Reinforcement learning signals in the anterior cingulate cortex code for others' false beliefs.NeuroImage. 2013; 64: 1-9Crossref PubMed Scopus (35) Google Scholar]. However, these control conditions are not always part of the experimental design. Sometimes it can be very difficult to create an equally matched non-social control that shares all or most attributes of a social stimulus except for its sociality – that is, a stimulus that is about or for another person or group. In the example of the computer condition, it may be that people anthropomorphise the computer and therefore still associate the stimulus with a social context, and it can be worth checking how participants perceive the condition. It is also well known that simple geometric shapes moving in a way that implies social interaction can be interpreted as social [92.Castelli F. et al.Movement and mind: a functional imaging study of perception and interpretation of complex intentional movement patterns.Neuroimage. 2000; 12: 314-325Crossref PubMed Scopus (1006) Google Scholar]. Therefore, a central factor in creating a social versus non-social condition appears to be the beliefs that the person has about whether the stimulus is social or non-social, rather than necessarily the observed behaviour. This is clearly shown in a study by Stanley and colleagues [93.Stanley J. et al.Effects of agency on movement interference during observation of a moving dot stimulus.J. Exp. Psychol. Hum. Percept. Perform. 2007; 33: 915-926Crossref PubMed Scopus (115) Google Scholar] who used a 2 × 2 design to probe the role of beliefs and behaviour in perceiving stimuli as social. Participants observed dot-motion animations and were instructed that they were either from prerecorded human movement or that they were computer-generated. They also manipulated whether the dot display followed biologically plausible or implausible velocity profiles. Participants experienced interference from the display when they were told that it reflected human movement, regardless of whether the velocity profiles were biologically plausible or not. This study therefore supports the idea that inducing beliefs that a stimulus is social versus non-social is crucial for creating social and non-social experimental conditions. Another major debate is whether social learning requires uniquely 'social' processes or arises from domain-general associative learning [14.Heyes C. What's social about social learning?.J. Comp. Psychol. 2012; 126: 193-202Crossref PubMed Scopus (252) Google Scholar, 15.Cook R. et al.Mirror neurons: from origin to function.Behav. Brain Sci. 2014; 37: 177-192Crossref PubMed Scopus (314) Google Scholar, 16.Catmur C. et al.Associative sequence learning: the role of experience in the development of imitation and the mirror system.Philos. Trans. R. Soc. B Biol. Sci. 2009; 364: 2369-2380Crossref PubMed Scopus (202) Google Scholar]. 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For example, lesion and brain stimulation approaches can clearly examine the impact of disrupting the implementational level on the algorithmic level. We can disrupt a specific implementation and test whether a social or non-social algorithm is changed. At the algorithmic level, we might examine a common algorithm, such as a reinforcement learning (RL) process, and then test whether it is differentially implemented in a social (learning about rewards for others) versus non-social (learning about rewards for self) condition. Considering how to dissociate different levels of analysis is therefore important in generating experimental designs that aim to address specificity. Moreover, we suggest that it is critical that additional 'non-social' control conditions are tested, if identifying social specificity is the aim (Box 1). In the following sections we examine this hypothesis by using key examples from the field and across research in humans, non-human primates, and rodents. A first question to ask is – why might we expect to find social specificity at any level of explanation? Evolutionarily, animal species are adapted to physical environments, and, for species that often interact with their conspecifics, they are adapted to social environments too [26.Gabay A.S. Apps M.A.J. Foraging optimally in social neuroscience: computations and methodological considerations.Soc. Cogn. Affect. Neurosci. 2020; (Published online March 30, 2020. https://doi.org/10.1093/scan/nsaa037)Crossref PubMed Scopus (9) Google Scholar]. The social brain hypothesis argues that the cognitive abilities required for navigating through social environments shaped the large brains of primates relative to other animals [27.Reader S.M. Laland K.N. Social intelligence, innovation, and enhanced brain size in primates.Proc. Natl. Acad. Sci. U. S. A. 2002; 99: 4436Crossref PubMed Scopus (783) Google Scholar, 28.Dunbar R.I.M. Neocortex size as a constraint on group size in primates.J. Hum. 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