The neuropeptide oxytocin (OT) is associated with modulation of a variety of cognitive functions, especially related to sexual preference and trust. This led to its image as the mediator of the “social brain” (Adolphs, 2009). In this blogpost, I will first outline recent research on the effect of oxytocin on social behaviour, focussing mainly on humans. Then I will present and critique the anxiety-reduction hypothesis of OT function as well as explanations of OT altering motivational states. Then I move on to a perceptual model which posits that OT affects the salience of social stimuli. I will attempt to place this idea within the predictive coding framework by arguing that oxytocin biases priors in social perception.
OT is a mammalian neuropeptide that together with vasopressin is engaged in homeostatic regulations associated with the parasympathetic autonomic nervous system (Ropper & Samuels, 2009). In virgin rodents, injection of OT induced nest building (Insel, 1992), and in mouse mothers to the initiation (but not maintenance) of maternal behaviour (Kendrick 2000; Insel, 2000). OT levels positively correlated with partner preference and lifelong bonding in rodents (Carter, et al., 1995). In the monogamous prairie vole, the OT and dopaminergic reward system are coupled through OT receptors in the nucleus accumbens, while these two systems are decoupled in the promiscuous montane vole (Edwards & Self, 2006). This led to the conclusion that the sequential activation of dopaminergic and OT pathways is crucial in long-term partner bonding and monogamy (Skuse & Gallagher, 2008). In other words, rodent studies support the idea that OT holds the key to understanding mating, bonding and attachment to infants in animals (review in Lim & Young, 2006).
In humans, evidence is more sparse and varied. There is a positive correlation between one’s propensity to share feelings and plasma OT (Tops, et al., 2007). There are higher cerebrospinal fluid levels of OT after childbirth, which has been linked to attenuated anxiety and feelings of positivity (Takagi, et al., 1985). In humans, the birthing process does not seem so critically bound to OT (Campbell, 2010); but OT is associated with early bonding of mother and child in the first weeks of life (Broad, et al., 2006). The release of OT in breast-feeding women leads to a milder endocrine response to physical stress associated with the sucking, which encourages positive contact (Heinrichs, et al., 2001). One of the facilitatory effects of oxytocin lies in the inhibition of automatic defence behaviour against others in close proximity, which allows approach behaviour (Carter, 1998). There are positive effects on emotion detection (Marsh, et al., 2010), empathy (Hurlemann, et al., 2010), approachability (Rimmele, et al., 2009) and perceived attractiveness (Theodoridou, et al., 2009). There is also an effect of oxytocin on the ability to perform theory of mind tasks. Specifically, when asked to identify emotion from pictures of the eye region of human faces, the administration of oxytocin led to a significant increase in performance. In other words, reading minds is contingent on or at least mediated by the availability of oxytocin (Domes, et al., 2007).
In a landmark study on Oxytocin and trust, Kosfeld et al. (2005) gave half their participants a single nasal dose of the neuropeptide. In a game where trust and social interaction is associated with greater gain, the OT group was much more willing to engage in prosocial behaviour. Specifically, 45% of the participants receiving OT maxed out on the trust displayable in the game, while only 21% of the control group did. Kosfeld et al. interpreted these results as showing a greater willingness to accept social risk, which is in turn the basis for much group behaviour and strategising. To summarise, OT may be a player in eliciting emotions and behaviours that have evolutionary value, including theory of mind, attachment, trust, and bonding.
This picture of correlation between OT and bonding is not without criticism though. Postmenopausal women’s report of marriage happiness actually negatively correlates with plasma OT (Taylor et al., 2006). While plasma OT levels rises as a result of a partner massage in both sexes (Grewen, et al., 2005), basal salivary OT does not (Light, et al., 2005). Also, massage alone did not alter plasma OT levels, however, players in a trust game who had a massage beforehand showed a correlation between their displayed trust and plasma OT (Morhenn, et al., 2008). In other words, the effect of OT was context dependent. Women who received neck and shoulder massages from their partner prior to a stressor showed lower heart rate and cortisol levels, but not a significant difference in oxytocin in comparison to a control group (Ditzen, et. al., 2007).
Massage results are difficult to interpret since physical contact produces a lot of endocrinological confounds. Still, also outside the sexual-bonding domain, studies on the role of OT produced mixed results. In a review, Bartz, et al. (2011) found that in a sizeable proportion of studies using oxytocin to influence trust, OT induced antisocial effects, such as envy & Schadenfreude (Shamay-Tsoory, et al., 2009) or mistrust (Declerck, et al., 2010). Also, in a study trying to replicate Kosfeld et al., (2005), cooperation in a trust game was diminished following administration of OT if the participant believed that the other player is a member of a social out-group (De Dreu, et al., 2010). There is no consistent evidence for the hypothesis that OT enhances social memory in humans. One study found that administration of OT led to better performance on a memory test 24hrs later for facial familiarity. It had however no effect on facial recall or memory of non-social stimuli (Rimmele, et al., 2009). However, no effect of OT on either accuracy or speed in facial memory was found in Di Simplicio et al. (2009). Paradoxically, OT administration impaired cued recall of words irrespective of their meaning or social salience (Heinrichs, et al., 2004). A study on emotion recognition in healthy adults found that only performance on difficult items was improved following OT administration (Domes, et al., 2007), whereas a similar study on autism spectrum disorder (ASD) patients found improvement only on simple items (Guastella, et al., 2010).
This inconsistency in findings may be due to severe methodological limitations of using OT in studies. The neuropeptide induces uterine contractions in sheep (Keverne & Kendrick, 1992), which is why many studies that exogenously increase oxytocin in participants use exclusively male samples, introducing a sex confound. If you exogenously inject a participant with oxytocin, only a small proportion surpasses the blood-brain barrier (Heinrichs & Domes, 2008). Also there is little evidence for coordination between OT release in the periphery and the central nervous system (Neumann, 2007). Hence studies using plasma oxytocin suffer from problem that it is not a good indicator of oxytocin availability in the cortex (Landgraf & Neumann, 2004; Heinrichs & Domes, 2008). Next to these measurement issues, there are also problems associated with experimental design. An extensive review of studies reporting prosocial effects of OT found that half of publications in the field report a non-significant main effect of OT, and most report an interaction with stimulus or task type (Bartz, et al., 2011). The review asserts that the question ought not be whether OT has prosocial or social behavioural effects, but under what circumstances it does. They posit that the effect of OT is not situation invariant, but rather a function of contextual factors.
To summarise, OT has many prosocial effects linked to trust, attachment and bonding in rodents and humans. However, human studies face methodological problems and often find socially negative effects of OT administration, which led to the hypothesis that OT function is context dependent. Any credible account of OT function must be more complex and multi-layered than asserting that OT mediates binding and trust.
One such attempt is the anxiety-reduction hypothesis, which posits that OT attenuates aversion to proximity and distrust of others in the in-group (Bartz & Hollander, 2006). In other words, OT is anxiety-reducing and relaxing (Carter, 2007). In rats, oxytocin is often released in response to stress in fearful situations (Neumann, et.al., 2000), where it inhibits stress-related activity of the hypothalamic-pituitary-adrenal (HPA) axis (Neumann, et.al., 2002). In rats, the suckling of the young led to a release of OT, which in turn is accompanied by a decrease in cortisol levels (Neumann, et.al., 2000). This has been replicated in humans (Heinrichs, et.al., 2002). Interestingly, this effect is not just immediate to breast-feeding. If breast-feeding occurred about half an hour before exposure to a stressor, HPA axis response is milder compared to a no-feeding condition (Altemus, et.al., 1995); and breast-feeding mothers have lower blood pressure following a verbal stressor after holding their baby, if their plasma OT level is high as well (Light, et.al., 2000). Also, in a placebo-design, participants received either intranasal oxytocin or a placebo and either received social support or not, 50 minutes before onset of a stressor (Heinrichs et al., 2003). Measure cortisol levels during stress, they found that those who received oxytocin and support showed the lowest concentrations, whereas those with the placebo and no support the highest. On a psychometric test afterwards, these results were reflected by participant’s self-evaluation of calmness and anxiety. Their conclusion was that oxytocin has a “stress-protective” effect in social interaction. Under this hypothesis, greater cooperation in trust games after OT administration is due to attenuated anxiety about social loss due to misplaced trust.
There is neuroscientific evidence for the anxiety-reduction hypothesis also. OT receptors can be found in a variety of brain areas associated with stress control, bonding, parental care, anxiety and aggression (Heinrichs & Domes, 2008). Specifically they appear in various parts of the limbic system, including the amygdala (Huber , et. al., 2005), which has been described as good candidate region for mediating the “anxiolytic” effects of OT (Heinrichs & Domes, 2008). In a functional MR study, Kirsch (et al., 2005) found reduced activation of bilateral amygdala following fearful faces after participants received nasal oxytocin as opposed to a placebo. This effect persists also regardless of the emotional content of the faces (Domes, et al., 2007). Huber et al. (2005) demonstrated that OT excites a specific portion of the central amygdala, thereby modulating the strength of fear responses. The implication of this evidence is that OT has a mitigating effect on amygdaloid function in reacting to fear, thereby reducing fear-related behaviour.
An alternative explanation of OT function posits that the peptide influences the perception of social cues by direction attention and changing the weight of salient aspects of a social percept. For instance, sheep only form attachments to their own offspring, a process facilitated by OT (Kendrick, et al., 1997). Mice with a null mutation of the OT production gene are severely impaired in recognising members of its own species even after multiple encounters, which crucially can be restored to baseline by OT injection before the first encounter (Ferguson, et al., 2002). In humans, OT has been linked to cross-modal integration (Lee, et al., 2013); and administration of OT leads to the activation of the fusiform gyrus under functional MR imaging (Petrovic, et al., 2008), cautiously suggesting an attentional redirection towards face perception. The number and length of gazes towards the eyes of emotionally neutral faces was increased following nasal administration of OT (Guastella, et al., 2008). These OT induced gaze shifts were associated with greater functional connectivity between amygdala and posterior colliculi, possibly indicating an effect on perception (Gamer, et al., 2010). In essence, OT might alter the salience of social percepts (Ross, et al., 2009). This explains why context matters in determining the effect of OT: Increasing attention towards social cues will magnify prosocial and antisocial behaviour patterns based on contextual information, e.g. whether the opponent in a trust game belongs to an out-group.
While increased attention to social cues is a promising hypothesis for OT function, it does not take a stance on how this modulation of cue salience comes about. Also, it is not able to incorporate some of the above empirical evidence. For instance, how does increased attention to social cues lead to recognition of offspring? The attenuated stress and discomfort following a rise in plasma OT during breastfeeding also have no obvious connection cue detection. I would like to propose that there is a plausible theoretical framework for explaining how OT modulates social perception, namely the predictive coding of social percepts. Under this framework, OT would bias the relative weight of specific sensory stimuli to bring about action enabling emotions that have survival value.
According to predictive coding accounts of perception, connections from higher perceptual areas (those involved in percept monitoring and awareness) carry predictions about the current state of the sensory input to lower visual areas in the hierarchy (those involved in low-level stimulus respresentation) via feedback connections. In the lower areas these predictions are compared to the incoming sensory stimulation and only error signals or divergences between prediction and actual state are being signalled upward the hierarchy to correct the prediction (Friston, 2005). This constant updating of predictions attempts to most efficiently explain the sensory input and is a neurally feasible way of updating internal models of the world (Rao & Ballard, 1999). Higher level representations of the sensory input mirror these probabilistic models, which we call priors. This framework has been rigorously applied to visual perception. Recently, also introspection has been described within it (Seth, et al., 2011; Seth, 2013). Interoception describes the process of monitoring one’s own internal bodily states, which aids in homeostatic regulation by eliciting action enabling emotions. Essentially, Seth posits that interoceptive predictive errors are passed upward the perceptual hierarchy through the limbic system, where they are integrated with exteroceptive and proprioceptive errors and predictions. It is these convergences that allow for the formation of top-down expectations of emotional internal states and our ‘gut feeling’ (Critchley & Seth, 2012). There is strong empirical evidence of the involvement of OT in modulating these interoceptive signals of various modalities (itching in dogs: Yaksh, et al., 2014; feelings of cold and warmth: review in Chaves et al., 2013, thirst: James, et al., 1995; see Quattrocki & Friston 2014).
A model asserting that OT changes the relative weight of interoceptive error signals can explain a great proportion of the previously mentioned evidence. For instance, in breast-feeding mothers, OT could enhance top-down expectations of positive feelings, which in turn attenuates pain signals coming from the breast as well as other negative interoceptive sensations which would deter a mother from feeding. More generally, OT could mitigate the salience of anxiety inducing interoceptive signals in stressful situations after massages. OT and sexual bonding could be associated if OT acts as biasing agent on the probability of negative consequences when in close proximity to another individual. Theory of mind tasks require filtering of relevant stimuli from perceptual noise, a process perhaps facilitated by OT. Sheep may only be able to recognise their offspring if the olfactory error signals pertaining to the offspring are given extra weight through OT release. Decreases in amygdaloid function contingent on OT level could be the result of greater top-down expectations of non-threatening stimuli, thereby mitigating the brain’s fear response. More generally, OT may alter the sensitivity of neural circuits to sensory input (Nusbaum & Blitz, 2012).
Another highly promising application of this predictive coding view links OT function and prediction with ASD. Specifically, impairment of the OT system would entail that ASD patients are unable to correctly assimilate social cues into an action enabling generative model (Quattrocki & Friston 2014). Support of this hypothesis comes from the similarity between the impairments of ASD and those abilities that are enhanced with greater OT (Carter, 2007). Indeed, ASD patients show lower plasma oxytocin levels (Green, et al., 2001); an oxytocin receptor is encoded on the same gene region previously linked to ASD (McCauley, et al., 2005); and injection of oxytocin had an attenuating effect on repetitive behaviours and improved emotion recognition in patients (Hollander, 2003; 2007).
The predictive coding account of OT function has a slightly harder time explaining it’s effect in trust games. In a social situation, such as a trust game, our decision to cooperate will be informed by a variety of reasons, gut feelings and perceived social cues pertaining to the trustworthiness of the opponent or reward value associated with trusting behaviour. OT may act as biasing agent on the probability of these perceived variables. For instance, if there is an in-group opponent in the trust game, OT could add relative weight to the perception of perceptual cues indicating trustworthiness. Specifically, knowledge of group membership creates strong top-down expectations as to the likely causes of the opponent’s perceived behaviour. The strength of these expectations increases with OT levels, which modulates an action enabling ‘gut feeling’. In other words, the neuropeptide may function by influencing top-down modulations of sensory salience of social cues (Quattrocki & Friston 2014). A big problem with this picture is a lack of explanation of how higher expectations of trustworthiness create interoceptive experiences. One possibility is that analogous to breastfeeding, OT attenuates interoceptive signals arising from the danger of trusting an unknown opponent, i.e. OT modulates our stress response to situations of potentially high social loss or gain. I have to acknowledge however that this explanation is somewhat far-fetched and requires focused empirical investigation.
The popular opinion that OT facilitates the social functions of the brain and aids in bonding and attachment, makes us monogamous and leads to greater reproductive success, is not supported by empirical evidence. Instead, we get a picture suggesting that the effect of OT on the brain is varied and context-dependent. Multiple frameworks have been proposed to explain why this is the case. Specifically, this essay considered the anxiety-reduction hypothesis as well as accounts of OT changing the salience of social cues. Predictive coding is a promising framework for explaining a variety of cognitive processes and can be applied to OT function. However, especially in explaining the effect of OT on trust, predictive processing is unable to tell a convincing story. A fully developed model of prediction in social perception applicable to trust games might be able to account for this in the future.
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