Sensory Processing Sensitivity: The Neuroanatomical and Hormonal Basis of Empathy

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Introduction

Sensory processing sensitivity (SPS) is a genetically and environmentally determined personality trait that can be described as an increased sensitivity of the central nervous system (CNS) and deeper cognitive processing of physical, social, and emotional stimuli (Aron, Aron, and Jagiel Lowicz, 2012). One such example of a behavior associated with deeper processing and responding to external social and emotional stimuli is empathy, or the ability to understand – and even experience – the feelings of others. Empathy can be evoked in many different ways, from watching someone stab their finger with a knife to looking at pictures of an abandoned child, but the outcome is always the same – people tend to feel a deep sense of distress and even sadness, almost as if it was happening to them directly. Some people, however, can physically experience the pain of others just by looking at them due to very pronounced differences in the central nervous system sensitivity; these people are commonly referred to as highly sensitive persons, or HSP.

In a study conducted by Acevedo, Aron, Aron, Sangster, Collins, and Brown (2014), the researchers estimated that approximately 20% of the world population react more strongly to external stimuli – both social and environmental. In the same study, the researchers examined the neural correlates of SPS, which was measured by the short-form Highly Sensitive Person scale and a consequent fMRI scan. The participants had to view color photos of their significant others and complete strangers displaying positive, negative, or neutral facial expressions. After one year since the initial examination, the study was conducted again and it was found that HSP scores were associated with increased activation of the cingulate and premotor area as well as inferior frontal gyrus – the brain structures responsible for empathy and integration of sensory information. In addition, the results obtained in this experiment provided evidence that empathy (as well as awareness and responsiveness) are essential features of SPS.

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Therefore, it is reasonable to propose that HSP are different from the neurotypical population due to unique patterns of activation of the brain areas responsible for empathy, as evident from the above-mentioned study. However, there are also social aspects to sensory processing sensitivity. One such difference is that they can physically understand the difference between empathy and sympathy – because the latter is only demonstrating compassion and sorrow while the former is physically experiencing these things by being unable to separate one’s feelings and sensations from those of someone else. Therefore, for highly sensitive people, showing compassion is not about morality – in contrast, they have no choice of being anything but empathetic. With that in mind, the question of how exactly this phenomenon is produced still stands.

Although the behavioral outcomes of empathetic behavior are very well understood, little is known about the exact physiological processes that govern this cognitive task. It can be proposed, based on previous research, that these physiological processes include both neuroanatomical structures necessary for empathy production, as well as the molecular substances such as hormones. In this paper, both aspects of empathy production will be analyzed to establish exactly what processes and structures are responsible for emphatic behavior.

Common techniques used to investigate neuroanatomical structures: lesions studies and voxel-based morphometry

A common way to scientifically analyze the neuroanatomical structures of the brain is by measuring grey matter density in the regions of interest. Therefore, to investigate the inter-individual ability to feel empathy, many researchers rely on the analysis of grey matter density in the regions that are thought to be responsible for empathy production such as the anterior cingulate cortex (ACC) and the insula among others. It is thus reasonable to predict that the grey matter density in these regions will be increased in people with sensory processing sensitivity as the synapses activated during the production of empathy will be more abundantly used, and, therefore, more neuronal connections will be generated.

One technique commonly used to measure grey matter density is the voxel-based morphometry which allows for an investigation of focal differences in brain anatomy. By using this technique, Banissy, Kanai, Walsh, and Rees (2012) analyzed various brain structures of different people concerning the inter-individual ability to feel empathy. In their experiment, participants completed the Interpersonal Reactivity Index (IRI), a popular multi-dimensional measure of empathy, following a magnetic imaging scan to determine what brain regions were activated during episodes of empathy. This study confirmed that there are two distinct types of empathy: affective empathy directed at another person and self-oriented affective empathy. This study was also able to determine that the former is associated with an increased volume of grey matter density in the left insula, while the latter is linked with reduced grey matter volume within the left precuneus, left inferior frontal gyrus, and left anterior cingulate. Because these findings suggest that both types of empathy are completely different processes, it raises a question if the brain of a highly sensitive person is unable to separate the pain experiences of oneself from those of another person due to a different grey matter composition of those key brain regions.

Lesion studies are another common way of analyzing functions of specific brain regions and structures. Because they have been historically utilized to investigate the necessity of a brain region for a specific behavioral output, this technique is frequently used by researchers investigating the physiological basis of empathy. In a lesion study, Shamay-Tsoory, Tomer, Goldsher, Berger, and Aharon-Peretz, J. (2004), studied the impairments in cognitive and affective empathy in patients with various brain lesions. The researchers examined the extent of impairment in cognitive and affective empathy among patients with brain lesions as well as the contribution of specific cognitive abilities, such as processing of emotional information and cognitive flexibility, to empathy. To do that, the affective empathetic and cognitive response of patients with localized prefrontal lesions was compared to those with impaired prefrontal lesions and healthy control individuals. The results indicate that the right hemisphere is important in mediating empathetic behavior because damage to the right hemisphere impairs empathetic capacities even if the damage is delocalized.

Neuroanatomical basis of empathy

The anterior cingulate cortex (ACC) is one of the most prominent brain regions linked with empathetic behavior. The anatomical structure of the cingulate cortex allows for functional differentiation because it is composed of distinct cytoarchitectonic zones that represent various cellular structures underlining a functional subdivision (Lockwood at el, 2016). The anterior cingulate cortex can be subdivided into two different parts based on their primary functions and the association with other brain regions. For example, the dorsal part of the ACC is connected with the parietal and prefrontal cortices which makes it a central station for top-down and bottom-up processing. The ventral part of the ACC, on the other hand, is connected to the amygdala and anterior insula which are responsible for emotion and other cognitive functioning.

A study done by Gu, Liu, Guise, Naidich, Hof, and Fan, (2010) endeavored to analyze the dissociation in functions of the ACC and the frontoinsular cortex associated with empathy. In this study, the functional roles of both the frontoinsular cortex (FI) and the anterior cingulate cortex (ACC) in empathetic responses were examined by the following procedure: after viewing pictures showing human body parts in painful/nonpainful situations, the participants had to perform either pain judgment or laterality judgment task. As a result, the researchers concluded that the activation of FI (and not the ACC) was significantly higher for painful vs non-painful images, suggesting the fact that the activation of ACC is not specific for automatic or controlled empathetic responses. It was thus established that FI is more domain-specific when processing empathy, signifying the fact that the frontoinsular cortex becomes more activated when a subject is witnessing pain, as opposed to other types of empathy. Moreover, this study proposed that the right FI also becomes more activated than the ACC because it works by influencing the ACC and triggering hierarchical voluntary control. Finally, it was concluded that while both regions are associated with empathy, FI works by initially identifying the most significant input of pain and then passing this information down to ACC which, in turn, transforms that input into voluntary control over behavioral decisions.

Although the ACC is one of the structures accountable for empathy, it is not the only brain area associated with it. According to existing research, no brain region is solely responsible for empathetic behavior; instead, there is an interconnection of different brain structures that provide humans with the ability to feel empathy towards another person. Lamm, Decety, and Singer (2011) examined a network of structures that are responsible for empathy – the bilateral anterior insular cortex and medial/anterior cingulate cortex. In this image-based analysis of nine independent functional magnetic resonance imaging investigations, the researchers were trying to establish what brain regions are activated during empathy for pain. The results showed that there is a core network consisting of the bilateral anterior insular cortex and medial/anterior cingulate cortex that is activated during the above-mentioned episodes of empathy for pain.

The physiological basis of empathy can also be examined during neurodevelopment, which is the brain’s power to establish pathways responsible for the normal functioning of the brain and its ability to learn, focus, and develop social skills. Empathy is crucial for establishing prosocial behavior, morality, and the regulation of impulsivity, which are very important for normal social functioning (Decety et al, 2010).

Various components of empathetic behavior rely on a complex network of interacting neural regions; therefore, these brain regions must develop correctly to produced desired behavioral output. Decety and Michalska (2009) examined developmental changes associated with the neural circuits responsible for empathy by using a functional MRI scan. This study examined the brains of individuals of various ages (ranging from 7 to 40 years old) to assess how neurodevelopmental characteristics of the key brain regions change over time. The test subjects were presented with cartoons containing painful and non-painful situations, and neural activation patterns were analyzed by fMRI. The findings obtained as a result of this experiment indicate that the brain regions responsible for empathy were the dorsolateral prefrontal cortex and ventromedial prefrontal cortex. Moreover, this paper indicates that lesions of the ventromedial prefrontal cortex are associated with socio-cultural disturbances, such as empathetic dysfunction. Therefore, it was proved that correct neurodevelopment is crucial for proper social functioning.

Hormonal basis of empathy

Sensory processing sensitivity also relies on neuroendocrine processes implicated in various social behaviors and emotional processing. Thus, empathy is characterized by unique patterns in levels of certain hormones such as estradiol, testosterone, oxytocin, and androstenedione.

Circulating levels of oxytocin (OT) are central to the empathetic behavior of individuals because it is associated with bonding behaviors in mammals. Oxytocin is a neuroactive hormone that is produced in the hypothalamus and projects to the brain areas linked with emotional processing and social behavior, such as the amygdala and the cingulate cortex. Abu-Akel, Palgi, Klein, Decety, and Shamay-Tsoory (2014) analyzed two possible mechanisms underlining the involvement of OT on empathy production in the brain. The first mechanism states that OT works by enhancing the difference between self and other in terms of the empathetic response that it triggers in the brain. The second one, on the other hand, states that OT makes it impossible for the brain to distinguish between self and other, therefore, the same amount of empathy will be produced in these two scenarios. The experiment was conducted by recruiting healthy individuals whose task was to imagine both themselves and other people in painful and nonpainful situations. The participants were then administered either OT or placebo and the task was repeated. The findings of this study indicate that the intranasal administration of OT increases empathetic responses to pain perception in others but not in self. It was therefore proposed that OT works by enhancing the difference between self and others thus enhancing the empathetic response.

Oxytocin is not the only hormone responsible for empathy. Moreover, there is evidence from contemporary research suggesting that no single hormone is solely responsible for any cognitive or behavioral outcomes. Conversely, a new model for explaining such outcomes was proposed – the dual-hormone hypothesis. According to Mehta and Prasad (2015), the dual-hormone hypothesis states that the role of testosterone in status-relevant behavior – such as aggression, social status, and risk-taking – depends on concentrations of cortisol, a hormone that is released in response to psychological and physiological stress. Although the exact mechanisms of such interactions are still unclear, it has been proposed that the effects of testosterone on aggressive behavior are inhibited when the cortisol levels are increased. When the circulating levels of cortisol are low, the dual-hormone hypothesis predicts that the effects of testosterone on status-seeking behavior will be amplified.

Because aggressive behavior can be viewed as the opposite of empathetic behavior, this idea of the dual-hormone hypothesis was analyzed concerning empathy by Zilioli, Ponzi, Henry, and Maestripieri, (2014). In their study, the association between basal cortisol, basal testosterone, and empathy was investigated. Empathy was assessed with a short version of the Davis’s Interpersonal Reactivity Index and with the Reading the Mind in the Eyes Test (RMET). The results have been identified as significant only for men, although the analyses were performed for the two sexes separately from each other. The findings indicate that, in men, testosterone levels were negatively correlated with self-reported empathy. Therefore, these results not only add empathy to the list of behaviors that are influenced by the joint action of testosterone and cortisol which is in agreement with the dual hormone hypothesis but also establish a relationship between the levels of testosterone and empathy.

The effects of testosterone, as well as other hormones, were also investigated by Pascual-Sagastizabal, Azurmendi, Sánchez-Martín, Braza, Carreras, Muñoz, and Braza (2013). In their study, they examined the nature of the relationship between circulating hormone levels (testosterone, androstenedione, and estradiol) and empathy in children. The subjects were 9-year-old Caucasian children whose levels of the circulating hormones were measured by using an enzyme immunoassay technique in saliva samples. To measure empathy, Bryant’s Empathy Index was employed. The findings indicate that estradiol-gender and testosterone-gender interactions do exist: girls with low testosterone levels scored higher in cognitive empathy than girls with higher testosterone. In contrast, boys with low levels of estradiol scored higher in affective empathy than boys with high levels of his hormone. Therefore, based on these findings, it can be reasonably proposed that the hormonal basis of empathy is sex-dependent due to the fact that testosterone, as well as other sex hormones, influence empathetic behavior in different ways.

Conclusion

As evident from the existing research articles about the physiological and molecular aspects of empathy, it can be reasonably concluded that the origins of empathetic abilities are not limited to a single source.

In addition, the environmental factors influencing the sensory processing sensitivity cannot be overlooked. Chen, Chen, Moyzis, Stern, He, Li, and Dong (2011) analyzed the environmental factors influencing the inter-individual empathetic capacity. In their study, a systematic genetic approach was used to investigate this personality trait by genotyping the research participants for 98 representative polymorphisms in all major dopamine neurotransmitter genes. In addition, two environmental factors were also examined – stressful life events and parental warmth. Add significant results Moreover, the results of this study demonstrate the fact that it is possible to use a two-step system in accessing genetic contributions to individual differences in human behavior because both the environmental and genetic factors seem to affect the above-mentioned trait.

By studying the phenomenon of sensory processing sensitivity – and its empathetic component in particular – it can become possible to obtain a better understanding of various disturbances in social functioning and personality disorders, such as psychopathy.

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