Article summary with The neurobiology of human attachments by Feldman - 2017

What is neurobiology of human attachments?

Neurobiology of human attachments is a new scientific field that integrates insights from other mammals with new tools available for human research to test the biological basis of human attachments.

Which are the four bonds observed across mammalian species?

The four bonds are parent-infant, romantic attachment, peers, and conspecifics (other humans). Attachment bonds in mammals are underpinned by functioning of two ancient systems, namely oxytocin and dopamine, which maintained basic organization across vertebrate evolution and supported group living in harsh ecologies and motivational goal-directed action throughout animal evolution. The integration of oxytocin and dopamine in striatum leads to mammalian bonding. In humans, attachment bonds are marked by selective and enduring features.

How does time sensitivity and pulsatility of the oxytocin and dopamine systems enable plasticity of neural networks to incorporate new attachments?

Oxytocin and dopamine are characterized by pulsatile release that supports time-keeping mechanisms implicated in patterned action and seasonal rhythmicity. The pulsatility and time sensitivity of oxytocin and dopamine enabled their involvement in neural plasticity. This neural plasticity is required for selective recognition and long-term memory, which are two key characteristics of human attachments.

• Pulsatility is a defining feature of oxytocin functionality across evolution. Oxytocin signals cause dendrite release without increasing electrical activity. It can become self-sustaining by creating auto-regulated release primed by salient experiences. Once it is activated, release is repeated in time-sensitive bursts. Special primed signals can trigger dendrite release, cause oxytocin to be relocated in vesicles from reserve to releasable stores. Future release is then shaped by the primed cue.
• Dopamine striatal neurons process the timing of reward and encode reward anticipation, building the internal sense of time in the brain. Dopamine neurons are sensitive to rewards that come from social interactions and can link rewards to attachment experiences, grounding rewards in cycles of caregiving actions. They can integrate social components into their temporal predictions. This permits social signals to act upon a pre-established synaptic tract and associate it with specific reward outcomes, and enables an increase in dopamine activity to become subjective and social.

What is bio-behavioral synchrony?

Bio-behavioral synchrony is the coordination of biological and behavioral processes between attachment partners during social contact and a key feature of human attachments. Temporal sensitivity of dopamine neurons enables humans to draw rewards from the experience of bio-behavioral synchrony, which is built on familiarity with the partner´s repeated social patterns. It evolved from the coordinated group activity of lower species where joint motor action (involving dopamine) is locked with coupled physiology to achieve survival-related collaborative goals (involving oxytocin).

What is trophallaxis?

Trophallaxis refers to the exchange of sensory signals among members of a social group. The term was extended to include social stimuli and to denote the reciprocal multisensory stimulation of low intensity that elicits approach response. Parenting is a form of trophallaxis. Three aspects of trophallaxis have been integrated into mammalian bonds:

• The low intensity and arousal-modulatory nature of attachments.
• The social reciprocity and online construction embedded in them.
• The trophallaxic process as charting a line from parent-child bond to life within social groups.

What is myoactivity?

Myoactivity refers to the stimulation of rhythmic tissue contraction. It is the most conserved function of the oxytocin system. Due to pulsatility there exists a dendritic release that supports oxytocin´s role in bonding. This leads to autoregulated, feed-forward release that is triggered by primed attachment experiences which, once activated, release repeated rhythmic bursts. These time-sensitive mechanisms make it possible for early attachment experiences to shape the organization of oxytocin in specific places in an infant´s brain.

What are the two key features of attachment bonds and how are they expressed in the four different bonds?

Two key features characterize attachment bonds, namely that they are selective (specific to attachment target) and long-lasting.

• The parent-child bond is selective and enduring.
• Romantic bonds are selective and enduring, but more precarious. Romantic attachments can terminate under normative conditions.
• Close friendships are selective and enduring, but in a weaker form. Humans can have numerous friendships at the same time and enduring friendships may dwindle without a clear breakup.
• Humans´ relationships to conspecifics is not selective nor enduring. However, humans have the unique ability to activate the behavioral and neurobiological systems of affiliation towards other humans.

What is unique about behavioral synchrony in humans?

During or following social contact, human synchrony is evident in four systems: behavior, autonomic, hormones, brain. Behavioral synchrony takes place in different ways in the human attachment bonds. Unlike other mammals, humans display behavioral synchrony toward strangers. They may coordinate gaze, use vocal turn-taking during conversations, display heart-rate coupling, or show brain-to-brain synchrony of alpha rhythms.

Which tools are used to measure human attachments?

• Autonomic response: measures of heart period and respiratory sinus arrhythmia, skin conductance.
• Peripheral measures of hormones, such as cortisol, oxytocin, testosterone, estradiol, progesterone, etc.
• Micro/macroanalysis of social behavior and behavioral synchrony: observations, global rating scales, and micro- and macro-level analysis.
• Peptide administration.
• Brain imaging, such as fMRI.
• Brain oscillations, such as EEG and MEG.
• Genetics and epigenetics.

Which main interconnected neural systems integrate to establish, maintain, and enhance our affiliative bonds with others?

• The reward-motivation system includes the striatum, amygdala, ventral tegmental area, orbitofrontal cortex, ventromedial prefrontal cortex, and anterior cingulate cortex. This system supports multiple attachment-related motivational behaviors, such as social seeking, social orienting, and maintaining contact across extended periods. The system is based on the fact that attachments have intrinsic motivational value that combine immediate hedonic response with approach motivation, goal-directed behavior, and learning.
• The embodied simulation/empathy network includes the insula, anterior cingulate cortex, inferior frontal gyrus, inferior parietal lobule, and supplementary motor area. This mechanism recreates other´s state in one´s brain via automatic interoception and internal representations. It relies on neural pathways that involve the experience of internal body formats and the perception of similar states in others via perceptual-motor coupling.
• The mentalizing system includes frontotemporal-parietal structures, particularly the superior temporal sulcus, posterior cingulate cortex, temporoparietal junction, temporal pole, and medial prefrontal cortex. Mentalizing processes involve complex top-down inferences of others´ mental states by attributing beliefs, thoughts, and intentions to others to create a full sense of ´togetherness´. They underlie the forming of attachments by building on one´s ability to appreciate multiple perspectives, understand the others´ goals and motives, and keep in mind the others´ values and concerns.