The personal experience of pain produces a reliable effect on facial behavior in humans and in nonhuman mammals. Why should pain have a face? What is it for? I will attempt to head towards answering this question by invoking a theoretical framework: polyvagal theory (Porges, 2001, 2006).
1 Polyvagal Theory
According to polyvagal theory (Porges, 2001, 2006), evolution of neural control within the autonomic nervous system (ANS) has tracked three stages, each revealing a specific behavior, and a specific function:
In the first stage, the ancient unmyelinated visceral vagus nerve that enables digestion could respond to danger and pain only by reducing metabolic output and producing immobilization behaviors.
In the second stage, the sympathetic nervous system (SNS) made it possible to increase metabolic activity and inhibit the visceral vagus nerve, thus allowing fight/flight behaviors following perceived threat or pain.
The third stage, which is uniquely mammalian, involves a myelinated vagus that can rapidly control cardiac and bronchi output to enable spontaneous interaction (i.e., engagement or disengagement) with the environment. The interaction of the autonomic nervous system (ANS) with the hypothalamo-pituitary-adrenal (HPA) axis, nervous and immune systems change to maximize response to stressors such as nociception. During nociception, the ANS operates together with nervous, endocrine and immune systems to produce stress (Chapman et al. 2008; Porges, 2001, 2006). In terms of polyvagal theory, pain facial expression is a dynamic autonomic response caused by noxious signaling. In terms of polyvagal-type identity mechanistic theory pain facial expression is a type of behavior that is identical to a type of neurophysiological mechanism; namely, the phylogenetically recent brain-heart-face mechanism.
The expansion of cortex in the third stage increased innervation and neural control of the mammalian face: upper face innervation is bilateral and arises from the supplementary motor area (M2) and the rostral cingulate motor area (M3). Lower face innervation is contralateral and arises from primary motor cortex (M1), ventral lateral premotor cortex, and the caudal cingulate motor cortex (M4) (Morecraft et al. 2004). Human pain facial movements of the eyebrows and upper lip are type identical with negative emotional aspects of pain and activation of M1, M2, M3, whereas facial movements around the eyes are type identical with somatosensory aspects of pain, and activation of M2 and M3 (Kunz et al. 2011). Thus, evolution of cranial anatomy enabled a highly integrated facial representation of the multidimensional experience of pain.
2 Why Pain Should Have a Face
In clinical and experimental settings, the pain face is observed to rapidly appear following noxious stimulation, and diminish concurrent with cessation of the noxious stimulus, or when analgesics are administered (e.g., Craig & Patrick, 1985). The brain-heart-face mechanism is an integrated system with both a somatomotor part controlling the striated facial muscles and a visceromotor part controlling the heart through a myelinated vagus nerve (Porges, 2001, 2006). When the vagal tone to the cardiac pacemaker is high, the myelinated vagus acts as a brake or restraint limiting heart rate. Rapid inhibition and disinhibition of vagal tone to the heart supports the rapid mobilization of facial muscles and formation of the pain face concurrent with pain onset. In humans and nonhuman mammals, the main vagal inhibitory pathways in the myelinated vagus originate in the nucleus ambiguus.
The vagal brake supports the low-metabolic requirements involved in the rapidly appearing and disappearing pain face. Withdrawal of the vagal brake is strongly correlated with the rapid appearance of the pain face; reinstatement of the vagal brake is strongly correlated with the rapid diminishing of the pain face. These correlations are not unique to pain facial expression; similar relationships hold with regard to the vagal brake and the timing and duration of aversive, but non-noxious emotional facial expressions (e.g., Pu et al. 2010), and positive emotional facial expressions (e.g., Kok & Fredrickson, 2010).
In terms of the function of rapid pain face onset and offset, the vagal brake makes it possible for the individual in pain to quickly disengage from source of wounding and pain, concurrent with the rapid appearance or diminishing of pain facial expression, which may offer temporary access to additional metabolic resources to aid healing, recovery and self-soothing behaviors, with likely involvement from care givers.
Concerning aid from others, the vagal brake reliably maps onto specific interaction types observed in mammalian pain events. In pain events comprising the individual in pain and care givers, mammalian behavior is typed according to interpersonal communication through facial expressions, vocalizations, head and hand gestures (Hadjistavropoulos et al. 2011; Porges, 2001, 2006; Williams, 2002). A relevant feature is the rapid ‘switching’ of temporary engagement to temporary disengagement behaviors between the individual in pain and care givers. This interaction type may involve care givers speaking to the one in pain, and then quickly switching to listening; for the one in pain, looking into the face of the care giver, and then quickly switching to vocalizing (Craig et al. 2011; Hadjistavropoulos et al. 2011; Porges, 2001, 2006; Williams, 2002). The brain-heart-face mechanism thus allows the one in pain and the care giver to get the timing right. Some philosophers and neuroscientists claim that evolutionary neurobehavioral solutions to timing problems such as these are implicated in the origin of empathy and ultimately consciousness itself (Churchland, 2002; Cole, 1998; Engen & Singer, 2012; van Rysewyk, 2011).
However, if pain is severe or chronic and the vagal brake is withdrawn (or dysfunctional), the concurrency of increased pain facial expression, cardiac output, and other mobilization behaviors (i.e., increased SNS and HPA output), means that, if care giving is to succeed in promoting healing and recovery, the care giver’s vagal brake must be dynamically reinstated. By applying their own vagal brake, care givers may regulate their own visceral distress and thereby succeed in allocating valuable metabolic resources to communicate safety to the one in pain (and themselves) through calming facial and head behaviors, eye gaze, and prosodic vocalizations (i.e., increasing the vagal brake decreases SNS and HPA output). Since the vagal brake of the person in pain has been provisionally withdrawn, the care giver is effectively an integrated external brain-heart-face mechanism (cf. Tantam, 2009, the ‘interbrain’).
Thus, the pain facial muscles function as neural timekeepers detecting and expressing features of safety and danger that cue the one in pain to quickly disengage from the source of wounding and pain, simultaneous with the rapid appearance or attenuation of pain facial activity, and also cue others who can help.
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