Evolved Physiological Reactions
KeywordsAutonomic Nervous System Sympathetic Nervous System Physiological Reaction Sweat Gland Skin Conductance Response
Species-specific physiological reactions have evolved in response to evolutionarily stable opportunities and challenges. Many of these responses are conserved across species and can be studied in humans as sets of interrelated, and coordinated, physiological reactions to stimuli with intrinsic and/or learned values.
The survival of a species relies on multiple motivational factors: appetitive, defensive, and reproductive. In mammals, each of these factors are characterized by specific patterns of physiological reactions designed to cope with challenges and opportunities in the environment. Research has described physiological profiles of a variety of motivational states, ranging from basic avoidance of predators and defense against threatening conspecifics (Blanchard and Blanchard 1990) to complex social states, such as deception (de Waal 1992) and empathy (Zaki 2014). Avoiding threats and, at the same time, maintaining opportunities of foraging and mating constitute strong environmental constraints that have favored the selection of physiological reactions that facilitate the detection of, responding to, and learning about, salient cues in the environment. The particular set of activated physiological reactions to, for example, threat and reward cues vary across species as a function of the evolutionary constraints operating on the species habitat. In addition to ecology-dependent variation between species, individuals within each taxon vary considerably in their physiological response profile. In humans, extreme physiological reactions to threat and reward cues constitute the bases for common psychiatric disorders.
In humans, the autonomic part of the nervous system coordinates a host of physiological responses that have evolved to cope with opportunities and challenges in the environment. In this entry, we focus on a series of physiological reactions to threat that have been extensively studied and described: endocrine, cardiovascular, pupillary, electrodermal, and respiratory. In the following, each of these evolved reactions will be briefly described together with their known key brain correlates.
Physiological Reactions to Stress and Reward
Physiological responses of an organism aid in survival, optimizing the trade-off between possible rewards and threats present in the surrounding. Mammals react to the presence of a threat, such as a predator, by reduced heart rate and immobilization (Mobbs et al. 2015). Other immediate reactions, among them enhanced orienting and arousal (Öhman 1986), increase the chances of survival by facilitating information processing and preparation for action. At any given moment, an animal must strike a balance between exploring possible resources and avoiding being attacked. The choice between passive and active coping strategies in the face of threat is regulated by its proximity (Mobbs et al. 2015). If escape is possible or fight is an option, the prey prepares for active coping. The fight-or-flight response has evolved to aid escaping and fighting the aggressor, and is therefore characterized by specific physiological changes to support this, such as an increasing blood flow, which supplies the muscles with energy. In turn, and as a part of the ongoing arms race between prey and predators, predator physiology is designed to maximize its chances of catching the prey by increasing mobility and readiness for action. The behavioral principles governing these decision processes across species have been discussed extensively in behavioral ecology (Stephens and Krebs 1986). Freezing, on the other hand, is a passive coping strategy and is used when escape is not possible or in immediate proximity to the predator. This state allows the animal to both avoid detection and assess defense strategies. For example, flight behavior can be initiated while freezing.
In the laboratory, researchers have predominantly used Pavlovian fear conditioning procedures to study reactions to threat (LeDoux 2012). During Pavlovian conditioning, an emotionally neutral stimulus (CS) is repeatedly paired with a naturally aversive unconditioned stimulus (US), such as an electrical shock. As a result of this pairing, the CS elicits an enhanced orienting toward the stimulus, and a slew of defensive responses, such as freezing and autonomic arousal (Lang and Davis 2006). In many social species, threat information is transmitted efficiently between individuals, offering a more safe learning strategy as compared to individual trial and error learning (Laland 2004). Social learning about threats is supported by partly the same physiological responses as Pavlovian fear conditioning (Olsson and Phelps 2007).
The emotional value of threats and rewards can be described along two key dimensions: (1) valence and (2) arousal, each one linked to partially different physiological response profile. Valence varies from negative to positive, whereas arousal ranges from low to high. In humans, the arousal component is commonly measured through autonomic activity, which can be quantified by, for example, changes in electrodermal activity (sweat production on the surface of the skin). Valence is commonly measured through the potentiation of the basic, brain-stem mediated, startle reflex, to sudden sensory stimuli (Lang and Davis 2006). This response can be measured across species and is therefore relatively well described in terms of its neural bases. Human research often uses electromyography (EMG) to record the amplitude of the eye-blink startle reflex in response to external stimuli. In comparison to a neutral baseline, aversive stimuli potentiate, and rewarding stimuli augment, the startle reflex.
Not only must the individual respond immediately to threats and opportunities. The value of past experiences should also be used to predict future encounters. Physiological responses have therefore evolved to facilitate the encoding of, for example, threats, thereby optimizing behavioral responses. Indeed, emotional arousal improves memory (McGaugh 2002), constituting an adaptive physiological function that has been preserved across many species. Human and animal studies have highlighted the hippocampus as the key region for declarative memory formation in the brain (Kim et al. 2015). The amygdala, which is located in close proximity to the hippocampus, is thought to facilitate plasticity in hippocampal regions, thereby promoting learning of emotionally salient information for future use. The amygdala promotes learning indirectly through modulating memory formation in different regions of the brain, such as the hippocampus (McGaugh 2002), and directly, through acquiring, storing, and expressing learned threat responses (LeDoux 2012).
Two parts of the peripheral nervous system orchestrate human physiological reactions: the somatic and the autonomic. The somatic nervous system consists of motor nerves, responsible for voluntary muscle control from the central nervous system (CNS) to the body and sensory nerves that are responsible for perception of external stimuli from the sensory organs and the environment to the CNS. The autonomic nervous system (ANS) coordinates the physiological reactions. These bodily responses are to a large extent not under voluntarily control and are unleashed before a behavior is performed to prepare for its execution.
The ANS controls human physiology through its two parts: the sympathetic nervous system (SNS) and the parasympathetic nervous system. Generally, these two systems work in opposition to each other. For example, the parasympathetic nervous system activity antagonizes the SNS and the body enters into relaxation (e.g., heart rate and digestion reduces) and restoration. The detection of, and learning about, threats are regulated by a combination of different physiological pathways in many different organs and tissues: the endocrine, the cardiovascular, the pupillary, the electrodermal, and the respiratory. Each of these subsystems will be briefly described below in relationship to their evolved functions and associated key brain structures that are involved in creating these responses.
The Endocrine System
The endocrine system consists of the glands distributed in an organism that secrete hormones in order to regulate physiology and behavior. Hormones are secreted into the circulatory system, allowing different organs to communicate with each other. Apart from maintaining homeostasis, hormones are responsible for creating responses to internal and external stimuli.
The hypothalamus links the nervous system, from which information about external stimuli is acquired, to the endocrine system through the pituitary gland. Together, hypothalamus and the pituitary glands are part of a larger complex called the hypothalamic-pituitary-adrenal (HPA) axis, which controls a range of states from sleep to stress. The main hormones that are secreted by the glands of the HPA-axis are glucocorticoids (“cortisol” in humans), the so-called stress hormone, through adrenal glands. Stress can be defined as a state, which systems adopt when facing a threat to the homeostatic balance. This threat can be a direct threat to the physical integrity of the individual, such as the presence of a predator, or of psychological kind, such as the perceived harmfulness of getting bankrupt. Glucocorticoids influence behavior through regulating neural pathways that facilitate or hinder certain behavior, depending on when they are released: before, during, or after encountering the stressor.
The onset of the stress response depends on the environmental conditions and the learning history of the organism. As mentioned earlier, the state of arousal resulting from the presence of threat augment learning, and a key structure is the hippocampus. The hippocampus is densely populated by glucocorticoid receptors (Kim et al. 2015). Mild stress has a transient effect and leads to changes in memory, yet strong and chronic levels can lead to permanent morphological changes (Conrad 2008). While it is important to remember sources of threat for survival, severe, and long-term stress leading to such permanent changes can create to unwanted outcomes, such as in the case of posttraumatic stress disorder (PTSD) (Kim et al. 2015).
The Cardiovascular System
The cardiovascular system is responsible for distributing oxygen and energy throughout the body. Blood flow is regulated by the demand for oxygen in active tissues, and the rate of delivery is the product of heart rate and blood pressure. Heart rate and blood flow are controlled by many factors, such as hormones and the autonomic nervous system. In times of stress, for example during fight-or-flight responses, cardiac output, respiration, and blood flow increase in response to the NA released from the SNS. During this active strategy, the amygdala mediates the changes in cardiac and respiratory outputs (Cacioppo et al. 2007). Increase in heart rate and blood flow to large muscles aids in survival by meeting the metabolic needs of the animal when flight is the best behavioral strategy.
In the case of a passive coping strategy, heart rate slows down. Decrease of heart-rate, fear bradycardia, has been associated with freezing in animal studies, including humans (Lang and Davis 2006). Immobilizing decreases the chances of being detected by the predator, increasing the chances of survival. Humans vary to the extent with which their heart rates are changed in response to fear conditioning; low heart rate variability is commonly believed to be a precursor for higher responses in fear-potentiated startle (Hamm and Weike 2005).
The Pupillary System
The diameter of the pupil regulates how much light enters the eye and is controlled by two muscles: sphincter and dilator pupillae, which are under the control of ANS. Pupil dilation does not only occur in low light conditions, but has been found to reflect cognitive and emotional processes. Pupils dilate more to emotional words (Siegle et al. 2003), auditory stimuli (Partala and Surakka 2003), and pictures (Bradley et al. 2008) than to neutral ones. The evolutionary advantage of dilated pupils is still debated, but one prominent idea is that increased light allows better vision in dark environments.
The dependence on other individuals and the group has been an important factor during primate evolution. This dependence is likely to have selected for various abilities to detect and respond to conspecifics’ intentions, emotions, and trustworthiness. Pupil size, along with characteristics such as facial expressions, is a social cue used to predict others’ behaviors by ascribing intentions and emotions to them. For example, exposure to large vs. small pupils activates the amygdala (Harrison et al. 2006), which regulates the arousal system, and helps preparing for defensive responses (see above). More complex social behaviors, such as in-group trust, have been linked to mimicry of the pupil. For example, partners with dilated pupils are generally more trusted then those with constricted pupils (Kret et al. 2015).
The Electrodermal System
One of the most widely used psychophysiological reactions used in human research on basic affective and survival responses is electrodermal activity (EDA). ANS regulates sweating through sweat glands: the eccrine sweat glands cover most of the body and are more concentrated on the palms and soles of the feet; and the apocrine sweat glands are located in the armpits and the genital areas.
Most researchers study the skin conductance property of the EDA, which measures the skin resistance or conductance. Tonic level of skin conductance is referred to as skin conductance level (SCL), whereas phasic changes overlapping with the tonic response is referred to as skin conductance response (SCR). In a laboratory setting, changes in EDA can be the signal of different level of activation of the SNS: sensory input, expectation/preparedness, increased attention, and arousal (Boucsein 1992). Along with interindividual variability in ANS responses, variations in SCR between individuals are related to differences in personality (e.g., introversion/extraversion) and psychopathology (e.g., anxiety disorders) (Boucsein 1992).
Many studies investigating threat responses report an increase in SCR and SCL in response to stimuli associated with threatening, compared to “safe,” stimuli. Pavlovian fear learning paradigms have shed light on mechanisms of threat learning and extinction learning (the updating of threat value when the CS is no longer dangerous (e.g., Phelps et al. 2004). Learning each and every threat source is inefficient for the organism, thus generalizing the threat information to an extent would be optimal for survival. SCR to stimuli from the same category as the CS+ do not need to be specifically conditioned in humans, generalization occurs most readily between typical members of categories and less between atypical ones (Dymond et al. 2015) Overgeneralization, however, is a characteristic of certain psychopathologies (i.e., PTSD, panic disorders and specific phobias) (Dunsmoor and Murphy 2015). Just as in the case of enhancement of memory upon stress (see above), this highlights how an adaptive physiological mechanism can become dysfunctional in its extreme cases.
The Respiratory System
The respiratory system provides the oxygen for the organism that is needed to carry out metabolic processes that create energy in cells. The respiratory system is closely linked to the cardiovascular system: variance in depth or rate of breathing affects heart rate. As heart rate in turn controls blood flow, thus the rate at which oxygen travels to required tissues is controlled by both respiratory and cardiovascular systems. Rate of respiration is controlled by the ANS and has therefore been used in research to investigate ANS activation, complementary to other physiological measures mentioned above (e.g., heart rate, SCR). When faced with threat, cardiac acceleration is accompanied by faster breathing, dilation of the airways (bronchioles), decrease in respiratory volume, and thus a decrease in blood CO2 levels (Kreibig et al. 2007) as the body gets ready for defense. Metabolic needs of the organism are met in the face of threat, allowing a successful flight or fight. Relief from threat decreases the respiratory activity and increases sigh frequency (Blechert et al. 2006).
The Motor System
When the threat is perceived and an appropriate response is initiated, it is the skelomotor system’s responsibility to enact any of the three basic behavioral responses; freezing, fleeing, or fighting. The startle response is defined as a series of rapid muscle movements throughout the whole body, in the face of an abrupt sensory input. In many animals, this facilitates escape (e.g., the whole body startle in mice), but it might also function to protect the organs. For instance, the eye-blink startle reflex causes a blink, which serves to protect the eye from damage. Mammals under threat elicit stronger startle reflexes, due to the state of arousal, which is also reflected in greater amygdala activation (Lang and Davis 2006).
A great part of nonverbal communication in humans and other primates involves facial expressions. Correctly interpreting, and responding to, others’ facial expressions is crucial for both successful social interactions and survival. In humans, six basic facial expressions have been identified to represent emotional states and assumed to have a universal meaning: happy, sad, fear, anger, surprise, and disgust (Ekman et al. 1983). Based on lacking support of neural activities corresponding to discrete emotional labels, recent researchers have emphasized the dimensional approach to emotions (Lindquist et al. 2012). The response of the ANS to perceiving facial emotional expressions is sensitive to the valence of the emotion and can distinguish among negative emotions (i.e., between anger, fear, disgust, and sadness).
Many physiological reactions have clear evolutionary advantages in the face of environmental opportunities or challenges. Internal physiological and environmental cues support the individual’s choice between different survival and foraging strategies, allowing the organism to behave adaptively and thus increasing its chances of survival. To this aim, different systems work in concert under the control of the nervous system. In this entry, we have described a selection of integrated physiological reactions, their survival functions, and associated key neurobiological bases.
- Blanchard, R. J., & Blanchard, D. C. (1990). Anti-predator defense as models of animal fear and anxiety. In Fear and defence (pp. 89–108). Retrieved from http://offcampus.lib.washington.edu/login?url=http://search.ebscohost.com/login.aspx?direct=true&db=psyh&AN=1990-98071-005&site=ehost-live
- Blechert, J., Lajtman, M., Michael, T., Margraf, J., & Wilhelm, F. H. (2006). Identifying anxiety states using broad sampling and advanced processing of peripheral physiological information. In Biomedical Sciences Instrumentation (Vol. 42, pp. 136–141).Google Scholar
- Boucsein, W. (1992). Electrodermal activity. Springer Science & Business Media. New York, NY: Plenum Press.Google Scholar
- Cacioppo, J. T., Tassinary, L. G., & Berntson, G. (2007). Handbook of psychophysiology. Cambridge: Cambridge University Press.Google Scholar
- Dunsmoor, J. E., & Murphy, G. L. (2015). Categories, concepts, and conditioning: How humans generalize fear. Trends in Cognitive Sciences, 1–5. doi: 10.1016/j.tics.2014.12.003.
- Olsson, A., & Phelps, E. A. (2007). Social learning of fear. Nature neuroscience, 10(9), 1095–1102.Google Scholar
- Siegle, G. J., Steinhauer, S. R., Carter, C. S., Ramel, W., & Thase, M. E. (2003). Do the seconds turn into hours? Relationships between sustained pupil dilation in response to emotional information and self-reported rumination. Cognitive Therapy and Research, 27(3), 365–382. doi: 10.1023/A:1023974602357.CrossRefGoogle Scholar
- Stephens, D. W., & Krebs, J. R. (1986). Foraging theory. Evolutionary Behavioral Ecology (Vol. 121). doi: 10.2307/2409049