engleski

Biological basis of emotions

Since being mediated by the multiple and overlapping neural networks, emotions represent complex behavioral states comprised of many components: autonomic (hormonal and visceral), cognitive (subjective feelings and thoughts), motor expressions (face, body, voice), conscious perceptive awareness, unconscious perception (of e.g.: pheromones and other molecules that can induce different emotions / fearful faces in the blind part of the visual field – “ fear blindsight” / infant facial cues that serve to elicit parental instincts and bonding accompanied by increased aggressiveness toward perceived threats, etc.), attention (external and internal), memories, etc. Different emotions have different adaptive functions and underlying neural mechanisms. According to Ekman, there are 6 basic emotions (being afraid, angry, disgusted, happy, sad, surprised) since these emotions are cross-culturally recognized regardless of experience. Emotions are not discrete ; rather they are having dimensions (e.g. low to high arousal, pleasant vs. unpleasant valence, approach vs. avoidance, etc.). Some emotions last for several seconds, while others (e.g. moods) can last from a few minutes to several weeks. Finally, personality traits of emotional nature may be life-long. For all of these reasons, emotions are relatively easily recognized, but difficult to quantify. There are three contemporary theories on the biological basis of emotions: somatic marker hypothesis, cognitive-emotional interaction theory, and lateralization theory. In short, according to the SOMATIC MARKER HYPOTHESIS, an emotion consists of a change in the body (i.e. the somatic markers) that, in turn, changes brain states in response to the evaluation of a particular external event. Hence, emotions engage those neural structures that represent body states and those that somehow link the perception of external stimuli to body states. There is accumulated evidence that the amygdaloid complex of nuclei (amygdala, AMY) represents the central “ emotional computer” within the human brain, which gives emotional (affective) tone to the input sensory information even before its conscious (cortical) processing (the so-called “ emotional filter” ). Sometimes, even when the original information is not retained, the emotional tone (e.g. in case of spoken language the tone of voice) remains (“ source amnesia” ). This happens particularly when the source of information is unreliable, such as media. Through direct reciprocal connections with the hypothalamus AMY influences regulation not only of endocrine functions, but also autonomic component of emotional expressions and behaviors aimed at individual (thirst, hunger) and collective (libido) survival. On the other side, one of the most important specializations of AMY is recognition of facial expressions. Facial expressions convey information about identity, mood, and intention. Thus, facial expressions are not a single stimulus - such diverse information is analyzed in an extensive network that, besides AMY, includes large regions of the inferior frontal, posterior temporal, parietal and occipital cortices. The two key aspects of the somatic marker hypothesis are that emotion is fundamental to the survival of the individual within a particular physical and social environment, and that emotion is a necessary experience to make rational decisions, especially in situations in which an individual faces risk, conflict or a potential partner. The COGNITIVE-EMOTIONAL INTERACTIVE THEORY is also evolutionary in a sense that emotions evolved to enhance the survival of the individual. However, according to this view, all animals inherently detect and respond to danger, and the related neural activities in the AMY evolve to produce a feeling – in this case, fear. Probably the most important implication of this theory is that our fear system includes both unconscious fear responses (through genetically-programmed neural networks that mediate “ instinctive” behaviors) and conscious awareness of subjective feelings of fear (through networks shaped by learning from experience about dangerous stimuli for which evolution could not prepare us). Today, the evidence is clear that damage of the hippocampal formation (HIPP) interferes with the development of contextual fear associations. Panic disorder, posttraumatic stress disorder, obsessive-compulsive disorder, anxiety and phobias illustrate the extreme power of fear-related events to affect cognition by classical fear conditioning, suggesting that evolution has crafted a powerful mechanism for forming such associations. The fact that limbic system size (AMY, HIPP and adjoining structures at the medial edge of the hemispheres including cingulated cortex and hypothalamus) is evolutionary quite conserved suggests that many emotional functions are also conserved. The orbitofrontal cortex (OFC) has massive reciprocal connections with AMY, suggesting that AMY-OFC circuits play a key role in the formation of thoughts about fearful stimuli. The OFC allows for emotional control and flexibility of decision making, e.g. patients with damaged OFC laughing out loud at a funeral. The meta-analysis of 267 areas of activation in stereotaxic space (Kringelbach and Rolls, Prog. Neurobiol. 2004) showed that there is a medial-lateral distinction in the human OFC, such that activity in the medial OFC is related to the monitoring, learning and memory of the reward value of reinforcers, whereas lateral OFC activity is related to the evaluation of punishers that can lead to a change in behavior. Similarly, a posterior-anterior distinction was shown, with more complex or abstract reinforcers (such as monetary gain and loss) being represented more anteriorly in OFC than less complex reinforcers such as taste. Many scientists agree that an universal brain rewarding system is composed of dopaminergic projections from ventral tegmental area (VTA) to AMY and OFC (the so-called mesolimbic and mesocortical projections), which release dopamine at higher rate during pleasant experiences of eating food, having sex, bonding with a child, etc. (behavioral activation, step 1). However, many drugs are acting the same way (cocaine, amphetamines, opioids, canabinoids, nicotine, partially alcohol ; investigators at Vanderbilt University even encountered people in whom aggressiveness could produce the same effect!). It has been shown that the firing frequency of these dopaminergic neurons is the signaling mechanism coding for an error value in prediction of reward that directly correlates with the motivational levels (Arrias-Carrion and Poppel, Acta Neurobiol. Exp., 2007). When a reward is greater than expected, the firing of certain VTA dopaminergic neurons increases, which consequently increases the desire or motivation toward the reward in OFC. It is the OFC glutamatergic projections that regulate activation of motor responses to such “ motivation-related events” (e.g. water vs. cocaine) through n. accumbens septi and thalamic motor nuclei, while blocking dopamine D1 receptors in OFC prevents seeking for a drug (direction of behavior or “ choice” , step 2). In the case of a failure to achieve planned goals, the reward mechanism can be activated by the “ compensatory” or the so-called “ displaced behavior” , such as overeating (chocolate being a common example). Due to change in regulation of motivation circuit and pathological long-term changes in synaptic plasticity of OFC neurons that project to ventral striatum, in the last phase (dependence and drug abuse, step 3) such patients are in a constant search for a drug. The most recent neuroimaging studies confirmed that (in normal people) the final decision making (affective “ go / no go” decisions) also relies strongly on interactions between the OFC and anterior cingulate cortex (ACC). When the function of ACC is no longer maintained because of e.g. inhibitory influence of AMY (possibly also via the dopaminergic projections from VTA), the phenomenon of “ learned helplessness” can be seen (Bauer et al., Neuroimage, 2003). The posterior cingulate cortex (PCC) mediates conditioning of negative (painful) stimuli (“ aversive conditioning” ) (Bromm et al., News Physiol. Sci., 2001). Self-control of motivation through OFC is considered the most important part of emotional intelligence and one of the three most important personality traits (e.g. a highly intelligent, but non-motivated person may have lower scores and results when compared to a motivated person without such abilities). Some scientists believe that long-term and learned motivation, which is independent from fulfillment of basic drives (aimed at survival and reproduction), is human-specific (in comparison to solely “ intrinsic” motivation in animals) and perhaps one of the most important steps in human evolution. The decreased activation of OFC has been documented in patients with schizophrenia and in non-premeditated murderers, while increased activity of OFC is often seen in depression. The OFC control can be so strong that even unwanted memories can be suppressed (originally this was a Freud’ s concept) by reduction of hippocampal activation during a conscious recall (of e.g. sexual abuse in childhood) (Anderson et al., Science, 2004). OFC is also one of the limited number of the brain areas specifically involved in recognition of mother’ s own infant distress. Infant cues, such as smiling or crying facial expressions, are powerful motivators of human maternal behavior that activate dopamine-associated reward circuits. It has been recently shown that mothers with secure attachment show greater activation of medial OFC, ventral striatum and the oxytocin-associated hypothalamic regions, while insecure/dismissing mothers showed greater insular activation in response to their own infant’ s sad faces (Strathearn et al., Neuropsychopharmacol., 2009). The early social attachment lie at the heart of emotional and social development of mammals. The newest data obtained using the monogamous prairie vole (Ahern and Young, Front. Behav. Neurosci., 2009) suggests that naturalistic variation in social rearing conditions mediated through various neuropeptides, mainly oxytocin, vasopressin and corticotropin-releasing factor systems, introduce diversity into adult nurturing and attachment behaviors. Slight modulation in serotonin levels, turnover and metabolism, or in receptor type activation, density and binding activity is particularly important in this respect because it may strongly affect aggressive behavior. The recent data obtained by Nelson and Trainor (Nature Rev. Neurosci., 2007) suggested that children carrying the short form of the MAOA promoter gene, which confers decreased MAOA activity, are more likely to develop conduct disorders and increased antisocial behaviour when exposed to abusive home environments (Casni et al., Science, 2002). LATERALIZATION THEORY is based on the fact that lesions of the right and left hemispheres have different effects on emotional behaviors. Left-hemisphere lesions produce “ catastrophic” reactions characterized by fearfulness and depression, while damage to the right hemisphere appears to produce larger effects, mostly “ indifference” (flattening of mood). It seems that the left AMY plays a special role in generating fear. Subjects with bilateral AMY lesions (such as in Rössle-Urbach-Wiethe syndrome due to selective calcification of AMY) are impaired at recognizing negative expressions (such as fear), but not at recognizing happy faces (besides loss of fear, they are also unable to comprehend social signals and modulate episodic memory emotionally). Emotions also enhance retrieval of past experiences, but there are sex differences. In recognition memory test 3 weeks after seeing pictures, highly emotional pictures were remembered best, and remembered much better by women than by men (Canli et al., PNAS USA, 2002). Men activated significantly more structures than women in a network that included the right AMY, while women activated significantly fewer structures in a network that included the left AMY (Canli et al., PNAS USA, 2002). Overall, the two sides of the brain play balancing and harmonizing roles in emotional behavior, the right being more engaged in the automatic components of emotion and generation of emotional feelings, whereas the left being engaged in the overall cognitive control of emotion (presumably because “ speaking” hemisphere acts like an “ interpreter” of these feelings) and affective behavior. In conclusion, all three briefly described theories are complementary in a way that emotion and cognition are intimately related and entail overlapping multiple neural systems. Emotions play a critical role in the evolution of consciousness and all mental processes. The key structures (“ epicenters” ) for emotional behavior include OFC, AMY, extended AMY (bed nucleus of stria terminalis, shell of nucleus accumbens septi) and associated paralimbic cortex (ACC, PCC, HIPP), and the hypothalamus. From a practical point of view, a physician should never forget that emotional behavior might appear abnormal not only because a person is unable to produce the appropriate behavior (expression, say), but also because patient may misinterpret the emotional signals from others. Neglect and abuse during early life may cause bonding systems to develop abnormally and compromise capacity for rewarding interpersonal relationships and commitment to societal and cultural values later in life. Other means of stimulating reward pathways in the brain, such as drugs, sex, aggression, and intimidating others, could become more attractive and less constrained about violating trusting relationships. Unmet needs for social bonding and acceptance early in life might also increase the emotional attractiveness of gangs or sects, with violent and authoritarian values and leadership. The relation of the (particularly right) fronto-parietal “ mirror neuron system” (a distinct class of neurons that transform specific sensory information to motor format and seems to be important to all aspects of social cognition) as a neural substrate for understanding emotions in others - empathy (Leslie et al., Neuroimage, 2004) and cultural influences on the development of socio-emotional skills is an unresolved issue that is likely to attract future research because “ therapies based on mirror neuron system could provide a non-invasive approach in treatments of emotional disorders” .

emotions ; attachment ; bonding ; orbitofrontal cortex ; amygdala ; mirror neuron system ; fear conditioning ; fear blindsight ; aggression ; oxytocin ; parental behavior ; facial cues

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