Physiology of emotions

Based on discoveries made through neural mapping of the limbic system, the neurobiological explanation of human emotion is that emotion is a pleasant or unpleasant mental state organized in the limbic system of the mammalian brain. If distinguished from reactive responses of reptiles, emotions would then be mammalian elaborations of general vertebrate arousal patterns, in which neurochemicals (for example, dopamine, noradrenaline, and serotonin) step-up or step-down the brain's activity level, as visible in body movements, gestures, and postures.

For example, the emotion of love is proposed to be the expression of paleocircuits of the mammalian brain (specifically, modules of the cingulate gyrus) which facilitate the care, feeding, and grooming of offspring. Paleocircuits are neural platforms for bodily expression configured before the advent of cortical circuits for speech. They consist of pre-configured pathways or networks of nerve cells in the forebrain, brain stem and spinal cord.

The motor centers of reptiles react to sensory cues of vision, sound, touch, chemical, gravity, and motion with pre-set body movements and programmed postures. With the arrival of night-active mammals, smell replaced vision as the dominant sense, and a different way of responding arose from the olfactory sense, which is proposed to have developed into mammalian emotion and emotional memory. The mammalian brain invested heavily in olfaction to succeed at night as reptiles slept—one explanation for why olfactory lobes in mammalian brains are proportionally larger than in the reptiles. These odor pathways gradually formed the neural blueprint for what was later to become our limbic brain.

Emotions are thought to be related to certain activities in brain areas that direct our attention, motivate our behavior, and determine the significance of what is going on around us. Pioneering work by Broca (1878), Papez (1937), and MacLean (1952) suggested that emotion is related to a group of structures in the center of the brain called the limbic system, which includes the hypothalamus, cingulate cortex, hippocampi, and other structures. More recent research has shown that some of these limbic structures are not as directly related to emotion as others are, while some non-limbic structures have been found to be of greater emotional relevance.

In 2011, Lövheim proposed a direct relation between specific combinations of the levels of the signal substances dopamine, noradrenaline and serotonin and eight basic emotions. A model was presented where the signal substances forms the axes of a coordinate system, and the eight basic emotions according to Silvan Tomkins are placed in the eight corners. Anger is, according to the model, for example produced by the combination of low serotonin, high dopamine and high noradrenaline.

Prefrontal cortex
There is ample evidence that the left prefrontal cortex is activated by stimuli that cause positive approach. If attractive stimuli can selectively activate a region of the brain, then logically the converse should hold, that selective activation of that region of the brain should cause a stimulus to be judged more positively. This was demonstrated for moderately attractive visual stimuli and replicated and extended to include negative stimuli.

Two neurobiological models of emotion in the prefrontal cortex made opposing predictions. The Valence Model predicted that anger, a negative emotion, would activate the right prefrontal cortex. The Direction Model predicted that anger, an approach emotion, would activate the left prefrontal cortex. The second model was supported.

This still left open the question of whether the opposite of approach in the prefrontal cortex is better described as moving away (Direction Model), as unmoving but with strength and resistance (Movement Model), or as unmoving with passive yielding (Action Tendency Model). Support for the Action Tendency Model (passivity related to right prefrontal activity) comes from research on shyness and research on behavioral inhibition. Research that tested the competing hypotheses generated by all four models also supported the Action Tendency Model.

Homeostatic/primordial emotion
Another neurological approach distinguishes two classes of emotion: "classical" emotions such as love, anger and fear that are evoked by environmental stimuli via distance receptors in the eyes, nose and ears; and "homeostatic" (or "primordial" ) emotions – feelings such as pain, hunger and fatigue, evoked by internal body states communicated to the central nervous system by interoceptors, that motivate behavior aimed at maintaining the body's internal milieu at its ideal state.

Derek Denton defines the latter as "the subjective element of the instincts, which are the genetically programmed behaviour patterns which contrive homeostasis. They include thirst, hunger for air, hunger for food, pain and hunger for specific minerals etc. There are two constituents of a primordial emotion--the specific sensation which when severe may be imperious, and the compelling intention for gratification by a consummatory act."