I have absolutely no knowledge of psychology. But I am wondering what causes emotions in human? Emotions such as happiness, sadness, love, hate etc. Is there some sort of hormones responsible for this? How does brain understands this and is there a way to control this?
Let's break this down into two terms: affect and emotion.
Affect corresponds to two dimensions that underlie emotional experience: valence and arousal.
- Valence - feelings of pleasantness and unpleasantness
- Arousal - feelings of activation (e.g., alert, energized) and deactivation (e.g., de-energized, sleepy)
So what are the sources of these two basic sensory dimensions?
One answer is... immunologic, metabolic, and autonomic changes in the body, whose sensory input flows up and is processed by primary interoceptive cortex (posterior insula) (Barrett & Simmons, 2015). Think of the body as a source of sensory input, just like any other modality (visual, auditory, etc.).
Additionally, there are top-down predictions descending from agranular interoceptive cortex, which are actually responsible for most of our interoceptive sensations (i.e., conscious affect). That is, most of your affective feelings are produced by top-down predictions about input from the body rather than bodily input itself. Weird!
Another part of the answer is... endogenous opioid and cannabinoid signals in certain "hotspots" of the brain (e.g., nucleus accumbens, ventral pallidum), which are thought to mediate pleasure (Berridge & Kringelbach, 2013).
But in general... this is still being figured out.
The answer to this part depends on your theory, about which I'll be very brief.
Basic emotion theorists (e.g., Ekman & Cordaro, 2011) argue that emotions correspond to discrete, circumscribed parts of your brain (e.g., the amygdala and fear, the insula and disgust). We have "affect programs," that when activated, produce a discrete emotion (fear, anger, etc.).
Constructionists (e.g., Lindquist et al., 2013) argue that emotions are essentially interpretations of your affect (valence and arousal) in a given context. So "fear" is just a concept that you deploy in certain situations (e.g., when you feel negative, highly aroused, and are running from a bear). On this view, emotions emerge from interactions among distributed brain networks that are involved in more basic psychological processes (e.g., exteroception, interoception, conceptualization, language).
The meta-analytic evidence seems to favor the latter view, but there's still a buttload of work to be done on this end.
As far as controlling emotions--well, that's an entirely different question. But see James Gross for more info.
It mostly depends which theory you go by.
Psychologists have proposed theories of emotion to explain what emotions are, by what they are caused and what effect they have on living beings. There're different ways to categorize those theories like on the basis of the content of emotions for example and the major theories can be grouped into three main categories: physiological, neurological, and cognitive.
James-Lange-Theory This theory suggests that when you see an external stimulus that leads to a physiological reaction. Your emotional reaction is dependent upon how you interpret those physical reactions. For example, suppose you are walking in the woods and you see a grizzly bear.
Cannon-Bard-Theory Suggests that the physical and psychological experience of emotion happen at the same time, and that one does not cause the other.
Schachter-Singer-Therory This theory suggests that the physiological arousal occurs first, and then the individual must identify the reason for this arousal to experience and label it as an emotion. A stimulus leads to a physiological response that is then cognitively interpreted and labeled which results in an emotion. Schachter and Singer’s theory draws on both the James-Lange theory and the Cannon-Bard theory of emotion. Like the James-Lange theory, the Schachter-Singer theory proposes that people do infer emotions based on physiological responses. The critical factor is the situation and the cognitive interpretation that people use to label that emotion.
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If you are looking for a general defintion of emotions go with this:
Emotions are complex patterns of psychological and physiological reactions. They are object-related (= require an external stimulus) and mostly short-lived (in contrast to moods).