There are two different "thinking" systems that predominate our brain. They are responsible for our first impressions and then later changing them, our in depth views and perspectives, in addition modulating our thinking with respect to a given context. System one is mainly the "fast" thinking system. System two is a relatively slower thinking system. Their remarkably different yet crucial functions are portrayed beautifully in this video.

I recommend you watch the whole video, but if you're anxious and eager to see what I'm referring to, skip to the fourth minute.

An informative recapitulation of the brain's complex physiological mysteries.

This video explains it quite well. The key hormone in question is serotonin. Serotonin is known to play a role in depression. Low serotonin levels are believed to be the reason for depression and associated symptoms of anxiety, apathy, fear, feelings of worthlessness, insomnia and fatigue. The opposite is true when a high-level of serotonin is present, you feel jovial and enthusiastic. MDMA promotes the formation of this hormone.

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Someone who greatly enjoys sweet foods is said to have a "sweet tooth." Experimental evidence now shows us that eating sweets forms memories that may control eating habits. In other words, people may enjoy eating sweets because the taste is correlated with positive memories.

The findings, published online in the journal Hippocampus, show that neurons in the dorsal hippocampus, the part of the brain that is critical for episodic memory, are activated by consuming sweets. Episodic memory is the memory of autobiographical events experienced at a particular time and place.

In the study, a meal consisting of a sweetened solution, either sucrose or saccharin, significantly increased the expression of the synaptic plasticity marker called activity ...