Science Daily:

Along with colleagues in the U.S. and Switzerland, Pfaus analyzed the results from 20 separate studies that examined brain activity while subjects engaged in tasks such as viewing erotic pictures or looking at photographs of their significant others. By pooling this data, the scientists were able to form a complete map of love and desire in the brain.

They found that that two brain structures in particular, the insula and the striatum, are responsible for tracking the progression from sexual desire to love. The insula is a portion of the cerebral cortex folded deep within an area between the temporal lobe and the frontal lobe, while the striatum is located nearby, inside the forebrain.
Love and sexual desire activate different areas of the striatum. The area activated by sexual desire is usually activated by things that are inherently pleasurable, such as sex or food. The area activated by love is involved in the process of conditioning by which things paired with reward or pleasure are given inherent value. That is, as feelings of sexual desire develop into love, they are processed in a different place in the striatum.

Somewhat surprisingly, this area of the striatum is also the part of the brain that associated with drug addiction. Pfaus explains there is good reason for this. "Love is actually a habit that is formed from sexual desire as desire is rewarded. It works the same way in the brain as when people become addicted to drugs."

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