Nostalgia is a sentimentality for the past, typically for a period or place with happy personal associations. The word nostalgia is a learned formation of a Greek compound, consisting of νόστος (nóstos), meaning "homecoming", a Homeric word, and ἄλγος (álgos), meaning "pain, ache", and was coined by a 17th-century medical student to describe the anxieties displayed by Swiss mercenaries fighting away from home. Described as a medical condition—a form of melancholy—in the Early Modern period, it became an important trope in Romanticism.In common, less clinical usage nostalgia can refer to a general interest in the past, their personalities and events, especially the "good old days" from one's earlier life. Boym argues that nostalgia is more prevalent during times of great upheaval.The scientific literature on nostalgia is quite thin, but a few studies have attempted to pin down its essence and causes. Smell and touch are strong evokers of nostalgia due to the processing of these stimuli first passing through the amygdala, the emotional seat of the brain. These recollections of our past are usually important events, people we care about, and places where we have spent time. Music and weather can also be strong triggers of nostalgia.