What is synesthesia?

The answer

Synesthesia is a neurological condition in which stimulation of one sense automatically triggers an involuntary experience in another, such as seeing specific colors when hearing music (chromesthesia) or associating particular colors with letters and numbers (grapheme-color synesthesia). It affects approximately 4% of the population and is highly consistent within individuals; if the letter A appears red today, it will appear red decades later. Neuroimaging studies show increased cross-activation between adjacent brain regions in synesthetes, along with stronger structural connections (white matter tracts) between sensory areas. Synesthesia runs in families with a genetic component, and synesthetes often report enhanced memory for information connected to their synesthetic associations.