Synesthesia is a neurological phenomenon where stimulation of one sensory pathway leads to automatic, involuntary experiences in a second sensory pathway. It is estimated to occur in 1 in 25,000 people. Common types include seeing colors in response to letters, numbers, or sounds. Research suggests it may be due to excessive or defective neural connections between brain areas in synesthetes. Studies have found synesthetes' perceptions are consistent over long periods and different areas of their brains are activated compared to non-synesthetes. While the exact causes are unknown, current theories explore issues of neural connectivity and cross-wiring in the brain.