Imagine a world of magenta Tuesdays, tastes that have shapes, and wavy green symphonies. At least 1% of otherwise normal people experience the world this way—in a harmless neurological condition called synaesthesia. In synaesthesia, stimulation of one sense triggers anomalous perceptual experiences.1 2 For example, a voice or music may be not only heard but also seen, tasted, or felt as a touch. Synaesthesia is a fusion of different sensory perceptions: the feel of sandpaper might evoke an F sharp, a symphony might be experienced in blues and golds, or the concept of February might be experienced above the right shoulder. Synaesthetes are typically unaware that their experiences are unusual. In the linked article (doi:10.1136/bmj.b3191), Logsdail reports one patient’s journey with synaesthesia.3

Synaesthesia comes in many varieties, and a person can have several different types. Experiencing letters and numbers with colours or textures …