Synesthesia, from the Greek for joined sensation, is an involuntary condition in which stimulating one sense or cognitive pathway triggers an experience in another. This episode covers some of the more than 80 documented types, from grapheme-color synesthesia, in which the letter A is overwhelmingly seen as red, to lexical-gustatory synesthesia, in which pub owner James Wannerton tastes earwax whenever he hears the name Derek. It explains the split between projectors, who see colors and shapes out in the room, and associators, who feel them internally, and the two leading theories: cross-activation between neighboring regions such as the grapheme area and the color region V4, and disinhibited feedback, in which the brain’s usual filter on cross-sensory signals is lowered.

About 4.4 percent of people have some form of synesthesia, with higher rates among autistic people. Russian reporter Solomon Shereshevsky, studied for decades by Alexander Luria, had five-fold synesthesia and a near flawless memory, yet struggled to grasp metaphor and abstract ideas. The episode also covers Vladimir Nabokov’s colored letters, musicians including Billie Eilish, Pharrell Williams and Lorde, the bouba-kiki effect, and The vOICe, a device by Peter Meijer that turns camera images into sound for blind users and trains the visual cortex to read it.

• Mirror-touch synesthesia, where watching someone tapped on the shoulder produces the feeling of a tap
• Ticker-tape synesthesia, which shows spoken words as floating mental subtitles
• Ideasthesia, where the same shape changes color depending on whether it is read as S or 5
• Spatial sequence synesthesia, which places years and months at fixed points in space
• Why 95 to 98 percent of people, including toddlers, call the spiky shape Kiki and the round one Bouba