What went on inside the mind of 20th-century French composer Olivier Messiaen? Nothing like what goes on inside yours or mine, dear readers. For one thing, he had chromesthesia, a form of synaesthesia whereby specific musical chords, scales, and transpositions triggered vivid mental visions of colours. Messiaen’s mental life would have been more urban bustle than pastoral repose. His famous fascination with bird song – he would notate their singing and include the melodies in his pieces – gives an entirel…