COLOURED HEARING, a particular form of synaesthesia, which manifests itself by the appearance in consciousness of colours (or various shades of grey) whenever certain sounds are heard. These colours are most frequently associated with tones of the musical scale. For some individuals each tone of the scale has its specific colour ; for others all of the tones within certain regions of the scale will have the same colour. It is seldom, how ever, that two people agree as to the colour they associate with the same tone or region of tone. It has been estimated that coloured hearing occurs in about 5% of the population.