The ancient Greeks had an interest in the color of sound. Philosophers pondered whether the color (chroia, now termed timbre) of music was a quantifiable quality. English scientist Isaac Newton proposed that musical tones and color hues shared common frequencies; a sentiment echoed by German scientist Johann Wolfgang von Goethe in his 1810 book Theory of Color.
German psychologist Gustav Fechner was the founder of psychophysics: investigation into the relationships between physical stimuli, sensations, and perceptions. His 1871 thesis was on the color of letters (printed in black ink) reported by synesthetes.
English polymath Francis Galton, cousin of Charles Darwin, reported in 1880 on synesthesia from picturing symbols, such as the number 6.
Research into synesthesia was brisk in several countries for a few decades. The difficulty in measuring subjective experiences cooled research ardor among empiricists, as did the bent toward behaviorism that arose in the early 20th century. Synesthesia research went moribund between 1930 and 1980.
Behaviorism is the ultimate “the surface is the depth” school of psychology. According to behaviorist philosophy, anything and everything any organism can do is a behavior, whether acting, feeling, or thinking. What can be observed is equivalent to private processes such as emotion or cogitation.
In the behaviorist creed, there is no need to consider hypothetical constructs such as mental processing. Never mind the mind. Psychological disorders are best treated by modifying behavior patterns or altering the environment.
Behaviorism was taken to its logical extreme by American behaviorist B.F. Skinner, who extended behaviorism into internal organic processes, where it didn’t matter what a cell was thinking or feeling.
The 1950s heralded a cognitive revolution in psychological philosophy: that the mind matters. In this there is some overstatement, as behaviorism was always an American fashion. The interdisciplinary approach betokening the cognitive sciences was a return to long-standing European sensibilities.
In the wake of the cognitive revolution came a revival of interest in synesthesia. Recent research indicates some 4% (1 in 23) of the human population are synesthetes of some sort.