[Why can we hear pure tones?]

This study is concerned with the role and significance of "hearing out" pure tones in many types of sound. While for decades the phenomenon had played only a secondary role in auditory psychophysics, it is becoming now recognized as a key to auditory information acquisition. It is suggested that the auditory spectral pitches (i.e., the subjective pitches of pure tones) are in several respects analogous to visual contours; that both in visual and auditory perception contours convey most important information on external objects; and that contourization requires a decision process in the peripheral part of the sensory system and such can be regarded as the first step of sensory information processing. Recent experimental results on part-tone synthesis of audio signals are outlined that strongly emphasize the significance of part-tone hearing.