PITCH PERCEPTION MECHANISMS IN MARMOSETS

The mechanisms of pitch perception have been one of auditory neuroscience’s central questions for over a century due to the importance of pitch in music and speech perception. Yet the evolutionary origins of pitch perception, and whether its underlying mechanisms are unique to humans, is unknown. For my dissertation, I have investigated the perceptual properties of pitch in marmoset monkeys. One of the most well-known phenomena of pitch perception, that of the missing fundamental, has suggested that humans do not simply use the fundamental frequency component to perceive pitch from harmonic complex sounds but can actively infer the pitch from the higher overtones or harmonics. It has been suggested that several non-human species are also sensitive to the pitch of missing fundamental sounds. However, none of these demonstrations has shown this sensitivity to pitch with a precision below three semitones. For humans to perceive Western music melodies, a precision of at least one semitone is necessary. The first step of my thesis was to confirm that marmoset monkeys can also perceive pitch through missing fundamental sounds with at least one semitone precision, using a behavioral generalization paradigm. The next step of my thesis is to determine the mechanisms behind pitch perception of harmonic complex sounds in marmosets. It has been shown that humans hear the pitch of harmonic sounds through spectral or temporal features. Over the last century of human psychophysics research, three primary features of human pitch perception mechanisms have been described: (1) Lower resolved harmonics have a stronger pitch strength compared to a pure tone at the fundamental frequency (F0) and also to higher unresolved harmonics; (2) pitch of resolved harmonics is sensitive to the quality of spectral harmonicity; (3) pitch of unresolved harmonics is sensitive to the salience of temporal envelope cues. Among these features, the first two have never been demonstrated in any other species besides humans. For this part of my thesis, I provided evidence that the marmoset, a highly vocal New World monkey species separated from humans by about 30 to 40 million years, exhibits all three primary features of pitch perception mechanisms as found in humans. Combined with previous neurophysiological findings of a specialized pitch processing region in both marmoset and human auditory cortex, these findings suggest that the mechanisms for pitch perception, which have long been thought unique to humans, may have originated early in primate evolution, before the separation of New World and Old World primates