Auditory brain areas in humans but not in macaque monkeys show a preference for harmonic sounds, finds a study published online this week in Nature Neuroscience.

Speech and music, considered unique to humans, contain harmonic frequencies that are perceived as having ‘pitch’. The capacity to discriminate pitch is crucial for speech and music. In humans, auditory brain regions thought to be involved in pitch perception respond more to harmonic tones than to noise. However, it is unknown if similar brain regions are present in other animals.

Sam Norman-Haignere and colleagues used functional MRI to measure human and macaque brain responses to natural and synthetic harmonic sounds, including recorded macaque vocalizations, and compared those responses with noise sounds that lack pitch. In one experiment, the authors observed strong responses to harmonic tones in four human participants, but these responses were largely absent in three macaques. In another experiment, the authors measured brain responses from six humans and five macaques to natural macaque vocalizations or to modified versions in which the harmonic tones were replaced with noise. Human brain responses showed stronger selectivity for harmonic vocalizations than macaques.

The authors conclude that auditory cortical organization differs between human and macaques, potentially due to the unique importance of speech and music to humans.