Speech and music contain harmonic frequencies which we perceive to have "pitch". The capacity to differentiate pitch from noise (sound that lacks pitch) is considered to be an intrinsic human quality -- but how unique is this ability? A new study suggests that although humans and macaque monkeys share a similar visual cortex, there are important differences in the auditory cortex which processes sound.
Sam Norman-Haignere and colleagues at Columbia University measured cortical responses to both natural and synthetic harmonic tones and noise in human subjects and macaque monkeys. These sounds also included recorded macaque vocalizations that were pitched in post-production.
During one experiment involving four human participants and three macaque monkeys, the researchers noticed strong responses to harmonic tones in humans and virtually no response in the monkeys. In another experiment that studied the brain responses of six humans and five monkeys to natural and modified macaque vocalizations, the researchers found that the human brain performs stronger selectivity for harmonic vocalizations. Meanwhile, the macaques seem to lack the capacity to discern the pitched version.
The team of researchers concludes that the auditory cortical organization differs between human and macaques. These differences are likely driven by human's propensity for speech and music.
Pitch allows us to convey mood or emphasis when speaking. For instance, read these sentences aloud:
- I never said she stole my money.
- I never said she stole my money.
- I never said she stole my money.
Each of the sentences above carries a different meaning due to the emphasis on certain words through pitch change. Previously, a study published in the journal Neuron involving epilepsy patients narrowed down the brain region responsible for pitch and its variations -- the dorsal laryngeal motor cortex. Such studies are particularly useful to sufferers of aprosodia, a neurological condition that some researchers have described as "a disruption in the expression or comprehension of the changes in pitch, loudness, rate, or rhythm that convey a speaker's emotional intent."
High and low pitches are created by the vibration of vocal cords, which are controlled by tension in the folds that comes from flexing muscles, causing a faster vibration.
"We speculate that the greater sensitivity of the human cortex to harmonic tones is driven in development or evolution by the demands imposed by speech and music perception," the authors concluded in the journal Nature.
