Previous studies have shown that the brains of humans and nonhuman primates process visual information in much the same way. Yet, researchers have remained unsure as to whether there are any differences in how we and our primate 'cousins' process different types of sounds.
This is precisely the area that scientists from the Massachusetts Institute of Technology in Cambridge, MA, and the Laboratory of Sensorimotor Research, of the National Eye Institute of the National Institutes of Health in Bethesda, MD, recently decided to investigate.
In their study paper, which appears in Nature Neuroscience, the researchers explain that the '[v]isual cortex is similar between humans and macaque monkeys, but less is known about audition' differences in the two species.
The research team thus set out to compare how the brains of humans and those of rhesus macaques reacted to auditory stimuli, particularly ones that we usually associate with humans, namely harmonic tones that characterise music and speech.
"Speech and music contain harmonic frequency components, which are perceived to have 'pitch,'" the authors explain in their paper. "Humans have cortical regions with a strong response preference for harmonic tones versus noise," But is the same true for nonhuman primates?
"We found that a certain region of our brains has a stronger preference for sounds with pitch than macaque monkey brains," said Senior Author Bevil Conway, Ph.D., commenting on the current study's findings.
"The results raise the possibility that these sounds, which are embedded in speech and music, may have shaped the basic organisation of the human brain." - Conway, Ph.D.
For the study, the researchers worked with three rhesus macaques and four human participants, playing them harmonic tones and noise that featured five different frequency ranges.
Using functional MRI images, the team measured the monkey and human brain responses to the different sounds and frequency ranges.
The first analysis of functional MRI scans seemed to suggest that there was not much difference in brain responses between humans and monkeys - both the human participants and the macaques showed activation of the same parts of the auditory cortexes.
But when the researchers assessed the scans in more detail, they saw that human brains appeared to be much more sensitive to 'pitch' in harmonic tones than the brains of rhesus macaques, which seemed not to distinguish between harmonic tones and regular noise.
"We found that human and monkey brains had very similar responses to sounds in any given frequency range. It's when we added tonal structure to the sounds that some of these same regions of the human brain became more responsive," explained Conway.
"These results suggest the macaque monkey may experience music and other sounds differently," he continued, noting that, "[i]n contrast, the macaque's experience of the visual world is probably very similar to our own."
"It makes one wonder what kind of sounds our evolutionary ancestors experienced," Conway ponders.
Even when they exposed the macaques to sounds with more natural harmonies - namely, recordings of macaque calls -the results remained the same, supporting the idea that human brains are more sensitive to "pitch."
"[The current findings] may also help explain why it has been so hard for scientists to train monkeys to perform auditory tasks that humans find relatively effortless," noted Conway.
To learn more about this research, you can watch an interview with the senior author below.