Communication amongst members of a species is a crucial aspect of social behaviour and is important for survival. Our ability to discriminate our own species' vocal sounds from other sounds and to distinguish between the vocalizations from someone we know and someone we do not has been described down to the specific brain regions responsible. It is recognized that humans have greater vocal range and linguistic capabilities than other non-human primates and our specialized ability to communicate may at first seem to set us apart from other species. New evidence suggests, however, that this may be a hasty conclusion. A recent study by Christopher Petkov and colleagues, from the Max Planck Institute in Tubingen, Germany and from the University of Manchester, UK, published in Nature Neuroscience in March, 2008, has identified a specific `voice' region in the brains of a species of Old World monkey, the macaque.
Functional magnetic resonance imaging (fMRI) studies in humans have identified a brain region that is sensitive to human voices and speech. It is unclear, however, whether this is a function of the linguistic processing of speech itself (because we communicate with words) or whether voice regions have been evolutionarily conserved in other primates. Behavioural evidence shows that in many primates, their attention is drawn to the sound of their own species' vocalizations, but no one had experimentally identified a specific auditory region dedicated to the recognition of vocalizations from their own species.
By using fMRI to measure brain activity while macaques listened to a variety of sounds, this study sheds light on a region of the macaque brain that shares a functional relationship with the human voice region, supporting the idea that different primate species possess brain regions adapted to recognizing communication signals from their own species.
First, the researchers investigated which cortical regions became active when the macaques listened to different sounds. Using four different categories of sound – specific macaque vocalizations, other animals'vocalizations, natural sounds and an acoustic control (consisting of sounds of a similar pitch and duration to those from the other categories) – they identified a discrete brain region that reacted robustly in response to the macaque sounds but exhibited only a modest response to the other three categories of signals. This region was identified as a hierarchically high-level processing region in the anterior of the auditory cortex. Importantly, other regions of the auditory cortex were also activated, but this study was particularly interested in identifying a region solely adept at recognizing the sounds of its own species. Further experiments determined that the anterior region, and only this region, became activated in macaques listening to the vocal sounds of familiar macaques and that the sounds of unfamiliar individuals did not elicit a similar activation pattern. This is evidence that the anterior region is sensitive to the identity of the`speaker'. Altogether, this study highlights the existence of a voice region in macaque monkeys that has functional properties comparable to those described in humans.
Social animals depend on communication with their own species for survival. If the voice region is not unique to us, evolution likely influenced different primate species and other animal species to evolve similarly. With this in mind, perhaps we should be careful what we say around our pets: who knows what they might be picking up.