The uniqueness of the human voice

Published in 2000, in the journal Nature (Belin, Pascal, et al., 2000, “Voice-selective areas in human auditory cortex,” Nature 43, 20 January 2000: 309-312), is a brief report on three experiments by a team of researchers, regarding the brain's response to human vocal sounds:

In conclusion, they write:

p.311
[T]hese experiments provide strong evidence that the human brain contains regions that are not only sensitive to, but also strongly selective to, human voices. ... it could lead to new comparisons between species, by suggesting that areas sensitive to species-typical vocalizations could be found in the homologous regions in other primates. Indeed, language is probably unique to humans, and its possible evolutionary precursors are hard to define and study in other animals. In contrast, we share the ability to reliably extract affective- and identity-related cues from the species-specific vocalizations with many other species, at least of primates. Finally, these data extend the current knowledge on the organization of the human auditory cortex, by identifying regions of the brain involved in the analysis of human voices, a class of auditory objects of high occurrence and ecological interest.

This represents a major shift in our approach to sound stimuli. During the 1960s and 1970s a technique known as dichotic listening was quite popular. Subjects were given stereo headphones, which presented different stimuli to each ear. A comparison between the processing abilities of listeners, based on which brain hemisphere received the data, and on what sorts of stimuli were presented, led to a great many conclusions regarding the localization of capacities in one side of the brain or the other. Mostly these conclusions described which hemisphere was better equipped to process certain types of information (known as hemispheric dominance.)

One major assumption in many of these studies regarded the nature of the stimuli that were used. Often they were arbitrarily classed as "linguistic" or "non-linguistic," and conclusions were hastily drawn regarding the brain's reaction to language stimuli. This new study calls into question these earlier assumptions, by noting an empirical distinction between vocal and non-vocal sounds produced by conspecifics, but not one between linguistic and non-linguistic sounds.

As Belin, et al. point out, language is likely uniquely the domain of humans, but the ability to produce and understand affective and indentifying information (which is clearly a part of language) is one we share with other creatures, certainly with our primate cousins.

Human children are born with the special propensity to draw out the human voice from a cacophony of sounds, and to abstract from the sounds of the human voice information regarding who is speaking, and what emotions they are expressing. What makes this capacity special is mostly that it focuses on human vocal sounds, rather than on any other sounds that it hears.

Just what specific features of the sound lead to the identification of a sound as originating from a human voice has not been fully explicated. However, it is clear that frequency structure (the timbre and pitch range of the sound) provide the needed cues. Of course, acoustic perception is not foolproof.

String players often attempt to produce a "singing" tone from their violins or 'cellos. The guitarist Peter Frampton and others went so far as to use a device called a talk box to impart a vocal timbre to electronic sounds.

The moral to be taken from this is that the human voice is special, regardless of how it is used. Infants are naturally drawn to our voices, whether we speak or sing or giggle or cheer. Shower your children with a variety of vocal sounds. They will lap them all up.