Belin, Pascal, et al. 2000. “Voice-selective areas in human auditory cortex.” Nature 43 (20 January 2000): 309-312.
This team of researchers reports on what they term voice selective areas of the brain, which selectively respond to the sounds of a human voice regardless of the sorts of behavior in which it is involved. This flies in the face of previous work on auditory perception that presupposed a distinction between linguistic and non-linguistic sounds. Much of the work on dichotic listening from the 1960s for instance (and even more so, later articles that uncritically cited this earlier work) presupposed as uncontentious the a priori distinction between linguistic and non-linguistic inputs. What is implied by this report is that a more valid distinction lies between vocal and non-vocal sounds.
Here we show, using functional magnetic resonance imaging in human volunteers, that voice-selective regions can be found bilaterally along the upper bank of the superior temporal sulcus (STS). These regions showed greater neuronal activity when subjects listened passively to vocal sounds, whether speech or non-speech, than to non-vocal environmental sounds. (309)
The authors conducted three experiments. The first consisted in passive listening by 8 right-handed subjects to two categories of stimuli:
1) vocal sounds produced by several speakers of different gender and age, either speech (for example, isolated words, connected speech in several languages) or non-speech (such as laughs, sighs and coughs); and (2) energy-matched, non-vocal sounds (for example, natural sounds, animal cries, mechanical sounds) from a variety of environmental sources. (309)
Experiment 2 further supported a supposition that
the voice-sensitive response was not entirely due to the presence of speech in the vocal stimuli. (310)
and further that
frequency structure plays a more prominent role in voice-sensitive activation than does amplitude envelope. (310)
The third experiment was conducted with a different group of subjects. In this case, a vocal/non-vocal decision task and a speaker’s gender-identification tasks were added, post-scanning.
In all three experiments, peaks of voice selectivity could be found in most subjects along the upper bank of the STS, a deep, long sulcus (>8 cm) running along the whole temporal lobe that is also found in many non-human primates. (310)
They observe that
voice-selective regions were found in the STS on both sides, but voice selectivity was stronger in the right hemisphere in the first two experiments… However, the same pattern was not found in expt 3, suggesting that the neural substrate of voice perception might be less clearly lateralized than in the case of speech perception. (310-311)
these experiments provide strong evidence that the human brain contains regions that are not only sensitive to, but also strongly selective to, human voices. (311)
In outlining the significance of these findings, among other points, they suggest that
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. (311)
Indeed, it would be fascinating to attempt such studies with non-human subjects (though the logistics of scanning an ape might be more than can be handled at the moment). I would suggest also that similar experiments should be conducted on deaf subjects, using passive viewing of sign and gestures. In the place of environmental sounds, environmental motions could be used. The point would be to ascertain 1) whether these voice-selective areas are indeed exclusively a part of auditory processing, or whether (as in the case of congenital deafness in particular) they might be shown to be part of a general human pattern-recognition process, coopted by the mind, in the absence of auditory input; and 2) even if these regions are sustained as exclusively (or primarily) part of our sound-processing mechanism, is it possible to uncover homologous regions in the brain that are selective, rather than to speech or vocal sounds, to (in the words of Petitto 2000) “aspects of the patterning of language …its temporal and distributional regularities”?