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Zusammenfassung:
A region of ‘voice’ clusters has recently been identified in the macaque auditory cortex with functional magneticresonance imaging (fMRI). These clusters show a strong fMRI activity preference for the voice of conspecific
individuals and appear to functionally correspond to those from the known human voice region. In the visual
system fMRI has been used to guide electrophysiological recordings from neurons in the monkey brain that were
shown to be extremely selective for faces, so-called ‘face’ cells. We investigated whether fMRI-guided
electrophysiology would reveal comparable levels of selectivity in one of the monkey voice clusters, and how the
functional properties of those ‘voice’ cells compares to those of their visual counterparts.
During fMRI acquisition and electrophysiological recordings, three categories of 12 sounds were used for
stimulation: macaque vocalizations (MVocs), other animal vocalizations (AVocs), and natural sounds (NSnds).
The sound categories were comparable in their low-level acoustical features, having been selected for this from a
large set of sounds. We first used the stimuli during fMRI, as we have previously done, to identify clusters with a
strong activity preference for MVocs. Then electrophysiological responses to the auditory stimuli were recorded
from the anterior voice cluster in two awake macaques (total of 186 responsive single- and multi-units). A
significant majority of the neurons (45%, Χ2-test: p = 0.0013) responded better to MVocs than to any of the other
two complex natural sound categories. The area’s preference for MVocs was also present in the population spiking
and local-field potential response, consistent with the fMRI results. Adapting the frequently employed criterion
used to define ‘face’ cells as responding at least two-fold stronger to faces than to other objects, 25% of the
neurons recorded could be classified as ‘voice’ cells. Finally, we evaluated the response selectivity to individual
stimuli within the MVocs, and found that units in the voice area responded to an average of 27% of the MVocs
stimuli.
Our results suggest that a strong fMRI activity preference need not result from a large proportion of highly
selective neurons, and describe a population of neurons with a preference for voices over other complex natural
sounds. The proportion of identified ‘voice’ cells is comparable to what the majority of studies on ‘face’ cells
report. However, ‘voice’ cells seem to be more selective for individual voices than ‘face’ cells, which have been
shown to respond to ~62% of the face stimuli. This divergence in functional properties between ‘voice’ and ‘face’
cells may reflect evolutionary differences that have affected voice- and face- specialization in primate brains.
Namely, the visual system appears to have specialized during vertebrate evolution to represent canonical facial
features (e.g., two eyes, a nose and a mouth). By contrast, the auditory system could have had less opportunity to
specialize for canonical auditory features, given that many animals modify the acoustics of their vocalizations to
be distinct from those of other animals and to circumvent environmental noise.
