Orienting asymmetries and the perception of sounds in nonhuman primates and humans

Shedding light on the origin of specific neuronal mechanisms for processing acoustic stimuli is important to understand the evolution of primate communication. One of those specialisations, lateralised processing of speech in humans, is a well established finding. A number of studies suggested that diverse animal taxa also show hemispheric asymmetries in the perception of conspecific sounds. Rhesus macaques (Macaca mulatta), for instance, exhibit a right head-turning bias in response to playback of natural conspecific vocalisations, and a left head-turning bias in response to one heterospecific stimulus and several manipulated conspecific calls. This finding was related to a hemispheric specialisation for processing conspecific versus heterospecific vocalisations. I will present the results of orienting experiments with Barbary macaques (M. sylvanus) living in the enclosure ‘La Foręt des Singes’ in Rocamadour, France. In contrast to rhesus macaques, the subjects showed no orientation preferences in response to conspecific or heterospecific vocalisations. These results added to the puzzling mosaic picture of orienting asymmetries in different mammalian species. Therefore, we initiated a study on humans to substantiate the assumption of a coupling of orienting bias and hemispheric asymmetry in the processing of sounds. We presented human speech, human non-verbal utterances, speech backwards and artificial sounds in an orienting task and also in a localizations task during which we assessed hemispheric lateralization using fMRI. In our experiments, orienting asymmetries in humans were not linked to a preferential processing of conspecifics or heterospecific sounds, indicating that the premise of the above mentioned studies in nonhuman primates cannot be supported.