Integration of auditory and visual information in human face discrimination in pigeons: Behavioral and anatomical study

Social stimuli are often multi-sensory. Animals commonly recognize a conspecific through visuo-auditory and olfactory sensory cues. In this study pigeons were trained to discriminate between a human face and the face of a Japanese macaque then tested with morphing images of human and macaque faces (Experiment 1). These images were also presented in conjunction with two different auditory stimuli, namely, vocalization of either a human or a macaque. Results showed that the human vocalization facilitated pigeons’ recognition of human faces, but a comparable facilitation of recognition of macaque faces accompanied by a macaque vocalization was not observed. The subjects had previous exposure to human vocalizations (due to living in laboratory cages) but not to the macaque face or vocalization. This difference in experience can explain differences that such auditory stimuli have on visual discrimination in pigeons. In primates, the primary auditory area and the primary visual area connect to each other in the telencephalon. An anatomical study (Experiment 2) provided evidence that visual and auditory pathways were independent at the level of the primary sensory area in the telencephalon (entopallium and field L) suggesting an anatomical difference in visuo-auditory integration between primates and pigeons. Thus, the visual and auditory systems should be integrated in higher associative area, such as nidopallium caudo-lateralis (NCL) or in an earlier stage, such as the midbrain in the avian brain.