Auditory and Multisensory Perception of Looming Signals by Rhesus Monkeys: A Naturalistic Behaviour Research

Brains have evolved to process information that is important for the survival of animals in their natural environment. Instead of using experimental paradigms that involve artificial or arbitrary stimuli and a high degree of training and conditioning, it would therefore be better to study animals’ natural behavior and reactions using behaviourally-relevant stimuli. One of the most basic needs of animals is to be able to deal with rapidly approaching dangerous objects (predators, competitive individuals or abiotic sources). Animals across the animal kingdom show a bias for detecting looming signals --signals that indicate the rapid approach of objects-- over receding signals, in the visual domain. However, when vision fails, for example in darkness, animals must rely on their auditory system to detect looming motion. Under conditions where both visual and auditory signals can be detected, their bimodal integration can enhance detection and discriminability. We studied rhesus monkeys’ (Macaca mulatta) spontaneous behaviour in response to auditory and multisensory looming stimuli. First, using the head-orientation response, we showed that in the auditory domain, monkeys also have a bias for detecting looming signals over receding signals. We tested the effectiveness of two different auditory motion cues --dynamic intensity and pitch change-- for detecting looming sound sources. Both cues proved effective in detecting looming motion. Second, using the preferential looking paradigm, we showed that monkeys have a natural capacity for integrating auditory-visual looming, but not receding signals. This ability was dependent on the spectral structure of the sound. The results suggest an evolved bias for detecting ecologically relevant looming signals and reveal the power of using naturalistic paradigms for investigating the perception of sensory signals in primates