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Abstract:
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.
