Spontaneous preference for visual cues of animacy in naïve domestic chicks: The case of speed changes

Animacy perception arises in human adults from motion cues implying an internal energy source to the moving object. The internal energy of the object is often represented by a change in speed. The same features cause preferential attention in infants. We investigated whether speed changes affecting adults' animacy ratings elicit spontaneous social preferences in visually-naïve chicks. Human observers evaluated the similarity between the movement of a red blob stimulus and that of a living creature. The stimulus entered the screen and moved along the azimuth; halfway through its trajectory it could either continue to move at a constant speed or linearly increase in speed. The average speed, the distance covered and the overall motion duration were kept constant. Animacy ratings of humans were higher for accelerating stimuli (Exp. 1). Naïve chicks were then tested for their spontaneous preference for approaching the stimulus moving at a constant speed and trajectory or an identical stimulus, which suddenly accelerated and then decelerated again to the original speed. Chicks showed a significant preference for the 'speed-change stimulus' (Exp. 2). Two additional controls (Exp. 3 and 4) showed that matching the variability of the control 'speed-constant' stimulus to that of the 'speed-change stimulus' did not alter chicks' preference for the latter. Chicks' preference was suppressed by adding two occluders on both displays, positioned along the stimulus trajectory in such a way to occlude the moment of the speed change (Exp. 5). This confirms that, for chicks to show a preference, the moments of speed change need to be visible. Finally, chicks' preference extended to stimuli displaying a direction change, another motion cue eliciting animacy perception in human observers, if the speed- and direction-profile were consistent with each other and resembled what expected for biological entities that invert their motion direction (Exp. 6). Overall, this is the first demonstration of social predispositions for speed changes in any naïve model or non-human animal, indicating the presence of an attentional filter tuned toward one of the general properties of animate creatures. The similarity with human data suggests a phylogenetically old mechanism shared between vertebrates. Finally, the paradigm developed here provides ground for future investigations of the neural basis of these phenomena.