Detection of direction of biological motion in domestic chicks is affected by lateralization caused by in-ovo light exposure of chicks' embryos.

Naïve chicks preferentially approach biological motion (a Johansson’s point-light walking-hen) rather than non-biological (i.e., random or rigid) motion displays (Vallortigara et al., PLOS Biology, 2005). Visually-inexperienced light-incubated chicks are spontaneously sensitive to the dynamic relations specified by the kinematics of the motion of the point-light walking-hen (Vallortigara & Regolin, Current Biology, 2006). Chicks oriented themselves in the same direction of the walking-hen only when the display was presented upright, suggesting that the interpretation of biological motion is constrained by a gravity bias which does not require any learning. We found that dark-incubated (i.e., not lateralised, as light exposure of the embryo modulates brain asymmetry in this species) chicks (N=309) also showed sensitivity to the direction of motion of the biological motion stimulus. However, differently from light-incubated chicks, they oriented their bodies in the opposite direction with respect to the upright stimulus walking direction (t(160)=3.981; p<0.001), as if facing rather than following the walking-hen. Similarly to light-incubated chicks, dark-incubated chicks did not respond to the upside-down display (t(147)=0.074; p=0.941).