Chicks (Gallus gallus) learned to run from a starting box to a target located at the end
of a runway. At test, colourful and bright distractors were placed just outside the starting box.
Dark incubated chicks (maintained in darkness from fertilization to hatching) stopped significantly
more often, assessing more the left-side distractor than chicks hatched after late (for 42 h during the
last three days before hatching) or early (for 42 h after fertilization) exposure to light. The results
show that early embryonic light stimulation can modulate this particular behavioural lateralization
comparably to the late application of it, though via a different route.
