Pre-hatching light stimulation initiates and/or modulates
visual lateralisation in birds (Rogers, 1982, 1990; Skiba et al.,
2002), but it is unclear how much light input is actually needed
to induce an asymmetry of brain function. According to Rogers
(1982), 2 h of light exposure within the last days before a hatch
was sufficient in dark-incubated chicken eggs to establish visual
lateralisation. Because this study did not aim to define the minimum
amount of required light stimulation, shorter periods were
not tested. As will be argued below, it is very likely that time
periods even considerably lower than that observed in the present
semi-natural study suffice to alter neural systems