Prior to hatching, embryos of most avian species keep their
head turned to the right such that the left eye is occluded by
the body and the right eye is close to the translucent shell and thus exposed to light stimulation (Kuo, 1932). Light exposure
of pigeon eggs induce the establishment of visual lateralisation
with a superiority of the right eye in object discriminations,
while dark incubation prevents the emergence of this asymmetry
(Rogers, 1982; Skiba et al., 2002). The same result is
achieved by right-sided monocular deprivation after hatching
(Manns and Gunt ¨ urk ¨ un, 1999b ¨ ). Thus, light incubated pigeons
show a population asymmetry (all or at least most individuals
are biased into the same direction) in diverse visual discrimination
tasks (see Gunt ¨ urk ¨ un, 2002 ¨ , for review). The ‘loss’ of
asymmetry in dark-incubated pigeons could be due to two phenomena.
The first is the replacement of population asymmetry
by individual asymmetry. In this case each pigeon is lateralised
but about half of them have a right eye bias while the other half
is skewed to the other side. The second possibility is that no
individual animal shows a substantial left–right difference. A
detailed analysis showed that indeed, dark incubation produces
the second alternative, i.e. these pigeons show no individual bias
to the right or the left eye in object discriminations (Skiba et al.,
2002).