Following an early suggestion byMach [Mach E. The analysis of sensations. Chicago: Open Court Publishing
House; 1897 [reprinted by Dover Publications, 1959]] it has been claimed that brain asymmetrywould be
crucial for biological organisms in order to discriminate left fromright.However, direct evidence in support
of this hypothesis is scanty. In the animal system model provided by the newly hatched domestic chick
(Gallus gallus) it has been proved feasible to manipulate lateralization on visual tasks by exposing the eggs
to light for a brief period before hatching. The light exposure leads to the development of lateralization of
some visual functions and generates asymmetry in the thalamofugal visual projections to the forebrain,
because the late-stage embryo is turned in the egg so that it occludes its left but not its right eye. Thus,
it is possible to obtain organisms with strong (light-incubated chicks, Li-chicks) or weak (dark-incubated
chicks, Di-chicks) lateralization. Li- and Di-chicks were trained to discriminate between two small beads
for food reward on the basis of their relative left–right position. Li-chicks performed better than Di-chicks.
In order to check whether Di-chicks showed a general impairment in discrimination learning, not confined
to left–right discrimination only, chicks were tested in a spatial re-orientation task in a square-shaped
arena, in which a target located in a corner could be identified using the left–right location of a conspicuous
cue (the colour of awall) which could be used as a landmark. Results showed that Di-chickswere impaired
with respect to Li-chicks in use of left–right information for re-orientation, but not in use of the cue as a
landmark. These results provide direct evidence that modulation of the strength of visual lateralization
may affect left–right discrimination abilities.
