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.
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