Left-right asymmetries in spatial numerical processing. Behavioural evidence from an animal model: the domestic chick (Gallus gallus)

Young domestic chicks, when trained to identify the 4th element in a series of identical elements, and then required to respond to a left/right oriented series, referred as correct the 4th element from the left end, and not the 4th from the right end (Rugani et al. 2007, 2010, 2011). To disentangle the engagement of either hemisphere in dealing with the ordinal task and in determining the leftward bias, visual input was restricted to one eye, so as to determine the functioning of the contralateral hemisphere. Four-days-old chicks (N=10) were binocularly trained to peck at the 4th target element in a series of 10 identical and sagittaly aligned (with respect to the chick’s starting position) elements. At test, the series was rotated by 90°. The test was conducted in three different conditions of vision: binocular, right monocular and left monocular. When binocularly tested, chicks generalized to the 4th element from the left (p<0.01). In right monocular condition chicks generalized to the 4th element from the right (p<0.01) and in left monocular condition to the 4th element from the left (p<0.01). These results indicate that ordinal information is bilaterally represented in the cerebral hemispheres. Whenever both hemispheres are processing the information an extra-activation of the right hemisphere would take place, favoring an allocation of attention into the left hemispace and thus producing a bias to ‘‘count’’ selectively from left to right.