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

Young domestic chicks and adult Clark’s nutcrackers can learn to identify a target element (i.e. the 4th) in a series of fixed and identical elements, sagittally oriented with respect to the birds’ starting point. A peculiar finding is that whenever birds are required to generalize on a left/right oriented series, birds refer the correct position starting from the left end (Rugani et al. 2007, 2010, 2011). Possibly this leftward bias is a result of right hemispheric dominance in visuospatial tasks, resulting in the left visual hemifield controlling the birds’ behavior (Diekamp et al. 2005; Regolin 2006). Here, to disentangle the engagement of either hemisphere in dealing with the ordinal task and in determining the leftward bias, we used the technique of restricting the visual input to one eye using a simple patch over the other eye (Rogers 1997; Gülbetekin et al., 2007). Since the avian brain does not have a corpus callosum and displays a virtually complete decussation of fibres at the optic chiasm (Csillag & Montagnese 2005), by restricting, the visual input to a single eye, we determine the functioning of the contralateral hemisphere. Four-days-old chicks (N=10) were binocularly trained to peck for food reinforcement at a target element (i.e. the 4th) in a series of 10 identical and fixed elements, sagittaly aligned with respect to the chick in its starting position. At test, the series was maintained identical with respect to the training, but it was rotated by 90° and consequently lying frontoparallel to the bird’s position. The test was conducted in three different conditions of vision: binocular, right monocular and left monocular. When binocularly tested, chicks identify as correct solely the 4th element from the left (Mean=26.00, ES=2.87, t=5.57, p<0.01). In right monocular condition chicks identify as correct the 4th element from the right (Mean=27.50, ES=2.50, t=6.79, p<0.01) and in left monocular condition chicks identify as correct the 4th element from the left (Mean=30.50, ES=3.20, t=7.00, 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.