Asymmetrical number-space mapping in the avian brain

Young chicks (Gallus gallus) are able to use ordinal information to identify the 3rd, the 4th or the 6th target element in a series of 10 identical, fixed and equally spaced elements, sagittaly oriented with respect to the chicks’ body. Interestingly, whenever, during a subsequent generalisation test, the target had to be identified on a series identical to the training one but oriented from left to right, chicks would refer the correct position starting from the left end of the series (Rugani et al., 2007; 2010). This asymmetry could be due to a right hemispheric dominance for the spatial processing required by the test. Though in those experiments numerical and spatial information were intertwined. We therefore devised a series of experiments to disentangle the information coded by the two hemispheres. In Experiment 1, birds were trained to identify a target element solely on the basis of ordinal information. To avoid any possible use of spatial information, the inter-elements distances were changed from trial to trial, throughout training and testing. When solely the ordinal information was available, chicks identified as correct both the target positions from the left (t(11)=4.532; p<0.001) and from the right t(11)=4.504; p<0.001) ends. In Experiment 2, subjects were trained to peck the 4th position in a series of 10 identical fixed and equally spaced positions, sagittaly aligned with respect to the chicks’ starting point. All subjects then underwent two generalization tests (one with a series of 10 and the other of 16 elements). All subjects manifested the leftward bias when 10 (t(11)=10.169; p<0.001), but not when 16 elements (left: t(11)=3.220; p=0.008; right t(11)=3.373; p=0.006) were employed. Data demonstrated that purely ordinal representation seems to be bilaterally represented in the chick brain and activation of this representation would not produce any imbalance in the activity of the two hemispheres. As a result, allocation of attention would be identically directed towards both the left and the right visual hemifields. In contrast, the purely spatial representation would be unilaterally represented in the right hemisphere.