The role of brain lateralization on ordinal tasks: An investigation on day-old domestic chicks (Gallus gallus)

Numerical abilities are a crucial aspect of cognition involved in many of the everyday choices. The studies in this field have grown rapidly and recent research with both human and non-human animals demonstrated that there is an association between the representation of numbers and space (spatial numerical association, SNA), that relates small numbers with the left and large numbers with the right as in the human mental number line (MNL). The effects of this phenomenon can be inferred from the SNARC effect and from a left-oriented bias in ordinal tasks in comparative research. Latest studies investigated in particular the association between the SNA and the hemispheric lateralization in the domestic chick (Gallus gallus). Chicks represent a great model for this type of work due to their precocious abilities and also the possibility of the lateralization manipulation through light exposure during the last incubation days. Previous collected data show that the degree of hemispheric lateralization affects ordinal-spatial-numerical performances. In particular, a lower lateralization degree prevents the left bias and doesn't affect overall numerical abilities even though higher lateralized chicks perform better. This findings are in line with the hypothesis that numerical information is processed bilaterally by both hemispheres while spatial information is preferentially processed by the right hemisphere. The aim of this study was to test how pre-hatching light stimulation affects chicks' performance in a purely ordinal task . The spatial cue in this study is absent, as spatial information is made uninformative through the constant variation (at each trial) of the inter-element distance, hence allowing only the numerical information processing. We expected a better performance by strongly lateralized chicks and the extinction of the bias in all of the subjects. Both experimental groups (strong and weak lateralized chicks) underwent four ordinal test: one in which target elements were sagittally-oriented and three with a fronto-parallel alignment. For the front-parallel disposition, besides a binocular condition, a monocular occlusion technique was used to disentangle each hemisphere processing for both sides. Interestingly, we found that there is no longer difference between hatch conditions without the spatial information while the prevention of left bias in both groups was observed.