At the root of the left–right asymmetries in spatial–numerical processing: From domestic chicks to human subjects

In this review, we discuss evidence showing that birds (Gallus gallus and Nucifraga columbiana) represent numerical magnitudes as being oriented from left to right. Subjects, trained to identify a target element (i.e. the 4th) in a series of sagittally oriented identical elements, when required to generalise on an identical series oriented spatially from left to right, correctly identified the target element “counting” from the left of the array. Moreover, chicks, when presented with sets of 5 vs. 10 or 6 vs. 9 imprinting objects, which were made to disappear one at a time behind one of two identical screens, spontaneously inspected the screen which occluded the larger set. Interestingly, chicks scored a higher percentage of correct choices when the larger of the two sets was on their right side. Similarities with the phenomenon of the spatially oriented (left to right) number line in humans are discussed. Animal models promise a fresh approach to the understanding of developmental mechanisms underlying the expression of knowledge, offering attractive arguments for doubting the uniqueness of human numerical cognition.