Is spatial-numerical association driven by numerical magnitude or ordinal position? Evidence from day-old domestic chicks

People tend to associate small numerosity with the left side space and large one with the right side. Such spatial-numerical association (SNA) is not only present in adults and infants, but also in many non-human species, including domestic chicks (Gallus gallus). Previous research demonstrated that chicks associate different ordinal positions and numerosity with space. Specifically, when chicks are trained to peck at target stimulus in a specific ordinal position, they tended to count from the left. Moreover, similar to human, they associate small numerosity with the left side and large numerosity with the right side of space. However, it remains unknown how numerical magnitude and ordinal position interact and contribute to the SNA effect. To address the question, we trained chicks using either an ascending (2-5-8) or a descending (8-5-2) numerical sequence. At test, two panels with stimuli composed of red squares randomly distributed on a white background. During the test phase, two panels with identical stimuli (2 vs.2, 5 vs.5, 8 vs.8) were placed laterally and symmetrically in front of the chicks. Their choices revealed that the ascending group exhibited a general preference to associate numerical stimuli with the left space, while the descending group reported SNA driven by numerical magnitude. Notably, from the analysis of the first two stimuli, SNA driven by numerical magnitude emerged also in the ascending group. In conclusion, both numerical magnitude and ordinal position influence the SNA effect. Crucially, different numerical sequence orders modulated the strength of SNA.