Lucia Regolin

Spatial-numerical association reflects an internal "mental number line" where numerosities are mapped from left to right in space. While evidence from 8- to 9-month-old human infants suggests a privileged link between number and space, these findings do not definitively establish whether discrete numerosity or continuous quantity is the primary driver of spatialization due to the relatively extensive postnatal experience. Using domestic chicks (Gallus gallus), a model testable with minimal postnatal experience, we investigated precocial predispositions to map both discrete numerosities and continuous quantity (physical size) onto space. In Experiment 1, we replicated spatial-numerical association: chicks associated relatively smaller numerosities with the left hemispace and larger ones with the right hemispace. In Experiment 2, however, spatial-quantity association for physical size was asymmetric: chicks showed a rightward preference for smaller sizes and no congruent small-left/large-right mapping. Together with infant data, these findings indicate that a generalized and congruent spatial-magnitude association does not emerge at the earliest stages of development, and challenge accounts positing that spatial mappings for continuous magnitudes precede and underpin the spatial organization of number.

Spatial-numerical association reflects an internal “mental number line” where numerosities are mapped from left to right in space. While evidence from 8- to 9-month-old human infants suggests a privileged link between number and space, these findings do not definitively establish whether discrete numerosity or continuous quantity is the primary driver of spatialization due to the relatively extensive postnatal experience. Using domestic chicks ( Gallus gallus ), a model testable with minimal postnatal experience, we investigated precocial predispositions to map both discrete numerosities and continuous quantity (physical size) onto space. In Experiment 1, we replicated spatial- numerical association: chicks associated relatively smaller numerosities with the left hemispace and larger ones with the right hemispace. In Experiment 2, however, spatial- quantity association for physical size was asymmetric: chicks showed a rightward preference for smaller sizes and no congruent small-left/large-right mapping. Together with infant data, these findings indicate that a generalized and congruent spatial-magnitude association does not emerge at the earliest stages of development, and challenge accounts positing that spatial mappings for continuous magnitudes precede and underpin the spatial organization of number.

Numerical cognition is supported by core knowledge that operates independently of language and formal instruction. Two systems have been extensively documented: the Object Tracking System (OTS), which supports exact representations of small sets, and the Approximate Number System (ANS), which encodes numerical magnitude approximately and exhibits ratio sensitivity. In naturalistic settings, individuals are frequently required to move flexibly between small and large numerosities within the same problem-solving context. However, it remains unclear whether numerosities can be simultaneously represented through the ANS system during arithmetic processing and whether transitions between them incur cognitive costs early in development. The present study investigated preschoolers' performance during a computerized non-symbolic arithmetic task involving small (≤ 4) and large (> 4) numerosities, observing the cognitive demands associated with switching between numerosity ranges. Ninety-three children aged 4 to 6 years participated in the study and were randomly assigned to one of three experimental conditions, differing in numerosity presentation orders: Small-to-Large, Large-to-Small or Mixed. Children demonstrated higher accuracy with small numerosities, which is consistent with the precise representational capacities typically associated with the OTS. Additionally, the numerical ratio influenced the performance across presentation orders, highlighting the substantial contribution of the ANS. Notably, preschoolers perform similarly across the different orders in which the problems were presented, suggesting that the transition between numerosities, and thus between systems, does not incur cognitive costs. This highlights the dynamic interactions between the two core numerical systems before formal schooling begins.

Humans across multiple languages spontaneously associate the nonwords “kiki” and “bouba” with spiky and round shapes, respectively, a phenomenon named the bouba-kiki effect. To explore the origin of this association, and whether it is unique to humans, we tested the bouba-kiki effect in baby domestic chickens ( Gallus gallus ). As a precocial species, chicks can be tested shortly after hatching, allowing us to control their pretest experiences. Similar to humans, both 3-day-old [Experiment 1 (Exp. 1)] and 1-day-old (Exp. 2) chicks spontaneously choose a spiky shape when hearing the “kiki” sound and a round shape when hearing the “bouba” sound. Results from naïve young animals suggest a predisposed mechanism for matching the dimensions of shape and sound, which may be widespread across species.

Humans order numerosity along a left-to-right mental number line (MNL), traditionally considered culturally rooted. Yet, some species at birth show spatial-numerical associations (SNA), suggesting neural origins. Various accounts link SNA to brain lateralization but lack evidence. We investigated brain lateralization effects on numerical spatialization in 100 newborn domestic chicks. In ovo light exposure yielded strongly lateralized brains in half the chicks and weakly lateralized in the other half. Chicks learned to select the 4th item in a sagittal array. At the test, the array was rotated 90°, with left and right 4th items correct. Strongly lateralized chicks outperformed weakly lateralized ones when ordinal and spatial cues were reliable (experiment 1), but not with unreliable spatial cues (experiment 2). Moreover, only strongly lateralized chicks showed left-to-right directionality, suggesting the right hemisphere’s key role in integrating spatial and numerical cues. We demonstrate that brain lateralization is fundamental for developing a left-to-right oriented SNA.

Humans order numerosity along a left-to-right Mental Number Line (MNL), traditionally considered culturally rooted. Yet, some species at birth show Spatial-Numerical Associations, SNA, suggesting neural origins. Various accounts link SNA to brain lateralization, but lack evidence. We investigated brain lateralization effects on numerical spatialization in 100 newborn domestic chicks. In-ovo light exposure yielded strongly-lateralized brains in half the chicks and weakly-lateralized in the other half. Chicks learned to select the 4th item in a sagittal array. At test, the array was rotated 90°, with left and right 4th items correct. Strongly-lateralized chicks outperformed weakly-lateralized ones when ordinal and spatial cues were reliable (experiment 1), but not with unreliable spatial cues (experiment 2). Moreover, only strongly-lateralized chicks showed left-to-right directionality, suggesting the right hemisphere’s key role in integrating spatial and numerical cues. We demonstrate that brain lateralization is fundamental for developing a left-to-right oriented SNA.

Humans order numerosity along a left-to-right Mental Number Line (MNL), traditionally considered culturally rooted. Yet, some species at birth show Spatial-Numerical Associations, SNA, suggesting neural origins. Various accounts link SNA to brain lateralization, but lack evidence. We investigated brain lateralization effects on numerical spatialization in 100 newborn domestic chicks. In-ovo light exposure yielded strongly-lateralized brains in half the chicks and weakly-lateralized in the other half. Chicks learned to select the 4 ^th item in a sagittal array. At test, the array was rotated 90°, with left and right 4 ^th items correct. Strongly-lateralized chicks outperformed weakly-lateralized ones when ordinal and spatial cues were reliable (experiment 1), but not with unreliable spatial cues (experiment 2). Moreover, only strongly-lateralized chicks showed left-to-right directionality, suggesting the right hemisphere’s key role in integrating spatial and numerical cues. We demonstrate that brain lateralization is fundamental for developing a left-to-right oriented SNA.

Newborn chicks can solve complex numerical discriminations through a purely perceptual mechanism based on the possibility of creating all equal-sized subsets. The present work investigates whether this is a prerogative of precocial birds or whether an analogous mechanism can be found in humans. We familiarized 8-month-olds with sets of even numerosities (i.e., decomposable in equal-sized subsets), and then we tested them for their spontaneous preference between two novel sets, one that cannot be divided into equal-sized subsets (i.e., a prime numerosity) and one decomposable in equal-sized subsets (i.e., a non-prime odd numerosity). Infants were tested with either the 7vs.9 comparison (i.e., the prime being smaller) or the 9vs.11 comparison (i.e., the prime being larger). Infants oriented their gaze more often toward the prime set of elements, irrespective of whether it was smaller or larger in the comparison, similar to the preference observed in baby chicks. Overall, our results suggest an early-emerging perceptual mechanism that can support complex numerical discriminations, that might be shared between distantly related and ecologically different species.

Sensitivity to numbers is a crucial cognitive ability. Numerical discrimination, defined as the ability to distinguish between different numerosities, allows animals to reduce predation risk, increase foraging efficiency, and maximize food intake, and has been documented in primates, amphibians, fish, birds, and insects. Evidence from day-old chicks suggests that basic numerical competencies are part of a precociously available cognitive toolkit that guides animals in their interaction with the environment. To discriminate between two numerosities, animals could employ either an absolute or a relative strategy. Absolute discrimination requires identifying a specific numerosity (e.g., 10) against others. Relative discrimination consists in selecting the higher (or the lower) numerosity in any pair. Which strategy is preferred differs across species: Humans and fish preferentially adopt a relative strategy, while honeybees apply an absolute strategy. This posed the question how strategy preferences evolve phylogenetically and develop ontogenetically. This study explores whether 3-d-old chicks (Gallus gallus) spontaneously adopt an absolute or relative strategy. During rearing, 124 chicks were exposed to a numerical comparison (5 vs. 10; experiment 1) and learned that one numerosity (10) was associated with food. During testing, which consisted of a 6-min unrewarded observation, each chick was presented with the numerosity associated with food (10, the choice of which would indicate an absolute strategy) alongside an unfamiliar numerosity consistent with the relative strategy (20, which is larger than 10). Based on previous evidence, we hypothesized that chicks would spend more time near the numerosity corresponding to their preferred strategy. Chicks did not show a significant preference on a group level, regardless of whether the higher (experiments 1 and 2) or the lower numerosity (experiment 3) had been associated with food during rearing. Interestingly, chicks exhibited significant individual preferences for one or the other strategy that cannot be explained by the novelty of the stimuli (experiment 4) or by the use of physical variables (experiment 2). These findings suggest either that diversity of numerical discrimination strategies was evolutionary favorable in domestic chickens’ natural environments, or that, at this early stage of ontogeny, young chicks’ strategies are still highly variable and flexible.