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At the roots of Spatial Numerical Association: from behavioural observation to Neural Basis

Periodic Reporting for period 2 - SNANeB (At the roots of Spatial Numerical Association: from behavioural observation to Neural Basis)

Reporting period: 2020-10-01 to 2021-09-30

Number knowledge and processing are fundamental for everyday living. In humans, numerical abilities are present pre-linguistically in infancy. Qualitatively and quantitatively similar mathematical performance has been observed in different species, including human adults, consistent with a shared, ancestral, nonverbal numerical mechanism. Uniquely human mathematical abilities appear to be scaffolded on this ontogenetically and evolutionarily ancient “number sense”. A peculiar characteristic of numbers concerns their strong association with space: humans represent numbers on a mental number line oriented from left to right. Traditionally, spatial numerical association, SNA, has been considered a by-product of culture. Recently, evidence on the SNA in animals suggests it originates from pre-linguistic and biologically determined precursors.
In the SNANeB project, I studied the origin of SNA in monkeys and day-old domestic chicks, thus addressing the SNA’s origin in the absence of language or culture. Better understanding of the brain mechanisms behind SNA would be critical for disentangling the wellspring of number-space mapping and for understanding how this could affect mathematical reasoning, providing a valuable early intervention for infants with potential problems in mathematical comprehension.
The first aim of our initial study was to understand whether numerical magnitude in monkeys prompts attention leftward or rightward. Young and naive monkeys learned to respond to a target, which appeared in different positions on a monitor. When the stimuli did not convey numerical information, monkeys equally responded on either side. Whenever they were presented with increasing numbers, monkeys showed either a left or a right bias. Though present in monkeys, spatial numerical association, SNA, is either left-to-right or right-to-left oriented, depending on the individual.

Our second objective was to determine whether action selection is modulated by numerical magnitude. New young and inexperienced monkeys learned to select a target, which could appear in a random location on a monitor. In probe trials, we varied the number of dots from two to 10, and the target’s ordinal position; 1st, 2nd, 3rd or 4th. When the series comprised a small number of dots (two), monkeys performed better when the target was on their left. When the series comprised a larger number (six or ten), they identified more correctly targets on their right side. A series of control experiments showed that monkeys use both spatial and numerical cues, supporting our hypothesis that spatial and numerical information are strongly associated.

Our third objective was to explore brain lateralization’s role in determining a left-to-right oriented SNA, by comparing performance of animals characterized by different levels of brain lateralization. We used the domestic chick as our animal model, because its level of brain lateralization can be easily and non-invasively determined, by controlling light exposure on chicken embryos: chicks hatched from light incubated eggs are strongly lateralized, while this lateralization is largely prevented in darkness-incubated chicks. Strongly lateralized chicks performed better in a numerical task than weakly lateralized birds. Remarkably, strongly lateralized chicks showed a left-to-right oriented bias — while weakly lateralized chicks showed no SNA-like bias. This evidence shows that prenatal experience modulating brain lateralization affects numerical cognition and SNA.

Our fourth objective was to disentangle the engagement of either hemisphere in dealing with numerical tasks and in determining the SNA-like bias. Hence, we exploited the monocular occlusion technique, restricting visual input to one eye using a simple eyepatch. Since avian brains have no corpus callosum and display a virtually complete decussation of fibres at their optic chiasm, by restricting visual input to a single eye, we determined the contralateral hemisphere’s functioning. When the left hemisphere oversaw processing, chicks succeeded but revealed no SNA-like bias. Whenever the right hemisphere oversaw processing, chicks succeeded and showed an SNA-like bias like those observed when the two hemispheres collaborate in processing information. This evidence indicates that in normal, binocular condition of vision, the right hemisphere takes control of animal actions.

Scientific results of these four main studies have been communicated through scientific peer reviewed papers, with other manuscripts under review or in preparation, and were presented at international scientific conferences and workshops.

To disseminate the most innovative findings of this research project, I issued a press release, which resulted in the dissemination of our results in the media. Dissemination was also conducted during the European Researchers' Night and in an interactive event, organized at the Esapolis Museum in Padua, Italy, directed to children and their families.
The outgoing phase of the SNANeB project enlarged our knowledge on spatial numerical association, SNA, in non-human primates. We demonstrated that monkey showed consistent tendencies for spatial representations of numerousness and that numerical magnitudes modulate responses on ordinal targets. Monkeys better identified left targets when responding to small numerical magnitudes and right targets when responding to larger ones. The first two studies were based on an innovative method, which integrates SNA and ordinal identification, providing evidence on how such a spatially oriented representation of numbers potentially supports counting acquisition.
The returning incoming phase of the SNANeB project meets the challenge to unveil SNA’s neural substrates.
We investigated whether SNA could be affected by brain-lateralization levels. We manipulated brain lateralization level by varying the pre-natal environment, by exposing chicken embryos to light or darkness, which results in strongly or weakly brain-lateralized birds, respectively. We predicted that strongly lateralized chicks would show a stronger SNA than weakly-lateralized chicks. Since traditionally both functional brain specialization and numerical competencies were considered a prerogative of our species, the idea to relate pre-natal experience with mathematical thinking and SNA was thus plausible — although ambitious. We demonstrated that brain lateralization level, determined by a prenatal experience as simple as light exposure affects SNA and numerical performance.
Moreover, we investigated dual-cerebral hemispheres’ role in SNA. In avian brains, their contralateral hemisphere mainly elaborates visual input to each eye. We found that both hemispheres can deal with spatial-numerical tasks. Remarkably, only when the right hemisphere or both hemispheres process information, chicks showed a left-to right oriented SNA.
Overall, this project helps in establishing SNA’s origin and the underlying neural representation. This facilitates optimized design of clinical applications to enhance numerical comprehension. Last but not least, the obtained outcomes as well as the experimental design might provide a valuable early intervention for infants with potential problems in mathematical comprehension, as occurs in Williams syndrome or dyscalculia among others.
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