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The influence of visual illusion perception on numerosity estimation could be evolutionarily conserved: exploring the numerical Delboeuf illusion in humans (Homo sapiens) and fish (Poecilia reticulata)

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Abstract

Discriminating between different quantities is an essential ability in daily life that has been demonstrated in a variety of non-human vertebrates. Nonetheless, what drives the estimation of numerosity is not fully understood, as numerosity intrinsically covaries with several other physical characteristics. There is wide debate as to whether the numerical and spatial abilities of vertebrates are processed by a single magnitude system or two different cognitive systems. Adopting a novel approach, we aimed to investigate this issue by assessing the interaction between area size and numerosity, which has never been conceptualized with consideration for subjective experience in non-human animals. We examined whether the same perceptual biases underlying one of the best-known size illusions, the Delboeuf illusion, can be also identified in numerical estimation tasks. We instructed or trained human participants and guppies, small teleost fish, to select a target numerosity (larger or smaller) of squares between two sets that actually differed in their numerosity. Subjects were also presented with illusory trials in which the same numerosity was presented in two different contexts, against a large and a small background, resembling the Delboeuf illusion. In these trials, both humans and fish demonstrated numerical biases in agreement with the perception of the classical version of the Delboeuf illusion, with the array perceived as larger appearing more numerous. Thus, our results support the hypothesis of a single magnitude system, as perceptual biases that influence spatial decisions seem to affect numerosity judgements in the same way.

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Data availability

The data that support the findings of this study are available from the corresponding author upon request.

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Acknowledgements

We performed the present work within the scope of the research grant ‘Dipartimenti di Eccellenza’ (art.1, commi 314–337 legge 232/2016) entitled ‘Innovative methods or technologies for assessment, intervention, or enhancement of psychological functions (cognitive, emotional or behavioural)’. We thank Giada Alessi for her help in testing the fish.

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Authors and Affiliations

  1. Department of Biology, University of Padova, Viale Giuseppe Colombo 3 - Via Ugo Bassi 58/B, 35131, Padua, Italy

    Maria Santacà

  2. Department of General Psychology, University of Padova, Padua, Italy

    Umberto Granziol

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  1. Maria Santacà
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  2. Umberto Granziol
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Contributions

MS conceived and designed both experiments, performed the experiments with fish, analysed the data, and drafted the manuscript. UG conceived and designed the experiment with humans, analysed the data, and drafted the manuscript. Both authors gave final approval for publication. All authors read and approved the final manuscript.

Corresponding author

Correspondence to Maria Santacà.

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Conflict of interests

The authors declare no competing interests.

Ethical approval

The experiment with humans was approved by the ethics committee (Protocol no. 3796) of the Department of General Psychology at the University of Padova (Italy). Regarding the experiment with fish, we followed all applicable international, national, and/or institutional guidelines for the care and use of animals (Italy, D.L. 4 Marzo 2014, no. 26). The experiment was in accordance with the ethical standards of the institution where the study was conducted and was approved by the relevant ethics committee (Organismo preposto al benessere animale) of the University of Padova (Protocol no. 32/2019).

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Santacà, M., Granziol, U. The influence of visual illusion perception on numerosity estimation could be evolutionarily conserved: exploring the numerical Delboeuf illusion in humans (Homo sapiens) and fish (Poecilia reticulata). Anim Cogn 26, 823–835 (2023). https://doi.org/10.1007/s10071-022-01721-6

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