Beyond numerical quantity: A study of the sub-base-five effect in single-digit comparison

Previous findings suggest that the mental representations involved in symbolic number comparison carry information not only about the numerical quantity they denote, but also about higher-order properties. While the time to compare pairs of digits increases continuously with numerical size, former studies also revealed a categorical effect characterised by a step increase in response times for pairs with at least one digit >5 compared to those including only digits ≤5. In line with the embodied cognition view, this step increase was interpreted as an inheritance from finger counting practices. This "sub-base-five" effect would find its origin in the need to generate the mental representation of two hands to process numbers >5, whereas one hand is sufficient to represent numbers ≤5. We provide evidence against this interpretation using new analyses designed to uncouple the continuous effect of numerical size and the categorical sub-base-five effect. We demonstrate that the time to compare two Arabic numerals is primarily determined by numerical size, with no steeper increase between pairs with or without number >5. We explain that the sub-base-five effect in digit comparison was likely an artefact of the modelling method previously used to control for numerical size. We nevertheless identified a robust response time increase when comparing 5 and 7, which could not be accounted for by numerical size. These findings question the existence of a sub-base-five effect, which was considered as a marker of embodied cognition, but prompt interest for a previously unnoticed effect emphasizing the special status of particular prime numbers 5 and 7 in symbolic number comparison.