Reading numbers is hard, and the difficulty is a syntactic one: a descriptive analysis of number-reading patterns in readers with and without dysnumeria

Number transcoding, the ability to convert digits to words and vice versa, is a critical skill in mathematical literacy and in everyday life. While transcoding is known to be difficult for children, it is unclear whether it challenges adults too, and if so, what the source of that difficulty is. Here, we analyzed the number reading performance of 172 neurotypical adults. Their mean error rate was 6.5%, considerably higher than that typically observed in word-reading tasks, indicating that transcoding is a relatively challenging task even in adulthood. To investigate the cognitive origin of this difficulty, we examined the error types produced by these participants, as well as by a second group of 51 adults with a number-reading deficit (dysnumeria, mean error rate of 28.7%). In both groups, most errors reflected corruptions of the number's syntactic structure. Moreover, among adults with dysnumeria, the most common subtype was syntactic dysnumeria. These findings indicate that, as in children, the primary challenge in adult number reading lies in the processing of the number's syntactic structure.