Syntax-driven number reading: The identification of digits is dominated by the number’s syntactic structure

Reading aloud multi-digit numbers is a surprisingly difficult cognitive operation that involves several visual and verbal processes. The main challenge is the need to handle the number’s syntactic structure, however, the extent to which this syntactic processing guides the number-reading process is still unknown. Here, I asked whether syntax-driven processing exists even in the initial visual stage that identifies each digit, via a mechanism that groups the digits according to language-specific conventions (triplets in most western languages: 23,456). Participants read aloud multi-digit numbers presented with a purely visual manipulation: the digits of each number were presented serially, and the inter-digit stimulus-onset-asynchrony (SOA) was constant except one prolonged SOA – either between the thousand and hundred digits, congruent with the standard division of numbers to triplets, or between the hundred and decade digits (incongruent grouping). Accuracy was higher in the congruent condition than in the incongruent one, indicating that the visual analyzer divides the digit string into triplets in the “standard” manner. Critically, this congruency effect was found not only when the participants read the numbers as syntactically-structured number names (“twelve”), but also when they said each number as a syntax-less series of digit names (“one, two”), i.e., triplet-grouping is not a top-down effect arising from the syntactic properties of the current verbal response. I conclude that the visual analyzer consistently divides the digit strings into “standard” triplets, and that this division guides the digit-identification process.