How short-term memory supports number processing

The involvement of Short-Term Memory (STM) in mathematical and numerical processing has been demonstrated repeatedly but its precise role remains unclear. Here, I examined the specific STM mechanisms involved in a fundamental number processing skill: handling symbolic numbers, in particular when writing multi-digit numbers to dictation. Participants heard 3- to 6-digit numbers and wrote down each number on paper with a digitizing pen. As shown previously, the inter-digit temporal gaps follow a consistent hierarchical pattern, reflecting the number’s internal hierarchical representation. Critically, when I imposed a delay between hearing the number and writing it down, thus forcing the participants to maintain the number briefly in an STM buffer, the hierarchical pattern diminished. I conclude that the STM buffer they used was (1) not hierarchical and (2) located in a processing stage that follows the hierarchical representation. When using the buffer (due to the imposed delay), the response was dominated by its flat structure rather by than the hierarchical structure of the preceding processing stage. I propose that this buffer is a Digit Output Buffer that maintains the digit string before writing it down. This means that, similar to the processing of words, number processing is supported by several format-specific STM buffers: some maintain phonological information, as shown in previous studies, and others maintain digits, as shown here.