Number line estimation under working memory load: Dissociations between working memory subsystems

The preverbal representation of quantity has been shown to associate with space, as quantities are spatially mapped on a mental number line. One traditional method to test this association is the number line estimation task that asks participants to locate a number on a number line. However, current approaches suggest that number line estimation task performance involves verbally mediated strategies such as reliance on reference points, questioning the nature of the task as a measurement of pure quantitative or spatial skills. To resolve this conflict, in the current study participants performed the number line estimation task in three dual task conditions under phonological, spatial or visual working memory (WM) loads. We found that phonological WM load and spatial WM load affected performance, while visual WM load did not. Model fits indicated that number line estimation task performance was based on the usage of reference points, and involved phonological WM and spatial WM which are required for the understanding of symbols and the spatial relation between them.