Prefrontal neuromodulation reverses spatial associations of non-numerical sequences, but not numbers

Numerical and non-numerical sequence items interact with spatial responding, pointing towards mental representations that are grounded in space and referred to as SNARC effects (spatial-numerical association of response codes). An ongoing controversy pertains to the universal origin of different SNARC effects and whether their underpinning is a spatial arrangement of cardinal magnitude (mental number line) or a sequential arrangement of ordinal elements in working memory. Recent results from prefrontal neuromodulation with transcranial direct current stimulation (tDCS) were supportive of the unified working memory account. The current tDCS experiment was designed to empirically test the generalizability of the prefrontal modulation effects previously found for numbers in a non-numerical sequence (weekdays) and to examine predictions from the universal account. Participants performed a series of classification tasks with numerical and non-numerical sequences (1-5, Monday-Friday) before and concurrent to a prefrontal stimulation with either anodal (N=24) or cathodal polarity (N=24). Results show a dissociation of SNARC effects for numbers and weekdays by anodal tDCS: Spatial associations of weekdays were reversed by stimulation, when order was relevant for the task, but SNARC effects with number symbols were emphasized in the regular left-to-right direction, corroborating previous results. A control experiment showed that the polarity-dependent neuromodulation effects were absent in order-irrelevant font color classification, supporting the tDCS principle of activity-dependence. We discuss differences in linguistic markedness between temporal and magnitude-related classifications in an integrative account explaining the full pattern. We suggest that stimulation-enhanced psycholinguistic processing can evoke space-number associations whose direction is opposite to cultural visuospatial experience.