# Does the Brain Care Which Direction We Read? A Cross‐Cultural tDCS Study on Functional Lateralization of Number Processing

## TL;DR

This study explores how cultural reading direction might influence brain lateralization in number processing using tDCS on Iranians and Germans.

## Contribution

It is the first cross-cultural tDCS study on the functional lateralization of number processing.

## Key findings

- No overall effect of tDCS stimulation was found on number processing.
- tDCS over the left and right IPS facilitated number processing more in Iranians than Germans.
- Number processing may be bilaterally represented in the IPS, independent of cultural reading direction.

## Abstract

The potential influence of culture on functional lateralization was rarely investigated, yet it may be an important factor in our understanding of the human brain. In numerical processing, evidence was found for differential directional preferences of space–number associations in cultures with opposite reading direction systems. This may affect finger‐counting preferences like the starting hand, which in turn have previously been associated with differing lateralization. Such studies raise the question of whether number culture may also play a distinct role in the lateralization of the intraparietal sulcus (IPS), the hallmark region of numerical magnitude processing.

In our preregistered cross‐cultural study, we applied anodal transcranial direct current stimulation (tDCS) over the left versus right IPS to investigate the effect of stimulation as compared to sham in Iranians (with right‐to‐left reading system) and Germans (with left‐to‐right reading system).

Results indicated no overall effect of stimulation; however, exploratory analyses revealed that tDCS over the left and right IPS facilitated number processing in Iranians compared to Germans after controlling for training effects. Finger‐counting direction was not found to be decisive for this effect.

At the end, number processing might be bilaterally represented in the IPS; however, our exploratory analyses emphasize the need for further investigation on the potential role of culture in the representations of numbers.

This is the first cross‐cultural transcranial direct current stimulation (tDCS) study on functional lateralization of number processing. The cultural differences in reading direction did not influence functional lateralization of the intraparietal sulcus (IPS), pointing at a bilateral representation of number processing independent of culture.