Inhibitory Control Influences the SNARC Effect in Tasks without Explicit Reference to Numerical Magnitude

Evidence for number-space associations comes from the spatial-numerical association of response-codes (SNARC) effect, consisting in faster reaction times to small/large digits with the left/right hand respectively. Although the SNARC effect has now been extensively replicated, it is characterized by high inter-individual variability (Wood et al., 2008). For instance, it has been shown to depend on inhibitory control as indexed by the color Stroop effect in the elderly, with individuals having weaker inhibitory control displaying stronger SNARC effects (Hoffmann et al., 2014). Apart from these well-documented inter-individual differences, number-space associations are also influenced by intra-individual factors. Georges et al. (2014) found that in a population of healthy young university students (n=85, 39 females, mean age=23.44 years), the SNARC effect was qualitatively different within single individuals depending on the number-processing task that they performed. While the strength of the SNARC effects were related in a parity and color judgment task (parity slope=-11.58; color slope=-6.79; r=0.36, p=0.001), as well as in the parity and a magnitude comparison task (magnitude slope=-6.98; r=0.36, p=0.001), no relation could be observed between number-space associations in the color and magnitude tasks (r=0.18, p=0.11). These findings indicate that two distinct factors seem to account for the variance related to number-space associations observed during the three tasks. In the present study, we built on these findings while investigating how inhibitory control influences variance in the SNARC effect observed during different numerical tasks. To this aim, we performed a principle component analysis followed by varimax rotation to combine the color and parity SNARC effects (i.e. number-space associations in tasks without explicit reference to numerical magnitude) and the parity and magnitude SNARC effects (i.e. number-space associations in tasks involving semantic number processing) into single factors (color-parity-SNARC and parity-magnitude-SNARC factors respectively). We then investigated how these two extracted SNARC factors were influenced by inter-individual characteristics such as inhibitory control. Inhibitory control was evaluated in a task that involved responding to the color (green or red) of a centrally presented arrow pointing either in the left or right direction by pressing on the left or right hand-side. To get a single inhibitory control measure for each individual, we calculated inverse efficiency scores on compatible and incompatible trials and computed performance differences between those two conditions. The scores of the extracted parity-color-SNARC factor significantly correlated with the inhibitory control measure (μ=109.98ms, SD=85.82ms; r=-0.26, p=0.02), while no relation was observed between inhibitory control and the parity-magnitude SNARC factor scores (r=-0.1, p=0.42). This suggests that individuals with better inhibitory control (i.e. smaller performance differences between compatible and incompatible trials) displayed weaker SNARC effects only in number-processing tasks that required the suppression of an irrelevant numerical (magnitude) code for successful task completion.
Number-space associations are characterized by high inter- and intra-individual variability. We determined how the SNARC effect observed in tasks with and without explicit numerical magnitude processing related to inhibitory control. Individuals with better inhibitory control displayed weaker SNARC effects only in tasks requiring the suppression of an irrelevant numerical magnitude.