The impact of inhibition capacities on number-space associations in young and elderly adults

Background: Numerical and spatial representations are tightly linked, i.e. when doing a binary classification judgment on Arabic digits participants are faster to respond with their left/right hand to small/large numbers respectively (SNARC effect, Dehaene et al., 1993). The SNARC effect has been extensively replicated but one of its characteristics remains inter-individual variability (Wood et al., 2006). Different sources have been proposed to account for the reported inter-individual variability, namely response speed (Gevers et al., 2006), inhibition capacities (Wood et al., 2008) and age (Wood et al., 2008). The present study aims to investigate the impact of inhibition capacities on the SNARC effect in young and elderly adults, controlling for individual general processing speed.
Methods: Two groups of participants were included: young adults, N=28, mean age: 23 years (SD=3.02) and elderly adults, N=46, mean age: 65.9 years (SD=3.9). Participants performed a parity judgment SNARC paradigm as well as inhibition tests (Stroop, Incompatibility). General processing speed was evaluated using a simple shape matching task.
Results: The two age-groups differed in the strength of the SNARC effect, inhibition capacities and processing speed, with the elderly adults displaying stronger SNARC effects, weaker inhibition capacities and slower processing speed. Correlation analysis including all participants confirmed these findings on an individual level by showing relations between the SNARC effect and age, as well as relations between the SNARC effect and both inhibition capacities (i.e. the Stroop effect) and processing speed. When controlling for processing speed, the relations between the SNARC effect and both inhibition capacities and age remained. Conversely, when controlling for inhibition capacities, only the relation between the SNARC effect and age (but not processing speed) remained significant, even when controlling in addition for processing speed.
Relevance: By combining the variables age, inhibition capacities and individual processing speed, the present data are the first to reveal a strong link between inhibition capacities and number-space associations. Importantly, we demonstrate that this link is not mediated by general processing speed. Interestingly, the robust relation between the SNARC effect and age remains after controlling for processing speed and inhibition capacities, pointing to a new source of inter-individual differences in the strength of the SNARC effect that will need to be clarified in future research projects.