Inhibitory control influences number-space associations in atypical young adults with ADHD

Evidence for number-space associations comes from the spatial-numerical association of response-codes (SNARC) effect, consisting in faster reaction times (RTs) to small/large digits with the left/right hand respectively. The SNARC effect is, however, characterized by high inter-individual variability, depending amongst others on inhibition capacities. Hoffmann et al. (2014) showed that individuals more sensitive to the interference of irrelevant information in the classical color-word Stroop task displayed stronger number-space associations. This relation was most pronounced in elderly, but did not reach significance in young healthy adults.

To determine whether the negligible correlation in the young resulted from their near ceiling performances on the color-word Stroop task, we recruited young adults featuring atypically weak and variable inhibitory control. Our study population consisted of individuals (n=32; 18 females; age=27.28 years) formally diagnosed with attention-deficit/hyperactivity disorder (ADHD; n=4) and/or displaying symptoms consistent with ADHD according to the Adult ADHD Self-Report Scale (ASRS-v1.1; n=29).

Within this population, a significant negative correlation (r=-0.45; p=0.009) could be observed between the parity SNARC effect (mean slope=-14.17; p<0.001) and Stroop interference, as indexed by the color-word Stroop ratio score (i.e. the difference in RTs between the color-word interference condition and the color naming condition divided by the RT in the word reading condition; mean ratio=0.82). The relationship remained significant even after controlling for arithmetic performance and general processing speed, as assessed using the arithmetic battery (Rubinsten & Henik, 2005; Shalev et al., 2001; mean accuracy=84.61%) and a speeded matching-to-sample task respectively (mean RT=671.86ms; r=-0.47; p=0.008).

Our findings thus reveal that stronger number-space associations are associated with weaker Stroop inhibitory control in young adults with atypical attentional profiles, thereby further confirming the similarities between SNARC effects and Stroop-like interference effects.