Impaired inhibition of prepotent motor tendencies in Friedreich ataxia demonstrated by the Simon interference task

Friedreich ataxia (FRDA) is the most common of the genetically inherited ataxias. We recently demonstrated that people with FRDA have impairment in motor planning - most likely because of pathology affecting the cerebral cortex and/or cerebello-cortical projections. We used the Simon interference task to examine how effective 13 individuals with FRDA were at inhibiting inappropriate automatic responses associated with stimulus-response incompatibility in comparison with control participants. Participants had to respond to arrow targets according to two features which were either congruent or incongruent. We found that individuals with FRDA were differentially affected in reaction time to incongruent, compared with congruent stimuli, when compared with control participants. There was a significant negative correlation between age of onset and the incongruency effect, suggesting an impact of FRDA on the developmental unfolding of motor cognition, independent of the effect of disease duration. Future neuroimaging studies will be required to establish whether this dysfunction is due to cerebellar impairment disrupting cerebro-ponto-cerebello-thalamo-cerebral loops (and thus cortical function), direct primary cortical pathology, or a possible combination of the two.