Seiss and Praamstra observed an increased PCE in PD patients relative to healthy controls, while the time course of the NCE seemed to be normal 9. To the best of our knowledge, only two studies from the same group examined the effects of PD on the PCE and NCE 9, 10. It is hypothesized that the former results from a prime-induced motor activation while the latter involves inhibition of the prime-induced activation 23. Two consecutive effects can be observed behaviorally when manipulating the direction compatibility between prime and target stimuli and the interstimuli interval (ISI): an initial positive compatibility effect (PCE) at ISI-0 (i.e., shorter RT for compatible than for incompatible trials), followed by a negative compatibility effect (NCE) at longer ISI (i.e., longer RT for compatible than incompatible trials). In this task, participants are asked to make rapid button presses with the left or right hand following leftward or rightward pointing arrows, which are preceded by a brief subliminal masked prime arrow. This task has been intensively used in order to study automatic and unconscious motor processes elicited by external stimuli presented below the threshold of awareness 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22. However, a number of studies using subliminal stimuli have questioned the assumption that the basal ganglia involvement in motor selection is restricted to situations in which the stimuli are perceived consciously 7, 8, 9, 10, by using a subliminal masked prime-arrow paradigm derived from the Eriksen Flanker Task. For example, in the Eriksen Flanker task 4 in which participants are asked to respond to the direction of a target arrow, appearing with flanking arrows that point to the same or opposite direction, greater response interference effects were observed in PD patients than in healthy aged controls 5, 6.
Indeed, basal ganglia disorders such as PD are associated with response selection impairments. The decrease in dopaminergic nigrostriatal terminals, which is typically more pronounced in the posterior aspects of the striatum in the hemisphere contralateral to the most clinically affected body side 2, is believed to lead to an imbalance between cortico-basal circuits that regulate movement facilitation and inhibition 3.Ī related role of the basal ganglia is to implement response selection. Severe striatal dopamine loss secondary to the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc) 1 is considered as the main pathological hallmark of PD. Parkinson's disease (PD) is the second most common adult-onset progressive neurodegenerative disorder.
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