We investigated response activation and suppression processes in Parkinson's disease patients with freezing of gait (FOG). Fourteen freezers, 14 nonfreezers, and 14 matched healthy controls performed the attention network task (ANT) and the Stroop task. The former task has more stimulus-response overlap and is expected to elicit stronger irrelevant response activation, requiring more inhibition. Congruency effects were used as a general measure of conflict resolution. Supplementary reaction time (RT) distribution analyses were utilized to calculate conditional accuracy functions (CAFs) and delta plots to measure response activation and suppression processes. In agreement with previous research, freezers showed a general conflict resolution deficit compared with nonfreezers and healthy controls. Moreover, CAFs pointed to a strong initial incorrect response activation in FOG. As expected, conflict resolution impairment was only apparent in the ANT, and not in the Stroop task. These results suggest an imbalance between automatic and controlled processes in FOG, leading to a breakdown in both motor and cognitive response control.

We examined the influence of task complexity on implicit sequence learning in secondary-school-aged children with developmental dyslexia (DD). This was done to determine whether automatization problems in reading extend to the automatization of all skill and depend on the complexity of the to-be-learned skill. A total of 28 dyslexic children between 12 and 15 years and 28 matched control children carried out two serial reaction time tasks using a first-order conditional (FOC) and second-order conditional (SOC) sequence. In both tasks, children incidentally learned a sequence of hidden target positions, but whereas FOC sequence learning could be based on knowledge about the immediate preceding position, SOC sequence learning required more complex knowledge about the previous two positions. The results demonstrated that sequence learning was highly comparable in dyslexic and control children, regardless of the sequence complexity. This shows that implicit sequence learning, as manifested in the present study, is maintained in DD and is unrelated to task complexity. We suggest that previous reports of sequence-learning deficits in DD can be accounted for by attenuated explicit sequence learning, possibly related to malfunctions in prefrontal processing. The present findings indicate that deficits in skill learning and automatization in DD are not general in nature, but task dependent.

No abstract

We investigated the influence of implicit learning on cognitive control. In a sequential Stroop task, participants implicitly learned a sequence placed on the color of the Stroop words. In Experiment 1, Stroop conflict was lower in sequenced than in random trials (learning-improved control). However, as these results were derived from an interaction between learning and conflict, they could also be explained by improved implicit learning (difference between random and sequenced trials), under incongruent compared with congruent trials (control-improved learning). Therefore, we further unraveled the direction of the interaction in 2 additional experiments. In Experiment 2, participants who learned the color sequence were no better at resolving conflict than participants who did not undergo sequence training. This shows that implicit knowledge does not directly reduce conflict (no learning-improved control). In Experiment 3, the amount of conflict did not directly improve learning either (no control-improved learning). However, conflict had a significant impact on the expression of implicit learning, as most knowledge was expressed under the highest amount of conflict. Thus, task-optimization was accomplished by an increased reliance on implicit sequence knowledge under high conflict. These findings demonstrate that implicit learning processes can be flexibly recruited to support cognitive control functions.

People react more quickly and more accurately to stimuli presented in locations corresponding to the response, as compared with noncorresponding locations, even when stimulus location is irrelevant (Simon effect [SE]). The explanation that SEs are caused by the automatic priming of a corresponding response has been questioned, because of the many exceptions to the effect. We replicated practice-induced and sequential modulations of the SE in two experiments-first, by training participants with blocks of location-relevant stimuli, and second, by mixing location-relevant and location-irrelevant trials. The decrease of the SE with incompatible training was relatively permanent in the blocked experiment, whereas the effect was temporary in the mixed experiment. The difference was caused by a more permanent reversal of the SE after incongruent trials, showing that sequential modulations depend on long-term practice effects. We suggest that there is a formation of a contralateral longterm memory stimulus-response link in blocked conditions and that short-term and long-term memory links are primed by preceding events.