In the Stroop matching task, a Stroop word is compared to a colored bar. The origin of the conflict presented by this task is a topic of current debate. In an effort to disentangle nonresponse and response conflicts, we recorded electromyography (EMG) and event-related potentials (ERPs) while participants performed the task. The N450 component was sensitive to the relationship of color surfaces, regardless of the response, suggesting the participation of nonresponse conflict. Incompatible arrays (e.g., incongruent Stroop stimuli during "same" responses) presented a substantial amount of double EMG activation and slower EMG latencies, suggesting the participation of response conflict. We propose that both response and nonresponse conflicts are sources of these effects. The combined use of the EMG and ERP techniques played an important role in elucidating the conflicts immersed in the Stroop matching task.
The Stroop-matching task is a variation of the Stroop task in which participants have to compare a Stroop stimulus attribute (color or word) to a second stimulus. The Stroop-matching response conflict (SMRC) represents an interference related to the processes involved in selection/execution of manual responses. In the present study we developed a variation of the Stroop-matching task in which the Stroop stimuli were matched to graspable objects (a cup) with intact or broken handles laterally oriented (Experiment 1) or to colored bars laterally presented (Experiment 2). It allowed testing the presence of the correspondence effect for lateralized handles and bars, and its possible influence on SMRC. Two different intervals (100 and 800 ms) were also included to investigate time-modulations in behavioral performance (reaction time and accuracy). Fifty-five volunteers participated in the study. In both experiments, significant SMRC was found, but no interaction occurred between SMRC and correspondence effect, supporting that the hypothesis of different and relatively independent psychological mechanisms is at the basis of each effect. Because significant facilitation for ipsilateral motor responses (correspondence effect) occurred for graspable objects but not for lateralized bars, the attentional shift/spatial coding view was not able to completely explain our data, and therefore, the grasping affordance hypothesis remained as the most plausible explanation. The time course of facilitation observed in the first experiment and by others indicates the importance of further studies to better understand the time dynamic of facilitation/inhibition of motor responses induced by graspable objects.
Inhibition is a broad construct referring to a set of functions responsible for suppressing stimuli or responses to prioritize more relevant ones. Several paradigms were used to assess different types of inhibition, such as the stop-signal and Stroop-like tasks. The horse-race model is the main theorical model that supports the stop-signal paradigm, which postulates that response inhibition depends on a competition between the processes involved in the emission and inhibition of a motor response. We investigated the assumptions of the horse-race model using a novel primary-task with different levels of difficulty (the Stroop-matching task) to understand how the stop-signal task performance varied according to the demands imposed by the different Stroop conditions. ANOVAs and planned comparisons revealed that the motor inhibition, assessed by inhibition rates and the inhibition-errors rates were influenced by the primary task reaction times (RTs) obtained in the different Stroop conditions. These results confirm that the primary task demand is a major factor that influences response execution/ inhibition in a stop-signal protocol. Moreover, they corroborate the horse-race model even under this new approach in which the primary task was not represented by a simple detection task, but instead required interference control. Altogether, our data indicate the potential use of our "Stroop/Stop" protocol to investigate the interaction between different inhibitory processes involved in each task and elucidate the relationship between different types of inhibition.
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