| To make effective response to environmental stimuli, human beings need to give selectively attention to different features of a stimulus or switch between different tasks quickly or frequently. Switching from a cognitive task to another is known as task switching which is a basic way of cognitive processing. It has been considered that the task switching is an important aspect of the central executive function, so the study of conversion process could be used to explore the mechanisms of executive control. The task-switching paradigm provides a laboratory tool for studying conversion process. In this paradigm, participants perform a discrete task according to the instruction. On some trials the task changes and on others it does not. It is constantly found that switching between tasks is slower and more error prone than repeating the same task. The increase of RT in task switching is termed a "switch cost", which is related to the dominance (or difficulty) of the task. The cost is greater when switching to the more dominant task than when switching to the less dominant task. This phenomenon is named as asymmetric switch cost (ASC). A few theories have been put forward to account for the mechanisms underlying the ASC. For instance, Allport et al (1994) propose a task-set inertia hypothesis which attributes the ASC to relatively differential negative priming as a form of persistence of previous task set inhibition. However, Yeung and Monsell (2003) argue that there is no inhibition of irrelevant information. They develop a computational model which lays stress on the difference in the benefit of repetition between two tasks of unequal dominance. The controversial problems cannot be solved through analyzing the behavior result simply. Based on the present situation, the current study combined the high temporal resolution of event-related potentials (ERP) technology and the time-frequency analysis to explore the neural mechanisms associated with the generation of the ASC.Previous studies have showed that the ASC is modulated by the type of stimulus. The ASC occurs when the stimulus is incongruent, but disappears when the stimulus is neutral. These studies only examined that the ASC could be impacted by the type of stimulus on current trial. However, it has been observed that the switch cost will be affected by the stimulus type on previous trial, indicating that switch costs are higher after trials with incongruent stimuli than with congruent stimuli. The present study was designed to investigate the relationship between preceding stimulus incongruency and the ASC. We adopted a Stroop-task-switching paradigm, in which each trial contained a pair of successive task stimuli. The first stimulus(S1) of a task pair is previous while the second stimulus (S2) is current. Each stimulus consisted of a word for color printed in black but superimposed on a colored shape. Participants were required to respond to either the color or word of the stimuli according to the rule indicated by the shape. The square indicated the Word task (i.e., respond depending on the name of the word and regardless of the color of the background), and the diamond indicated the Color task (i.e., respond depending on the color of the background and regardless of the name of the word). In repeat trials, the shape of S1and S2are the same. In switch trials, they are different.In the current study, both behavioral and EEG data were recorded from eighteen healthy participants when switching between two tasks. The behavior results showed the interaction between task rule and task transition was only significant in the incongruent S1condition, indicating the ASC was modulated by the incongruency of the previous stimulus. The ERP results showed a larger negative frontal deflection (N2) for switch trials relative to repeat trials (300-400ms after S2onset), which was only present following incongruent stimulus. This finding reflects the involvement of overcoming persisting inhibition at early stage of the task execution. Moreover, a fronto-central SP was larger in switch trials than in repeat trials for the Color task at late stage of task execution (600-900ms after S2onset), indicating that the cognitive control is triggered to resolve the task-set conflict between the more automated word processing and color identify. However, the SP was lower in switch trials for the Word task. This SP modulation may reflect the updating or activation of task-set to select the appropriate S-R mapping before motor execution, after recovering from the persistent inhibition. The time-frequency results reveal lower alpha power (9-11Hz) ERD over right-frontal region was visible for the Word task, suggesting the requirement of attentional resources to overcome persisting inhibition, the right frontal region is associated with this process. A fronto-central upper alpha power (10-13Hz) increase was observed exclusively during the Color task switching may indicate that the fronto-central region is involved in the resolution of task-set conflict and the updating or activation of task-set. In conclusion, our findings provide neural evidence for dissociable cognitive processes, which occur at early or later stage of the task execution, contributing to the asymmetrical switch cost. |
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