Abstract: Task switching is the basic method in human cognitive activities, and the switching process is an important aspect of executive control, which is responsible for controlling the switching course between two tasks competing for the same cognitive resources in working memory. Switching costs refer to longer reaction times for switching items (e.g., AB), as compared to repeating items (e.g., BB) as an indicator of executive control. The course of switching is involved in both endogenous preparation without an external stimulus and exogenous adjustment in response to the external stimulus.
The event-related potentials (ERPs) of participants were recorded to investigate task switching, by using the “task-switching” paradigm. Tasks included a simple classification of Chinese characters (noun/verb) or that of digits (odd/even). For each task, the stimulus picture consisted of one Chinese character and a two-digit natural number on the same background (red and blue were used for the digit and character tasks respectively). We aimed to detect task switching under foreknowledge conditions. Participants consecutively performed two tasks (tasks 1 and 2) that involved either task repetition (i.e. AA、BB,…) or task switching (i.e. AB、BA,…), including four blocks respectively. All these blocks were presented with the sequence ABBAABBA. The participants were 17 healthy and right-handed undergraduates (10 males and 7 females). Their eyesight was either normal or corrected-to-normal, and their ages ranged from 19 to 23 (mean age = 20.24). An ANOVA with repeated measures was used to analyze the data.
The ERPs of task 1 in repeating trial and switching trials (e.g., AA to AB) were analyzed, and the ERPs of task 2 in repeat tasks and switching tasks (e.g., AA to BA) were also analyzed. The authors found that the differences between ERPs in task 1 were significant at about 300 ms after the stimulus onset. There was a late slow negative-going wave that was larger for switching trials than for repeating trials during 500~800 ms. A larger P3b deflection was found for repeating tasks than for switching tasks, and the Chinese characters were different from numerals in the cerebral regions. In Cz, at about 320 ms, the switching trials evoked a larger negative wave as compared to the repeating trials in task 1; and in task 2, the switching tasks produced a larger negative-going wave as compared to the repeating tasks. The difference wave was a N320.
These results suggest the following: (1) under foreknowledge conditions, for simple task switching, the endogenous preparation for task 2 started at 300 ms after the appearance of task 1; (2) the P3b in the switching tasks may reflect the differences in the frequency of the occurrence of these events but not the differences in the cognitive resources of task switching. Moreover, the N320 reflected the cognitive conflict that may be the main source of the switching cost; (3) the material specificity during task switching may be one of the reasons that lead to the difference in the same experimental paradigm

Key words: task switching, switching cost, endogenous preparation, exogenous adjustment, ERPs