OBJECTIVETo investigate the effects of task probability on context processing using AX-type continuous performance test (AX-CPT) and event-related potential (ERP).

METHODSWe set 3 groups of complementary probabilities (AX/AY: 0.55/0.15, 0.35/0.35, and 0.15/0.55) for AX task and AY task and kept the probability 0.15 for both BX and BY tasks. Eighteen subjects were asked to press the button only when the go-probe (X: green "<" or ">") followed the go-cue (A: blue "ooo") and withdraw response when the go-probe or a nogo-probe (Y: red "<" or ">") followed the nogo-cue (B: blue "xxx").

RESULTSAs the task probability increased, the reaction time of AX task was shortened and the rate of false alarm of BX task increased significantly. The probability effects of ERP revealed by statistical parametric mapping occurred in many regions during both cue and probe stages: cue A in the parietal central (300-350 ms) and the left occipital area (450-500 ms); cue B in the right frontal pole (350-500 ms) and the left frontal pole (750-950 ms); AX probe in the right frontoparietal area (200-250 ms), the left frontal pole, the right dorsal prefrontal and bilateral parietal region (300-400 ms), and the bilateral occipital area and parietal region (400-650 ms); AY probe in the fronto-central area (350-500 ms) - the P3 effect; BX probe in the right temporal and the left occipital areas (300-350 ms); BY probe in the left temporal area (150-250 ms).

CONCLUSIONThe task probability not only influences both the encoding of go-cue and expectancy of nogo-cue but also modulates the neural assemblies of probe processing differentially.