Contrast Effect on the P300 Component: Passive Versus Active Tasks

OBJECTIVE: Contrast perception could be influenced by top-down factors. We measured P300 component, a well-documented index for top-down influence in visual functions, to examine such effect. METHODS: Twelve participants, 18- to 50-years old, with normal or correct-to-normal visual acuity participated in the study. Standard two-stimulus oddball paradigm was employed. Target stimuli were Gabor patch with 4 cy/degree spatial frequency and 90% contrast. In an attempt to mimic clinical population, event-related potential (ERP) was recorded in (1) the passive viewing condition, in which the participants were instructed to fixate at the center of screen; and (2) the active condition, in which the participants were required to press a button to indicate the presence of a target. Participants performed passive task before active task. The probability that a target was presented in a trial was either 50% or 30% in a run. P300 oddball effect was defined as the difference in ERP between 30% and 50% probability conditions. RESULTS: A reliable P300 oddball effects was found under active condition. For passive viewing condition, P300 oddball effects were not apparent. CONCLUSION: The findings support the hypothesis that P300 reflects the task-dependent top-down effect in visual functions.