Time–frequency analysis of the event-related potentials associated with the Stroop test

• Three time-frequency components are sensitive to the interference in Stroop test.
• The frontal total theta amplitude is larger in the incongruent condition.
• The evoked delta amplitude is larger in the congruent condition.
• The occipital alpha desynchronization persisted longer in the incongruent condition.

Multiple executive processes are suggested to be engaged at Stroop test, and time–frequency analysis is acknowledged to improve the informative utility of EEG in cognitive brain research. We aimed to investigate event-related oscillations associated with the Stroop test. EEG data was collected from 23 healthy volunteers while they performed a computer version of Stroop test. Both evoked (phase-locked) and total (phase-locked + non-phase-locked) oscillatory responses in the EEG were analyzed by wavelet transform. Data from the congruent (color–word matching) and incongruent stimuli (color–word non-matching) conditions are compared. In the incongruent condition, N450 wave was more negative and amplitude of the late slow wave was more positive. In the time–frequency plane, the fronto-central total theta amplitude (300–700 ms) was larger in the incongruent condition. The evoked delta (250–600 ms) was larger in the congruent condition particularly over parieto-occipital regions. The larger frontal theta response in the incongruent condition was associated with the detection of interference and inhibition of the response to task-irrelevant features, while the larger evoked delta in the congruent condition was suggestive of the easier decision process owing to congruency between the physical attribute and the verbal meaning of the stimuli. Furthermore, in the incongruent condition, amplitude of the occipital total alpha in the very late phase (700–900 ms) was smaller. This prolonged desynchronization in the alpha band could be reflecting augmentation of attentional filters in visual modality for the next stimulus. These multiple findings on EEG time–frequency plane provide improved description of the overlapping processes in Stroop test.