Differential impact of attention on the early and late categorization related human brain potentials

Previous human studies have shown that object-selective attention enhances neural activities evoked in response to target stimuli. However, it is not clear whether the magnitude of activity enhancement is constant or varies according to the level of stimulus visibility. To examine the effect of attentional modulation on different event-related potentials (ERPs) and the relationship between attentional modulation and level of stimulus visibility, subjects were instructed to perform face detection and leaf detection tasks in separate blocks while the level of stimulus visibility varied randomly from trial to trial. As a result of object-selective attention, N170 and P400 ERP components were both modulated in response to the task target category compared to when the same category was used as distracter. We found that the magnitude of modulation of the N170 component was independent of the stimulus visibility level, while the P400 ERP component showed increased enhancement as the level of stimulus visibility increased. These findings demonstrate that attention impacts the neural populations, indexed by early (N170) and late (P400) evoked potentials, differentially and that attentional modulation becomes increasingly dependent on the level of stimulus visibility through the course of brain activities.