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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.

Face perception relies on activation of a complex set of different neural modules. In this study, we assessed the stimulus selectivity of the occipitotemporal N250 ERP component and the possible link between its neural substrates and modules underlying preceding (N170/VPP) and following (P400) category selective ERPs. We recorded N250 during face and leaf detection tasks while we varied stimulus visibility from trial to trial by using a backward masking paradigm. Our results revealed that N250, but not the other tested potentials, was exclusively sensitive to the visibility of faces even when the non-face stimuli served as the task target. We also found a correlation between evoked N170 and N250, in response to face stimuli and to a lesser extent in response to other non-face objects, irrespective of the subjects' task. Besides N250, P400 also showed a strong correlation with N170, but here, the amount of correlation was not affected by stimulus category. Interestingly, despite N250 and N400 correlation with N170, we did not find any correlation between N250 and P400, suggesting that modules underlying these ERP components belong to two different face-processing pathways. We suggest that N250 is initiated by N170 and indexes processes exclusively responsible for encoding faces.